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Water and Environmental EngineeringDepartment of Chemical EngineeringMaster Thesis 2013

Mahan Amani Geshnigani

Methane production from separate digestion of primary and

biological sludge

Postal address Visiting address TelephoneP.O. Box 124 Getingevägen 60 +46 46-222 82 85SE-221 00 Lund, Sweden +46 46-222 00 00Web address Telefaxwww.vateknik.lth.se +46 46-222 45 26

Methane production from separatedigestion of primary and biological sludge

by


Master’s thesis number: 2013-03

Water and Environmental EngineeringDepartment of Chemical Engineering

Lund University

April 2013

Supervisor: Research associate Åsa DavidssonCo-supervisor: PhD student Hamse KjerstadiusExaminer: Professor Jes la Cour Jansen

Picture on front page: Batch reactors (2-liter glass bottles) and gas chromatograph for determination of themethane potential. Photo by Mahan Amani Geshnigani.

www.vateknik.lth.se

AcknowledgementsI would like to thank my supervisors, Åsa Davidsson and Hamse Kjerstadius and myexaminer Jes la Cour Jansen for all the help and constructive comments and suggestionsduring this work.

I would also like to thank Gertrud Persson who learned me a lot about the laboratory worksand helped me with all the problems happened in the laboratory procedures.

I would like to appreciate VA SYD and especially Disa Sandström and Max Grankvist atSjölunda WWTP and NSVA, especially Marinette Hagman and Beata Matulaniec whoprovided me with the needed data and the sludge samples.

My thanks also go to all the Water and Environmental Engineering group at the ChemicalEngineering Department for their friendship and support.

SummaryRecently a great amount of attention has been paid to anaerobic digestion of sludge fromwastewater treatment plants to pursue two goals; handling of the sludge in a harmless way forthe environment and producing more biogas (mainly methane and carbon dioxide). Theproduced biogas is a renewable energy source that can be used as a substitute for fossilvehicle fuels (needed to be upgraded to separate the methane content) also for production ofheat and electricity.

To investigate possible ways to increase the methane production from sludge digestion atwastewater treatment plants the digestion of primary sludge, biosludge and the mixture of theboth was investigated. To evaluate the relationship between the type of sludge and methaneproduction, the primary sludges and biosludges of two municipal Swedish wastewatertreatment plants were used.

One plant produced primary sludge with in situ hydrolysis and the biosludge from anextended activated sludge process including biological nitrogen and phosphorus removal(Öresundsverket WWTP), the other plant produces primary sludge through pre-precipitationand biosludge from a high loaded activated sludge process (Sjölunda WWTP).

The laboratory procedure for measuring the methane production from the sludge was based onanaerobic digestion of the sludge followed by periodical measurements of the producedmethane in the head space of the batch reactors (2-liter glass bottles). The methane productionis expressed as volume of produced methane per mass of substrate in terms of the initial VS(Volatile solids) content (Nml CH4 /g VS).

The results showed that the primary sludge of Sjölunda WWTP with pre-precipitation (ferric-based precipitation agent) resulted in a higher methane production per gram of added VS(about 9%) and per gram of degraded VS (about 21%) than the primary sludge ofÖresundsverket WWTP with hydrolysis.

The biosludge from high loaded activated sludge process at Sjölunda WWTP resulted in ahigher methane production per gram of added VS (about 35%) and per gram of degraded VS(about 24%) as well as higher biodegradability than the biosludge from Öresundsverket takenat the extended activated sludge process including biological nitrogen and phosphorusremoval.

The methane production from mixed sludge corresponds to the share of the primary sludgeand biosludge and their separate methane production. Thus no synergy was seen.

According to COD/VS analyses, the correlation between the COD and VS values for theprimary sludge from both WWTPs was linear although these two primary sludges differ fromeach other.

AbbreviationsAD Anaerobic digestion

BOD7 Biochemical oxygen demand, 7 days

GC Gas chromatograph

COD Chemical Oxygen Demand

CSTR Continuously-stirred tank reactor

HRT Hydraulic retention time

Nml Normal milliliter, gas volume at standard temperature (0ºC) and pressure (1 atm)

PE Population equivalent

SRT Solids retention time

TS Total solids

VFA Volatile fatty acids

VS Volatile solids

WAS Waste activated sludge (surplus biological sludge)

WWTP Wastewater Treatment Plant

Table of contents

1. Introduction..................…………………...……………...………………………............1

1.1. Background.................................................................................................................1

1.2. Aim...................................……………………………………………………….......4

2. Sludge production at WWTP.....................................................................………………5

2.1. Öresundsverket WWTP.....................................................................……………….5

2.2. Sjölunda WWTP......................................…………………………………………...7

3. Materials and methods......................................................... ……….……...…………...11

3.1. Sludge characteristics and analyses..........................................................................11

3.2. Bio-methane potential test (BMP)............................................................................11

3.3. Inoculum and substrate.............................................................................................11

3.4. Procedure of the BMP test........................................................................................12

3.4.1. Methane measurement.......................................................................................14

3.5. Calculation of the methane production.....................................................................14

3.6. End of the BMP test..................................................................................................16

4. Results and discussions....................................................................................................19

4.1. COD versus VS analyses….……………………….................................................19

4.2. Methane production from the first BMP experiment................................................19

4.3. Methane production from the second BMP experiment...........................................22

4.3.1. Primary sludge...................................................................................................24

4.3.2. Biosludge...........................................................................................................24

4.3.3. Mixed sludge......................................................................................................25

4.3.4. Cellulose as reference material..........................................................................26

4.4. End of the BMP test...................................................................................................27

5. Conclusions......................................................................................................................33

6. Future work......................................................................................................................35

