Combination of sonophotolysis and aerobic activated sludge processes for treatment of synthetic pharmaceutical wastewater

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Chemical Engineering Journal 255 (2014) 411–423

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Combination of sonophotolysis and aerobic activated sludge processesfor treatment of synthetic pharmaceutical wastewater

http://dx.doi.org/10.1016/j.cej.2014.06.0641385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +1 416 9795000x6555; fax: +1 416 9795083.E-mail address: [email protected] (M. Mehrvar).

Amir Mowla, Mehrab Mehrvar ⇑, Ramdhane DhibDepartment of Chemical Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada

h i g h l i g h t s

� Aerobic activated sludge process was only able for partial TOC removal from a SPWW.� UV/US/H2O2 process at optimum condition was able to remove more than 90% TOC in SPWW.� Effects of various operational parameters on the UV/US/H2O2 process were studied.� Combined UV/US/H2O2 and aerobic AS resulted in higher mineralization while lower oxidant consumption.� Combined processes reduced retention time in both sonophotoreactor and bioreactor.

a r t i c l e i n f o

Article history:Received 16 April 2014Received in revised form 11 June 2014Accepted 13 June 2014Available online 21 June 2014

Keywords:SonophotolysisActivated sludgeSynthetic pharmaceutical wastewaterTotal organic carbon removalCombined processesAdvanced oxidation processes

a b s t r a c t

The performance and effectiveness of sonophotolytic process, aerobic activated sludge (AS) process, andtheir combination in reduction of total organic carbon (TOC), chemical oxygen demand (COD), and bio-logical oxygen demand (BOD) from a synthetic pharmaceutical wastewater (SPWW) are evaluated. Batchmode experiments are performed to obtain optimal experimental operating conditions of the sonophot-olytic process. An ultrasonic power of 140 W, initial pH solution of 2, and air flow rate of 3 L min�1 arefound to be optimal operating conditions. The initial optimum molar ratio of H2O2/TOC was found tobe 13.77 for the sonophotolytic process operated in batch mode. In continuous mode, a 90% TOC reduc-tion was obtained in the sonophotolytic process after 180 min retention time, whereas only 67% in anaerobic AS process for retention time of 48 h. However, combined sonophotolytic and aerobic AS pro-cesses improved the biodegradability of the SPWW with 98% TOC and 99% COD removal while reducingthe retention time in sonophotoreactor and aerobic AS bioreactor to 120 min and 24 h, respectively.Besides, the consumption of H2O2 was reduced significantly in the combined processes.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Pharmaceutical industries are characterized by a large numberof products, processes, plant sizes as well as the magnitude andthe quality of produced wastewater. For manufacturing each typeof product, several processes and raw materials may be required[1]. During the last few decades, the production and consumptionof pharmaceutical compounds have been increased significantly,mainly due to the developments in medical science and also theconsiderable growth in the world population. Nowadays, a hugeamount of medicines is manufactured each year for human andanimal consumptions [2]. Therefore, an enormous amount ofwastewater is generated in pharmaceutical industries [3]. Pharma-ceutical wastewaters are generally categorized as one of the main

complex and toxic industrial wastewaters with high BOD, COD,total suspended solid (TSS), toxicity and odor as well as lowBOD/COD ratio. Moreover, wastewater from pharmaceutical indus-try might contain various amounts of organic solvents, catalysts,raw materials, and reaction intermediates which make their treat-ment procedure complicated [1,4,5].

Most treatment methods of pharmaceutical wastewater arephysico-chemical and conventional biological processes. Coagula-tion-flocculation and activated carbon adsorption are frequentexamples of physico-chemical mechanisms. Suarez et al. [6]applied coagulation-flocculation as pre-treatment for hospitalwastewater. The treatment was able to reduce TSS by about 92%and COD up to 70%. However, the removal of most pharmaceuticalcomponents such as antibiotics were marginal. Activated carbon inboth powdered (PAC) and granular (GAC) forms was also used forthe removal of micropollutants. More than 90% removal of estro-gens was reported by both GAC and PAC processes [7]. Also, up

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to 90% removal of endocrine disrupting material by PAC wasobserved [8]. Biological methods are known as the most commonand cost-effective choices of the treatment. In the case of industrialpharmaceutical wastewater, aerobic AS process with long hydrau-lic retention time (HRT) is a very frequent treatment option [9].Membrane bioreactors (MBR) are also aerobic technologies whichhave been used alone or in combination with AS process to treatpharmaceutical wastewaters. About 99% COD and 95% BOD of areal pharmaceutical manufacturing wastewater was removed byMBR [10,11].

Even though conventional biological methods are economicalchoice of treatment, several types of industrial wastewater suchas those from petrochemical, pharmaceutical, leather, dye, pulpand paper and pesticide manufacturing plants contain consider-able amount of organic compounds which are nonbiodegradableand refractory to microorganisms applied in biological treatmentsystems. These pollutants cannot be removed by conventionalwastewater treatment plants and the standard regulations cannotbe reached. Also, the release of these substances into the environ-ment and their presence in drinking water may have harmfuleffects on both humans and ecosystems [12–14]. Considering theaforementioned issues, additional treatment steps seem to beindispensable. Among technologies used to remove nonbiodegrad-able substances, advanced oxidation processes (AOPs) are influen-tial treatment methods for degrading recalcitrant materials ormineralizing stable, inhibitory, or toxic contaminants [15]. AOPsare of great interest and used by several researchers to treat differ-ent types of pollutants during past few decades [16–22]. AOPs suchas UV/H2O2, Fenton, etc. could be described as an oxidation methodbased on the intermediacy of highly reactive species such as hydro-xyl radicals (�OH) in a procedure leading to the degradation of tar-get contaminants [23]. The application of ultrasound irradiation(US) or sonolysis in water and wastewater treatment has receiveda lot of attention in recent years and several studies have beenreported [24]. Several advantages of sonolytic process such asavoiding consumption of chemical oxidants or catalysts, safety,and lower demand for the clarification of aqueous solution, maketheir application simple and desirable [25].

