HIGH LOADED ANAEROBIC MESOPHILIC DIGESTION OF SEWAGE SLUDGE An evaluation of the critical organic loading rate and hydraulic retention time for the anaerobic digestion process at Käppala Wastewater Treatment Plant (WWTP). IBRAHIMA SORY GÄRDEKLINT SYLLA School of Business, Society and Engineering Course: Degree Project in Energy Engineering Course code: ERA403 Credits: 30 hp Program: Master of Science in Engineering in Energy Systems Supervisor at Mälardalens University: Monica Odlare Supervisor at Käppalaförbundet: Jesper Olsson and Sofia Bramstedt, Examiner: Eva Thorin Costumer: Käppalaförbundet Date: 2020-08-21 Email: [email protected][email protected][email protected]
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HIGH LOADED ANAEROBIC MESOPHILIC DIGESTION OF SEWAGE SLUDGE
An evaluation of the critical organic loading rate and hydraulic retention time for the anaerobic digestion process at Käppala Wastewater Treatment Plant (WWTP).
IBRAHIMA SORY GÄRDEKLINT SYLLA
School of Business, Society and Engineering Course: Degree Project in Energy Engineering Course code: ERA403 Credits: 30 hp Program: Master of Science in Engineering in Energy Systems
Supervisor at Mälardalens University: Monica Odlare Supervisor at Käppalaförbundet: Jesper Olsson and Sofia Bramstedt, Examiner: Eva Thorin Costumer: Käppalaförbundet Date: 2020-08-21 Email:
1.3 Research questions .................................................................................................................................................. 2
2.2.1.3. HYDRAULIC RETENTION TIME, ORGANIC LOADING RATE, AND PROCESS AID .................................. 7
2.2.2 Inhibition of the anaerobic digestion process .................................................................................................. 8
2.3 Importance of sludge recycling for the environment and society ......................................................................... 9
3 METHODS AND MATERIALS ......................................................................................................................................... 10
3.1 Experimental set-up and operational protocol ...................................................................................................... 10
3.1.1 Automatic Methane Potential Test SYSTEM (AMPTS) ................................................................................. 11
3.1.2 Set up of the system .................................................................................................................................... 12
3.1.3 Feeding process .......................................................................................................................................... 13
3.2 Substrates and inoculums ..................................................................................................................................... 13
3.2.1 The outcome of the experimental setup and experiment period ................................................................... 15
3.3 Analysis parameters and methods ........................................................................................................................ 16
3.3.1 pH, TS, and VS ............................................................................................................................................ 16
3.3.7 Degree of degradation ................................................................................................................................. 19
3.3.8 Process stability ........................................................................................................................................... 19
3.5 Mass balance .......................................................................................................................................................... 21
3.6 Forecasting of the future OLR at Käppala ............................................................................................................. 21
3.7 Process aid .............................................................................................................................................................. 22
3.8 Dewaterability study of digested sludge ............................................................................................................... 22
3.9 Description of the anaerobic digestion at Käppala WWTP .................................................................................. 23
4 RESULT ........................................................................................................................................................................... 26
4.2 The anaerobic pilot-scale experiment ................................................................................................................... 26
4.2.1 OLR and HRT in the six pilot-scale reactors ................................................................................................. 26
4.3 Stability analysis of the process ............................................................................................................................ 28
4.3.1 CH4 and pH ................................................................................................................................................. 28
4.3.2 VFA and ratio VFA/TA ................................................................................................................................. 29
4.4 Energy potential and performance of the experiment .......................................................................................... 31
4.4.1 Methane yield in the pilot-scale experiment .................................................................................................. 31
4.4.2 The degree of degradation ........................................................................................................................... 33
4.4.3 Chemical oxygen demand COD ................................................................................................................... 34
4.4.4 Mass balance ............................................................................................................................................... 35
4.5 Result of the Chemical additive ............................................................................................................................. 37
4.6 Dewaterability study of the digested sludge ......................................................................................................... 38
4.7 Result of the excel prediction model ..................................................................................................................... 39
5.1 Suitability of the substrate for biogas production ................................................................................................ 40
5.2 Maximum organic loading rate (OLR) .................................................................................................................... 40
5.4 Gas production ....................................................................................................................................................... 42
5.5 Process aid .............................................................................................................................................................. 43
5.6 Dewaterability study ............................................................................................................................................... 44
7 SUGGESTIONS FOR FURTHER WORK ......................................................................................................................... 48
Figure 1 Stages of anaerobic digestion process for biogas production Kumar & Samadder, (2020). .......................... 6
Figure 2: The laboratory-scale biogas production process set up. .............................................................................. 12
Figure 3: Description of the feeding process. ............................................................................................................... 13
Figure 4: The sludge dewatering thickening process with the aid of a compact moisture analyzer (first object from
the left) and a high molecular filter (second object from the left) ......................................................... 15
Figure 5: Alkalinity analyses of the digested sludge by the titration robot. 50 mL digested sludge liquid in the
different test tubes are being analyzed with a blue pH-meter, a white mixer, a nitrogen gas tube, and
a 0.05 M hydrochloric acid (HCl) tube. ................................................................................................... 17
Figure 6: Measurement of methane concentration in the produced biogas, using the membrane gas sampling port
and the NaOH solution containing the pH indicator, in an Einhorn pipe meter. ................................. 18
Figure 8 Process chart over Käppala WWTP used with permission Kappala (2011) ................................................. 25
Figure 9 Organic loading rate (OLR) [g VS/day] and hydraulic retention time (HRT). ............................................ 27
Figure 10 pH and CH4 of the digesters during the experiment................................................................................... 29
Figure 11 The VFA level and the ratio VFA/TA during the pilot-scale experiment. .................................................. 30
Figure 12 Ammonia in the different digesters during the pilot-scale experiment ...................................................... 31
Figure 13 Specific methane production (methane yield) and the accumulated methane produced .......................... 33
Figure 14 Degree of degradation for reactors R1, R2, R3, and R6. ............................................................................. 34
Figure 15 CODs, and total COD in primary and digested sludge during the experiment. .......................................... 35
Figure 16 Theoretical methane content converted into methane gas ......................................................................... 36
Figure 17 Comparison of the theoretical and the specific methane yield .................................................................... 36
Figure 18 Effect of the Sodium Carbonate on the pH and CH4 of R4 ......................................................................... 37
Figure 19 Result of the CST analyses. ...........................................................................................................................38
Figure 20 Result of the prediction model. .................................................................................................................... 39
Figure 21 Accumulated methane production ................................................................................................................. 2
Figure 22 Daily methane production per day ................................................................................................................. 2
Figure 23 Methane production flow per day .................................................................................................................. 3
LIST OF TABLES
Table 1: The scenarios that, according to the first plan, were to be studied in the six lab-scale reactors. The organic
loading rates and hydraulic retention times are written as OLR and HRT ............................................ 11
Table 2: Expected values and real outcome in the six pilot-scale reactors between February 17th and March 1st.
