13 CHAPTER 2 REVIEW OF LITERATURE 2.1 GENERAL The use of solar energy for the production of food, fibre and heat has been known to mankind for a long time. Research over the last five decades has also made it possible to produce mechanical and electrical power with solar energy. Although the potential of solar radiation for disinfection and environmental mitigation has been known for years, only last two decades, has this technology been scientifically recognized and researched. When sunlight is used to cause a chemical reaction by direct absorption, the process is called photolysis. If the objective is achieved by the use of catalysts, it is known as photocatalysis. Photocatalysis by titanium dioxide has been demonstrated to be an inexpensive and effective method for treating a variety of organic pollutants in water (Augugliaro et al 2004). The UV radiation required for photocatalytic processes may come from an artificial source or the sun. The artificial generation of UV radiation contributes to a large portion of the operating, capital and maintenance costs of a photocatalytic reaction system because of the utility consumption and periodic replacement of the UV lamps. There is, therefore, a significant economic incentive to develop solar powered photocatalytic reactors. In addition, the environmental impact induced by the use of solar energy is minimal and this renders the photocatalytic process environmentally attractive (Chan et al 2003). The
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13
CHAPTER 2
REVIEW OF LITERATURE
2.1 GENERAL
The use of solar energy for the production of food, fibre and heat
has been known to mankind for a long time. Research over the last five
decades has also made it possible to produce mechanical and electrical power
with solar energy. Although the potential of solar radiation for disinfection
and environmental mitigation has been known for years, only last two
decades, has this technology been scientifically recognized and researched.
When sunlight is used to cause a chemical reaction by direct absorption, the
process is called photolysis. If the objective is achieved by the use of
catalysts, it is known as photocatalysis. Photocatalysis by titanium dioxide has
been demonstrated to be an inexpensive and effective method for treating a
variety of organic pollutants in water (Augugliaro et al 2004). The UV
radiation required for photocatalytic processes may come from an artificial
source or the sun.
The artificial generation of UV radiation contributes to a large
portion of the operating, capital and maintenance costs of a photocatalytic
reaction system because of the utility consumption and periodic replacement
of the UV lamps. There is, therefore, a significant economic incentive to
develop solar powered photocatalytic reactors. In addition, the environmental
impact induced by the use of solar energy is minimal and this renders the
photocatalytic process environmentally attractive (Chan et al 2003). The
14
application of solar powered photocatalytic reactors to treat water
contaminated with organic pollutants holds promise for regions receiving
strong sunlight throughout the year, like India.
Advanced Oxidation Processes (AOPs) involve high cost if compared
to biological treatment processes. However, the use of AOPs is more suitable
when the wastewater to be treated is not easily biodegradable. One
economically viable option to treat wastewater containing non-biodegradable
pollutants consists of combining an AOP, for instance photocatalysis and a
biological post treatment. In this case, the photocatalysis step is used to
enhance the biodegradability of the wastewater, so that it can be more easily
treated biologically. Photocatalysis has been suggested to be feasible and
promising to treat wastewaters containing non-biodegradable wastewaters,
being used as a pre-treatment method to increase the biodegradability (Sarria
et al 2003).
2.2 ADVANCED OXIDATION PROCESSES (AOPs)
Advanced Oxidation Processes (AOPs) generally involve
generation and use of powerful but relatively non-selective transient oxidizing
species, primarily the hydroxyl radical (˙OH), in some vapour-phase
advanced oxidation processes, singlet oxygen has also been identified as the
dominant oxidizing species. Table 2.1 shows that ˙OH has the highest
thermodynamic oxidation potential, hence ˙OH based oxidation processes
have gained the attention of many advanced oxidation technology developers.
In addition as shown in Table 2.2, most environmental contaminants react
1 million to 1 billion times faster with ˙OH than with O3, a conventional
oxidant (US EPA 1998).
15
Table 2.1 Oxidation potential of several oxidants in water
Sl.No. Oxidant Oxidation Potential (eV)
1 Hydroxyl radical ( .OH) 2.80
2 Singlet oxygen O(1D) 2.42
3 Ozone (O3) 2.07
4 Hydrogen peroxide (H2O2) 1.77
5 Perhydroxy radical 1.70
6 Permanganate ion 1.67
7 Chlorine dioxide (ClO2) 1.50
8 Chlorine (Cl2) 1.36
9 Oxygen (O2) 1.23
Table 2.2 Rate constants for O3 and ˙OH Reactions with organic
compounds in water
Sl.No. Compound Type Rate Constant (M-1 s-1)
O3 ˙OH
1 Acetylenes 50 108 to 109
2 Alcohols 10-2 to 1 108 to 109
3 Aldehydes 10 109
4 Alkanes 10-2 106 to 109
5 Aromatics 1 to 102 108 to 1010
6 Carboxylic acids 10-3 to 10-2 107 to 109
7 Chlorinated alkenes 10-1 to 103 109 to 1011
8 Ketones 1 109 to 1010
9 Nitrogen-containing organics 10 to 102 108 to 1010
10 Olefins 1 to 450 103 109 to 1011
11 Phenols 103 109 to 1010
16
2.2.1 Classification of Advanced Oxidation Processes
The AOP may be classified depending on the source to generate the
oxidizing species. The commonly used classification is shown in Figure 2.1.
The Advanced Oxidation Processes are Photolysis, AOPs based on ozone,
AOPs based on hydrogen peroxide, Hot AOPs, Photocatalysis, electro-
chemical oxidation, Ultrasound technologies and electron beam oxidation
(Peratitus et al 2004, and Gogate and Aniruddha 2004 a). This research work
mainly focused on photocatalysis using solar light as irradiation source.
Figure 2.1 Classifications of advanced oxidation processes
Advanced Oxidation Processes
- Photolysis - Ozone based AOPs - H2O2 based AOPs - Hot AOPs - Photocatalysis - Ultrasound technologies - Electro-chemical oxidation - Electron beam oxidation
The pH of the aqueous solution significantly affects TiO2, including
the charge of the particle and the size of the aggregates it forms. The pH at
which the surface of an oxide is uncharged is defined as the Zero Point
Charge (pHzpc), which for TiO2 is around 7. Above and below this value, the
23
catalyst is negatively or positively charged according to Equations (2.15) and
(2.16).
-TiO2H2+ ↔ TiOH + H+ (2.15)
-TiOH ↔ TiO- + H+ (2.16)
In consequence the photocatalytic degradation of organic
compounds is affected by the pH (Tsong and Wan 1991).
2.2.4.4 Effect of catalyst dosage
Either in static or in slurry or dynamic flow photoreactors, the
initial reaction rates was found to be directly proportional to catalyst dosage.
This indicates a truly heterogeneous catalytic regime. However, above a
certain value, the reaction rate levels off and becomes independent of catalyst
dosage. This limit depends on the geometry and working conditions of the
photoreactor and is for a definite amount of TiO2 in which all the particles,
i.e. the entire surface exposed, are totally illuminated. When catalyst dosage is
very high, after travelling a certain distance on an optical path, turbidity
impedes further penetration of light in the reactor. In any given application,
this optimum catalyst dosage has to be found in order to avoid excess catalyst
and ensure total absorption of efficient photons (Velegraki and Dionissions
2008).
2.2.4.5 Effect of temperature
Because of photonic activation, photocatalytic systems do not
require heating and operate at room temperature. The true activation energy Et
is nil, whereas the apparent activation energy Ea is often very low (a few
24
kJ/mol) in the medium temperature range (20º-80°C). However, at very low
temperatures (-40°- 0°C), activity decreases and activation energy Ea becomes
positive. By contrast, at high temperatures (>70-80°C) for various types of
photocatalytic reactions, the activity decreases and the apparent activation
energy becomes negative. When temperature increases above 80°C, nearing
the boiling point of water, the exothermic adsorption of reactants is
disfavoured and this tends to become the rate-limiting step (Gaya and
Abdullah 2008). In addition to these mechanical effects, other consequences
of plant engineering must be considered. If temperature is high, the materials
used for the plant should be temperature-resistant (more expensive), and
oxygen concentration in water decreases. Consequently, the optimum
temperature is generally between 20 and 80°C. This absence of need for
heating is attractive for photocatalytic reactions carried out in aqueous media
and in particular for environmental purposes (photocatalytic water
purification).
2.2.4.6 Effect of light intensity
At low light intensity and corresponding low carrier concentrations,
the rate of oxidation of a particular compound is proportional to light
intensity, while at higher light intensity the rate is dominated by second-order
charge carrier combination and has a square-root dependence on light
intensity. The transition from one regime to the other depends on the
photocatalyst material, but is typically above 1 sun equivalent. This transition
depends on the catalyst configuration and on the flow regime in the
photoreactor, and varies with each application. The optimal light power
utilization corresponds to the domain where the destruction rate is
proportional to light intensity (Chen and Ray 2001).
25
2.2.4.7 Effect of initial pollutant concentration and kinetic model
In heterogeneous catalysis, the pollutants have to be adsorbed on
the catalyst surface sites for bond breaking or formation. The adsorption of
pollutants and the availability of sites are hence important parameters in
photocatalytic reactions. The rate of pollutants conversions is proportional to
the active sites. As the reaction proceeds, the amount of pollutants adsorbed
on the catalysts surface will decrease until the pollutants is completely
converted (Velegraki and Dionissios 2008).
Kinetic models are often formulated to describe photocatalytic
reactions with respect to the initial pollutant concentrations. The kinetic
models for photocatalytic reactions are derived based on the classical
heterogeneous catalysis model, which is the Langmuir-Hinshel wood (LH)
kinetic model. This model assumes that the reaction occurs on the surface and
the reaction rate (r) is proportional to the fraction of surface coverage by the
pollutant is given by Equation (2.17).
r tdc Kcr k kdt 1 Kc
(2.17)
where kr is the reaction rate constant, K is the adsorption constant and C is the
pollutant concentration at any time t. Integrating the above Equation (2.17)
yields
rColn K (Co C) k KtC
(2.18)
where Co is the initial pollutant concentration. At high Co, the Equation
(2.18) can be simplified to a zero order rate law (Equation 2.19)
r(Co C) k Kt kt (2.19)
26
At low Co, a first order rate Equation (2.20) can be obtained from
Equation (2.17)
rColn k Kt ktC
(2.20)
where k is the apparent rate constant.
2.2.4.8 Effect of anions and cations
Industrial wastewater contains apart from the pollutants, different
salts at different concentrations. The salts are generally ionized under the
conditions of photocatalytic degradation (Robert et al 1999 and Alhakimi et al
2003 b). The anionic and cationic parts of the salts have different effects on
the photocatalytic degradation process. The presence of anions such as
chlorides, sulphate, carbonate and bicarbonate is common in industrial
wastewater. These ions affect the adsorption of the degrading species, act as
hydroxyl ion scavengers and may also absorb UV light (Kamble et al 2004).
2.3 OVERVIEW OF WOR KDONE ON PHOTOCATALYSIS
Photocatalysis, i.e. using semiconductor particles under bandgap
irradiation as little microreactors for simultaneous reduction and oxidation of
different redox systems has been intensively studied during the last 30 years,
since the pioneering work of Carey et al in 1976. The photocatalytic
degradation processes are gaining importance in the area of wastewater
treatment, because these processes result in complete mineralization with
operation at mild conditions of temperature and pressure (Gogate and
Aniruddha 2004 b). A major advantage of the photocatalytic oxidation based
processes is the possibility to effectively use sunlight or near UV light
(Kanmani et al 2003). Many researchers (Bhatkhande et al 2003, Marcpera
et al 2004, Moon et al 2005, Linda et al 2005, Kaniou et al 2005, Yuan et al
27
2006, Priya and Giridhar 2006, Muruganandham et al 2006 a, and Tamer et al
2007) have shown that most of the organic pollutants like 4-cholorophenol,
nitrobenzene, alachlor, Lanasol blue dye, sulfamethazine, microcystin-LR,
phenol, reactive yellow and p-nitrophenol are completely oxidizated into non
toxic products of carbon dioxide and water. Various catalysts such as TiO2,
ZnO, CdS, ZnS, etc have been used as photocatalyst so far in different studies
reported in the literature (Siemon et al 2002, Sarria et al 2003, Marcpera et al
2004, Gogate and Aniruddha et al 2004, Linda et al 2005).
The surface area and the number of active sites offered by the
catalyst for adsorption of pollutants plays an important role in deciding the
overall rates of degradation as usually the adsorption step is the rate
controlling. It should be noted that the best photocatalytic performances with
maximum quantum yields have been always with titanium dioxide. In
addition, Degussa P25 catalyst is the most active form among the various
ones available and generally gives better degradation efficiency (Chan et al
2001 and Linda et al 2005). The energy needed to activate TiO2 is 3.2 eV or
more, which corresponds to near UV radiation of a wavelength of 388 nm or
less. As 4-6 percent of sunlight reaching the earth’s surface is characterized
by these wavelengths, the sun can be used as the illumination source.
The stages involved in the development of photocatalysis assume
cyclic form. The cycle, shown in Figure 2.3 starts and ends with the analysis
of needs by the end-user. Each stage of the development cycle is critical for
successful operation of the next immediate step. Some of the illustrative work
done in the recent years in the area of photocatalysis applied to wastewater
treatment has been furnished in Table 2.3 with discussion about reactor
operating conditions and the important findings obtained in the work. It was
observed that, most of the Photocatalytic Oxidation (PCO) studies reported in
the literature, used single-constituent model solution with a concentration up
28
to 100 mg/L. But, the concentration of phenolic compounds in industrial
wastewaters varies from 100 to 500 mg/L and also contains inorganic
pollutants.
Figure 2.3 Photocatalytic system development cycle
Majority of academic studies reported, have used artificial UV light
source for photocatalytic oxidation. Solar light intensity in our region is quite
suitable and can be used as a light source in the PCO process. By employing
solar light, the common drawback of relatively high cost of UV lamps and
electricity can be overcome. Hence it is necessary to study, the PCO process
using solar energy and prove it to be an economically and technologically
feasible option for degradation of non-biodegradable organic pollutants in
industrial wastewaters. Though, advanced oxidation processes oxidize almost
all pollutants, considering the economical aspects, the use of advanced
oxidation process alone as treatment may not look lucrative. Thus, a hybrid
method consisting of using advanced oxidation process to reduce the toxicity
of the wastewaters up to a desired level followed by biological treatment is
perhaps needed for the future.
29
Table 2.3 Typical findings observed in the representative works related to photocatalytic degradation of wastewaters
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
1. Acid Green 16 (910-4 M)
Experiments were conducted in an immersion type photochemical reactor equipped with 8 W low pressure mercury lamp.
At 5 10-4 M of dye the degradation efficiency was 97.5 % whereas for 10 x10-4 M the efficiency was 79.45 %. The photodegradation efficiency increases rapidly with increasing amounts of TiO2 from 100 mg to 250 mg and then decreases on further increase of TiO2. It was observed that photodegradation efficiency was more in acidic pH. A solution containing 9 10-4 M of the dye could be completely decolourised in 3 min and 90 % degradation could be achieved in 3 h.