Sonochemical reactions are principally due to a phenomenonnamed acoustic cavitation. The phenomenon is the process of for-mation, expansion, and sudden implosion of gas microbubbles. Theacoustic cavitation leads to the generation of high local pressure(as high as 1000 atm) and high temperature (as high as 5000 K).It is known that under these extreme conditions, the pyrolysis ofwater molecules results in the formation of hydroxyl radicals asfollows (Eq. (1)) [26,27]:

H2O�!US�Hþ� OH ð1Þ

Generally, US waves at frequencies in the range of 20-1000 kHzcan produce cavitation in aqueous solutions [28]. The cavitationacts as a means of concentrating the diffusing energy of ultrasoundinto microbubbles. During sonolysis, three types of sonochemicalreactions can take place. First, the pyrolytic reactions which hap-pen due to the high pressure and temperature inside the cavitationbubbles; second, the free radical attack which is performed by theproduced reactive radicals in the interfacial area between the bub-bles and the liquid phase, and third, the generation of hydroxylradicals in the liquid bulk solution [29,30]. Organics componentswith low solubility and/or high volatility are expected to gothrough fast sonochemical degradation since they have a tendencyto accumulate inside or around the gas–liquid interface. Therefore,sonolysis may be a proper method for the removal of pharmaceu-tical micropollutants.

Even though AOPs are very effective in treating almost allorganic compounds, some flaws prevent their commercial applica-

tions. The high requirement of oxidant/catalyst dosage, high elec-trical power consumption, and precise pH adjustment are someof these drawbacks which make operational cost of AOPs high[31]. Therefore, to overcome the aforementioned problems and tofind efficient and economical treatment, the combination ofadvanced oxidation and biological processes as a potential alterna-tive has attracted attention of many researchers. Carballa et al. [32]combined ozonation and anaerobic digestion for the removal of 11pharmaceutical components and reported that the ozonation pre-treatment improved the efficiency of the biological post treatment.In another study, Sitori et al. [33] achieved 95% dissolved organiccarbon removal (DOC) from an industrial pharmaceutical waste-water by combining solar photo-Fenton and biological treatments.Fenton was also combined with sequential batch reactor to treat areal pharmaceutical wastewater containing two antibiotics where89% COD removal was achieved [34]. In these studies, generally,AOPs are applied as a pre-treatment to degrade refractory com-pounds and to improve the biodegradability level of the wastewa-ter. The produced biodegradable intermediates could bemineralized in a subsequent low cost biological step. Finding theoptimum retention time of the wastewater in an AOP reactor is achallenging issue. On one side, in order to reduce the cost of AOPs,lower dosage of chemicals and lower retention times should beapplied to achieve small percentage of mineralization. On the otherhand, a very low mineralization causes the formation of intermedi-ates which are still toxic and similar to the parent compounds.Therefore, the selection of the point to transfer the effluent of anAOP reactor to the bioreactor should be performed carefully. Twofactors are important in combined processes, the biodegradabilityof the wastewater after photochemical oxidation and the presenceof residual oxidants such as H2O2, which are inhibitory to microor-ganism in biological treatment systems.

In this study, the remediation of a synthetic pharmaceuticalwastewater was carried out by means of a sonophotolytic process(UV/US/H2O2) alone and sonophotolysis as a pre-treatment for theaerobic AS process. The effects of operating parameters, such as USpower, H2O2 concentration, pH, and HRT in both sonophotolysisand aerobic AS processes were investigated. Furthermore, theeffluent of the sonophotoreactor was analyzed to evaluate thechanges in the biodegradability of the wastewater as well as resid-ual concentration of H2O2. Based on the results obtained, the opti-mal HRT for both the sonophotoreactor and the bioreactor arefound and the combined treatment was performed under optimaloperating conditions. To the best of the authors’ knowledge, thisis one of the first reports studying the combination of the sonop-hotolytic process and biological treatment without using H2O2

neutralizer. Results of this study can help having an efficient treat-ment of industrial pharmaceutical wastewater.

2. Materials and methods

2.1. Materials

The SPWW was prepared based on a list of componentsreported in a study by Badawy et al. [35]. The components weredetected in the wastewater generated by a pharmaceutical andchemical company in Cairo, Egypt. The wastewater containedchloramphenicol, diclofenac, salicylic acid, and paracetamol whichwere the main products of the production plant. Also, some by-products including p-aminophenol, nitrobenzene, benzoic acid,and phenol were detected in the raw wastewater [35]. Three setsof concentrations in distilled water were chosen to conduct theexperimental runs. Characteristics of these three sets are shownin Table 1. The 30% (w/w) H2O2 (Sigma–Aldrich) was used asreceived. Also, NaOH and H2SO4 solutions (VWR, Canada) were

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Table 1Characteristics of the SPWW used in this study.