(Experimental period 2) ........................................................................................................................... 15
Table 3 The scenarios applied in the six lab-scale reactors during the different periods of the experiment ............ 16
Table 5 Substrate composition in the primary sludge and the digested sludge .......................................................... 26
Table 6 The needed quantity of polymer per mass TS sludge. ....................................................................................38
Table 7 Weekly protocol of the pilot-scale experiment .................................................................................................. 1
NOMENCLATURE
Symbol Description Unit
𝐵𝐴 Bicarbonate alkalinity [mgBasic ions L−1]
%𝐶𝐻4 Methane concentration [%]
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝐻4 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 Specific methane production/ Methane yield
[𝑁𝑚𝐿 𝑔−1𝑉𝑆−1]
d Number of days [d]
HRT Hydraulic Retention Time [d]
NH3 − N The free ammonium in the digested sludge
[%]
𝑀𝐿 Nitrogen mineralization [%]
𝑚𝑁𝑎2𝐶𝑂3 Mass of Sodium Carbonate [g]
𝑀𝑁𝑎2𝐶𝑂3 The molar mass of Sodium Carbonate
[g mol-1]
𝑁𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟 Number of people connected to the Käppala WWTP
[-]
Qin Organic feeding load measured in volume or mass
[g d-1]
OLR Organic Loading Rate [g VS dm-3 d-1] or [kg VS m-3 d-1]
𝑉𝐹𝐴s Volatile Fatty Acids [mg L-1]
𝑇𝐴 Total Alkalinity [mgBasic ions L−1]
TSDesired_PS Desired total solids [%]
𝑉 Digester’s working volume [dm3]
𝑉𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 The desired volume of the collected primary sludge
[dm3]
𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑 The assumed volatile solid [%]
𝛼 Degree of degradation [%]
ABBREVIATIONS
Abbreviation Description
AD Anaerobic digestion
ALK Alkalinity
BMP Biochemical methane potential
COD Chemical Oxygen Demand
GWP Global warming potential
HRT Hydraulic Retention Time
NL Normal liter
OLR Organic Loading Rate
TAN Total Ammonia Nitrogen
TKN Total Kjeldahl Nitrogen
TS Total Solids
VS Volatile Solids
WWTPs Wastewater treatment plants
DEFINITIONS
Definition Description
Anaerobic digestion
The biotechnological oxygen-free process to degrade organic material extracting biogas
Acetogenins A key enzyme in energy metabolism
Acetoclastic methanogens
Microorganisms produce methane by fermenting acetate and H2-CO2 into methane and carbon dioxide
Free ammonia nitrogen (FAN)
NH3-N, the undissociated form of ammonia, free ammonia nitrogen (FAN)
Ammonium ion NH4-N is the dissociated form of ammonia
Biogas Gaseous product from fermentation consisting of methane and carbon dioxide, and depending on the substrate used, ammonia, hydrogen sulfide, and water vapor
Biogas yield Quantity of biogas produced per quantity of substrate feed
Biogas formation potential
Maximum possible biogas yield from a defined quantity of substrate
Total biogas production
The quantity of biogas formed in units of volume
Biogas production rate
Biogas quantity produced per unit of time
Total ammonia nitrogen (TAN)
Total ammonia nitrogen (TAN) is the sum of FAN and Ammonium in water
Chemical Oxygen Demand (COD)
The measure of the content of oxidizable compounds in a substrate
Colloidal particles Microscopic solid particles suspended in a fluid
Degree of degradation
Reduction in the concentration of organic substance due to anaerobic degradation
Dewaterability The ability in digested sludge to let go of water
Fermentation Anaerobic process in which a product, in this case, biogas is produced by the activity of microorganisms
Global warming potential (GWP)
Amount of heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide
Hydraulic Retention Time (HRT)
Average time for which the substrate remains in the fermenter
Inflow load Mass fed daily into the fermentation installation
Definition Description
Inhibition Hindering of fermentation due to damage to the active micro-organisms or reduction in the effectiveness of enzymes
Loading rate per unit volume
The ratio of the daily load to the fermenter volume
Methane formation potential
Maximum possible methane yield from a defined quantity of substrate
Methane yield Quantity of methane produced per quantity of substrate feed
Methane production rate
Methane quantity produced per unit of time
Organic Loading Rate (OLR)
The amount of organic material per unit reactor volume, which is subjected to the anaerobic digestion process in the reactor in a given unit period
Solids retention time (SRT)
The time the solid fraction of the wastewater spends in a treatment unit
Total solids (TS) TS is the substance contained in the sludge that is left after dying the sludge at 105 degrees for 24 hours.
Volatile solid (VS) The amount of organic solids in water
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1 INTRODUCTION
The volume of wastewater in the world is expected to increase during the coming decades due
to the continuously growing world population and increasing industrialization (Duan et al.,
2012). According to the Swedish Environmental Protection Agency, about 1 million tons of
sewage sludge is produced every year in municipal wastewater treatment plants in Sweden
(Arthurson, 2008). The optimal management of sewage sludge is, therefore, an important
issue worldwide, since the sludge can cause severe damage to both the environment, animals,
and humans (Chen et al., 2020). However, since the sludge contains organic matter, it can
serve as a renewable resource. It also contains un-degradable particles and living organisms,
and in order to reduce biological activity and odor, it must be stabilized (Chen et al., 2020).