A Pyrex cylindrical reactor with total volume of 3.5 L equipped with 365 nm UV lamp was used.
The experimental results indicated that with the TiO2 as catalyst, the optimal condition was 0.25 g/L. Both increased light intensity and continuous aeration increased COD removal efficiency, particularly under continuous aeration for significantly raising the ratio of BOD/COD to improve efficiency of subsequent biological treatment.
Hsieh et al (2000)
3. Azo dye (10 mg/L)
Experiments were conducted in a 1 L pyrex glass batch reactor equipped with 4 nos. of UV lamps.
A Pseudo first order reaction kinetic was proposed to simulate the photocatalytic degradation of orange G in the batch reactor. More than 80% of 10 mg/L orange G decomposition in 60 minutes reaction time was observed in this study and fast decomposition of orange G only occurred in the presence of both TiO2 and suitable light energy. Faster degradation of orange G was achieved under acid conditions. The degradation rates of orange G at pH 3 were about two times faster than those at pH 7. The reaction rates were proportional to TiO2 concentration and light intensity with the power order of 0.726 and 0.734 respectively.
Hung et al (2001)
30
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
4. EDTA (5.0 mM)
Irradiations were performed using a high-pressure xenon arc lamp.
Experiments with 5.0 mM EDTA were performed for 3 hours irradiation under different conditions. Under N2 bubbling, depletion of EDTA was very low. Under O2 bubbling, the concentration of EDTA decreased around 90 %. However, the corresponding decrease of TOC ranged only between 4.5 % and 9 %.
Babay et al (2001)
5. Humic acid (TOC = 32 mg/L)
Irradiation experiments were carried out with 9 numbers of 15 W low-pressure UV lamps.
It was confirmed that the technique used in this study was effective to remove TOC at 38 % and colour at 89 % within 150 min oxidation. The experiment results showed that low concentrations of hydrogen peroxide dosage (less than 0.016 M) to UV/ TiO2 system accelerated the TOC and colour removal rate from 9 % to 38 % and 40 % to 89 % respectively, while over dosage made this positive effective decline.
Tay et al (2001)
6. Methyl orange (2x10-3-1x10-5 M)
0.5 L three necks round bottom flasks were used as reactors. solar radiation was used as an irradiation source.
It was found that 0.4 % of TiO2 gave the highest degradation rate constant (0.619 h-1). In the second set of experiments TiO2 concentration was fixed at 0.4 % and the Methyl Orange was varied, the highest rate constants was obtained with the concentration of Methyl Orange was 4x10-5 M and it was found to be 0.639 h-1. The degradation becomes negligible in the presence of high concentrations of Methyl Orange. The highest degradation rate was obtained at pH =3 with a rate constant k = 2.6683 h-1 followed by that at pH=9 with a rate constant k = 0.7585 h-1.
Alqaradawi and Salman (2002)
7.
Humic acids (100 mg/L)
Experiments were carried out in a solar box. The light source was Xenon lamp.
The adsorption study of Humic acids at three pH Solution (1.9/7.5/11) indicated that at acidic pH, Humic acids are adsorbed on TiO2. It was obtained 88 % of TOC removal after 6 h of irradiation with optimum TiO2 loading 1.0 g/L. It was shown that Photocatalysis process improved the biodegradability of Humic acids.
Wiszniowski et al (2002)
31
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
8. Landfill leachate (COD =1260-1673 mg/L)
Quartz photoreactor 190 mL was equipped with 8 W Mercury lamp.
Under the acidic condition, the photocatalytic removal efficiency of landfill leachate was relatively high because there was very low concentration of inorganic carbons that could inhibit the photocatalytic oxidation. The effect of TiO2 concentration was obtained from experimental results and it could demonstrate the relationship of amounts of TiO2 dosage and reaction rate.
Cho et al (2002)
9. Benzidine and 1,2-dipehnylhydrazine (1 mM)
Irradiation experiments were conducted in a lab scale reactor of 250 ml capacity equipped with 500 W high-pressure mercury lamp.
The maximum efficiency for the degradation and the TOC depletion was observed at pH 5. The efficiency for the degradation as well as for the mineralization of the compound increases with the increase in the substrate concentration from 0.1 – 1 mM. The catalyst concentration was investigated using different concentrations varying from 0.5 to 5 g/l. The degradation and mineralization rate increases with the increase in catalyst concentration from 0.5 to 2 g/l. Bromate ions found to enhance the rate of degradation.
Muneer et al (2002)
10. Potassium hydrogen phthalate (200 ppm)
The photo reactor used for the degradation process was a 3L beaker made of Pyrex glass, equipped with a magnetic stirrer and an oxygen-purging device.
The optimum catalyst concentration was found at 3 g/L of Degussa P25 and 10 g/l of Hombikat UV 100. The optimum degradation rate was obtained at pH 5.0 for both Catalysts. The complete degradation of organic pollutants achieved after 5 h of irradiation using the optimized reaction conditions.
Alhakimi et al (2003 b)
11 Phenol and substituted phenols.
A Pyrex reactor open to air equipped with 125 W lamp.
Direct UV-irradiation of TiO2 catalyst suspended in aqueous solutins is able to destroy hydroxyphenol and nitrophenols. The photodegradation of these phenolic compounds follows Pseudo-first-order kinetics. It was found that the degradation of nitrophenols could be accelerated in acidic medium whereas hydroxy phenols are less sensitive to pH variations. It was also found that the BOD5 in the phenolic solutions increased with the COD decreased during photodegradation reaction.
Ksibi et al (2003)
32
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
12 4-Chlorphenol (100 ppm)
Photoreactor consisted of a 3L beaker made of Pyrex glass, equipped with a magnetic stirring bar and an oxygen purging device.
The comparison of the degradation of 4-chlorophenol, using sunlight and a UV lamp was carried out using two different catalysts, Degussa P25 and Hombikat UV 100. The optimum concentrations for Degussa P25 and Hombikat UV 100 occurred at 7 and 10 g/L, respectively. The optimum initial pH was found to be 5 for both catalysts. The degradation rate of 4-chlorophenol is 6.4 times and 1.6 times higher when using sunlight compared to the artificial UV lamp for Degussa p25 and Hombikat UV 100, respectively. The degradation rate of 4-chlorophenol is six times higher, compared to Hombikat UV 100 at the optimum conditions, when using sunlight and Degussa P25 as the catalyst.
Alhakimi et al (2003)
13 Inactivation of E. coli
Pyrex glass bottle of 50 mL was used as batch reactor. Irradiation by simulated sun test lamp.
Parameters such as light intensity, extend of continuous irradiation, catalyst concentration and temperature have a positive effect on disinfection. Intermittent illumination results in an increase in the time required for E. coli inactivation. No illumination of a contaminated TiO2 suspension. In contrast, without catalyst, illuminated bacteria recovered it initial concentration after 3 h in the dark. Bacterial inactivation in the absence of Catalyst was more affected than that with catalyst. When increasing light intensity from 400 to 1000 W/m2. TiO2 concentrations higher than 1 g/L do not increase the initial inactivation rate for both intensities. Water turbidity negatively affects the photocatalytic inactivation of bacteria.
Rincon and Pulgain (2003)
33
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
14 Sulfonylurea herbicides
Irradiation experiments were carried out with 125 W mercury medium pressure UV lamps, in a Pyrex Cylindrical flask opened to air.
Results show that adsorption is an important parameter controlling the apparent kinetic order of the degradation. In respect to the Langmuir – Hinshelwood model, the photocatalytic reaction is favoured by a high concentration. At moderate light fluxes (13.3 mW/cm2), the rate is proportional of photonic flux () giving optimum quantum efficiency. For higher values (39.8 mW/cm2), the electron – hole recombination becomes predominant decreasing the quantum efficiency although the reaction rate still increases.
Vulliet et al (2003)
15 Imazaquin (herbicide) (50x10-6 mol/ L)
Irradiation experiments were carried out with 125 W Mercury lamp in a 70 ml cylindrical borosilicate glass reactor.
A lower solution pH in the 3-11 range was found to be favourable to degradation. The addition of H2O2 up to 10-3 mol / L enhanced the degradation rate and decreased it at higher concentrations. The photocatalytic effect was more efficient in a suspension containing 2.0 g/L TiO2 with 1 h sonication in the dark, rather than with 20 min. sonication before irradiation. Solar radiation decomposed the herbicide faster than an artificial light source.
Garcia and Keiko (2003)
16 Biocides (2 mg/L)
Lab-scale photoreactor consisted of 1500 W Xenon arc lamp.
The primary degradation of the micro pollutants follows a Pseudo-first-order kinetics following the Langmuir-Hinshelwood model. The total disappearance of chlorothalonil and dichlofluanid was achieved in 90 and 20 minutes respectively, whereas the materialization of organic carbon to carbondioxide after 240 minutes of irradiation was found to be 100 % for chlorothalonil and 78 % for dichlofluanid.
Sakkas and Albanis (2003)
17 Dicamba (2,5-Dichloro-6-methoxybezoic acid)
Experiments were conducted in a quartz reactor of 500 mL capacity and illuminated with 150 W UV lamps.
Photolysis reactions were slow but the corresponding photocatalysis rates were increased by about 3 and 5 times in the presence of TiO2 at 300 and 350 nm UV, respectively. Photocatalytic rates were increased with the pH at acidic to neutral ranges. The results of H2O2 assisted photocatalysis experiments showed that a low H2O2 dosage in photocatalysis would enhance the decay rate of dicamba by 2.4 times, but an overdose of H2O2 will retard the rate.
Chu and Wong (2004)
34
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
18 Azo dyes (20 mg/L)
Experiments were conducted in a 3 L cylindrical glass reactor equipped with a 8W low pressure Mercury lamp.
The study examined degradation of azo dyes using photocatalytic oxidation. The photocatalyst used were TiO2, ZnO and SnO2. The reaction rate constants fits a first order reaction model and the reaction rate constant for TiO2 + SnO2 (0.31 h-1) is larger than that of TiO2 (0.24 h-1). The reaction rate constants had higher values at pH 10 than pH 7. At pH 10, the trend of the reaction rate constants of azo dyes followed the order: ZnO > ZnO + SnO2 > TiO2 + ZnO > TiO2 + SnO2 > SnO2.
Wu (2004)
19 Reactive azo dye (50 mg/L)
Irradiation experiments were carried out in an open batch reactor of 1000 mL capacity equipped with an 300 W UV lamp.
The results obtained from the experiments adding H2O2/ TiO2 show that the highest decolorisation rate is provided by the combination of (UV+ TiO2 +H2O2). The decolorisation efficiencies were 18 %, 22 %, 34 % and 52 % in the runs UV, UV+H2O2, UV+ TiO2 and UV+ TiO2 + H2O2
after approximately 100 min irradiation respectively. The decolourisation rate constant was 0.018 min-1 in the 1 g/L TiO2 while it was 0.004 min-1 in the presence of 0.125 g/L TiO2. The results of the obtained oxygen uptake rate measurements in biological activated sludge have shown that the photocatalytically treated dye was easier to degrade than untreated dye.
Nevim (2004)
20 Bisphenol-A (10 mg/L)
Experiments were performed in an immersion type reactor equipped with a 6 W lamp. The reactor capacity was 1.5 L.
The effects of immobilized TiO2 –film thickness, UV radiation intensity and pH on the photodegradation were investigated. Apparent rate constant of the first order increased with increasing TiO2 coating time from 1 to 3, however, decreased over 4-coating time. Rate constant increased as the pH value shifted from basic to acidic regions.
Lee et al (2004)
21 Protozoan, fungal and bacterial microbes
Solar irradiation was simulated using a 1000 W Xenon arc Solar simulator.
The ability of solar disinfection (SODIS) and solar photocatalytic disinfection (SPC-DIS) batch-process reactors to inactivate water borne protozoan, fungal and bacterial microbes waste evaluated. After 8 h simulated solar exposure both SPC-DIS and SODIS achieved at least a 4-log unit reduction in viability against Protozoa, fungi and bacteria. The inactivation is approximately 50 % faster in batch process SPC-DIS reactors than SODIS reactors under comparable conditions.
Lonnen et al (2005)
35
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
22 Alachlor (herbicides) (2 mg/L)
Experiments were conducted in an immersion type reactor of 1L capacity equipped with 400 W metal halide lamp.
The photodegradation rate of alachlor could be described as an apparent first order. The rate constant of alachlor increased from 0.021 to 0.060 h-1 as the number of coating times increased from 1 to 5 times in the absence of ferric ion, where the corresponding thickness of TiO2 film were 67 and 174 nm. The rate constant increased from 0.030 to 0.060 h-1 as pH value 9 to 5. The rate constant increased slightly from 0.031 to 0.050 h-1 as the concentrated of ferric ion increased from 0.75 to 7.5 mg/L in the absence of TiO2. However those increased from 0.051 to 0.110 h-1 in the presence of both TiO2 and ferric ion. In situ photodeposition of ferric ion onto the TiO2 surface enhanced the rate constant of photodegradation of alachor by about 80 % with an adding 7.5 mg/L.
Kim et al (2005)
23 Reactive Black (3.85 10 -4 M)
Open borosilicate glass tube of 50 mL capacity was used as reactor. solar as light source
The rate of degradation was maximum at catalyst amount 2 g/L, pH 9. A complete removal of 3.85 10 -4 M dye solution under solar irradiation was observed in 3.5 h. It was observed that after 90 min. the removal efficiency was 70 and 57 % for photocatalytic and photochemical process.
Muruganan-dham et al (2006 b)
24 p-toluenesulfonic acid (100 mg/L)
Experiments were carried out in a quartz cylindrical reactor of 100 mL Irradiation by solar.
The rate of degradation was maximum at catalyst amount 1 g/L, pH 3 and concentration 50 mg/L. It was also observed that in the presence of anions and cations, the rate was decreased.
Kamble et al (2006)
36
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated
Experimental condition and Type of equipment Important findings of the work Reference
25 Reactive yellow (5 10-4 mol/L)
Multilamp photoreactor of capacity 50 mL with 8 W lamp was used.
The rate of degradation was maximum at catalyst amount 4 g/L, pH 5.5 and dye concentration 5x 10-4 mol/L. The increase of radiation intensity from 16 to 62 W increases the decolourisation from 32.9 to 87 % in 20 min. The presence of electron acceptors like H2O2, KBrO3 enhanced the degradation. Negative effects are observed in the presence of NaCl and Na2CO3.
Muruganan-dham et al (2006 a)
26 Textile dye house wastewater (COD = 404 mg/L)
Immersion batch reactor of capacity 1 L with 400 W high pressure Hg lamp was used.
At pH 3 and catalyst amount 0.5 g/L, complete removal of colour and 40-60 % COD after 4 h of treatment was observed. It was observed that efficiency of catalyst marginally deteriorated on repeated use. H2O2 enhanced the reaction rate was observed.
Pekakis et al (2006)
27 Dye solutions (50 mg/L)
Irradiation experiments were carried out in a 1.5 L UV reactor with 125 W high pressure Hg lamp.
The rate of degradation was maximum at catalyst amount 1g/L, pH 7 and concentration 50 mg/L. Complete removal of dye was achieved with in 100 min.