Compound Molecularformula

Molecular weight(g mol�1)

Concentration in 1st SPWW(mg L�1)

Concentration in 2nd SPWW(mg L�1)

Concentration in 3rd SPWW(mg L�1)

4-Aminophenol C6H7NO 109.13 6.25 12.5 25Paracetamol C8H9NO2 151.17 2.5 5 10Phenol C6H6O 94.11 12.5 25 50Chloramphenicol C11H12Cl2N2O5 323.132 7.5 15 30Benzoic acid C7H6O2 122.12 6.25 12.5 25Salicylic acid C7H7O3Na 160.11 28.75 57.5 115Diclofenac

sodiumC14H11Cl2NO2Na 318.1 0.5 1 2

Nitrobenzene C6H5NO2 123.06 7.5 15 30TOC (mg L�1) 44.83 ± 0.25 89.75 ± 0.37 179.33 ± 0.35TN (mg L�1) 2.56 ± 0.08 5.12 ± 0.08 10.25 ± 0.12COD (mg L�1) 127 ± 1 252 ± 1 515 ± 1

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used for pH adjustment while required. For the biological processexperiments, aerobic sludge seed was obtained from AshbridgesBay Wastewater Treatment Plant, Toronto, Canada. The inoculumwas acclimatized to the SPWW samples.

2.2. Experimental setup and procedure

Fig. 1 depicts the schematic diagram of the experimental setup.The sonophotoreactor is an airlift external loop reactor. The riser is9.72 cm in diameter and 110 cm in height. The height and thediameter of the downcomer are 90 and 3.25 cm, respectively. Also,the total volume of the photoreactor is 7 L. The sonophotoreactorwas employed in both batch and continuous modes. As shown inFig. 1, the setup is equipped with a single ended UV lamp (UshioAmerica Inc.) and a commercial ultrasonic processor (Branson, S-250D Sonifier). The UV lamp, located in the centerline of the riser,is 84.6 cm in height and 1.55 cm in diameter with the wavelengthof 253.7 nm and 13 W output power. The sonifier had a 13 mmdiameter tip which was capable of working in continuous andpulse modes. It had a constant frequency of 20 kHz and a variableoutput power up to 200 W. All experiments were performed withsonifier at continuous mode. Since the sonophotoreactor was anairlift external loop reactor, the contents of the wastewater werecirculated during the sonication time. Therefore, all parts of thewastewater were exposed to sonication at various time periodsboth in batch and continuous modes. There was also a perforatedcircular tube air sparger 5 cm above the reactor bottom. The

Fig. 1. Schematic diagram of the external loop air lift sonop

temperature in the reactor was also monitored. During batch modeexperiments, the reactor was initially filled with the wastewaterand while required with the oxidant. Then the UV and/or US radi-ations was started and after the desired time, samples were takenfor analysis. Therefore, there was no input/output to the systemduring the radiation period. In the continuous mode experiments,the inlet flow of the wastewater was adjusted with desired resi-dence time. This means that the wastewater continuously enteredand left the reactor. The samples were taken from effluents of thereactors.

The aerobic AS bioreactor was a continuous flow completelymixed activated sludge reactor with an effective working volumeof 25.5 L under ambient condition. The activated sludge reactorwas composed of two parts; the aeration tank and the clarifierfor separation of liquid and biomass. Diffused air was used to pro-vide aeration and also to mix the content of the bioreactor. Theconcentration of dissolved oxygen (DO) must be kept over 2 mg L�1

in order to prevent anaerobic reactions [36].Before conducting experiments, 8 L of the aerobic sludge seed

with initial suspended solid concentration of 1800–1900 mg L�1,were loaded into the aerobic AS bioreactor and was acclimatizedwith the SPWW samples. The inoculum was acclimatized by feed-ing the SPWW continuously into the bioreactor. The flow rate ofthe inlet SPWW for the acclimatization process was set at20 mL min�1. Acclimatization period was performed for 28 days.This period could be divided into four periods of 7 days. The influ-ent concentration of the SPWW to the bioreactor was increased

1-Riser

2-Downcomer

3-UV lamp

4-US horn

5- US processor

6-Feed tank

7-Perlistatic pump

8-Sonophotoreactoroutlet

9-Compressed air

10-Aerobic AS bioreactor

11- Air flow Meter

hotoreactor and aerobic activated sludge (AS) reactor.

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gradually. In the first period (1st to 7th day), the initial TOC of theSPWW was set to 22.5 mg L�1. The initial TOC was increased to44.83, 89.75, and 179.33 mg L�1 on the 8th, 15th, 22nd day, respec-tively. Nutrients were fed to the reactor as well to maintain theCOD:N:P molar ratio of 100:5:1 [37]. The nutrient medium con-sisted of KH2PO4, K2HPO4, NaHPO4�7H2O, NH4Cl, MgSO4, FeCl3

and CaCl2 [38]. During 28 days, samples were collected from thebioreactor to measure the mixed liquor suspended solid (MLSS)and mixed liquor volatile suspended solid (MLVSS) concentrations.These parameters were used to determine the growth of microor-ganism and to observe the acclimatization process.

During experiments, DO and temperature were measured byusing a DO meter (YSI 58 Dissolved Oxygen Meter) and the pHwas also measured using a pH meter (Thermo Scientific, Ottawa,Ontario, Orion 230A+). By changing the air flow rate, the DO wasmaintained at least 2 mg L�1. The TOC and total nitrogen (TN) weremeasured by a TOC/TN analyzer (Apollo 9000, Teledyne Tekmar,USA). The concentrations of MLSS, MLVSS, BOD5 and COD weremeasured according to the American Public Health Association[38].

The residual concentration of H2O2 in the effluent of the sono-photoreactor was measured using 2,9-dimethyl-1,10-phenathro-line (DMP) (Alfa Aesar (USA)) method as described in the openliterature [39]. The method had a detection limit of 0.8 lM.