Municipal wastewater treatment plants (WWTP) generate sewage sludge from mechanical,
biological, and chemical treatment. A common treatment used to stabilize the sludge is
anaerobic digestion (AD). It is an efficient and well-studied process that biologically converts
the chemical energy of sewage sludge into combustible biogas that contains 60-70% methane
(Appels et al., 2008), and makes it a carbon-neutral alternative to fossil fuels.
Simultaneously, it reduces dangerous pathogens and odor in the process (Zhen et al., 2017).
Thus, transforming a waste problem into an essential renewable energy resource. As biogas is
a renewable energy source, the expansion of biogas production systems is an essential
contributor to the global conversion from fossil fuels to renewable energy systems
(Tchobanoglous & Burton, 2014).
To meet the increasing volume of sewage sludge, upgrading existing wastewater treatment
plants and the sludge digesters is essential. This is the case at Käppala WWTP in Stockholm,
which purifies wastewater from approximately 550,000 inhabitants northeast of Stockholm
(Käppala Association, 2020).
1.1 Background
Käppala WWTP has, during the last years, observed an increase in organic loading rate
(OLR) in the mesophilic anaerobic digestion process, due to the increased load to the WWTP.
This has resulted in a decrease in alkalinity and pH in the digested sludge. Since pH and
alkalinity are two of the anaerobic process stability parameters, this shows a risk for
instability in the digestion process. The digestion at Käppala WWTP is today high loaded,
with a high organic loading rate and low hydraulic retention time (HRT). In the future, this
problem will be noticeable due to the increased population in the region. R100 at Käppala
WWTP is currently operating with an organic loading rate of 3.9 [kg VS m-³, d-1], and a
hydraulic retention time of 13,2 days. The experience from Käppala show that this is working
well, but how close to the process limits it is possible to run a stable anaerobic digestion
2
process? If nothing is done, the digester R100 will be overloaded, which will lead to process
imbalance and process failure. Studies by, for example, Halalsheh et al., (2005); Duan et al.,
(2012); Olsson et al., (2018) show that the operational experience with similar process
conditions exists from other wastewater treatment plants. However, process conditions differ
from facility to facility, and local conditions are usually governing. Evaluating the mesophilic
digestion conditions for Käppala WWTP has never been done before.
1.2 Purpose
For Käppala to encounter the higher load of sewage sludge in future sludge strategies, this
thesis aims to evaluate the effect of the maximum organic loading rate and the minimum
hydraulic retention time for the anaerobic digestion of sewage sludge. Moreover, to
investigate further actions that can be taken in case of process problems in the digestion. The
focus of this work is to strive for a stable digestion process, which is vital for optimal biogas
production.
1.3 Research questions
The study is aiming at answering the following research questions:
1. Maximum load: What is the maximum organic loading rate (OLR) and the minimum
hydraulic retention time (HRT) to achieve a stable digestion process?
a. Which range of process parameters is acceptable?
b. What happens to the process parameters when the process is overloaded?
c. What measures can be taken in case of overload or risk of overload in the mesophilic
anaerobic digestion process? Could chemical additives be used?
d. When will R100 at Käppala WWTP reach maximum load, and a third anaerobic digester
will be needed?
2. Energy potential: How is the gas production affected when the organic loading rate
increases and the hydraulic retention time in the digesters decreases? How is the gas
production affected by instability in the digester?
3. Dewatering process: How are the sludge dewatering properties affected by changes in the
organic loading rate and hydraulic retention time in the digester?
1.4 Delimitation
The work in this master thesis is limited to a laboratory-scale experiment, studying six pilot-
scales anaerobic digestion reactors. Each one of the six reactors is representing the full-scale
reactor R100 at Käppala WWTP, with continued stirring of sewage sludge, in the mesophilic
conditions of 37±1 ˚C temperature. The study only deals with the sludge digestion for biogas
3
production at Käppala WWTP, not the treatment of wastewater. No economic analyses or life
cycle analyses of the process are included in the study.
4
2 THEORETICAL FRAMEWORK
2.1 Sewage sludge stabilization
Sewage sludge is produced while treating wastewater in municipal wastewater treatment
plants. It contains water, degradable organic material, living organisms, and un-degradable
particles. The sludge is generally treated as a waste product. However, since it contains
renewable resources such as carbon, that can be converted into biogas, and phosphorus and
nitrogen, that can be used as fertilizer for cultivation (Wawrzynczyk, 2007), it has become
more and more useful as a resource. Nevertheless, to be useful, it needs to be stabilized to
reduce biological activity, odor production, and the release of harmful chemicals substances
into the environment (Boušková et al., 2005).
Anaerobic digestion (AD) is an efficient method for sludge stabilization. Mainly because of
the high efficiency in organic matter degradation when producing biogas with 60-70%
methane content, which can be upgraded to biofuel (Cha & Noike, 1997; Chen et al., 2020). It
reduces greenhouse gas (GHG) and provides clean and renewable energy in the process.
According to Chen et al., (2008b), anaerobic digestion is one of the most promising sludge
stabilization methods. Mainly because it involves both controlling the pollution from
industrial and agricultural waste and a way to recover energy in the process. Other
advantages using anaerobic digestion for sludge stabilization are, for example:
• High reduction of volume in the process, between 30-50%.
• Reduction of offensive odors in the sludge.
• High rate of pathogen destruction in the sludge when using the thermophilic digestion for
sludge stabilization (Rubia et al., 2005).
Anaerobic digestion can be performed at two different temperatures, the mesophilic
digestion process at 30-40˚C, and the thermophilic digestion process at 45-60˚C (Cha &
Noike, 1997; Kumar & Samadder, 2020). Mesophilic digestion is the method used at Käppala
WWTP, investigated in the present study. However, thermophilic digestion is becoming more
and more popular due to its potential when it comes to the reduction of pathogens in the
sludge (Watanabe et al., 1997).