Bizani et al (2006)
28 Basic dye (50 mg/L)
Irradiation experiments were carried out in a 1 L reactor with 12 nos. of 8 W lamps.
The results showed that agitation speed varying from 50 to 200 rpm has an influence on the dye decomposition rate. The decomposition rate increases with TiO2 amount up to 0.98 g/L and then decreases. And also it was observed that the rate increases with the UV power intensity up to 64 W and then decreases.
Wu et al (2006)
29 Tannery wastewater (COD = 800 mg/L)
Annular batch reactor of 2 L capacity irradiated by 80 W high pressure Hg lamp was used.
The application of AOP (H2O2/UV, TiO2/H2O2/UV and TiO2/UV) was investigated. The COD removal increased in the order UV < H2O2/UV < TiO2/H2O2/UV < TiO2/UV treatments. In H2O2/UV treatment, the COD removal reached around 60 % in 4 h reaction.
Sauer et al (2006)
30 Estrone and 17 β estradiol (500 mg/L)
Batch reactor of 400 mL capacity with 150 W high pressure UV lamp.
The maximum removal of 97 % was observed at pH 7.6, catalyst amount 1 g/L and pollutant concentration 500 mg/L with in 4 h of irradiation. The presence of humic acid increases the degradation rate.
Zhang et al (2007)
37
Table 2.3 (Continued)
Sl. No.
Compounds / Wastewater
Treated Experimental condition and
Type of equipment Important findings of the work Reference
31 Olive mill wastewater (1300 mg/L)
The reactor had a working volume of 100 cm3. It composed of water cooled double-walled Pyrex glass tube. It was illuminated by a vertical UV tube of 415W.
Olive mill wastewater (OMW) was treated by photocatalysis using TiO2 under UV irradiation on the laboratory scale. The photocatalyst was TiO2 Degussa P25. It was added at a level of 1 g/L. The chemical oxygen demand, the coloration at 330 nm, and the level of phenols all showed decreases which, after a 24-h treatment, reached 22 %, 57 %and 94 %, respectively.
Hajjouji et al (2008)
32 Benzoic acid (150 mg/ L)
Experiments were carried out in a cylindrical reaction vessel, batch type, coupled with a double-walled immersion well. A medium pressure mercury lamp providing 25mW/cm2 was used as light source.
Experiments were conducted at benzoic acid initial concentrations between 25 and 150 mg/ L, catalyst loadings between 0.2 and 1 g/ L and initial solution pH values between 2 and 10.6. Conversion increased with increasing catalyst loading up to about 0.6 g/L and it was favoured at alkaline or neutral conditions but impeded at extremely acidic conditions. Increasing initial substrate concentration led to decreased benzoic acid conversion, which was found to follow a Langmuir–Hinshelwood kinetic expression. Complete conversion can be achieved in less than about 60 min.
Velegraki and Dionissios (2008)
33 Black table olive wastewater (COD = 1-8 g/L)
Experiments were conducted in an immersion well, batch type, laboratory scale photoreactor, equipped with a 400W high pressure mercury lamp.
Initial organic load, expressed in terms of chemical oxygen demand (COD), was studied in the range 1–8 g/L, anatase TiO2concentrations in the range 0.25–2 g/L and H2O2 concentrations in the range 0.025–0.15 g/L. Treatment efficiency, which was assessed in terms of COD, total phenols, aromatics and colour reduction, generally increased with decreasing initial COD and increasing contact time, catalyst and H2O2 concentrations. Depending on the conditions employed, nearly complete decolouration (>90%) could be achieved, while mineralization never exceeded 50 %.
Chatzisymeon et al (2008)
34 Winery wastewater (800 mg/L)
Experiments were carried out in batch operation in an annular type reactor. The lamp used was a medium pressure mercury arc UV lamp of 435 nm.
The performance of the reactor was studied as a functional of various operating variables, such as gas flow rate, pH and catalyst loading. It was found that the optimum gas flow rate was 6 L/min whereas the optimum pH value is 6.5. The highest photodegradation rate and the maximum COD removal were achieved at zero catalyst loading with COD removal of about 84 %. Lower rates of chemical reaction in photocatalysis compared to photolysis were possibly because of the shielding of UV light by catalyst particles.
Agustina et al (2008)
38
2.4 SOLAR PHOTOCATALYTIC REACTORS
The artificial generation of photons required for the treatment of
wastewater is the most important source of costs during the operating of
photocatalytic wastewater treatment plants. This suggests using the sun as an
economically and ecologically sensible light source with a typical UV-flux
near the surface of the earth of 20 - 30 W/m2 the sun puts 0.2 - 0.3 mol
photons/m2/h in the 300 - 400 nm range at the process disposal (Bahnemann
2004). Principally these photons are suitable for destroying water pollutants in
photocatalytic reactors. Over the last two decades several reactors for the
solar photocatalytic water treatment have been developed and tested.
Designs of solar photocatalytic reactors have followed the well-
known designs of solar thermal collectors including concentrating and non
concentrating designs. The key differences are viz., the fluid to be treated in
the reactors must be exposed to UV solar radiation, therefore, the absorber
must be transparent to UV solar radiation; and no insulation is needed, since
temperature does not play a significant role in the photoreaction. As a result,
the first engineering scale outdoor reactor developed was a simple conversion
of a parabolic trough solar thermal collector. The conversion replaced
absorber/glazing tube combination of the thermal collector with simple pyrex
glass tube through which contaminated water can flow. This reactor was used
to treat water contaminated with Trichloroethylene (TCE). The catalyst, TiO2
powder was mixed with contaminated water to form slurry which was passed
through the pyrex glass tube (reactor tube) located at the focal line of the
parabolic trough. Since that time, a number of reactor concepts and designs
have been advanced by researchers all over the world (Sagawe et al 2003 a
and Sagawe et al 2003 b). Based on the method of collecting sunlight, two
reactors systems are designed viz., concentrating and Non-concentrating
39
reactor systems. Based on the condition of the catalyst, two reactor systems
are designed and they are suspended and fixed catalyst systems.
2.4.1 Concentrating Collectors
Solar photochemical processes are based on the collection of only
high-energy short-wavelength photons to promote photochemical reactions.
Most of the solar photochemical processes use UV or near-UV sunlight (300-
400 nm), but in some photochemical synthesis processes, upto 500 nm
sunlight can be absorbed and photo-Fenton heterogeneous photocatalysis uses
sunlight up to 580 nm. Sun light at wavelengths over 600 nm is normally not
useful in any photochemical process. Nevertheless, the specific hardware
needed for solar photochemical applications has much in common with those
used for thermal applications. As a result, both photochemical systems and
reactors have followed conventional solar thermal collector designs, such as
parabolic troughs and non-concentrating collectors.
The original solar photoreactor designs for photochemical
applications were based on line-focus parabolic-trough concentrators (PTCs).
In part, this was a logical extension of the historical emphasis on trough units
for solar thermal applications. Furthermore, PTC technology was relatively
mature and existing hardware could be easily modified for photochemical
processes. The first outdoors engineering – scale reactor developed (in USA)
was a converted solar thermal parabolic-trough collector in which the
absorber/glazing-tube combination had been replaced by a simple pyrex glass
tube through which contaminated water could flow. The first engineering-
scale solar photochemical facility for water detoxification in Europe was
developed by CIEMAT using 12 two-axis PTCs, each consisting of a turret
and a platform supporting four parallel PTCs, with an absorber at the focus of
each collector. The platform has two motors controlled by a two-axis
40
(azimuth and elevation) tracking system. Thus, the collector aperture plane is
always perpendicular to the solar rays, which are reflected by the parabola
onto the reactor tube (concentrating ratio ≈ 10) at the focus through which
circulates the contaminated water to be detoxified (Curco et al 1996, Malato
et al 1996).
Typical overall optical efficiencies obtained in this PTC were
around 50 %. Parabolic-trough collectors make efficient use of direct solar
radiation and, as an additional advantage, the thermal energy collected from
the concentrated radiation could simultaneously be used for other
applications. The reactor is small, while receiving a large amount of energy
per unit volume. The flow is turbulent and volatile compounds do not
evaporate, so that handling and control of the liquid to be treated is simple
and cheap. The main disadvantages are that the collectors (i) use only direct
radiation, (ii) are expensive, and (iii) have low optical and quantum
efficiencies. Several different substances have been successfully degraded
with these collectors viz. chromium (VI), dichloroacetic acid, phenol,
and atrazine. In contrast, non-light concentrating reactors utilizing both the
direct and diffuse components have greater potential for process development.
Non-concentrating collectors are cheaper than PTC as they have no moving
parts or solar tracking devices. Wyness (1994 a) developed Flat plate reactor
and tested the reactor in an out-door solar photocatalytic oxidation facility.
Goswami (1994) reported that all the three non- concentrating reactors viz.
Flat plate, Shallow pond and Tubular reactors demonstrated satisfactory
performance in solar photocatalytic oxidation facilities when tested over a
wide range of operating conditions. Goslich et al (1997) developed Double
Skin Sheet Reactor (DSSR) and Thin Film Fixed Bed Reactor (TFFBR). But
it requires large catalyst area for purification of wastewater and also
constrained by mass transfer limitations due to laminar flow conditions. Feitz
et al (2000) developed fixed catalyst reactors viz. coated mesh and packed
bed reactor and its efficiency was very less compared to suspended catalyst.
Kanmani et al (2003) developed two water fountain solar
photocatalytic reactors of suspended catalyst system for treating textile dyeing
wastewaters. Chan et al (2003) constructed a solar photocatalytic cascade
reactor to study the photocatalytic oxidation of benzoic acid. Though a lot of
studies have been reported so far, still the efficient use of reactors at large-
scale are lacking due to opacity, light scattering and depth of radiation
penetration. Engineering design and operation strategies are lacking for
efficient use of reactors at large scale. Moreover, the requirement of at least
one side to be transparent to UV light significantly poses size limitations
along with breakage risk. Hence, studies are needed in terms of design of
simple reactor achieving uniform irradiation and minimal losses of incident
light due to opacity, light scattering and adsorption by liquid.
48
Table 2.4 Typical findings observed in the representative works related to solar photocatalytic reactors
Sl. No. Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
1. Coated mesh reactor (CMR)
Phenol (2 mg/L)
A freshly coated 6.5 m x 0.5 m TiO2 impregnated woven glass fibre mesh supported on an inclined corrugated support forms the main element of the reactor. The reactor operated in a continuous recirculatory mode. The flow rate was 5 L/min and irradiation was by solar light
The total removal of phenol from the solution for an exposure period of 200 min was only 36 %. Although the overall processing rate is slow, the removal rate over the length of the reactor surface was rapid indicating the deficiency of the rector arrangement, i.e., long non-contact periods due to the small reactor to tank volume ratio.
Feitz et al (2000)
2. Packed-bed reactor (PBR)
Phenol (2mg/L) The reactor vessel was constructed from mirror finish stainless steel and approximately 1x2m2 in aperture area. The reactor contains 6.5 cm deep TiO2 – coated Raschig rings and perforated baffles. The flow rate was 3 L/min and irradiation was by solar light.
PBR achieved 98 % removal of phenol from a 2 mg/L concentration, 100 L solution after 3 h irradiation. The air injection system forms an important component of PBR treatment. Under clear sky conditions with no air injection, 75 % of phenol was removed over 4 h compared to 99.3 % removal with additional air input.
Feitz et al (2000)
49
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
3. Solar Thin film Cascade rector
Benzoic acid (25-100 mg/L)
Nine stainless steel flat places were arranged in a Cascade configuration. The plates were coated with TiO2 catalyst. The plates were positioned 2.5 cm vertically apart from each other and inclined at an angle of 5˚ to the horizontal. The area of each plate exposed to solar light was 17.5 cm 28 cm.
The water fall effect introduced by the cascade design can promote mass transport and aeration in the solution film. The percentage removal of TOC in 7 L of 100 mg/L benzoic acid solutions increased from 30 % to 83 % by adding 10 mL of hydrogen peroxide solution. The average TOC removal rates did not demonstrate significant dependence on TOC0 and the intensity of the light was found to be the dominant factor affecting the degradation process.
Chan et al (2003)
4. Thin film cascade rector (UV)
Benzoic acid (100 mg/L)
Photoreactor consists of six fluorescent UV lamps and a cascade of three flat plates coated with TiO2 catalysts. The plates were positioned 2.5 cm vertically apart from each other and inclined at an angle of 5˚ to the horizontal. The area of each plate exposed to solar light was 17.5 cm 28 cm
The percentage removal of TOC for the flow rates from 2 L/min to 5 L/min was 7.34, 8.15, 7.14 and 7.95 respectively, which seemed to show no dependence of the removal of TOC on the flow rate. Increasing temperature from 291K to 300 K was found to decrease the DO level in the solution, hence reducing the percentage removal of TOC. When the number of lamps decreases from six to two the UV intensities decreases from 1.39 mW/cm2 to 0.56 mW/cm2 and the removal of TOC also decreases from 19.43 % to 11 %. The removal of TOC slightly decreased from 19.43 % for the cascade reactor to17.1 % for the single plate reactor.
Chan et al (2001)
50
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
5. Solar immobilized-catalyst photocatalytic reactor
Microcystin-LR (100 mg/L)
The reactor vessel was constructed from mirror-finish stainless steel to provide maximal internal respectively and was approximately 12 m2 in aperture area. It contains 6.5 cm deep randomly packed TiO2 coated Rashig ring and perforated bottles to direct the flow and maximize contact time.
The true quantum efficiency of the reactor ranges from 2.4-2.8 % for dichloroacetic acid mineralization. Rapid removal (96 %) of MLR was observed once exposed to sunlight with in 4 hours little difference was detected between the removal rates (70%) under cloudy or sunny conditions. The virtual independence of degradation rates for low to moderate levels of cloud cover is due to the ability of the non-concentrating reactor to harness diffuse light and minimize reflection losses.
Feitz et al (2002)
7. Batch mode plate film reactor
Diuron (810-5 mol/dm3)
Photoreactor was constructed from rectangular polymethyl methacrylate trays with a through at either end. The trays were dimensioned to accommodate a glass plate 60 cm long and 30 cm wide. The plates were positioned at an angle of 100C to the horizontal position. The plates were coated with TiO2. The flow rate was 1cm3/min
Dependence of the reaction rate on the diuron concentration (in the range of 0.8-8.010-5 mol/dm3 and the light intensity but independence on the flow rate (2.5-3.6 dm3/min) were found.
Krysova et al (1998)
51
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
8. Novel slurry bubble column reactor
2,4-Dichloro-Phenoxyacetic acid (100 mg/L)
Continuous experiments were carried out in borosilicate glass slurry bubble column reactors of 0.1m dia. x 3.0 m length and 0.15 m dia. 3.0 m length with capacities of 20 and 54 L respectively. A metering pump was used for delivering 2,4-D solutions to the top of the column air compressor was used to sparge the air at the bottom of the column. A parabolic reflector (3.0 m height with a total surface area of 6.0 m2) was used to concentrate the solar radiation.