3. Results and discussion

3.1. Assessment of parameters’ changes during the processes

The initial temperature of the SPWW was in the range of 24.8–25.3 �C. Effluents of the sonophotoreactor had temperatureincrease up to 33.1 �C. For the activated sludge reactor, during bothacclimatization period and experiments, the temperature variedbetween 24.5 �C and 25.6 �C. Therefore, there was a temperatureincrease in the effluents of the sonophotoreactor during experi-ments, which is obviously due to the heat generation caused byUV lamp and US probe. However, due to the SPWW circulation inthe external loop air lift sonophotoreactor, the effect of tempera-ture increase which was small, could be partially neutralized. Inorder to evaluate the thermal evaporation of the SPWW,

Fig. 2. Evolution of mixed liquor suspended solid (MLSS) and mixed l

experiments were performed at 25 �C and 30 �C. However, lessthan 2% TOC reduction was observed after 2 h. Therefore, thermalevaporation did not contribute significantly to the total removalrate and therefore, this effect was neglected. The initial pH of theSPWW was in the range of 3.80–4.0. Before introducing the SPWWto the bioreactor, the pH was increased to 7 using NaOH solution,which is the ideal range for the growth of microorganisms. Duringthe acclimatization of the biomass, the pH values in the biologicalreactor were fluctuating significantly. This may be due to metabo-lism and enzyme reactions during the growth of the microorgan-isms. Also, according to the literature, the rate of production orconsumption of hydrogen ions might be affected by the changesin the CO2 production rate, ammonia removal rate, and phosphorusremoval rate [40]. The pH values in the aerobic AS bioreactor werein the range of 5.53–7.81. On the other hand, during the experi-ments, the pH values in the aeration tank varied between 6.82and 6.95. While conducting the combined processes experiments,the pH of the effluents from the sonophotoreactor was adjustedto 7–7.50. The DO was also monitored in the bioreactor. In the aer-ation tank DO was in the range of 2.1–3.4 mg L�1.

3.2. MLSS and MLVSS concentrations of aerobic activated sludge

Fig. 2 shows concentration profiles of the MLSS and MLVSS inthe aerobic AS bioreactor. As this figure shows, there is a quick rateof adaptation of the biomass to conditions in the aeration tank. Thetrend in Fig. 2 indicates a rapid growth of microorganism untilreaching a plateau which is the stabilization phase. As shown inthe figure, by increasing the inlet TOC to the system due to the ele-vated food availability, the rate of reproduction of microorganismsincreased. However, after the second week, only the microorgan-isms that got adapted to the higher dosage of pharmaceutical com-ponents survived and their accumulation decreased. Finally, After28 days of acclimatization, a considerable amount of sludge wasproduced, therefore, the MLSS and MLVSS of the AS were reachedto approximately 3200 and 2300 mg L�1, respectively. The goal ofthe acclimatization period was to adapt the microorganisms tothe components of the SPWW. Therefore, during this period, somereduction of the TOC in the SPWW was expected. Experimentswere started after reaching a steady state phase in the growth

iquor volatile suspended solid (MLVSS) in the aerobic AS process.

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curve of microorganisms. The results are in accordance with previ-ous studies [41,42].

3.3. Treatment of SPWW using aerobic activated sludge process

Biological treatment using the aerobic AS process in continuousmode in a laboratory scale was studied to treat the SPWW. FourHRT of 12, 24, 36 and 48 h were used with the TOC loading ratesof 0.93–15 mg L�1 h�1 and TN loading rates of 0.064–0.85 mg L�1 -h�1. The experimental results for TOC and TN removals are depicted

Fig. 3. Total organic carbon (TOC) removal for different synthetic pharmaceutical watreatment in continuous mode without recirculation.

Fig. 4. Total nitrogen (TN) removal for different synthetic pharmaceutical wastewater (continuous mode without recirculation.

in Figs. 3 and 4, respectively. The optimum retention time for aero-bic AS process was 24 h since there was no significant changes inthe TOC removal for retention times over 24 h. The TOC removalafter 24 h retention time was 65%, 69%, and 73% for the three SPWWinitial concentrations. For HRT of 48 h, 67%, 71%, and 76% TOCreduction were observed which showed only a slight improvement.Also, in the case of TN removal, the results were in the ranges of 32–60% and 52–65% for HRT of 24 and 48 h, respectively. Furthermore,it was also observed that at higher influent TOC and TN concentra-tions, the TOC and TN removal rates were higher.

stewater (SPWW) concentrations at various HRT using aerobic activated sludge

SPWW) concentrations at various HRT using aerobic activated sludge treatment in

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The treatment ability of the aerobic AS process was not satisfac-tory since more than 95% TOC removal was the goal to meet thestandard regulations. These unsatisfactory results were attributedto the existence of nonbiodegradable components. In order toachieve high removal efficiency, the application of advanced oxida-tion technologies as pre-treatment seems crucial. However, inorder to make the treatment process practically economical, thecombination of AOPs with biological process could be a goodalternative.

3.4. TOC removal in SPWW using UV/US/H2O2 process alone in batchrecirculation mode

The slow mineralization rate is a major problem associated withthe application of sonolysis in wastewater treatment [43,44]. Also,the formation of toxic intermediates, which are sometimes moretoxic than parent components, is a great concern while applyingphotolysis [45]. On the other hand, during the sonophotolytic pro-cess, due to the simultaneous US and UV irradiation, more reactiveradicals are produced and consequently the rate of degradationwould increase [46]. Additionally, this elevated rate of mineraliza-tion causes the reduction of the formation of intermediatecomponents.