However, there are some obstacles to consider using anaerobic digestion. For example,
several studies have reported of foaming and low efficiency in the degradation of the volatile
solids in the anaerobic digestion process (Li & Noike, 1992; Halalsheh et al., 2005). Low rates
of VS degradation of colloidal particles in the waste have been reported due to the physical
limitations of low biodegradability (Elmitwalli et al., 2001). The degradation of insoluble
substances has also been mentioned as rate-limiting steps for the anaerobic digestion
(Eastman & Ferguson, 1981). For a high degradation of colloidal particles, there is a need for
long retention times, up to 20-30 days according to Parawira et al., (2004), even 35 days in
some full-scale operations for waste stabilization. Here thermophilic digestion has been
mentioned as more advantageous than mesophilic digestion, with higher VS degradation
5
efficiency, higher biogas production, less foaming, and better dewaterability (Rimkus et al.,
1982).
2.2 The anaerobic digestion processes
The anaerobic digestion process is a complex microbiological process that occurs in an
oxygen-free environment. The process involves a series of metabolic reactions that oxidize
organic matter into biogas and organic fertilizers (Kumar & Samadder, 2020). Sewage
sludge, produced from wastewater treatment plants, is a well suited organic material as a
substrate for a stable anaerobic digestion process since its nutrient content varies very little
(Bramstedt, 2015). According to Kumar & Samadder, (2020), the whole anaerobic process is
divided into four different stages (see Figure 1):
• Hydrolysis is the first stage in which complex organic compounds like carbohydrates,
proteins, and fats are broken down into soluble organic molecules such as amino acids, sugar,
fatty acids, and other related compounds. This stage is the slowest step due to the large size of
the molecules, the volatile fatty acid formation, and other toxic by-products (Zhang et al.,
2014).
• Acidogenesis or fermentation is the stage during which the produced organic compounds
from the previous stage are further broken down into intermediate products, such as short-
chain fatty acids along with hydrogen, carbon dioxide, and other by-products. Acid formation
under this stage takes place with the help of acidogenic bacteria. This stage is sometimes
divided into two stages. If the breakdown of fatty acids goes slowly, they accumulate and can
lead to a decrease in pH and instability in the process (Tchobanoglous et al., 2014).
• Acetogenesis is the third stage during which the organic acids formed in the acidogenesis
stage gets converted into acetic acid as well as hydrogen and carbon dioxide.
• Methanogenesis is the fourth stage during which two different groups of methanogens
produce methane. One group splits the acetic acid into methane and carbon dioxide, while the
other uses the intermediate products, H2 and CO2, for the methane formation (Appels et al.,
2008).
6
Figure 1 Stages of anaerobic digestion process for biogas production (Kumar & Samadder, 2020).
2.2.1 Anaerobic digestion process affecting parameters
Several essential parameters affect the speed of the different stages of the digestion process in
the anaerobic environment, namely pH, alkalinity, temperature, and the hydraulic retention
time.
2.2.1.1. pH, alkalinity, and volatile fatty acids
The pH level is one of the most crucial anaerobic process stability parameters. Every
microorganism group has a different optimal pH range. Methanogenic bacteria responsible
for methane formation in the anaerobic digestion process are susceptible to the pH level, with
an optimum pH for methane formation between 6.5 and 7.2 (Kanokwan, 2006; Kumar &
Samadder, 2020). When it comes to the fermentation microorganisms, they are less sensitive
and can function within a broader range of pH, between 4.0 and 8.5. Acetics are the main
products at a low pH level, while acetic and propionic acids are mainly produced at a high pH
value (Kanokwan, 2006).
The production of volatile fatty acids (VFA) in the anaerobic digestion process tends to lower
the pH level. This pH reduction is usually countered by the activity of methanogenic bacteria,
due to their capacity also to produce alkalinity in the form of carbon dioxide, bicarbonate,
and ammoniac (Appels et al., 2008; Kumar & Samadder, 2020). Together the two create a
balance in the reactor. The pH level in the anaerobic process varies depending on in which
state the process is. The process pH usually increases when the ammonia concentration
7
increases due to the reduction of proteins present in the substrate and decreases when the
A stable operating temperature in the digester is essential for the anaerobic digestion process
since it is one of the most critical parameters in the anaerobic reactor. It affects the
performance in general, but especially the methanogenesis (Turovskiy & Mathai, 2006;
Kumar & Samadder, 2020). The temperature also has a significant effect on the
physicochemical properties of the components in the digestion substrate. Besides the growth
rate and metabolism of micro-organisms, the whole population dynamic in the anaerobic
reactor is affected. Sharp or frequent fluctuations in the temperature affect the bacteria,
especially the methanogens, and process failure can occur if the changes are more than 1 o
C/day. Any changes in temperature of more than 0,6 o C/day are to be avoided (Appels et al.,
2008).
2.2.1.3. Hydraulic retention time, organic loading rate, and process aid
According to Svenskt Vatten, (2019), hydraulic retention time is the most crucial process
parameter for the anaerobic digestion process. The digestion result depends primarily on the
HRT, followed by the OLR and the temperature (Svenskt Vatten, 2019). Stabilization in the
digestion process also depends on the HRT, since an HRT less than ten days (10 d) under
mesophilic conditions can give rise to wash-out of the organisms and thus inhibit the process
according to Forkman, (2014). Each time the digested sludge is withdrawn from the digester,
a fraction of the bacterial population is also removed. The cell growth must then compensate
for the removed bacteria from the digester to ensure a steady-state in the process and avoid
process failure (Appels et al., 2008).
The OLR is, as mentioned, another essential parameter in the anaerobic digestion, due to the
process imbalance risk connected to an overload in the digesters. Increasing the OLR in the
anaerobic digestion process can lead to operational disruptions due to process imbalances.