As the size of the reactor increase the PCO was found to decrease. An increase in the residence time yields higher photocatalytic degradation. The presence of the sieve plates with down comers increases the percentage degradation by about 10 % over that obtained in their absence. A reduction in axial mixing has a beneficial effect on the extent of photocatalytic degradation.
Kamble et al (2004)
9. Novel aerated cascade photoreactor (ACP)
Dichloro acetic acid (DCA)
It consists of a double skin sheet channeling system made of UV transparent acrylic glass. By drilling a hole at alternate ends of the channels, a meandering flow is obtained. A porous aeration tube installed at the bottom of the sheet feeds an air –stream to each channel. ACP can be installed at any angle from 0 to 900 in order to optimize the hydrodynamic state and ensure optimal irradiation of the sunlight.
For the DCA degradation using TiO2 as the catalyst, the initial photonic efficiency was 14.5 % and 23.6 % and the initial degradation rate 3.8 and 1.8 g TOC/m2/h under artificial light and sunlight respectively. When treating the textile wastewater, the removal rates of TOC and COD are almost identical with about 42 % and 29 % at an average residence time of 2 hours under artificial light and sunlight respectively.
Xi et al (2001)
52
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters Description of Reactor Important findings Reference
10. Parabolic Trough reactor (PTR)
Phenolic wastewater
The PTR consisted of 6 HELIOMAN-modules connected in row, its concentration ratio was 6, and the effective collecting mirror aperture area was 32 m 2. Each module was equipped with two stepper motors. Flow rates between 500 and 3000 L/h was maintained and experiments were performed in single pass mode. The total volume of the pipe reactor was 838 L of which 484 L correspond to the borosilicate glass absorber tubes.
Only 30 % of the relative TOC was degraded after 250 min. of treatment. The major engineering problem is to run the PTR maintaining a sufficient concentration of molecular oxygen throughout the reactor. Using small flow rates to achieve high residence times results in an accumulation of oxygen bubbles in the tubes thus reducing the illuminated volume and lowering the degradation rates.
Goslich et al (1997)
11. Double skin sheet reactor (DSSR)
Dichloro acetic acid (DCA)
It consisted of a flat and transparent structured box of PLEXIGLAS in which a suspension of the photocatalyst circulates driven by a pump. This reactor employs both the direct and diffuse portion of the solar radiation. It consisted of a modified double-skin sheet, length = 1400 mm, height = 980 mm, 30 channels 28.5x12 mm, total inner volume = 144 L as the photoreactor and a reservoir (30 L), connected by PVC-tubes. All experiments were operated in a recirculation mode. The volume flow was 11.8 L/min, i.e. the residence time in the reactor was 73.2 S. The flow rate being 0.57 m/s resulted in a Reynolds number of about 9000, therefore the flow conditions are turbulent. The reactor was adjusted with an inclination angle of 450C. The reactor was irradiated by the sun.
Within 60 min. more than 80 % of the initially present DCA have been degraded. Photonic efficiencies up to about 12.5 % could be achieved. Purging the suspension with molecular oxygen results in an increase in photonic efficiencies.
Goslich et al (1997)
53
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters Description of Reactor Important findings Reference
12. Thin-Film-Fixed –Bed Reactor (TFFBR)
Phenolic wastewater
It consisted of a TiO2 coated glass plate with an effective surface area of 0.69m2. The plate was fixed by a metal frame and the reactor was aligned to facing south, sloping angle was always adjusted at 200 against the horizon. The flow rate was adjusted between 1 and 6 L/h, the total volume of the container was 5L. The thickness of the liquid film was between 80 and 140 m, resulting in a total illuminated reactor volume of 60-100 cm3
More than 70 % of relative initial TOC was degraded after 250 min. of treatment. In this reactor there is always a sufficient amount of oxygen present, since it is permanently present in the reactor due to the large surface area of the liquid in contact with the surrounding air.
Goslich et al (1997)
13. Shallow pond Reactor
4-chlorophenol (8 x10 -5 M)
The experimental facility consisted of three adjacent shallow pond reactors open to atmosphere was fabricated. All three ponds rectors were initially have the same aperture 106.7 x 53.3 cm while the respective depths of the ponds were 5.1, 10.2 and 15.3 cm. The reactors were fabricated from plywood and lined with polyethylene sheets. Each pond was equipped with a mixing facility which consists of a circulating pump and a submerged spray bar. The circulating rates were 6, 10 and 18 L/min respectively.
It was found that 4CP was successfully oxidized with the photocatalyst, TiO2, suspended in slurry or adhered to a fiber glass mesh. The reactor, perform better with the slurry. It has also been found that the first-order rate constant for oxidation of 4CP increases with decreasing initial UV intensity, catalyst loading, and initial solute concentration, the oxidation rate of 4CP is invariant provided the aperture to volume ratio is fixed. It has been determined that the 4CP solution contains sufficient dissolved oxygen to support the PCO.
Wyness et al (1994 b)
54
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
14. Flat-Plate trickle reactors
4-chlorophenol (1 10-4M)
It consisted of a rectangular stainless steel flat backing plate with dimensions 243.8 111.8 1.3 cm. A spray bar, which serves to evenly distribute the solution, is located near the top of the reactor. The spray bar has 39 evenly spaced 2.4 mm holes drilled along its length. Two 12.7 mm drain holes have been located at the bottom of the reactor.
The reaction rate constant for the slurry mode is typically two to five times greater than that for the fixed catalyst mesh tested at similar condition. In addition, the reaction rate constant appears to vary linearly with the UV isolation, and it shows no dependence on liquid film thickness in the slurry mode, but appears to vary linearly with the inverse of film thickness in the fixed catalyst mode.
Wyness et al (1994 a)
15. Rotating disk reactor
Phenol (22 mg/dm3)
The reactor consisted of three major components, a reactor vessel, a rotating disk loaded with TiO2 catalyst, and UV sources. It also included a control system for angular velocity and a controller for the UV radiation. The body of the reactor was a semicircular vessel with an inner diameter of 52 cm and a gap thickness of 3.5 cm constructed of stainless steel, had a diameter of 49.5 cm and a thickness of 0.32 cm. The capacity of the reactor was 3.5L. Four 15 W low pressure mercury UV lamps was used for artificial irradiation and solar for natural irradiation.
The solar reactor can receive solar light and oxygen from the atmosphere effectively. The phenol can decomposed rapidly under solar light than UV light. It was observed that 100 % removal was observed within 60 min.
Zhang et al (2000)
55
Table 2.4 (Continued)
Sl. No.
Types of Reactor
Pollutant/ Wastewaters
Description of Reactor Important findings Reference
16. Compound parabolic collector (CPC)
Synthetic Municipal Wastewater (COD = 250 mg/L)
The reactor consisted of two sunlight collectors, fixed on a platform inclined at 370. Each collector was consisting of three CPC modules mounted in series. The total surface area of each collector as 3m2. One module consists of eight parallel CPC reflectors with UV transparent tubular receivers. The three modules are connected in series and the wastewater flows directly from one to the other and finally from one to the other and finally to a tank. The total volume of the wastewater used during the experiments was 35L.
By an accumulation energy of 50 KJ/L the synergetic effect of TiO2 P 25 with H2O2 and Na2S2O8 leads to a 55% and 73% reduction of the initial organic carbon respectively. The photo-Fenton process shows to be more efficient when compared to TiO2/H2O2 system. An accumulation energy of 20 KJ/L leads to 80% reduction of organic content.
Kositzi et al (2004)
17. STEP reactor
4-Chlorphenol (45mg/L), formetanate (50 mg/L) and dye (50 mg/L)
The reactor was composed of 21 stainless steel stairs height: 70mm, width: 500 mm) covered with a 1m2 Pyrex sheet to limit water evaporation. The effluent flowed over the steps before being collected in a tank, from which it was elevated with a pump to the top of the steps for recirculation. The effluent volume, collector surface and flow rate was 25 L, 1 m2 and 450 m3/h respectively.
4-CP adsorption in the dark was higher. Complete removal of 4-CP was observed with in 1 h. Oxygen transfer was more due to formation of STEP. It was found that STEP reactor was more efficient when compared to CPC, for the removal of 4-CP and formetanate.
Guillard et al (2003)
56
2.6 BIOLOGICAL WASTEWATER TREATMENT-
SEQUENCING BATCH REACTOR (SBR)
Biological treatment has been the main technology capable of
reducing the contaminant level of wastewater for many years. The overall
objectives of the biological wastewater treatment are to transform
biodegradable compounds into acceptable end products, transform or remove
nutrients, capture suspended solids, and incorporate non-settleable colloidal
solids into biological flocs. The objective of the industrial wastewater
treatment is to remove and reduce the concentration of organic and inorganic
compounds. Although some of the organics are toxic or inhibitory to
microbial growth, a preliminary chemical oxidation step may eliminate
refractory or toxic substances. The main benefit of the biological wastewater
treatment is its relatively low operating cost and handling huge masses of
compounds (Ha et al 2000).
The successful design and operation require an understanding of
the type of microorganisms and organic compounds, the environmental
factors that affect the performance, and the types of reactors involved. The
successful operation and removal of dissolved compounds in wastewater are
done by a variety of microorganisms, principally bacteria. Microorganisms
oxidize the dissolved and particulate carbonaceous organics into simple
products and extra biomass. Among the environmental factors affecting the
treatment process, temperature, and pH have important effects on the
selection, survival, and the growth of microorganisms. The optimal growth of
a specific microorganism occurs in a fairly narrow range of temperature that
differs from one group of bacteria to the other. Most bacteria cannot tolerate
pH levels above 9.5 or below 4.0. Generally, the optimum pH for the growth
and survival of the bacteria lies between 6.5 and 7.5 (Tabrizi and Mehrvar
2004).
57
The activated sludge process has been traditionally applied to treat
industrial wastewater, but nature of such discharges often operational
problems in continuous flow systems. This is the case of wastewater
containing toxic compounds generated by several industries. In such
wastewaters, the mass of toxic pollutants could vary in time and space, thus,
to efficiently biodegrade these pollutants the treatment plants must be
designed with excess capacity. The main problem is that continuous reactors
are designed to work under steady-state conditions but, in reality, industrial
wastewater present great variability, despite equalizer tanks, giving transitory
conditions. Recently, innovative strategies like the discontinuous processes
(controlled unsteady-state processes) have been explored in order to increase
the degradation efficiencies of inhibitory wastewaters. The term sequencing
batch reactor (SBR) is used as a synonym of the wastewater treatment
technology, where the volume of the reactor tank is variable in time.
2.6.1 Sequencing Batch Reactor
The Sequencing Batch Reactor (SBR) is a fill and draw activated
sludge system for wastewater treatment. In this system, wastewater is added
to a single batch reactor, treated to remove undesirable components and then
discharged. Equalization, aeration and clarification can all be achieved using a
single batch reactor (Ketchum 1997). The sequence of operations carried out
for effective treatment involves five phases viz. Fill, React, Settle, Draw and
Idle as presented in Figure 2.4. A brief discussion of the five phases of the
SBR treatment is discussed herein under.
58
Figure 2.4 Typical operating sequences for sequencing batch reactor
2.6.1.1 FILL phase
The first phase in the sequence is filling of the reactor. The influent
to the tank may be either raw wastewater or primary effluent. Filling could be
achieved by pumping or through gravity. When a number of tanks are
operated, the wastewater is added by gravity, devices like an operated weir or
an automated valve are operated to divert the flow within the tanks. The tanks
are generally an earthen or oxidation ditch, a rectangular basin or any other
59
concrete or metal type structure. In the Fill phase, the wastewater is added to
the biomass, which is present in the tank from previous cycle. The liquid level
in tank increases from the set level to the maximum of 100 %. The set volume
may be as little as 25 % or as great as 70 %. The Fill phase can be designed to
terminate before the maximum level is reached, by either limiting the FILL
time to a predetermined maximum or eliminating IDLE phase.
Aeration is carried out during this phase by various diffusing
systems such as surface aerator and diffusers. A phase in Fill period, where no
mixing or aeration is carried out is called Static Fill. This phase utilises
minimum energy input and high substrate concentration at the end of the
FILL. Similarly, Mixed Fill imparts mixing of wastewater without aeration.
Level sensing devices, timers, or online probes can switch the aerators and/or
mixers on and off as desired. Fill is stopped, when the tank is full or diverted
into another tank in the SBR treatment scheme (Ketchum 1997).
2.6.1.2 REACT phase
Reactions are initiated during Fill, and are continued during this
phase. The react period is the time during which the tank receives no flow.
Alternating conditions of low dissolved oxygen concentration and high
dissolved oxygen concentration is adopted as per the requirement of
wastewater treatment. The liquid level remains at maximum throughout
React. Wasting of sludge could be carried out as means of controlling sludge
age. The duration of this phase can vary from zero to more than 50 % of cycle
time depending on the level of treatment achieved. React phase completes the
reactions initiated during the Fill phase (Ketchum 1997). Table 2.5 illustrated
bases of design for common operating policies to meet selected treatment
objectives.
60
Table 2.5 Common-operating strategies to meet the treatment objectives
Sl.No. Treatment Objective Fill policies React Policies
1 Organic carbon and suspended solids reduction, minimum energy consumption or sludge production
Static, mixed, then aerated
Aerated
2 Organic carbon and suspended solids reduction and nitrification
Static, mixed, then aerated
Aerated
3 Organic carbon and suspended solids reduction and denitrification
Static, mixed, then aerated
Aerated, followed by mixed then aerated
4 Organic carbon and suspended solids reduction and biological phosphate reduction
Mixed (short period), then aerated
Aerated
5 Industrial organic wastewater, toxic at high concentration
Mixed (short period), then aerated
Aerated (long period)
2.6.1.3 SETTLE phase
The reaction completed in the React phase is allowed to settle
down, wherein the solid liquid separation takes place under quiescent
conditions. Generally no inflow or outflow from the tank is practised. The
major advantage in the SBR is clarification process which is carried out in the
same tank. The biomass is retained in the tank until some fraction is decided
to be wasted. The time in Settle typically ranges between 0.5 and 1.5 h.
Sludge wasted at the end of Settle periods is harmful, as sludge may rise to
the surface (Ketchum 1997).
61
2.6.1.4 DRAW phase
This phase clearly means removal of clarified effluent. The various
mechanism for withdrawal of effluent are, i) a pipe fixed at some
predetermined level with the flow regulated with an automatic valve, ii) a
pump, depending upon the hydraulic grade line of the system, iii) an
adjustable or floating weir at or just beneath the liquid surface can be used.
The period of draw can range from 5 to more than 30 % of the total cycle time
(Ketchum 1997).
2.6.1.5 IDLE phase
The phase between Draw and Fill is termed Idle. Sludge can be
wasted effectively during Idle. The frequency of sludge wasting can be
designed to range between each cycle to once in every 2 to 3 months.
Aeration or mixing can be provided, after sludge wasting. Idle can be avoided
depending upon treatment policy by filling the tank, as soon as the tank in
Draw reaches the minimum liquid level (Ketchum 1997).