Operating factors such as US frequency and the output power,the pH, and the type, and the amount of dissolved gas have impor-tant role in a sonophotolytic process [2]. Additionally, the UV lightintensity and the concentration of H2O2 have key role in photolysisprocess. In this section, batch mode experiments were performedto determine the operating parameters for the maximum efficiencyfor the treatment of the SPWW.

3.4.1. Optimal H2O2 dosage for the UV/US/H2O2 processSeveral studies reported that the addition of H2O2 to UV or US

irradiation or their combination (US/UV) increases the treatmentefficiency [47–49]. However, it should be considered that there isan optimum concentration of H2O2 that should be determinedcarefully. Overdose of this oxidant causes a reduction in organicsremoval effectiveness due to the recombination of hydroxyl

Fig. 5. Optimal initial concentration of H2O2 for different synthetic pharmaceutical wasbatch mode (pH = 3.9, US power = 140 W, and air flow rate = 2 L min�1).

radicals (�OH) as well as the reaction of produced �OH with theexcess H2O2 molecules to generate other radicals, such as hydro-peroxyl radicals (HO2

�) which have less oxidizing power than �OH(Reactions (2) and (3)) [50]. Increasing the cost of the process isanother problem associated with the overdose of H2O2. The lowoxidant concentration, on the other hand, leads to the lack of �OHin the solution and decreases the degradation effectiveness.

�OHþ �OH! H2O2 ð2Þ

H2O2 þ �OH! H2OþHO�2 ð3Þ

In order to determine the optimum dosage of H2O2, various con-centrations in the range of 0–3000 mg L�1 were used. Three waste-water samples with the initial TOC concentrations of 44.83, 89.75,and 179.33 mg L�1 were used for experiments in the batch recircu-lation mode for 90 min reaction time. The results are shown inFig. 5. All curves in Fig. 5 show the same trend and indicate anenhancement in the TOC removal by increasing H2O2 concentra-tions up to an optimum dosage. The maximum TOC removal forthe samples with TOC of 44.83 mg L�1 was 63.95% after 90 minusing 1750 mg L�1 H2O2. The optimal oxidant dosage for the othertwo SPWW with higher initial TOC was found to be 2250 mg L�1

which eventuated in 44.72% and 19.8% TOC removal, respectively.Fig. 5 also confirms that at a higher organic loading of the waste-water, the TOC removal capacity decreases due to the presenceof more organic matter ready to compete for reaction with �OH.

In order to declare the results in a more practical form, it is rec-ommended to determine the optimal molar ratio of [H2O2]/[TOC][17,47]. The ratio of [H2O2]/[TOC] is an important parameter tooptimize the wastewater treatment which could be used to adjustthe H2O2 concentration based on the concentrations of organicmatters present at any given time. Consequently, this factor assiststo maximize the efficiency and diminish chemical and electricalexpenses. Fig. 6 presents the optimal initial molar ratio of[H2O2]/[TOC] for three wastewater samples. Molar ratio of 13.77was found as the optimum value for the SPWW with the initialTOC of 44.83 mg L�1. The results are in accordance with data foundin the open literature, where the molar ratios between 0 and 100

tewater (SPWW) concentrations after 90 min treatment by UV/US/H2O2 process in

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Fig. 6. Effect of the initial molar ratio of [H2O2]o/[TOC]o on the treatment of different synthetic pharmaceutical wastewater (SPWW) concentrations within the UV/US/H2O2

process in batch mode (pH = 3.9, US power = 140 W, and air flow rate = 2 L min�1).

A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423 417

were reported [17,51]. As shown in Fig. 6, UV/US process (no H2O2),reduced the TOC concentration of all the SPWW samples by lessthan 8%. This is shown in Fig. 6 where [H2O2]/[TOC]o is zero. TheSPWW was also treated using UV alone (the results are not shown).The UV alone was only able to reduce the TOC concentrationbetween 0 with [TOC]o = 179.33 mg L�1 and 7% with[TOC]o = 44.83 mg L�1.

3.4.2. Effect of ultrasonic power on TOC removalThe effect of ultrasonic power on the UV/US/H2O2 process was

also studied in the batch recirculation mode. Output powers of 20,

Fig. 7. Ultrasonic power effect on total organic carbon (TOC) removal during batch US/UVrate = 2 L min�1).

60, 100 and 140 W were applied with the initial TOC of 44.83 mg L�1.Optimum initial H2O2 concentration of 1750 mg L�1 was used whileother parameters kept constant (pH = 3.9, air flow rate = 2 L min�1).As Fig. 7 shows, increasing the US power from 20 to 140 W improvedthe TOC removal by about 19%. Elevated US power causes higher rateof breakage of H2O2 molecules in the aqueous solution [52].Consequently, the concentration of hydroxyl radicals was increased.Furthermore, an increase in the ultrasonic power contributed tohigher mixing intensity due to the turbulence and microstreamingwhich are generated during the cavitational microbubble collapse.To recap, higher ultrasonic power results in higher number of

/H2O2 process ([TOC]o = 44.83 mg L�1, [H2O2]o = 1750 mg L�1, pH = 3.9, and air flow

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cavitation, number of microbubble generated, formation of hydro-xyl radicals, mass transfer, and more degradation of pollutants[53–55]. The results of this study as well as data from literature con-firm the importance of US output power on competence of both son-olytic and sonophotolytic processes.

3.4.3. Effect of initial pH on TOC removalThe initial pH of solution is a critical parameter which affects

the efficiency of many AOPs. The data regarding the effect of pHon the UV/US/H2O2 process is limited. Durán et al. [28] reportedan increase in TOC removal while increasing pH from 2 to 8 forthe treatment of food processing industry wastewater treatmentusing the UV/US/H2O2 process. Another recent report by Xu et al.[46] studied the degradation of dimethyl phthalate by US/UV pro-cess and stated a systematic reduction in dimethyl phthalate deg-radation by increasing the pH in the range of 2–10.