They are commonly detected as disruptions in gas production, an increase in the CO2 level in
the produced biogas, and a decrease in the pH level (Svenskt Vatten, 2019). However, high
organic loaded digestion can result in the reduction of the needed digester tank volume and
improved process stability and process efficiency if all the conditions are in place (Appels et
al., 2008). According to Svenskt Vatten, (2019), a well-stirred anaerobic digester can be
loaded with up to 2-3 kg VS (organic material) per m3 and day. However, raw feeding sludge
should not exceed 6% TS. At higher TS level in the sludge, there is a risk of poisoning the
process with mainly ammonium ions or ammonia gas. High TS content also makes
mechanical stirring difficult.
In order to regain the process balance while keeping the high organic load in the digester,
conceivable measures to consider are, among other things, the use of chemicals. According to
Svenskt Vatten, (2019), the following chemicals are commonly used:
• Sodium hydroxide (NaOH) can be used in most cases.
8
• Calcium hydroxide (Ca (OH) 2) should be used when the sludge contains a high content of
inorganic material (a residue level higher than 60%).
• Sodium carbonate (Na2CO3) and sodium hydrogen carbonate (NaHCO3) should only be used
when the pH level is higher than five, due to high CO2 production, which can lead to foaming
problems in the digester.
Kasali et al., (1989) managed to fully recover a failed anaerobic digestion process, using
sodium hydrogen carbonate (NaHCO3) as a pH controller. Sodium carbonate (Na2CO3),
recommended by Svenskt Vatten, (2019), is used in the present study to recover a failed
overloaded pilot-scale digester at Käppala WWTP.
2.2.2 Inhibition of the anaerobic digestion process
Several studies on the anaerobic digestion process have shown considerable variations in the
inhibition levels reported for most substances. Some of these inhibitory substances are
ammonia, organic compounds sulfide, light metal ions, and heavy metals. The complexity of
the anaerobic digestion process, where mechanisms such as antagonism, synergism,
acclimation, and complexing could affect the inhibition phenomenon significantly, is the
primary reason for these variations in the inhibition levels (Chen et al., 2008a). The two most
common inhibitory substances reported by most studies are ammonia and volatile fatty acids
(VFA).
• Ammonia exists in two primary forms, ammonium ion (NH4) and ammonia nitrogen (FAN),
often called free ammonia (Chen et al., 2008a). Combined in water, the two becomes total
ammonia (TAN). Ammonia is a significant inhibitor of microbial activities in the anaerobic
reactor (Akindele & Sartaj, 2018). It can support the system, acting as a buffer, but it can also
become a problem if the level of concentration is too high. Then it becomes toxic to the process
and reduces methane production (Browne et al., 2014; Rajagopal et al., 2013; Sprott & Patel,
1986). The outcome depends, for example, on pH, temperature, C/N ratio, and the type of
substrate and inoculum. Yenigün & Demirel, (2013) concludes that a FAN value higher than
100 mg L-1 is the threshold value for ammonia in the anaerobic digestion process. Inhibition
of the anaerobic process has been reported by Zhang & Angelidaki, (2015) to start at a TAN
level of 1.5 g-N L-1.
• VFA:s are, like ammonia, one of the main in-between compounds in the metabolic pathway of
methane fermentation and can cause microbial stress if they are present in high
concentrations in the anaerobic digestion process (Buyukkamaci & Filibeli, 2004). The VFA
level in the process generally indicates the metabolic state of the obligate hydrogen-producing
acetogenins and the acetoclastic methanogens. Accumulation of VFA occurs when the
methanogens are unable to break down the VFA to methane, but the fermenting bacteria
continue to form VFA. As a result, the methanogens are held back even more. Monitoring the
evolution of the VFA level is thus key to detecting process imbalance (Aymerich et al., 2013). A
high concentration of VFA can also result in a pH decrease, which can lead to process failure.
The VFA concentration is regularly monitored in the present study to examine the optimal
conditions and the efficiency of the pilot digesters.
9
2.3 Importance of sludge recycling for the environment and society
To reach the global sustainable development goals (SDG) (The Global Goals, 2015),
systematic procedures for the treatment and recycling of sewage sludge, converting it into
energy and other renewable resources, is crucial (Arthurson, 2008). Anaerobic digestion of
sewage sludge is an efficient way of transforming the challenge into an opportunity. Not only
producing biofuel and methane-rich biogas that can be utilized as fuel but also to produce
offset heat and electricity for the wastewater treatment sector itself. Renewable energy
sources that reduce the need for externally produced heat, electricity and fosil fuel in the
process significantly (Cao & Pawłowski, 2012). The produced heat can also serve as an
essential contributor to the local energy demand, which reduces the need for non-renewable
energy sources in the district heating system, warming up houses, and producing hot water.
In Käppala WWTP, the sewage sludge-to-energy process results in several different energy
outcomes. The purified wastewater is lead to a heat pump to recover heat from it to use
internally in the WWTP to heat the sludge in the digester. The excess part is then delivered to
the district heating system (see point 13 in Figure 8). In the vehicle gas plant (see point 10 in
Figure 8), the carbon dioxide is removed, upgrading the biogas into vehicle fuel with a
minimum of 97% methane, used in the local busses (SL) (see point 12 in Figure 8). Biogas
can also be used to produce electricity via a water-to-steam-system that makes turbines turn,
thus creating energy. However, this is not the case in Käppala WWTP (Käppala 2018).
10
3 METHODS AND MATERIALS
The study consists of four parts:
• A practical laboratory experiment using six pilot-scale anaerobic digesters of 2.5 dm3, to
investigate how the process stability in the digested sludge is affected when the OLR increases
and the HRT decreases.
• A mass balance calculation based on the specific methane yield and the energy potential in the
feeding and the digested sludge.
• A study of the digested sludge’ filterability, to investigate how the sludge dewatering properties
are affected by changes in OLR and HRT in the digester.
• The construction of a forecasting model in Excel that can predict when the studied digester
R100 at Käppala WWTP will reach its maximum OLR and minimum HRT.