2.6.2 Design Criteria of SBR
Design of SBR systems to treat industrial wastewaters always
requires the use of treatability studies to determine appropriate SBR operating
times, including total aeration times and rates, and the need for chemical
additions for nutrients or pH control. Table 2.6 shows the design key
parameters for a wastewater system (US EPA 1999).
62
Table 2.6 Key design parameters for a conventional load
Sl. No.
Parameter Municipal wastewater
Industrial wastewater
1 Food to Mass (F:M) 0.15 – 0.4 / day 0.15 – 0.6 /day
effluent. Hence, there is a limitation associated with the biological treatment
due to the toxicity exerted by phenol itself at higher concentration. SBR
suffered by several constraints when used for toxic wastewater degradation,
inhibition of the microorganisms, problems with shock loads of toxic
compounds, deacclimation of the microorganisms and low efficiencies
regarding the removal of toxic compounds.
Shock loads appear when the toxic concentration in the influent
greatly increases as a consequence of changes in the manufacturing process,
for example, during the cleaning of the production units. A toxic shock
produces an increase in reaction time, diminishes the efficiency and may
posion the biomass (Watanabe et al 1996). Research efforts have been made
to overcome the difficulties associated with the treatment of an inhibitory
compound like phenol. The use of advanced oxidation processes in
conjunction with the biological oxidation has been a recent innovation in the
treatment strategies for wastewater treatment.
71
Table 2.8 Typical findings observed in the representative works related to biological treatment
Sl. No.
Compound/wastewater Experimental condition and Type of equipment Important findings of the work Reference
1. Bleached Kraft mill wastewater (COD=1400 mg/L)
Two activated sludge bioreactors were constructed out of 6 mm thick plexiglass. The height of each reactor was 25.5 cm and inner diameter was 19 cm. The inner volume 10 L contained 5 L of activated sludge. Aeration was supplied by passing compressed air. The MLSS was maintained in the range of 1500-2000 mg/L
At steady state (HRT 10-12 h, SRT 12-15 d), the percentage removal of BOD, COD, and toxicity averaged 87, 32, 97 respectively. Varying HRT between 12 and 4 h and SRT between 5 and 15 d indicated that HRT had more of an effect on treatment performance than SRT. Longer HRT (12 h) led to improved BOD (87 %), COD (50 %), toxicity (90%) removal, while longer SRTs were not shown to significantly affect the performance.
Barr et al (1995)
2. Piggery waste water (TOC = 5860 mg/L)
The anaerobic reactors had an active volume of 1.5 L. The temperature was kept constant at 350 C. Two aerobic reactors were used in this study. The first one had an active volume of 1.5 L. When the organic carbon load of the system was double, a 4 L reactor capacity was used. Aeration was provided by compressors to plastic tube placed at the bottom of the reactors.
Combined anaerobic-aerobic system was investigated using two Lab-scale sequencing batch reactors. The cycle length was 24 h. Three recycle-to-influent ratios from 1 to 3 were tested. Average performances of the overall process, in the different conditions tested, were a TOC removal of 81 to 91% and 85 to 91 % of TKN.
Bernet et al (1999)
3. Oil shale ash leachate (COD = 2000-4600 mg/L)
The anaerobic reactors were glass upflow anaerobic sludge blanket reactors (height 24 cm, diameter 3.2 cm) with total liquid volume of 0.2 L. The aerobic reactors were plastics activated sludge reactors, each with n aeration unit (liquid volume 0.2 L) and a separate settling unit (liquid volume 0.2 L). Aerobic conditions (>2 mg/L) and mixing in the aerobic reactors were assured through continuous aeration with submerged diffusers in each vessel.
In the sequential anaerobic – aerobic processes, the COD removals were most of the time slightly lower at 100C (67 %) than at 200C (78 %) while the BOD for 7 days removals was 97-99 % at both temperatures. In the single aerobic process at 200 C, the COD removal was 65 % while at 70 C the COD removal was 54 %. After the feed was changed from leachate to phenol in the 70 C aerobic reactors, the COD removal stabilized to about 95 %. In all the leachate treatment processes studies, the total phenols removals were on average 78-86 %. The anaerobic stages removed total phenols minimally
Kettunen et al (1996)
72
Table 2.8 (Continued)
Sl. No.
Compound/wastewater Experimental condition and Type of equipment Important findings of the work Reference
4. Paper mill process water (COD=2113 mg/L)
Two 4 L stainless steel activated sludge reactors were used. Both reactors were divided in five compartments, each compartment containing 800 mL of mixed liquor. Each compartment was aerated with pressurized air, sparged through an aeration stone plug flow was maintained. COD: N: P = 100:2.6:0.4
Two lab-scale plug flow ASP reactors were run in parallel for 6 months, a thermophilic reactor at 550 C and a reference reactor at 300 C. Both reactors were operated simultaneously at 20,15 and 10 days SRT. COD removal percentages were 58 5 % at 300C and 48 10 % at 550 C. The effect of the SRT on the total COD removal was negligible. At 300 C, COD removal percentages were 65 %, 75 % and 86 % at 20, 15, and 10 days SRT respectively. At 550C, these percentages were 48 %, 40 % and 70 % respectively. The mesophilic activated sludge showed a higher removal of total COD as compared to the thermophilic reactor.
Vogelaar et al (2002)
5. Pulping whitewater (COD=2500 mg/L)
A lab-scale Plexiglas reactor with a total liquid volume of 8.55 L was used in the study. The reactor was filled with 58 % Kaldnes carrier elements, occupying 11 % of the reactor’s liquid volume. The reactor was kept in a kept in a temperature-controlled water batch at 550C. Mixing and aeration were provided by pressurized air through ceramic aerators in the bottom of the reactor.
The continuously operated Lab-scale Kaldnes moving bed biofilm reactor (MBBR) was used for aerobic treatment of pulping white water. Inoculation with mesophilic-activated sludge gave 60-65 % COD removal from the first day onwards. During 107 days of experiment the 60-65 %. SCOD removals were achieved at organic loading rates of 2.5-3.5 Kg COD/m3/d, the highest loading rates applied during the run and HRT of 13-22 h.
Jahren et al (2002)
6. Sugar beet wastewater (COD=2500 mg/L)
Lab-scale mesophilic (20-350C) and thermophilic (550C) ASP was made of PVC and consisting of aeration tank of volume 1.5 L and settling units (0.4 L). Aquarium aerators were used for aeration. SRT was 116 d in the mesophilic ASP and 53 d in the thermophilic ASP. The MLSS and MLVSS were 5.6 g/L and 3.7 g/L respectively.
In the ASPs, the HRT was 12 h in both processes, corresponding to a volumetric loading rate of 3.2 1.0 kg COD/m3/d. The Mesophilic ASP gave 79 18 % and the thermophilic ASP gave 50 6 % of COD removals. Both ASPs gave 90 % COD removals at 24 h HRT.
Suvilampi et al (2005)
73
Table 2.8 (Continued)
Sl. No.
Compound/wastewater Experimental condition and Type of equipment Important findings of the work Reference
7. Phenol (500 mg/L)
Experiments were performed in a column type SBR @ 250C. The reactor had a working volume of 20L, with an internal diameter of 50.0 mm. Air at a flow rate of 3.5 L/min was introduced by a fine bubble aerator in the bottom of the column. The reactor was operated sequentially in 4 h cycles.
The reactor was started at a loading rate of 1.5 kg phenol/m3/d with phenol-enriched activated sludge as inoculums. The phenol loading was then adjusted stepwise to a final value of 2.5 kg phenol/m3/d. At this high loading, phenol was completely degraded and high biomass concentration was maintained in the rector. High specific phenol degradation rates exceeding 1 g phenol g/VSS/d were sustained up to phenol concentration of 500 mg/L and significant rates continued to be achieved up to a phenol concentration of 1,900 mg/L.
Tay et al (2004)
8. Paper mill wastewater (COD=1000 g/m3)
Four-bench scale SBRS were fed with pre-settled wastewater from a paper mill. The volume of each reactor was 6.5 L. All reactors were operated with the total cycle time of 8 hours, i.e. three cycles daily. In addition a fourth reactor was operated with total cycle time of 24 hrs. The COD: N: P was 100:3:1. MLSS was maintained in the range of 2600-5000 mg/L.
The influence of process conditions, like duration of filling phase (Fill) or duration of reaction phase (REACT) could be demonstrated. The highest COD removal of 92 % and the best sludge settling properties for the paper mill wastewater were obtained at a sludge age of 20 days with REACT period of 12 hours and duration of FILL phase of 0.5 hours.
Franta et al (1996)
9. Phenolic wastewater (1300 mg/L)
The bench-scale reactor comprised a Perspex cylinder with a detachable base-plate secured to one end with screws and an O-ring seal. The top of the cylinder was enclosed with a removable cover in two halves. Aeration was provided at the base of the cylinder through two air-stones at opposite ends. A magnetic stir-bar ensured complete mixing in the reactor. MLSS was maintained in the range of 3500-3900 mg/L.
The investigation demonstrated the capability of a bench-scale sequencing batch reactor (SBR) to biodegrade an inhibitory substrate at a high loading rate of 3.12 kg phenol/ m3/d (2.1 g COD /g MLVSS/ d) with a COD removal efficiency of 97 % at a SRT of 4 days and a HRT of 10 hours was achieved. The SBR was operated at 4 hours cycle, including 3 hours react phase.
Yoong and Lant (2001)
74
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment
Important findings of the work Reference
11. 2,4-Dichloro-Phenoxyacetic acid (100 mg/L)
Four identical lab-scale SBRs were used. Each rector was made of 6 mm thick Plexiglas cylinders with an internal diameter of 100 mm. The operating liquid volume was 2.0 L. Aeration was supplied through submerged diffuses placed at the bottom of the rectors. Temperature was kept constant at 220 C. The SRT was kept constant at 20 days HRT was varied in the range of 12-48 hours.
A bench scale study using SBR was conducted to investigate the effects of HRT, the presence or absence of supplemented substrate and variation in feed concentration on the degradation of 2,4-D was studied. A long acclimation period (about 4 months) was observed before 2,4-D biodegradation was established. At steady-state operation, all reactors achieved complete removal (>99 %) of 2,4-D and the corresponding supplemental substrate, regardless of the HRT applied, ranging from 12 to 48 h. The 2, 4-D specific removal rates were affected by the type of substrate used (Phenol or dextrose) being significantly lower (30 to 50 %) in the case of dextrose.
Magnat and Elefsiniotis (1998)
12. Tannery soak liquor (COD =1500 – 3600 mg/L)
The lab-scale SBR had a volume of 10 L. An air compressor delivering airflow of 1.2 L/min supplied aeration. Each cycle lasted for 24 h, the reaction took place in 22 h, the settling in 1 h 30 min and the withdrawal and filling of the treated effluent and influent in 30 min.
Once the acclimation of the microorganisms was achieved. Optimum removal efficiencies of 95 %, 93 %, 96 % and 92 % on COD,PO4
3-, TKN and SS, respectively, could be reached with 5 days hydraulic retention time (HRT), an organic loading rate (OLR) of 0.6 kg COD m3/d and 34 g NaCl/L. The organisms responsible for nitrogen removal appeared to be the most sensitive to the modifications of these parameters.
Lefebvre et al (2005)
13. Brewery wastewater (COD= 212 mg/L)
Experiments were carried out in a cylindrical column reactor (110 cm filling height, 10 cm dia.) with a working volume of 8.6 L. Aeration was provided by means of air bubble diffusers at a volumetric flow rate of 500 L/h. Temperature was controlled at 25 ± 2 _C. The reactor was operated in SBR mode with total cycle duration of 6 h.
Aerobic granular sludge was cultivated in a sequencing batch reactor fed with brewery wastewater. After nine-week operation, stable granules with sizes of 2–7 mm were obtained. After granulation, high and stable removal efficiency of 88.7 % COD was achieved at the volumetric exchange ratio of 50% and cycle duration of 6 h. The average total COD and soluble COD of the effluent were 212 and 134 mg/L, respectively, and the average effluent ammonium concentration was less than 14.4 mg/L. Nitrogen were removed due to nitrification and simultaneous denitrification in the inner core of granules.
Wang et al (2007)
75
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment Important findings of the work Reference
14. Tannery wastewater (COD = 4800 mg/L)
A bench-scale SBR made of plexiglass and of working volume 8 L was used for the study. Aeration was provided from a diffuser at the base of the reactor.
Measurement of oxygen uptake rates (OUR) and corresponding COD uptake rates showed that a 12 h operating cycle was optimum for tannery wastewater. At a 12 h SBR cycle with a loading rate of 1.9– 2.1kg/m3/d, removal of 80–82 % COD, 78–80 % TKN and 83–99 % NH3-N were achieved. These removal efficiencies were much higher than the conventional aerobic systems.
Ganesh et al (2006)
15. 4-chlorophenol (200 mg/L)
A fermenter with 5 L working volume was used as the sequencing batch reactor (SBR). Aeration was provided by using an air pump. Agitation speed was varied from 25 to 300 rpm depending on the operation phase.
Percent nutrient removals increased with increasing sludge age and decreasing 4-CP concentrations. Low nutrient removals were obtained at high initial 4-CP concentrations especially at low sludge ages. However, high sludge ages partially overcome the adverse effects of 4-CP and resulted in high nutrient removals. COD, NH4-N, PO4-P and 4-CP removals were 76 %, 72 %, 26 % and 34 % at a sludge age of 25 days and initial 4-CP concentration of 200 mg /L. Sludge volume index (SVI) also decreased with increasing sludge age and decreasing 4-CP concentrations.
Kargi and Konga (2006)
16. Paper factory effluent (COD = 1100 mg/L)
Continuous treatment of the phenolic effluent was performed with a packed bed reactor in a glass column (30 cm length and 6 cm diameter). Immobilized cells in beads were packed to a height of 25 cm in the glass column. The reactor was fed with effluent at varying flow rates of 2.5, 5.0, 7.5 and 10 mL/h.
Treatment of the paper factory effluent was done with free and immobilized cells of a phenol degrading Alcaligenes sp. d2. The free cells could bring a maximum of 99 % reduction in phenol and 40 % reduction in COD after 32 and 20 h of treatment, respectively. In the case of immobilized cells, a maximum of 99 % phenol reduction and 70 % COD reduction was attained after 20 h of treatment under batch process. In the continuous mode of operation using packed bed reactor, the strain was able to give 99 % phenol removal and 92 % COD reduction in 8 h of residence time The optimum flow rate was 2.5 mL/h and the half life period was 76 h.
Nair et al (2007)
76
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment
Important findings of the work Reference
17. Electroplating wastewater ( COD = 5l6 mg/L)
Six 10 L reactors made from acrylic plastic were used. The size of each reactor was 18 cm diameter and 40 cm height. The total volume and working volume of the reactor were 10.0 and 7.5 L, respectively.
SBR system showed the highest COD, BOD5, TKN and cyanide removal efficiencies of 79 %, 85 %, 49 % and 97 %, respectively with 4-times diluted rice noodle wastewater (RNWW) containing 10% EPWW and 138 mg/L NH4Cl (BOD5: TN of 100:10) at SRT of 72 ± 13 days (under organic and cyanide loadings of 0.40 kg-BOD5/m3 d and 0.0023 kg-CN/m3 d, respectively).