In the present study, the TOC removal was observed under thesonophotolytic process by changing pH in the range of 2–8. Theoptimal initial H2O2 concentration of 1750 mg L�1 and optimalUS power of 140 W were applied while the air flow rate was keptat 2 L min�1. The results are shown in Fig. 8. It was found that byincreasing the pH, the TOC reduction was decreased. An explana-tion to this trend may be due to the reduction of oxidation poten-tial of hydroxyl radicals while pH is elevated [56]. In addition, thefast consumption of hydroxyl radicals in alkaline medium must beconsidered (Reaction (4)).

�OHþ OH�¢ H2Oþ O�� ð4Þ

In Reaction (4), kforward and kbackward are 1.2 � 1010 M�1 s�1 and9.3 � 107 s�1, respectively [57]. The effect of pH on the degradationrates in sonophotolytic process is also reliant on the state of thecontaminant molecules, whether the pollutants are present asionic species or as molecules. Several studies reported that the deg-radation rate is low at the pH range which the pollutant is in itsionized form [54,57]. This behavior is due to the fact that compo-nents are nonvolatile and more stabilized at their ionized form.Therefore, they react with �OH only at microbubble surfaces. Onthe other hand, in molecular form, they can enter the vapor phase;consequently, they decompose by both thermolytic cleavage andreaction with �OH in aqueous solution. In current study, the pKa

value of most of the components are more than 3 (phenol: 9.95;diclofenac sodium: 3.80; salicylic acid: 3; paracetamol: 9.9; andbenzoic acid: 4.2) which implies that these compounds are mostly

Fig. 8. Effect of initial pH of the synthetic pharmaceutical wastewater (SPWW) ontotal organic carbon (TOC) removal after 90 min in batch mode by US/UV/H2O2

process ([TOC]o = 44.83 mg L�1, [H2O2]o = 1750 mg L�1, US power = 140 W, and airflow rate: 2 L min�1).

in ionic form in pH values greater than 3 [58–60]. Therefore,greater degradation rate could be expected at higher acidiccondition.

3.4.4. Effect of air flow rate on TOC removalAir flow rates in the range of 0–5 L min�1 were applied while

other parameters adjusted to their optimal values found in previ-ous steps ([H2O2] = 1750 mg L�1, US Power = 140 W, and pH = 2).The results are demonstrated in Fig. 9. Increasing the air flow ratefrom 0 to 3 L min�1 improved the TOC removal from 45% to 73%.Further increase in the air flow rate did not affect the TOC removalsignificantly and around 57% TOC removal was achieved for flowrates of 4 and 5 L min�1.

Several researchers stated a positive effect of gas sparging onthe enhancement of sonochemical process [55,61]. Adverse effectof high air flow rates (4 and 5 L min�1) could be described by con-sidering the lower residence time of organics in the sonophotore-actor which is caused by higher liquid circulation during thetreatment with elevated air flow rates.

3.4.5. TN removal in SPWW using UV/US/H2O2 processIn order to observe the ability of the sonophotolytic process on

TN removal, the SPWW with initial TN of 2.52 mg L�1 was treatedunder sonophotolysis. All optimal condition found in previous sec-tions ([H2O2] = 1750 mg L�1, US Power = 140 W, and pH = 2, airflow rate = 3 L min�1) were applied. After 90 min, less than 3% TNremoval was achieved. These poor results imply the disability ofthe UV/US/H2O2 process in the elimination of nitrogenouscompounds.

3.5. Combined UV/US/H2O2 and aerobic AS processes for treatment ofSPWW

It was observed that the SPWW samples were approximatelynonbiodegradable and the aerobic AS process was only able forthe partial treatment of the wastewater. On the other hand, theUV/US/H2O2 process using the optimal operating conditionsshowed a significant efficiency in the TOC removal in a short periodof time. However, achieving complete removal of pollutantsrequires longer reaction time and higher consumption of chemicalswhich make the treatment expensive. Therefore, the combinationof the two processes is studied in order to accomplish greaterremoval of TOC, COD, and TN while trying to make the treatmenteconomical by reducing the usage of chemicals and finding an

Fig. 9. Effect of air flow rate on total organic carbon (TOC) removal after 90 min inbatch mode by US/UV/H2O2 process ([TOC]o = 44.83 mg L�1, [H2O2]o = 1750 mg L�1,US power = 140 W, and pH = 2).

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A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423 419

optimum retention time for transferring the wastewater fromsonophotoreactor to the bioreactor.

The experiments were performed in continuous mode to inves-tigate the biodegradability of treated SPWW for different HRTunder the UV/US/H2O2 process. Continuous mode studies wereperformed only on the SPWW with the inlet TOC of 44.83 mg L�1.Biodegradability studies were carried out based on the evolution ofBOD5/COD ratio and the average oxidation state (AOS) of thewastewater samples during the treatment. Additionally, since theeffluent of the sonophotoreactor would be introduced to the biore-actor, the concentration of H2O2 should be minimized. Hence, theconcentration profile of H2O2 was determined during the UV/US/H2O2 process. According to the results from biodegradability stud-ies and H2O2 concentration profile, the optimum retention time totransfer of the SPWW from the sonophotoreactor to the bioreactorwas determined to be 120 min. In the next step, using the resultsfound from continuous mode studies, the SPWW was pre-treatedin the sonophotoreactor and the effluent was introduced to theaerobic AS bioreactor.