3.1 Experimental set-up and operational protocol
The pilot-scale experiment contained six pilot-scale anaerobic digesters of 2.5 dm3, running
parallelly. They were filled with 2 dm3 of mesophilic inoculum from the full-scale anaerobic
digester R100. The reactors were then operated at the same HRT as the full-scale reactor
R100, during one retention time (13.2 days). OLR and HRT were then changed in reactor R2
to R6, while reactor R1 was maintained at the same conditions and operated as a reference.
In digester R2, the change was made gradually from the conditions in R3 to R6. Each OLR
and HRT in R2 was maintained for at least two retention times. In R3 to R6, the conditions
were changed directly. The HRT for R2 to R6 was assumed as presented in Table 1, and the
feeding sewage quantity was determined using equation 2. The OLR for digesters R2 to R6
was determined using equation 1. VS% was assumed before each primary sludge collection to
calculate the desired TS (see equation 5) and then measured using the standard method
(APHA et al., 1995). The incoming proportion of TS% into the digester was controlled by
thickening the sludge to keep the OLR and HRT constant in the different reactors.
𝑂𝐿𝑅 =𝑄𝑖𝑛∗𝑇𝑆∗𝑉𝑆
𝑉 Equation 1
𝑄𝑖𝑛 =𝑚
𝐻𝑅𝑇 Equation 2
𝐻𝑅𝑇 =𝑚
𝑄𝑖𝑛 Equation 3
• 𝑂𝐿𝑅 is the organic loading rate in the digesters [g VS dm−3d−1].
• 𝑇𝑆 is the total solids matter in the sludge [%]
• 𝑉𝑆 is the volatile solids in total solids matter the sludge in [%].
• V is the working volume of the reactors, which is the part of the 2.5 𝑑𝑚3 that is filled with
mesophilic inoculum from the full-scale anaerobic digester R100 in [𝑑𝑚3]
• 𝑄𝑖𝑛 is the hydraulic flow into the digestor per day, in [g d−1]. 𝑄𝑖𝑛 was measured in grams (as a
mass) to ensure that the same amount of solid matter was obtained in the daily feeding sludge
(see Figure 3). It was assumed that 1L = 1dm3 = 103cm3 = 103g = 1kg. And the sludge density
was also assumed to 1000 [𝑔 𝑑𝑚−3]
• HRT is the hydraulic retention time in [d].
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• 𝑚 is the quantity of the mesophilic inoculum from the full-scale digester R100 in the pilots scales
reactors [g]
Table 1 presents the original scenario protocol that was planned to be investigated in this
study, where the presented parameters were used to determine the volume and TS content of
the incoming sludge to the reactors. The VS content of the incoming feeding sludge to the
reactors was assumed to 80%.
Table 1: The scenarios that, according to the first plan, were to be studied in the six lab-scale reactors. The organic loading rates and hydraulic retention times are written as OLR and HRT
• 𝑉𝐷𝑒𝑠𝑖𝑟𝑒d_PS is the desired volume of the collected primary sludge in [dm3].
• 𝑉𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 is the known initial volume of the collected primary sludge in [dm3].
• 𝑇𝑆𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 is the measured initial TS value of the collected primary sludge in [%].
• 𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 is the desired TS value of the collected primary sludge, determined in the next
section in [%].
▪ The collected sewage sludge was filtered to obtain the desired volume, determined by equation
6, using a high molecular filter (see the second object from the right in Figure 4).
▪ The thickened sludge was then stored at a temperature of 4˚C for feeding the reactors.
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Figure 4: The sludge dewatering thickening process with the aid of a compact moisture analyzer (first object from the left) and a high molecular filter (second object from the left)
3.2.1 The outcome of the experimental setup and experiment period
The experimental outcome could be divided into four periods. The first period (see Table 3)
of the pilot-scale experiment started on February 3rd with all six reactors running at the same
OLR and HRT for one retention time (13.2 days), before the OLR and HRT were changed, on
February 17th. Due to calculation errors and false assumptions of the VS values during the
start-up of the experiment, the outcome did not follow the expected original protocol. Thus,
the obtained OLR and HRT in all the digesters (R1 to R6) turned out differently than
expected for the second period (February 17th to March 2nd), especially in R4, R5, and R6, the
values deviated significantly from the expected (see Table 2). Table 2 presents the expected
and the real OLR and HRT for the period.
Table 2: Expected values and real outcome in the six pilot-scale reactors between February 17th and March 1st. (Experimental period 2)
Expected outcome Real outcome
Digester OLR [g VS dm-3 d-1] HRT [d] OLR [g VS dm-3 d-1] HRT [d]
R1 3.9 13.2 4.2 13.1
R2 4.3 12 4.7-4.5 12
R3 4.3 12 4.7-4.5 12
R4 4.7 11 12.6 - 8.3 4.4 – 6.5
R5 5.2 10 8.8 - 6.7 6.3 - 8
R6 5.5 9.5 6.5 – 5.4 8.6 – 9.9
The errors were corrected, resetting the new OLR and HRT values as close as possible to the
original protocol during period 3 (see Table 3). The experiment could then continue following
a new plan (see Table 3). The affected reactors were expected to recover during the third
period (March 2nd to April 16th) after the correction of the calculation errors. During period 3,
the chemical additive Na2CO3 was also used once a week to investigate if the anaerobic
digestion in reactor R4 could recover, while still keeping the high OLR and short HRT.
The last experimental period (period 4) (see Table 3), starting on April 17th, is the period most
affected by instability in all the reactors. The feeding of the digesters was then stopped twice,
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in an attempt to allow the digesters to recover and avoid process failure. Between April 17th
and 21rst, the feeding was stopped entirely because of total process failure. On April 22nd, the
feeding started again, but with only half of the OLR level. Between May 1st and 12th, the
feeding was stopped again because of process imbalance. Table 3 presents how the OLR and
HRT varied in the reactors during all four periods.