A continuously fed stainless steel anaerobic UASB and aerobic CSTR reactor were used in sequence for the experimentation. The UASB reactor had 2.5 L of effective volume with an internal diameter of 6 cm and a height of 100 cm. The CSTR reactor consisted of an aeration tank (effective volume = 9 L) and a settling compartment (effective volume = 1.32 L).
Hydraulic retention times were changed to determine the effect of HRT on removal efficiencies of colour, COD and total aromatic amine (TAA) through 186 days containing 46 days of steady-state and acclimation periods. COD and colour removal efficiencies varying between 97 % and 91 % and between 84 % and 91 % were obtained at a total HRT of 19.17 and 1.22 days in combined anaerobic/aerobic system, respectively. In the sequential aerobic stage the significant part of TAA was removed successfully while the colour removal slightly increased with TAA removal efficiencies of 70–85 % at total HRTs of 8.85 and 6.05 days, respectively. Increases in HRT provide enough time for partial mineralization of COD and intermetabolites in anaerobic and/or anaerobic/aerobic systems.
Isik et al (2008)
19. Textile wastewater (COD =1000-4000 mg/L)
The aerobic system used was a combined CSTR and FFB bioreactor. The continuous stirred tank reactor with a 700 mL working volume was used. The FFB is a 1.5 L continuous flow with three compartments packed with a rippled cylindrical polyethylene support.
This system gives high COD and colour removal efficiencies of 97.5% and 97.3 %, respectively, obtained with a total hydraulic retention time (HRT) of 4 days and total OLR of 0.29 g / L /d by the sloughing of biofilm, and the washout phenomena.
Khelifi et al (2008)
77
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment
Important findings of the work Reference
20. Phenol ( 250 mg/L)
The pre-denitrification system consisted of an anoxic upflow sludge blanket reactor (0.8 L) and an aerobic activated sludge reactor (1.8 L). The system was provided with a liquid displacement biogas measurement device.
The effect of phenol overloads on the removal of organic matter and nitrogen compounds was studied. During the overloads from 250 to 4000 mg/L, phenol was detected in the effluent of the anoxic reactor but the system recovered fast after stopping the overloads. TOC removal remained unchanged during phenol addition (91.9 % at 0.20 kg TOC/m3/d), except for the highest overload. Phenol concentrations from 250 to 4000 mg/L were added to the feed. Phenol was completely removed despite the presence of other carbon sources in the wastewater. In spite of the presence of phenol, a TOC removal around 91.3 % was achieved at an average organic loading rate of 0.11 kg TOC/m3 d.
Eiroa et al (2008)
21. Paper mill effluent
Four identical 4 L reactors were constructed with acrylic boards. MLSS concentration was between 3000 and 5000 mg/L with a sludge age of 5-10 days.
Laboratory scale research on the effects of operating parameters, including mixed liquor suspended solid (MLSS) concentration, volumetric exchange rate (VER), aeration time, temperature and daily operation cycle on biological treatment of the pulp and paper mill effluent was studied using four 4 L sequencing batch reactors (SBR). The results showed that COD removal efficiency was up to 93.1±0.3 % and the volumetric loading reached 1.9 kg BOD m3 /day under optimal operating conditions.
Tsang et al (2007)
22. Fish market wastewater
The four reactors were made of acrylic cylinders having an outer diameter of 24 cm and a height of 34 cm. The effective working volume was 10 L, with 6 L of biomass. SRT was varied between 20 and 100 days.
The effects of COD/N ratio (3 – 6) and salt concentration (0.5 – 2 %) on organics and nitrogen removal efficiencies in three sequencing batch reactors (SBRs) with synthetic wastewater and one SBR with fish market wastewater were investigated under different operating schedules. The solids retention time (SRT, 20 – 100 days) and aeration time (4 – 10 h) was also varied to monitor the performance. For synthetic wastewater, COD removal efficiencies were consistently greater than 95 %, irrespective of changes in COD/N ratio, aeration time and salt concentrations. Increasing the salt concentrations decreased the nitrification efficiency, while high COD/N ratio’s favored better nitrogen removal (>90 %). The treatment of real saline wastewater (>3.2 %) from a fish market showed high COD (>80 %) and nitrogen (>40 %) removal efficiencies despite high loading rate and COD/N fluctuations, which is due to the acclimatization of the biomass within the SBR.
Rene et al (2008)
78
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment
Important findings of the work Reference
23. Combined pharmaceutical and tannery wastewaters (COD = 2200 mg/L)
The 4 L fill-and-draw reactor, equipped with diffused aeration devices was operated at steady state with a sludge age of 10 days and a hydraulic retention time of 1 day.
The tannery sample was a plain-settled effluent having a total COD of around 2200 mg/L with a readily biodegradable fraction of 15 %. The same fraction was 57 % in the pharmaceutical wastewater sample having a much stronger total COD content of 4043 mg/L. Consequently, mixing of the pharmaceutical effluent with the tannery wastewater up to 38 % of the total COD in the mixture increased the readily biodegradable COD fraction but had an inhibitory effect on the biodegradation kinetics. This effect was relatively lower on growth, but quite significant on the hydrolysis of the slowly biodegradable COD decreasing the maximum hydrolysis rate from 2.0 day−1 to 1.2 day−1.
Cokgor et al (2008)
24. o-cresol (100 – 600 mg/L)
A slurry batch reactor of capacity 2 L was used as bioreactor. Aeration was provided at the base of the reactors. MLSS was maintained in the range of (2000 – 3000 mg/L)
The biodegradation kinetics of o-cresol was examined by varying initial o-cresol concentrations (30 – 600 mg/L), MLSS (1000 – 11500 mg/L) and aeration rates (0.05 – 1 L/min). It was found that about 7 h were required to biodegrade a phenol of 600 mg/L completely. The biodegradation rate was independent of the aeration rate Q ≥ 0.25 L/min. when Q ≤ 0.25 L/ min oxygen supply by the aeration was not sufficient to degrade the O-cresol.
Maeda et al (2005)
25. Phenol (100 – 1000 mg/L)
Batch reactor of dimensions 20 x 15 x 25 cm with total working volume of 5 L operated at a cycle time of 12 h .MLVSS was maintained in the range of 2000 – 4000 mg/L
It was observed that the inhibitory effect seemed to be more pronounced with the increase in the influent phenol concentration from 100 – 1000 mg/L. The k value was reduced from 290 x 10 -3 min-1 to 4.3 x 10 -3 min-1 when the phenol concentration was increased from 100 mg/L to 1000 mg/L. The phenol removal efficiency of 99 % was observed.
Chan et al (2007)
79
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment
Important findings of the work Reference
26. Reactive textile dye (60 – 100 mg /L)
The experimental system used was composed of two 1 L reactors operating in a sequencing batch mode (SBR) in 24 hour cycle with five discrete periods fill – 50 min, react-21 h , settle -1 h , draw -55 min and idle -15 min.
When comparing with sludge retention time of 15 days the total COD removal was around 80 % with 30 % removed anaerobically. After 40 – 50 days of acclimatization the colour removal efficiency reached a maximum, stable value of 90 % from a feed by concentration of 90 mg/L. Reduction of the SRT value from 15 to 10 days reduced the biomass concentration from 2.0 to 1.2 VSS/L and lowered colour removal levels from 90 % to 30-50 % then the SRT value was increased back to 15 days the colour removal efficiency of the system was completely recovered, suggesting that with a SRT of 10 days the adequate microbial population could not be installed in the reactor.
The activated sludge reactor had a rectangular shape with 650 mm length x 400 mm width x 350 mm liquid depth. The aeration was carried out using a diffused aeration system. The MLSS was maintained in the range of 3900 - 4200 mg/L.
The initial studies on the efficiency of the activated sludge reactor were carried out using diluted raw PoME for varying the HRT viz. 18, 24, 30 and 36 h and influent COD concentration, viz , 1000, 2000, 3000, 4000 and 5000 mg/L , respectively. The results showed that at the end of 36 h HRT, the COD removal efficiencies were found to be 83 %, 72 %, 64 %, 54 % and 42 % where at 24 h HRT, the COD removal to 57 %, 45 %, 38 %, 30 % and 27 % respectively.
A cylinder shaped reactor of acrylic plastic with 12 L maximum capacity and 175 mm in diameter and 500 mm in height was used. Air was supplied to the reactor through the air diffuses and controlled by solenoid valve. SBR cycling time was 8 hours
Treatment performance of the mixture of phenol and 2, 4 – DCP by the BAC-SBR and the SBR systems was studied by changing influent concentration and SRT. By increment of the influent COD concentration from 480 to 560 mg/L, the COD removal efficiency in BAC-SBR decreased from 92.1, 95.9 and 97.0 % to 90.8, 94.9 and 96.8 % for SRTs of 3, 5 and 8 days respectively. Meanwhile the COD removal efficiency in the SBR system changes from 85.9, 91.6 and 95.1 % to 83.6, 92.2 and 95.3 % for SRTs of 3, 5 and 8 days. When the SRT was increased from 3 to 8 days the COD removal efficiency was increased from 90.8 – 97 % in BAC-SBR and 83.6 – 95.3 % in the SBR.
Ha et al (2000)
80
Table 2.8 (Continued)
Sl. No.
Compound/wastewater
Experimental condition and Type of equipment Important findings of the work Reference
29. Kraft mill wastewater
Aerobic reactor of 4.5 L capacity was used. The OLR was varied from 0.4 to 1.42 g BOD/L/d and HRT was varied from 48 to 4.5 h SRT varied between 25 and 30 days
The result showed that high BOD (90 %) and COD (58 %) removal was observed when HRT varied from 16 to 6 h. Degradation of phenolic compounds was seriously affected by HRT variation, obtaining the highest removal efficiency at HRT of 48 h (33.5 %) and a minimal efficiency at HRT of 4.5 h (3.6 %). When HRT less than 6 h, the system showed destabilization and COD, BOD and SS removal decreased.
Diez et al (2002)
30. Bleached Kraft mill effluent (COD=880 mg/L)
Aerated Lagoon with an aerated (0.44 L) and a settling zone (0.22 L) was used as biological treatment. HRT was 44.5 and 45.2 h.
An aerobic lagoon was able to remove over 98 % and 80 % of the BOD and abietic acid respectively, when operated at HRT of 2 days. Under these conditions, the COD removal efficiency of 67.3 % was observed
Belmonte et al (2006)
31. Coke wastewater (COD = 3275 mg/L)
Aerobatic reactor consisted of 20 L volume made up of PVC. Oxygen was introduced through the bottom. MLVSS was maintained in the range of 1000 – 2000 mg/L
When bicarbonate was added, the maximum removal efficiencies of 71 %, 65 % and 97 % were obtained for ammonium nitrogen, COD, Phenols respectively for HRT of 54.3 h. When bicarbonate was not added this efficiency were 71%, 75% and 98% respectively. The biodegradation of phenol (504 mg/L) improves with increasing pH, achieving 96 % at pH 8 in 15 h.
A Lab-scale biological plant composed of two aerobic reactors operating at 35˚C. HRT was varied from 27-98 h.
When an effluent recycling ratio of 2 is employed, average removal efficiencies of 86.2, 98.8, 97,9 and 99.3 % for COD, phenol, SCN- and NH4
+-N respectively was obtained for a total HRT of 184 h.
Vazquez et al (2006 b)
33. Pulp and Paper mill wastewater (COD = 650 mg/L)
The study was carried out in a subsurface flow wetland (30.7 m2). It was divided into eight cells each size of 3.2 x 1.2 x 0.8 m. The depth was 2.3 m.
Initial 15 months results, indicate that removal efficiencies for phenol were 60 % at 5 day HRT and 77 % at 3 day HRT. It was thought that the longer retention time might have caused oxygen and nutrient deficiencies.
Abira et al (2005)
81
2.8 COUPLED ADVANCED OXIDATION AND BIOLOGICAL
PROCESSES FOR WASTEWATER TREATMENT
Wastewaters from chemical, pharmaceutical, and dye industries
most often contain significant amount of non-biodegradable organic
compounds. The elimination of these non-biodegradable toxic contaminants is
required before biological treatment. Although the biological treatment of
wastewater is often the most economical alternative process when compared
to other treatment options, the ability of a compound to undergo biological
degradation depends on a variety of factors. Such factors include the
concentrations, chemical structures, and the biodegradability of the target
molecules. Characteristics of the wastewater, such as pH, alkalinity, or the
presence of an inhibitory compound matrix, could also pay an important role
in the biological degradation of pollutants. Although many organic molecules
are readily biodegradable, many other synthetic and naturally existing organic
molecules are biorecalcitrant i.e., resistant to biodegradation (Tabrizi and
Mehrvar 2004).
Depending on the nature of the pollutants and the level of
contaminants, detoxification might be difficult and/or expensive to achieve by
conventional biological methods. In such cases, biological processes alone are
not able to reach effluent standards for the discharge into municipal sewer or
into surface water, therefore a pre-treatment or post-treatment is required. The
choice of the correct combination system must be carried out considering
several factors, both technical (treatment efficiency, plant simplicity,
flexibility, etc.) and economical (capital and operating costs including reagent
and energy consumption , sludge and gas disposal, maintenance, etc.) aspects.
In several cases, specific experimental tests are required in order to assess
actual efficiency and proper treatment conditions. Moreover, advanced
oxidation processes (AOPs) such as UV, UV/H2O2, UV/O3, UV/H2O2/O3, and
82
UV/TiO2 have been used as an attractive alternative for the treatment of these
types of wastewaters. AOPs are technologies for the production of highly
reactive intermediates, mainly hydroxyl radicals (˙OH), which are able to
oxidize almost all organic pollutants. Advanced oxidation processes can
reduce pollutant concentrations, and some processes produce more oxidized
compounds, which are in most cases more easily biodegradable than the
former ones. Although AOPs are expensive to install and operate, they may
be unavoidable for the tertiary treatment of refractory organics present in
industrial effluents to allow safe discharge of industrial contaminants. Despite
the effectiveness of AOPs, there are several scenarios that make them
economically disadvantageous. Effective treatment of a particular industrial
wastewater may require a combination of AOPs and biological processes in
order to exploit their individual quantities and, thus, reach the desired quality
within reasonable economical limits.
On one hand, AOPs have shown their worthiness for toxic
compounds elimination in water and wastewater treatment, however, the total
mineralization through these processes is very expensive. On the other hand,
biological treatment is relatively cheap and reliable process but there are
substances, which are unable to deal with. A combination of both processes
would mean a cheaper option for total organic degradation from a toxic
wastewater or a wastewater containing refractory organics (Tabrizi and
Mehrvar 2004).
It has been shown that the combination of advanced oxidation and
biological processes has the following advantages:
i) Advanced oxidation and biological processes are
accompaniments of each other.
83
ii) AOP pre-treatment can protect the microorganisms from
inhibitory or toxic compounds.
iii) Decrease in chemical cost by using cost-effective biological
pre or post-treatment.
iv) Total residence time is flexible as a result of different choices
that are possible for photocatalytic and biological reactor
residence times in a constant efficiency.
v) Achieving total mineralization for the organics while
minimizing the total cost.