3.5.1. UV/US/H2O2 as pre-treatment in continuous modeAccording to the results from batch mode studies, continuous

mode experiments were conducted using US power of 140 W, pH2, air flow rate of 3 L min�1, and various H2O2 concentrations at dif-ferent HRT from 30 to 180 min. The TOC profile is depicted inFig. 10. The results are in accordance with batch mode experi-ments. Increasing the inlet H2O2 concentration increased the TOCremoval. Under the optimum condition, approximately 90% TOCremoval was observed which confirmed the high treatment abilityof the UV/US/H2O2 process.

In order to achieve high treatment efficiency in bioreactorswhile combining with AOPs, the effluent of the AOP reactor shouldbe biodegradable. Also, it has been proven that high concentrationsof hydrogen peroxide are harmful to bacterial activity [62]. To findan optimum point to transfer the wastewater from sonophotoreac-tor to the aerobic AS bioreactor, the biodegradability and residualhydrogen peroxide concentration in the effluent of the sonophoto-reactor need to be determined. As Fig. 10 shows, the TOC removal

Fig. 10. Total organic carbon (TOC) removal using different H2O2 concentrations dpower = 140 W, and air flow rate = 3 L min�1).

using 250 mg L�1 H2O2 is marginal. Only 4% TOC removal wasobserved. Therefore, further studies were focused on inlet H2O2

concentrations of 500, 750, 1000, 1250, 1500 and 1750 mg L�1.

3.5.2. Biodegradability studiesCommon parameters to study the biodegradability of a waste-

water include the ratio of BOD5/COD or BOD5/TOC [23,63,64]. Typ-ically, the BOD5/COD ratio of 0.4 or more in a wastewater sampleimplies that the sample could be considered as biodegradable[23]. Another parameter is the AOS which is a measure of the oxi-dation state of the wastewater [33,65]. The AOS is a very helpfulparameter to estimate the oxidation degree of mixed solutionsand provides indirect data about biodegradability of the solutions.AOS may vary between 4 (for CO2, the most oxidized state of car-bon) and �4 (for CH4, the most reduced state of carbon). TheAOS could be calculated as follows [62]:

AOS ¼ 4 ðTOC� CODÞTOC

ð5Þ

The TOC and COD are in molar concentrations in Eq. (5). Theevolution of the BOD5/COD ratio and the AOS for the wastewaterwith inlet TOC of 44.83 mg L�1 were investigated during the UV/US/H2O2 process. Results are illustrated in Figs. 11 and 12. Increas-ing the HRT from 0 to 120 min improved the AOS significantly.However, the AOS remained almost constant for HRT of 150 and180 min. The elevation of the AOS implies that the intermediateswhich are formed during the treatment are oxidized easier [66].Constant AOS after 120 min HRT means that the chemistry of theproduced intermediates does not change notably after that time.

The BOD5/COD ratio of the SPWW was also improved byincreasing HRT during the UV/US/H2O2 process. Applying HRT of120 min for all experiment except the one with inlet oxidant con-centration of 500 mg L�1 made the BOD5/COD ratio cross 0.4 whichmeans the wastewater samples could be considered biodegradable.However, samples with higher inlet concentration of the oxidantbecame biodegradable in lower HRT.

uring continuous mode UV/US/H2O2 process ([TOC]in = 44.83 mg L�1, pH = 2, US

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Fig. 11. Evolution of average oxidation state during the UV/US/H2O2 process at various HRT, ([TOC]in = 44.83 mg L�1, pH = 2,US power = 140 W, and air flow rate = 3 L min�1).

Fig. 12. Evolution of BOD5/COD ratio during the UV/US/H2O2 process at various HRT,([TOC]in = 44.83 mg L�1, pH = 2, US power = 140 W, and air flow rate = 3 L min�1).

420 A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423

3.5.3. Residual H2O2 concentration profileAs mentioned earlier, high concentrations of H2O2 affect the

performance of bacterial communities negatively. In many studiesconcerning the application of H2O2 in AOPs prior to the biologicaltreatment, the residual H2O2 was removed or neutralized fromthe wastewater before introducing it to bioreactor. Several compo-nents are used to remove H2O2 such as catalase enzyme[17,35,49,67] and sodium sulfite [68]. Continuous addition of

catalase to the wastewater, especially in the industrial scale, wouldincrease the treatment cost significantly. In order to avoid usingcatalase, the residual concentration of H2O2 should be minimizedin the effluent of the AOP reactor. This may achieve whether bydecreasing the initial dosage of the oxidant or increasing the HRT.

Fig. 13 illustrates the residual H2O2 concentration under the UV/US/H2O2 process at various HRT. The trend of residual oxidant con-centration is approximately the same for all experiments. Reducing

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A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423 421

the inlet dosage of H2O2 caused lower residual concentration in theeffluent. Using the optimum dosage of the oxidant found earlier(1750 mg L�1) resulted in more than 130 and 110 mg L�1 H2O2 inthe effluent of sonophotoreactor after 120 and 180 min HRT, respec-tively. In the case of inlet H2O2 concentrations of 750 and500 mg L�1, experiments with HRT of 120 min and more, less than11 mg L�1 H2O2 in the effluent were observed. This low concentra-tion caused in a low efficiency of the treatment which can be seenin the TOC removal curve (Fig. 10). Therefore, there was no need

Fig. 13. Residual H2O2 concentration in the effluent of the sonophotoreactor at various HR

Fig. 14. Comparison of total organic carbon (TOC) and chemical oxygen demand (COD) rethe UV/US/H2O2 process alone in continuous mode, aerobic AS process alone in[COD]in = 127 mg L�1, air flow rate in sonophotoreactor = 2 L min�1, pH = 2, [H2O2]in =bioreactor = 24 h).

for retention times higher than 120 min while working with 500and 750 mg L�1 H2O2.