Table 3 The scenarios applied in the six lab-scale reactors during the different periods of the experiment
Period Period (1)
2020-02-03
2020-02-16
Period (2)
2020-02-17
2020-03-01
Period (3)
2020-03-02
2020-04-16
Period (4)
2020-04-17
2020-05-12
R1 OLR [g VS dm-3 d-1] 3.9 4.2 4.1-4.5 3.5-1.8-3.5-0.0
During the experiment, primary sewage sludge from Käppala WWPT was collected and
thickened once every two weeks, to be analyzed for relevant parameters for the anaerobic
digestion process, like TS, VS, COD, CODs, pH, NH4-N, Kjeldahl-N. The first feeding and the
last digested sludge were also analyzed for fat and proteins to determine the fat and protein
content in the primary sludge. The raw sludge was kept at a temperature of 4˚C for the
feeding of the digesters. The digested sludge was daily analyzed for pH. The methane
concentration in the produced biogas was also daily measured. The digested sludge was
weekly analyzed for TS, VS, COD, CODs, NH4-N, VFA, and ALK.
3.3.1 pH, TS, and VS
The pH, TS, and VS were analyzed according to the standard method APHA et al., (1995)
before and after the digestion, to evaluate the physicochemical change in the sludge
characteristics.
3.3.2 Alkalinity
The alkalinity of the digested sludge was analyzed once a week through a titration robot
connected with an 896 Compact Sampler Changer (Metrohm, 2020). The digested sludge
was first centrifuged at 4000 rpm for 20 minutes (Thermo Scientific, 2020) to separate the
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liquid part of the sewage sludge from the substantial part, and then analyzed by the titration
robot (see Figure 5). The titration robot mixed the sample with 0.05 M hydrochloric acid
(HCl) to reduce the pH of the sample to 5.4 to determine the BA. The pH of the sample was
reduced to 4.5 to determine the TA. BA and TA were then calculated using the following
equations 7 and 8 (SS-EN ISSO 9963-1 & SS-EN ISSO 9963-2 ) (Jarvis & Schnurer, 2009).
𝐵𝐴 = 380 ∗ 𝑉𝐻𝐶𝑙 Equation 7
𝑇𝐴 = 380 ∗ 𝑉𝐻𝐶𝑙 Equation 8
• BA is the bicarbonate alkalinity in [mgHCO3− dm−3].
• VHCl is the volume of the hydrochloric acid in [ dm−3].
• TA is the total alkalinity in [mgBasic ions dm−3].
Figure 5: Alkalinity analyses of the digested sludge by the titration robot. 50 mL digested sludge liquid in the different test tubes are being analyzed with a blue pH-meter, a white mixer, a nitrogen gas tube, and a 0.05 M hydrochloric acid (HCl) tube.
3.3.3 VFA, NH4-N, COD and CODs
The VFA of the digested sludge was analyzed, filtering the sludge with a suction filter with a
pore size of 0.45 μm Tisch Scientific, (2020). The filtered sludge was then analyzed for VFA
using LCK 365 cuvette test from HACH LANGE, which was later measured by a
spectrophotometer. The total chemical oxygen demand COD and the filtered CODs were
analyzed the same way as the VFA, except that the sample of the sludge analyzed for the total
COD was not filtered but diluted to a specific volume. The total COD and the filtered CODs
were analyzed using LCK 114 cuvette test from HACH LANGE, which was later measured by a
spectrophotometer. The ammonium NH4-N was analyzed the same way as the COD, with an
LCK 303 cuvette test from HACH LANGE.
3.3.4 Methane concentration
The methane concentration in the produced biogas was analyzed every day before the feeding
process. This was done taking 5 milliliters of the produced gas (with a 5 mL syringe) through
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the membrane gas sampling port, and injecting it into the NaOH solution containing the pH
indicator in an Einhorn pipe meter (see Figure 6). The percentage of the methane contained
in this biogas was then calculated using equation 9 below:
%𝐶𝐻4 =𝑉𝐶𝐻4
𝑉𝐵𝑖𝑜𝑔𝑎𝑠 Equation 9
• CH4 is the methane content in the produced biogas [%].
• 𝑉𝐶𝐻4 is the volume of methane read on the Einhorn pipe meter [mL].
• 𝑉𝐵𝑖𝑜𝑔𝑎𝑠 is the total volume of the sampled biogas [mL].
Figure 6: Measurement of methane concentration in the produced biogas, using the membrane gas sampling port and the NaOH solution containing the pH indicator, in an Einhorn pipe meter.
3.3.5 Free ammonium NH3-N
The amount of free ammonium NH3-N in the primary and the digested sludge was
determined from the measured NH4-N in the sludge, according to equation 10 Gallert &
Winter, (1997); Olsson et al., (2018).
𝑁𝐻3 −𝑁 = 𝑁𝐻4
+−𝑁∗10𝑝𝐻
𝑒(6344273+𝑇)+10𝑝𝐻
Equation 10
• 𝑁𝐻3 −𝑁 is the concentration of free ammonia in the digested sludge in [g L-1].
• 𝑁𝐻4+ −𝑁 is the concentration of free ammonium [g L-1].
• 𝑇 is the temperature in [oC].
• 𝑝𝐻 is the value of pH.
3.3.6 Nitrogen mineralization
The nitrogen mineralization was determined using the equation below:
𝑀𝐿 = ((𝑁𝐻4
+−𝑁)𝐷𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
−(𝑁𝐻4+−𝑁)
𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒
(𝑁𝑂𝑟𝑔)𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒) ∗ 100 Equation 11
19
• 𝑀𝐿 is the nitrogen mineralization [%].
• (𝑁𝐻4+ − 𝑁)Digested sludge is the concentration of free ammonium in the digested sludge [g L-1].
• (𝑁𝐻4+ − 𝑁)Primary sludge is the concentration of free ammonia in the primary sludge [g L-1].
• (𝑁𝑂𝑟𝑔)Primary sludge is the organic N in the primary sludge [g L-1].
3.3.7 Degree of degradation
The degree of degradation was determined according to Schnurer & Jarvis, (2017).
𝐷𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 = 𝛼 = (1 − (𝑇𝑆𝑂𝑢𝑡∗𝑉𝑆𝑂𝑢𝑡
𝑇𝑆𝐼𝑛∗𝑉𝑆𝐼𝑛)) ∗ 100 [%] Equation 12
• 𝛼 is the degradation degree in [%].