2.8.1 Strategy for Coupling AOP and Biological Treatments
As a general treatment strategy, four types of treatment for a
chemical compound are possible. In some cases only biological treatment
alone is sufficient to enhance the effluent quality. In the presence of some
refractory or toxic compounds in wastewater, chemical pre-treatment is
required. In case biological treatment is not sufficient for biodegradable
compounds, chemical post treatment is also necessary. In some rare cases,
combination of chemical and biological treatment in multi-stages is necessary.
A general strategy that can be used to develop a coupled advanced oxidation
and biological processes for the treatment of a certain wastewater, which
might contain non-biodegradable or toxic organics are, as a first step to avoid
utilization of high cost due to AOPs, it must be confirmed that whether the
wastewater contains recalcitrant or toxic organics. If the wastewater is
biodegradable, conventional biological reactors are used to treat the
wastewater.
If it is confirmed that wastewater contains recalcitrant or toxic
organic, it would be pre-treated by AOPs to modify the structure of pollutants
by transforming them into less toxic and easily biodegradable intermediates,
84
which are degraded in the subsequent biological reactor in a shorter time. This
method can also prove to be less expensive in comparison to the AOPs alone
and less time consuming compared to the biological process. Moreover, if the
wastewater from the final biological reactor has met the requirements, it will
leave the treatment plant, otherwise it has to go through the previous cycle
(Parra et al 2002). The information about the toxicity and the biodegradability
of wastewater treated by AOPs allows us to determine an optimum treatment
time in the AOPs reactor of the coupled system. The time should be the best
compromise between the efficiency of the chemical reactor and its cost. The
shorter reaction time avoids the high electrical cost of the treatment. At longer
photo-treatment time, the photochemical efficiency is improved by the
unnecessary photo-degradation of pollutants that are biological degraded.
However, the overall efficiency remains almost constant. This implies higher
energy consumption without beneficial effect, as about 60 % of the total
operational cost is electricity (Sarria et al 2003). However, if the reaction time
is too short, the intermediates remaining in the system could still have
toxicological or bio-recalcitrant effects. The four types of wastewater, which
have potential for increasing treatment efficiencies by coupled processes are
recalcitrant compounds, biodegradable wastes with small amounts of
recalcitrant compounds, inhibitory compounds and intermediate dead-end
products (Gogate and Aniruddha 2004 a).
2.8.1.1 Recalcitrant compounds
Chemical pre-treatment with oxidants is a capable method of
converting recalcitrant pollutants into easily biodegradable compounds.
However, biodegradable compounds, which are not oxidized by chemical
means, may slow down the whole process. This group consists of large
molecules such as polymers that cannot be degraded easily due to their large
size and not having enough reactive sites. Partial oxidation of COD is
85
favourable for subsequent biodegradation, whereas further mineralization can
reduce the efficiency of the biological treatment. If the following step is
biological treatment, pre-treatment favours the organic pollutants by an
increase in the maximum microbial specific growth rate and decrease in the
inhibitory effect by microorganisms. Isoproturon was removed by using
photocatalysis followed by fixed bed reactor successfully. The reaction time
with TiO2 was 60 min with complete removal of Isoproturon and 5% of
dissolved organic carbon remained in solution after the biological process.
But AOPs alone could only remove dissolved organic carbon by 20%
(Pulgarin et al 1999).
2.8.1.2 Biodegradable wastes with small amounts of recalcitrant
compounds
To achieve most effective cost effective treatment, only recalcitrant
compounds should remain after biological pre-treatment. The aerobic
treatment as a first step in treating black olive mills wastewater enhances the
later ozonation by removing the most biodegradable compounds. The post-
ozonation is capable of degrading the remaining non-biodegradable matters as
well as most of the phenolic compounds not removed before (Benitez et al
2003).
2.8.1.3 Inhibitory compounds
Using chemical treatment alone may not be an effective technique
to remove some highly toxic and stable compounds. The pretreatment is
necessary to modify the structure of contaminants by changing them into less
toxic and easily biodegradable by-products. This enhances the following
biological treatment to degrade the organics in a short period of time. Acute
toxicity measured as luminescence inhibition of Vibro fischeri was affected
86
by ozone as a first process. Therefore, the wastewater from ozonation can be
treated by biological process without the risk of having any inhibitory
compounds in the pre-treated tannery wastewater (Jochimsen and Jekel 1997).
Olive mill wastewater contains some organics that are toxic to some
methanogenic bacteria, and their elimination by pretreatment reduces the
toxicity of the subsequent aerobic treatment (Beltran et al 2001). Textile
wastewater was also pretreated by photochemical technology followed by an
activated sludge process successfully (Ledakowicz and Gonera 1999). The
pretreatment step decreased the inhibitory effect of wastewater and made it
suitable for biological treatment.
2.8.1.4 Intermediate dead-end products
Generally, to make sure that chemical pre-treatment can be utilized
as a first step followed by a biological process, it is important to obtain
information concerning the chemical nature of intermediates formed during
the pretreatment as well as to know the toxicity, biodegrability, and the
evolution of ions. Although toxicity may be decreased with chemical pre-
treatment, some compounds depending on the nature of the intermediates
produced and the chemical oxidants used may also increase the toxicity. It
was shown that after photo treatment, the solution resulting from the
degradation of metobromuron was not appropriate for the biological
treatment, as the intermediates were more toxic than the parent compounds. It
has also been shown that choosing the correct AOPs or biological treatment
had an important effect on the final efficiency of the wastewater. Moreover, to
have a much more effective treatment of wastewater, the first chemical
pretreatment has to be chosen properly, therefore it will facilitate the next
biological treatment. It was observed that different oxidants have different
influence on TOC removal of isoproturon and metobromuron as a function of
irradiation time (Parra et al 2000).
87
2.9 OVERVIEW OF WORK DONE ON COUPLED ADVANCED OXIDATION AND BIOLOGICAL TREATMENT
PROCESSES
Recently, the combination of advanced oxidation (AOP) and
biological treatment processes has been proposed. The coupling of
photocatalysis and a biological treatment (activated sludge) has been studied
by some authors, but literature is still scarce. From the Table 2.9 it was
observed that the combination of photocatalysis followed by activated sludge
was mainly used, showing that biodegradability of the effluents has always
been enhanced by the photocatalytic treatment with periods of irradiation
ranging from minutes to several hours. In most cases there was just one
photochemical reactor followed by biological reactor in series. However,
there are three cases in which there is a biological pre-treatment followed by a
chemical treatment step.
The AOP followed by the biological treatment (AOP-biological
treatment) could be justified if bio-recalcitrant compounds are easily
degradable by the AOP and the resulting intermediates are easily degradable
by the biological treatment. As the AOP-biological treatment, photocatalytic-
biological treatment of atrazine (Chan et al 2004), pulp mill bleaching
wastewater (Yeber et al 1999), 6-chlorovanillin (Yeber et al 2000), and azo
dyes and wool textile wastewater (Chun and Yizhong, 1999), photocatalytic/
photofenton–biological treatment of isoproturon (Sarria et al 2002),
ozonisation–biological treatment of phenolic acids (Amat et al 2003), and
UV-biological treatment of polycyclic aromatic hydrocarbons (Guieysse and
Viklund, 2005) and chlorophenols (Tamer et al 2006) have been reported. The
comparisons must be performed whether these combined treatments are better
than the AOP treatment only.
88
Table 2.9 The combined AOP and biological treatment of organic pollutants in water and wastewaters
Sl.No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
1. Dyeing wastewater (COD = 900 -3000 mg/L)
Biological -AOP
An Intermittently Decanted Extended Aeration (IDEA) reactor was used as an aerobic process, which was constructed of Perspex with a height of 1m and 1D of 100 mm. The influent was pumped into reactor at a flow rate of 1L/d and retained for 3 days (HRT = 3 days) and SRT = 20 days. Recirculating batch plate type photocatalytic reactor was used and size of the reactor was 60 x 8 x 3 cm. The recirculated flow rate was in the range of 100 to 130 L/min. 4 g of TiO2 was coated on the Zeolite (0.8 to 1.8 mm) and 30 W/m2 blacklight lamp was used as light source.
The catalysed photooxidiation process can degrade those non-degradable organic substances in the effluent treated by the IDEA process and also decolorize the effluent completely. It was also found that some non-biodegradable organic substances can be converted to biodegradable forms by the sensitized photo-oxidation reaction. A bio-photoreactor system was designed to combine this photocatalytic reactor with the IDEA reactor for the treatment of dyeing wastewater. The performance of this combined bio-photoreactor system with and without recirculation was investigated and compared. The system with recycle has similar efficiency for decolourization and COD removal to that without recycle, but has a high capacity to eliminate the effects caused by a shock loading, and also the system can treat dyeing wastewater with a higher organic concentration.
Li and Zhao (1997)
89
Table 2.9 (Continued)
Sl. No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
2. Cork processing wastewater (COD = 1900 mg/L)
AOP-Biological Biological -AOP
The continuous ozonation reactor was a 5000 cm3 glass bubble column operating at 200C. The aerobic process was performed in an activated sludge system. The HRT was kept as 96 h with constant temperature of 200 C. The MLVSS was maintained in the range of 1.2 – 1.8 g/L
The removal obtained in ozonation alone were 12-54 %, 65-81 % and 55-89 % for the COD, total phenolics, and absorbance at 254 nm respectively, the biodegradability varied from an initial 0.60 to final values of 0.93. The optimum hydraulic retention time and ozone partial pressure were 3 h and 3 Kpa, respectively. The COD removal obtained in aerobic treatment was between 13 % and 37 % for HRT between 24 and 96 h and the Contois model gave values of qmax = 0.14g COD /g VSS/ h and K1 = 22.6 g COD/g VSS . The sequential processes increased the substrate removal efficiencies in comparison with the individual processes. The enhancements were greater in the aerobic degradation-ozonation.
The coil photochemical reactor with 400 W, medium pressure Hg-lamp was used. The solution was recirculated in batch mode at 22 L/h through the illuminated part of the photoreactor. The fixed bed biological reactor consists of a column of 1 L capacity containing biolite colonized by activated sludge was used. The wastewater was recirculated at 6 L/h through the column.
The coupled reactor was operated in a semi-continuous made and an optimal pretreatment time of 300 min was found. The initial concentration of 0.2 mmol/L AMBI was completely removed and about 40 % of DOC was remained after 300 min in iron photoassited process. H2O2 and O2 were compared as electron acceptors in the pretreatment process, the higher biological efficiency was observed in the system with O2. In the coupled process complete removal of DOC was achieved. High concentrated 4000 mg/L real AMBI wastewater was also successfully degraded using solar photo-Fenton process with 40 KJ/L of accumulated solar energy, 80 % of AMBI was eliminated and a positive value of the AOS was reached.
Ledakowicz and Solecka (2000)
90
Table 2.9 (Continued)
Sl. No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
4. Textile wastewater (COD = 2154 mg/L)
AOP-Biological
The ozonation process was carried out in a 1.5 dm3 stirred gas-liquid rector equipped with UV lamps of 150 W. The optimum ozone dose, hydrogen peroxide and UV irradiation were 100 mg/dm3, 1cm3/dm3 and 1 h respectively. Biodegradation was carried out at 250 C in 250 cm3 shaken flasks. The MLVSS was maintained in the range of 1-2 g/L
The preozonated wastewater with initial COD of 2154 mg/L leads to an increase of the maximum specific rate of substrate elimination from 0.04 to 0.06 mgO2/mgVSS/h and to a significant decrease of the Monod constant from 3378 to 759 mg O2/dm3. This change in kinetic parameters suggested a faster biodegradation and that pretreated pollutants are more available to biological oxidation. And, also the inhibitory action of microbial growth in the untreated wastewater decreased from 47 % to 10 % after O3/ H2O2/ UV pretreatment.
Ledakowicz and Ganera (1999)
6. Common industrial wastewater
AOP-Biological
The thin film reactor of 1.44 m length with 0.52 m width and fixed bed height of 0.1 cm was used in photocatalytic treatment. The up-flow anaerobic sludge blanket reactor was operated at 350 C with a working volume of 1.6 L capacity continuously. HRT was maintained at 38.5 hrs.
The photocatalytic treatment under sunlight for 40 hours reduced the colour and COD removal by 74 % and 62 % respectively, whereas the COD removal in biological treatment was only 18 % after 120 hrs. treatment. The photocatalytic treatment improved the BOD/COD ratio from 0.21 to 0.56. After photocatalytic and biological treatments the no. of peaks in GC analysis was reduced to 31, whereas the original sample has 121 peaks.
Pratapreddy et al (2002)
7. Isoproturon (0.2 mmol/L)
AOP-Biological
Coaxial photocatalytic reactor with TiO2
supported on glass rings and 36 W black lights was used. The reactor had a total volume of 1.5 L. The solution was fed continuously at 90 L/h. The fixed bed reactor consists of a column of 1 L capacity containing biolite colonized by activated sludge. The wastewater was recirculated at 6L/h through the column.
Isoproturon was completely eliminated and about 80% of DOC remained in solution after 60 min. of phototreatment. The biodegradability and toxicity tests performed during a photodegradation show that the solution becomes biocompatible. The optimum time to stop the phototreatment before feeding the treated water to the biological reactor was found to be 1 hour. In this coupled system, 100 % of the initial concentration of Isoproturon and 95 % of DOC were removed.
Parra et al (2002)
91
Table 2.9 (Continued)
Sl. No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
8. Tannery wastewater AOP-Biological
Fenton’s process was carried out in a stirred batch reactor of 1 L capacity. Biological treatment was carried out in 1.8 L aerobic reactors and 0.6 L settling tank. HRT of 1 day was maintained. Biomass concentration up to 35 g MLVSS/L corresponding to an F/M ratio of about 0.01 to 0.02 d-
1was maintained.
The relationships of H2O2/Fe2+ and H2O2/COD were 9 and 4 respectively, reaching an organic matter removal of about 90%, subsequently; the oxidized effluent was fed to an ASP, The biological organic matter removal of the pretreatment wastewater ranged between 35% and 60 % for COD and from 60 % to 70 % for BOD. The sequential AOP pretreatment and biological treatment increased the overall COD removal up to 96% compared to 60 % without pretreatment.
Vidal et al (2003)
9. Herbicide/Pesticide wastewater (COD =540 mg/L)
AOP-Biological
Ozone treatment was performed in two stainless steel columns of volume 450 L each operated at dosage of 100-120 mg/L a HRT of 1.5 hrs. at ozone. The bioreactor of active volume about 200 L and the support media was Biolite. HRT was maintained in the range of 20 to 30 min.
The synergy between chemical and biological oxidation allowed removing herbicide and pesticides from starting concentrations in the order of 107-108 g/L to zero. Pre-ozonation improved biodegradability, so that the following biological treatment was above to contribute significantly to overall performance. The final polishing of the effluent was achieved by a post-ozonation step.