It has been reported in the open literature that very low concen-trations of H2O2 does not cause drastic problems on microorgan-isms employed in biological treatment [62,69]. Also, Laera et al.[3] reported that H2O2 in low range of 3–7 mg L�1 did not affectthe membrane bioreactor biomass while integrated with the UV/H2O2 process. These findings would be useful to determine thestage that the wastewater samples could be transferred from thesonophotoreactor to the aerobic AS process.

T, ([TOC]in = 44.83 mg L�1, pH = 2, US power = 140 W, and air flow rate = 3 L min�1).

moval using different alternatives in continuous mode without recycling, includingcontinuous mode and combination of both processes ([TOC]in = 44.83 mg L�1,750 mg L�1,US power = 140 W, HRT in sonophotoreactor = 120 min, and HRT in

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422 A. Mowla et al. / Chemical Engineering Journal 255 (2014) 411–423

3.5.4. Combined UV/US/H2O2 and aerobic AS processes for SPWWtreatment

Biodegradability studies demonstrated that except the experi-ment with inlet oxidant dosage of 500 mg L�1, after 120 min reten-tion time, the wastewater samples became biodegradable andsignificant efficiency from aerobic AS process could be expected.Additionally, considering residual H2O2 profile and data found inthe open literature regarding the tolerance of microorganisms toH2O2, it seems that only effluents from experiments with inletH2O2 concentrations of 750 and 500 mg L�1 and the retention timemore than 120 min could be transferred to the bioreactor withouthaving adverse effect on microorganisms. Since experiments with500 mg L�1 oxidant did not yield an acceptable TOC removal,750 mg L�1 H2O2 was chosen for combined experiments. Also,HRT of 120 min was selected due to limited residual H2O2 and sat-isfactory BOD5/COD ratio. For the aerobic AS process, the HRT of24 h was chosen for combined processes.

In the final step of this study, the SPWW with inlet TOC loadingof 44.83 mg L�1 was first treated under the UV/US/H2O2 processwith the HRT of 120 min. The inlet H2O2 concentration was750 mg L�1. Other operational parameters were adjusted to theoptimum values found in batch experiments (pH = 2, USpower = 140 W, air flow rate = 3 L min�1). The pre-treated waste-water was transferred to the bioreactor and the flow rate wasadjusted to 24 h retention time in the bioreactor. The results areshown in Fig. 14. The TOC and COD removal were 98% and 99%,respectively. The inlet molar ratio of [H2O2]/[TOC] in the combinedprocesses was 5.9 which showed a significant reduction comparedto the 13.77 optimal value found in the UV/US/H2O2 process alone.From the 98% TOC removal, about 31% was due to the sonophoto-lytic process and the rest was attributed to the aerobic AS process.In the case of COD reduction, 43.5% was eliminated in the sonopho-toreactor and 55.5% was deducted in the bioreactor. The resultsconfirm that the combination of advanced oxidation and aerobicAS processes could contribute to a high treatment efficiency andreduce the chemical dosage consumption.

In addition, 39% TN removal was observed. Almost all TNremoval was occurred in the aerobic AS process. However, higherTN removal was achieved in the combined processes comparedto the aerobic AS process alone. This may be due to change inthe structure of nitrogenous compounds. Also, the increased CODavailability and removal in the effluent of the UV/US/H2O2 process,has led to higher demand for nitrogenous sources in the biologicalprocess.

4. Conclusions

The efficiencies of the UV/US/H2O2 process, the aerobic AS pro-cess, and their combination for the treatment of a nonbiodegrad-able SPWW were studied. The biological system was able toremove TOC in the range of 65–73% under 24 h HRT. Increasingthe HRT to 48 h did not make any significant change on the effi-ciency of the treatment. The ability of the biological process aloneto treat the SPWW was not sufficient since more than 95% TOCremoval was set as the goal of this study.

A set of experiments in batch recirculation mode was per-formed to determine the optimum operational parameters of theUV/US/H2O2 process. An ultrasonic power of 140 W, initial pH solu-tion of 2, and airflow rate of 3 L min�1 were found as optimal val-ues. Also, in case of H2O2 concentration, 1750 mg L�1 for thewastewater with initial TOC of 44.83 mg L�1 resulted in highestdegradation which contributed to optimum experimental [H2O2]/[TOC] molar ratio of 13.77. The sonophotolytic process in continu-ous mode using the optimum operating condition under 180 minHRT resulted in more than 90% TOC removal. In order to reduce

the consumption of oxidants and the retention time in the AOPreactor, combined processes were studied. According to the datafrom biodegradability studies and also those from H2O2 concentra-tion profile, 750 mg L�1 of the oxidant was selected to be used inthe combined processes. The HRT in the sonophotoreactor and bio-reactor was selected as 120 min and 24 h, respectively.

In the combined processes, the SPWW was pre-treated in thesonophotoreactor using the optimum operational parameters and750 mg L�1 H2O2. The effluent was collected and transferred tothe bioreactor. Combined UV/US/H2O2 and aerobic AS processeswere able to treat the SPWW successfully. Over 98% TOC and99% COD removal were observed. The consumption of the oxidantwas reduced considerably.

Acknowledgments

The financial support of Natural Sciences and EngineeringResearch Council of Canada (NSERC) and Ryerson University is dee-ply appreciated. Also, the authors would like to thank AshbridgesBay Wastewater Treatment Plant in Toronto, Canada, for providingthe aerobic activated sludge.

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