• 𝑇𝑆𝑂𝑢𝑡 is the TS amount in the digested sludge in [%].
• 𝑉𝑆𝑂𝑢𝑡 is the amount of VS in the digested sludge in [%].
• 𝑇𝑆𝐼𝑛 is the TS amount in the primary sludge in [%].
• 𝑉𝑆𝐼𝑛 is the amount of VS in the primary sludge in [%].
3.3.8 Process stability
The process stability was examined, looking at the ratio between VFA and TA as proposed by
Svenskt Vatten, (2019).
• 𝑉𝐹𝐴
𝑇𝐴< 0.3 stable process.
• 𝑉𝐹𝐴
𝑇𝐴= 0.3 𝑡𝑜 0.5 small instability in the process.
• 𝑉𝐹𝐴
𝑇𝐴> 0.5 real instability in the process.
• 𝑉𝐹𝐴
𝑇𝐴> 1 There is a significant risk for a sudden reduction of gas production.
• 𝑉𝐹𝐴 is the volatile fatty acid in [mg L-1].
• 𝑇𝐴 is the total alkalinity in [mg L-1].
3.4 Theoretical methane potential
For the estimation of the theoretical methane yield in the primary feeding sludge and the
digested sludge, the substrates were analyzed for lipids, protein, and carbohydrates. The
protein content was determined using the Kjeldahl method for organic nitrogen analysis,
according to Väänänen & Koivistoinen, (1996). The Kjeldahl method for organic nitrogen
analysis consists of multiplying the deducted nitrogen content by 6.25, which is the
conversion factor used for the protein determination in food samples.
The primary purpose of this was to evaluate how the different components in the primary
feeding sludge had contributed to the production of the generated methane gas. The purpose
was also to investigate the wasted methane potential, in connection with the OLR and HRT,
in the digesters. According to the German Standard Verein Deutscher Ingenieure, (2006),
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lipids in primary sludge produce 1000.8 NmL g-1 VS-1 methane, protein produce 480 NmL g-1
VS-1 methane, and carbohydrates generate 375 NmL g-1 VS-1 methane. The theoretical
methane production values from the different substrates were determined from the given
theoretical biogas yield (see Table 4).
Table 4 Theoretical methane yield assumption
Substrate type
Theoretical biogas
yield [NL kg-1 VS-1]
Theoretical CH4/CO2 composition [%/Volume]
Carbohydrate 750 50%CH4 50%CO2
Fats (Lipids) 1390 72%CH4 28% CO2
Proteins 800 60%CH4 40% CO2
The carbohydrates, estimated as the remaining portion of organic material in each substrate,
were determined using equation 13, the theoretical methane potential in the sludge using
equation 14, and the methane yield (or specific methane production) using equation 15.
Zhang, & Angelidaki, I. (2015). Submersible microbial desalination cell for simultaneous
ammonia recovery and electricity production from anaerobic reactors containing high
levels of ammonia. Bioresource Technology, 177, 233–239.
https://doi.org/10.1016/j.biortech.2014.11.079
Zhang, C., Su, H., Baeyens, J., & Tan, T. (2014). Reviewing the anaerobic digestion of food
waste for biogas production. Renewable and Sustainable Energy Reviews, 38, 383–
392. https://doi.org/10.1016/j.rser.2014.05.038
Zhen, G., Lu, X., Kato, H., Zhao, Y., & Li, Y.-Y. (2017). Overview of pretreatment strategies for
enhancing sewage sludge disintegration and subsequent anaerobic digestion: Current
advances, full-scale application and future perspectives. Renewable and Sustainable
Energy Reviews, 69, 559–577. https://doi.org/10.1016/j.rser.2016.11.187
APPENDIX 1: EXPERIMENT PROTOCOL
Table 7 Weekly protocol of the pilot-scale experiment
Volym (cm3)
Måndag Tisdag Onsdag Torsdag Fredag Antal prov och budget
Temperatur X X X X X
pH - X X X X X -
Metan X3 - X X X X X -
VFA (HAC) rötslam X3
2 ml /prov X 270 kr/prov – Totalt 270 prover = 73 000 SEK
ALK rötslam X3
150 ml/prov
X 18*15 = 270 prov
COD rötslam X3
1 ml/prov X 18*15 = 270 prov
CODs rötslam X3
1 ml/prov X 18*15 - 270 prov
COD råslam X3 1 ml/prov X 3*8 = 270 prov
CODs råslam X3
1 ml/prov X 3*8 = 24 prov
GR/TS rötslam X3
15-20 g/prov
X 18*15 = 270 prov
GR/TS råslam X3 varannan vecka
15-20 g/prov
X 3*8 = 24 prov
NH4-N råslam varannan vecka
1000 ml X 8 st – 300 kr/prov = 2 400 SEK
Kjeldahl-N råslam varannan vecka
1000 ml X 8 st – 300 kr/prov = 2 400 SEK
NH4-N rötslam
? X 15 prover - körs internt av labbet
HPLC- fettsyror
100 ml/prov Eventuellt mindre
Slam från fredagens prover samlas och fryses. 10 g slam blandas upp med 40 ml vatten, centrifugeras, klarfasen filtreras först genom ett 0,45 mikronfilter och sedan genom ett 0,2 mikronfilter. Provet sparas i 10 ml behållare och fryses. Skulle rötkamrarna börja ackumulera VFA skickas valda prover för analys. Eventuell budget 1 gång/månad 1000 kr/prov = 24 000 SEK
Fett, proteiner och gödselprov – råslam – Gödselprov
1200 kr /prov- Genomförs av Agrilab
Gödselprov på råslam 3 prover 1200 kr/prov = 3600 kr
Total budget 105 400 SEK – Osäkerhet för eventuellt fler VFA prover
APPENDIX 2: EXTRA DATA ON METHANE PRODUCTION
The following Figures present extra data on the methane production obtained during the