Mezzanotte et al (2003)
10. Ethylene di amin-e tetra acetic acid (EDTA) (2.5x10-3 M)
AOP-Biological
The photocatalytic studies were carried out in a 3L annular photoreactor equipped with a 30 W germicidal lamp. The titania 0.85 g was coaled on glass raschig rings. The activated sludge reactor was used for biological treatment.
About 50 % of EDTA degradation was reached after 150 min irradiation. Simultaneously a four-time increase in the biodegradability, measured as BOD5/COD ratio was observed. The activated sludge is not capable to degrade the complex EDTA but remove partially the COD and efficiently the BOD5 of the pretreated solution.
Carla et al (2003)
92
Table 2.9 (Continued)
Sl. No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
11. 2,4-Dichlorophenol (100 mg/L)
AOP-Biological
The photochemical reactor consists of a Jacketed thermostatic 1.5 L equipped with three black blue lamp. Biodegradation reaction was completed in a 1.5 L aerobic sequencing batch reactor
Initial concentration of 65 mg/L H2O2 and 10 mg/L Fe (II) during 35 min irradiation time was sufficient for total 2, 4-Dichlorophenol removal. At these conditions, biodegradability was increased from 0 to 0.15. In a sequencing batch reactor, results showed that at cycle duration of 12 h, a TOC removal efficiency of 64 % was obtained.
The bioreactor was an activated sludge reactor with aeration (7.5 L) and settling chamber (2.5 L). The HRT was 2 days, 3 days, 1 day for landfill leachate, pulping, and phenolic wastewater respectively. Ozone treatment was performed in a stainless steel column of volume 2.3 L.
The combined method, aerobic bio-oxidation with ozonation of recycled biologically treated wastewater, increases the degradation efficiency when compared to the conventional aerobic bio-oxidation method. COD removal for landfill leachate increased from 61 % to 95 % at an ozone dose of 30 mg/L, from 76 % to 89 % at 52 mg/L for pulping and from 60 % to 75 % at 60 mg/L for phenolic wastewaters. The activated sludge was not deteriorated and the specific oxygen uptake rate constant increased 15-20 % as a result of a small ozone dose of 2 mg/L. Thus combined process should be a prospective method in the purification of recalcitrant wastewaters.
Photocatalytic experiments were carried out in an immersion well photoreactor equipped with 400 W Medium-Pressure mercury vapour lamp. 1 g of TiO2 was used as catalyst. Biodegradation experiments were carried out in 250 mL. Erlenmeyer flasks.
In photocatalytic treatment about 70-90 % COD reduction was attained in 5 hours. The degradation obeys Pseudo-first-order. Photocatalytic pre-treatment of H-acid for 30 min ensures enhanced biodegradation of effluents. Complete removal of H-acid was achieved in the coupled system.
Mohanty et al (2005)
93
Table 2.9 (Continued)
Sl. No.
Chemicals/ Wastewater
treated Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
14. Pulp mill effluent AOP-Biological
Ozonation experiment carried out in a semi batch bubble column reactor made up of plexiglass of height 21 m and diameter 0.1 m. The ozone generator produced ozone consistently at a concentration of 0.11 mg/mL. Batch biological treatment was conducted in shake flasks. The initial MLSS of 1000 mg/L was maintained.
At an ozone dosage of 0.7-0.8 mg O3/mL wastewater, integrated treatment showed about 30% higher TOC mineralization compared to individual ozonation or biotreatment. Ozone treatment enhanced the biodegradability of the effluent (21 % COD reduction and 13 % BOD5 enhancement) allowing for a higher removal of pollutants. Ozonation at pH 11 was more effective that that at pH 4.5 in terms of generating more biodegradable compounds.
Bijan et al (2005)
15. Table olive manufacturing wastewater
AOP-Biological
Wet air oxidation experiments were conducted in a 0.6 L, stainless steel autoclave batch reactor. The system was pressurized with air to 1 pa. Copper (II) was used as catalyst. Aerobic biological experiments were carried out batch wise in a cylindrical 3L glass reactors immersed in a theromostatic bath. The MLVSS was maintained in the range of 1000-2000 mg/L.
COD removal in the range of 30-60% at 6 h of treatment has been achieved by using mild conditions (443-483 K and 3.0-7.0 Mpa of total pressure using air). The rate of the COD biodegradation was compared to the kinetics of the aerobic process without a previous chemical pre-oxidation. The calculated kinetic parameters showed the positive effect of the pre-treatment (maximum growth rate of 0.030.006 h-1 and 0.014 0.00014 h-1). Acclimation of microorganisms to oxygenated species formed in a chemical pre-oxidation step enhanced the efficiency of the biodegradation.
Rivas et al (2001)
16. p-nitrotolunee-ortho-sulfonic acid (p-NTS)
AOP-Biological
The coil photochemical reactor with 400 W, medium pressure Hg-lamp was used. The solution was recirculated in batch mode at 22L/h through the illuminated part of the photoreactor. The fixed bed biological reactor consists oef a column of 1L capacity containing biolite colonized by activated sludge was used. The wastewater was recirculated at 6 L/h through the column.
The photo Fenton treatment generates intermediates with very oxidized functional groups being non-toxic and as biodegradable in 30 min. operated in semi-continuous mode, it was found that the optimal time to stop the photochemical treatment before leading the treated water into biological reactor was 70 min. At this moment appropriate efficiency was reached for the best compromise between time and energy invested in both biological and overall treatment.
Pulgrain et al (1999)
94
Table 2.9 (Continued)
Sl. No.
Chemicals/ Wastewater
treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
17. 6-Chlorovanillin (186 ppm)
AOP-Biological
The photocatalytic degradation was carried out in a 1 L glass reactor, TiO2 was supported on glass Raschig rings. 125 W high pressure lamp was used for irradiation. Biodegradation was carried out with two 250 mL Erlenmeyer flasks with P. paucimobilis and B. cepacia.
About 50% degradation was obtained after 30 min with recirculation of the solution. Then, oxidized samples were submitted under aerobic conditions to bacterial degradation in P. paucimobilis (S37) and B. cepacia (PZK). Both selected aerobic bacteria degrade more efficiently the photocatalysed samples, A first-order kinetic was observed in both systems.
Yeber et al (2000)
19. Logyard run-off (COD = 8050 mg/L)
AOP-Biological Biological -AOP
Batch biological studies were carried out in a 15 L cylindrical jacketed Plexiglas reactor. Ozonation was carried out in a 3 L jacketed glass vessel equipped with ports for ozone inlet and outlet.
Batch biological treatment of logyard run off reduced BOD (1250), COD (8050) and tannin (1550) mg/L and lignin (TL) concentration of 99%, 80% and 90 % respectively. The efficiency of ozone as a pre and post – biological treatment stage was assessed. During ozone pretreatment TL concentration and acute toxicity were rapidly reduced by 70 % and 71 % respectively. Pre-ozontion reduced BOD and COD concentration by less than 10 %. Biological treated effluent was subjected to ozonation, it was observed that a reduction in COD and TL concentration, however no further improvement in toxicity was observed. Ozonation increased BOD by 38 % due to conversion of COD to BOD.
Zenaitis et al (2002)
20. 2,4,6-Trinitro-Toluene (TNT)
AOP-Biological
Photocatalytic experiments were performed in a batch recirculating annular ring photoreactor 15 W fluorescent lamp. TiO2 0.3g/L was used as catalyst. The fungal mineralization by Phanerochaete Chrysosporium in a 250 ml Erlenmeyes flask.
The extent of TNT mineralization was approximately 14 % by biological transformation alone and improved to approximately 32 % using the combined photocatalytic and fungal treatment. Six hours of photocatalytic pre-treatment resulted in the greatest extent of biological mineralization in the combined process.
Hess et al (1997)
95
Table 2.9 (Continued)
Sl. No.
Chemicals/Wastewater treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
21. Herbicides (Diuron and Linuron)
AOP-Biological
Photo-Fenton experiments were conducted at 250.20 C in a cylindrical Pyrex thermostatic cell of 0.25 L capacity with 6 W black light was use as light source. Biological experiments were conducted in the 1.5 L capacity sequencing batch reactor. HRT 2 days and MLVSS in the range of 600-1000 mg/L was maintained.
The combined photo-Fenton and biological process can completely remove the herbicides from water, treated with Fe (II) 15.9 mg/L and H2O2 202 mg/L during 1 h of UV irradiation. And complete removal of TOC was achieved after biological treatment in a SBR.
Maria et al (2006)
22. Procion Red (250 mg/L)
AOP-Biological
Photo-Fenton experiments were carried out using a cylindrical pyrex thermostatic cell of 300 mL capacity with 6 W fluorescent lamp. Biological experiments were conducted in 1.5 L SBR. MLSS and MLVSS were maintained in the range of 2.85 g/L and 1 g/L respectively.
Best pre-treatment results were obtained in 60 min of photo-Fenton process with 10 mg/L Fe (II), 125 mg/L H2O2. At these conditions, BOD5/COD was increased from 0.1 to 0.35, with 39 % mineralization. Complete mineralization was achieved in SBR with 1 day HRT.
Garcia et al (2006)
23. Reactive azo dye (100 mg/L)
AOP-Biological
UV/H2O2 experiments were carried out in a cylindrical rector of 585 mL capacity with 60 W low-pressure mercury lamp. Biological experiments were conducted in 5 L cylindrical reactor. Plastic carriers were suspended in the bioreactor to faster growth of biofilms.
The UV/H2O2 treatment demonstrated that the dye and COD removal efficiency increased from 20 to 86 % and 3 to 39 % respectively, by modifying the most influential parameters like UV irradiation time from 10 to 30 min, initial H2O2 dosage from 100 to 500 mg/L and recirculation ratio from 600 to 0 %. Complete degradation was achieved in biological treatment with 1 day HRT.
Sudarjanto et al (2006)
96
Table 2.9 (Continued)
Sl. No.
Chemicals/ Wastewater
treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
24. Pesticides (100 mg/L)
AOP-Biological
Chemical oxidation process was carried out in a 5 L volume tank. The UV emitting device typically consisted of a stainless steel tube with coaxial Mercury vapour lamp (14 W). Ozone rate was of 1.2 g O3 /h. Biodegradation studies were conducted in 5 L stirred tank. COD: N: P ratio was maintained around 100:5:1
The O3 and O3/UV oxidation systems were able to reach 90 and 100 % removal of the pesticide respectively in a period of 210 min. The combined O3/UV system can reduce COD up to 20 % if the pH of the solution is above 4. Both pesticide degradation and COD removal in the combined O3/UV system follow Pseudo-first-order kinetics. More than 95% COD removal was achieved when treated wastewater by the O3/UV system was fed to the bioreactor.
Lafi and Alqodah (2006)
25. Hospital wastewater (COD=1350 mg/L)
AOP-Biological
Photo-Fenton experiments were carried out in a 1L cylindrical quartz reactor with low germicide lamp. Biological degradations studies were conduced in the activated sludge system, consists of cylindrical aeration glass vessels with a total volume of about 500 mL. COD:N:P ratio was maintained as 100:5:1 and the MLSS = 3000 mg/L was maintained
At the optimum conditions, a dosage ratio of COD:H2O2:Fe (II) at 1:4:0.1 and at pH 3, the biodegradability, in terms of BOD5/COD ratio increased form 0.3 to 0.52 and the oxidation degree, (AOS) levelled up from –1.14 to +1.58. In biological process, a maximum COD removal of about 30 % after a 72 h treatment time. In contrast, the COD of pre-treated wastewater by Photo-Fenton processes, attaining a COD removal of higher than 90% at the end of a 72 h treatment time.
Kajitvichya nukul and Nattapol (2006)
97
Table 2.9 (Continued)
Sl. No.
Chemicals/ Wastewater
treated
Order of Scheme
Experimental condition and type of equipment Important findings of the work Reference
26. Phenol (100 mg/L) P-Nitrophenol (50 mg/L)
AOP-Biological
UV-irradiation tests were conducted in 25 aliquots of 6ml of mineral salt medium supplied with 50 mg/L P-NP and 100 mg/L phenol were transferred into 25 x 10ml glass tubes placed beside each other on a rocking shaker, 1g of TiO2 was added to each tube. Two 18W UV blue lamps were used for irradiation. Biological treatments were conducted in a glass flask of 35 mL capacity. 25 mL of irradiation solution with 1mL of acclimated consortia. The flasks were flushed with N2 gas to remove O2 and incubated for 14 d under continuous agitation and illumination.
Photocatalytic degradation of phenol and P-NP was well described by Pseudo-first-order kinetic with removal rate constants of 1.9 x 10-4 and 2.8 x10-4 min-1 respectively, when the pollutants were provided together and 5.7 x 10-4 and 9.7 x 10-4 min-1, respectively, when they are provided individually. Pre-treatment of the wastewater during 60 h removed 50 1 % and 62 2 % of phenol and P-NP but only 11 3 % of the initial COD. Subsequent biological treatment of the pre-treated samples removed the remaining contaminants and 81-83 % of COD.
Tamer et al (2007)
27. Phenol (200 mg/L)
Biological -AOP
In bioreactor 0.2 L of the adapted sludge was mixed with 0.8 L of solution A (250 mg/ L of phenol, 62.5 g/ L of chloride, and pH 6.5) and the system aerated for 3 h. Photocatalytic experiments were carried out in a 0.25 L thermostated cylindrical Pyrex reactor with medium pressure mercury vapour lamp of 250 W.
The degradation of phenol by a hybrid process (activated sludge + photocatalysis) in a high salinity medium (50 g /L of chloride) has been investigated. The sludge used from a municipal wastewater facility was adapted to the high salt concentrations prior to use. The initial phenol concentration was approximately 200 mg/ L and complete removal of phenol and a mineralization degree above 98% were achieved within 25 h of treatment (24 h of biological treatment and 1 h of photocatalysis).
Amour et al (2008)
98
In the biological treatment followed by the AOP (biological-AOP
treatment), biodegradable compounds are removed by the biological treatment
and non-biodegradable compounds are degraded by the AOP. However, in the
reported studies of biological–photocatalytic treatments of kraft pulp
bleaching wastewater (Balcioglu and Cecen 1999) and dyeing wastewater (Li
and Zhao 1997), the concentration of pollutants in biologically treated water
was high and not suitable for TiO2 photocatalytic degradation. Although
phenol is toxic, 200 mg/L and 1000 mg/L could be decomposed by biological
treatment in 40 h (Prieto et al 2002) and 340 h (Gonzalez et al 2001),
respectively. On the other hand, 40 and 80 mg/L phenol could be decomposed
by the photocatalytic treatment in 6 h (Augugliaro et al 1988) and 8 h
(Sivalingam et al 2004), respectively. However, it required a long time to
mineralize concentrated phenol with only biological or photocatalytic
treatment. Therefore, the combined biological-photocatalytic treatment of
phenolic wastewater has been proposed. Studies reported on coupled system
are very limited hence more focus is needed in terms of optimization of pre-
treatment stage and its consequent effect on the biological process. The
oxidant dosage must be so adjusted that it is completely utilized in the pre-
treatment stage only. It is also important to note that a detailed analysis of the
oxidation products must also be done, as sometimes it may happen that the
intermediates formed might be toxic towards the microorganisms.