7. References........................................................................................................................37

Appendix...................................................................................................................................39

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1. Introduction1.1. Background

Water is a necessity for all kind of life and the supply of water is limited on Earth. It is clearlyimportant to look after the existing supply, to ensure a safe and sustainable environment andthat future generations will not encounter problems regarding the quality of the water. Thus itis important to take care of the produced wastewater and its treatment in a way that theenvironment can handle it without any disturbance to natural processes. Wastewater containssuspended solids, organic matters, nutrients, heavy metals, bacteria, viruses, parasite sporesand toxic substances. One way to classify contaminants in wastewater is to divide them intoinorganic and organic substances. The organic contaminants like carbohydrates, proteins,amino acids, higher fatty acids and soluble organic acids mainly comprise one-third ofparticulate, dissolved and suspended solids. The inorganic constituents of wastewater mainlycomprise dissolved salts, (Kemira Kemwater, 2003). Nitrogen and phosphorus are two maininorganic substances existing in wastewater and excessive concentration of them in theeffluent of the treatment plant causes eutrophication in receiving water bodies. Also largequantities of existing organic material in the wastewater cause oxygen depletion as a lot ofoxygen is needed for degradation of organic substances. Therefore removing excess amountsof nitrogen, phosphorus and organic material are the main reasons for treating wastewater.During the treatment process at a wastewater treatment plant (WWTP), the complex mixtureof contaminants is separated from the wastewater as sludge. Sludge formation is due toaggregation of the small particles and dissolved substances into larger particles that have theability to be settled. In general the sludge is made up of water and clustered larger particles.As there are different steps for treating the wastewater, the waste product of each step will bedifferent in properties and components. There is a common classification for different types ofsludge that is based on the step of wastewater treatment in which the sludge was produced.

Primary sludge: The residue formed during mechanical treatment and collected from thebottom of the primary clarifiers. Primary sludge contains easily digestible carbohydrates andfats (Gary et al., 2007).

Secondary sludge: also known as biological sludge (biosludge) or waste activated sludge(WAS) formed during biological treatment. Biosludge consists of complex carbohydrates,long chain hydrocarbons and complex proteins (Gary et al., 2007). Active bacterial cells arethe main constituent of the degradable fraction of biosludge while dead bacteria and theirremnants or refractory organics form the non biodegradable fraction of the biosludge(Davidsson, 2007).

Tertiary sludge: the residue formed during post-precipitation process which is carried out byaddition of chemicals.

Digested sludge (inoculum): the residue of anaerobic digestion of primary and biologicalsludge is called digested sludge. This contains less in mass, odor and pathogens in comparisonto primary sludge and biosludge.

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All different types of sludge have to undergo further treatment for hygienic and economicreasons. Sludge treatment consists of various processes such as: thickening, stabilization anddewatering. About 40-60% of the total costs of a treatment plant are related to the sludgetreatment process although the sludge volume is around 1% of the total influent wastewater’svolume (Kemira Kemwater, 2003). For the sludge treatment, firstly the sludge is thickened bysedimentation, flotation or belt filtration depending on the sludge type in order to reduce thevolume and consequently the costs of the treatment process. Subsequently, the thickenedsludge is stabilized to decompose the biodegradable compounds of the sludge. Stabilization iscarried out by different methods such as anaerobic digestion, aerobic stabilization,composting, thermal or chemical oxidation, incineration or liming (Kemira Kemwater, 2003).At the end, dewatering is carried out to reduce the volume as the sludge contains more than90% water. The most common method is to dewater the digested sludge in centrifuges. Apolymer can be added before the centrifuge to make the water release more readily from thesludge flocs.

Anaerobic digestion

Anaerobic digestion (AD) consists of several stages in which microorganisms break downbiodegradable material in the absence of oxygen, which lead to biogas production (mainlymethane and carbon dioxide). Nowadays in an energy demanding lifestyle, the producedbiogas as a renewable and eco-friendly energy source can be used as a substitute for fossilvehicle fuels (needed to be upgraded to separate the methane content) and also for productionof heat and electricity.

Anaerobic digestion process can be divided into four stages (Figure 1): 1) hydrolysis, 2)acidogenesis, 3) acetogenesis and 4) methanogenesis (Brinch et al., 1994; Gujer & Zender,1983; Henze et al., 1997; Appels et al., 2008).

In the hydrolysis stage, the organic matter consisting of particulate and large dissolvedmolecules are converted into soluble molecules with the aid of enzymes (Brinch et al., 1994).

In the acidogenesis stage, fermentative bacteria or anaerobic oxidizers convert the solubleorganic matter to volatile fatty acids (VFA), alcohols, acetate, hydrogen and carbon dioxide,(Appels et al., 2008).

In the acetogenesis, hydrogen-producing acetogens degrade long chain fatty acids and volatilefatty acids to acetate, hydrogen and carbon dioxide (Brinch et al., 1994).

In the methanogenesis stage, methanogens convert acetate, carbon dioxide and hydrogen intomethane and carbon dioxide (Appels et al., 2008).

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Carbondioxide

HydrogenCarbondioxide

Methane

Acetate

Figure 1. Summary of anaerobic digestion (Outline from Gujer & Zender, 1983; Appels et al.,2008).

Some environmental factors like nutrients availability for microorganisms, presence oftoxicants, temperature, pH, alkalinity and moisture content of the substrate have effects on theanaerobic digestion process (Davidsson, 2007). Two common temperatures for operation ofanaerobic digesters are mesophilic (with a range of 15-45°C and optimum temperature of 35°C) and thermophilic temperature (with a range of 45-75°C and optimum temperature of 55°C), (Prescott et al., 1999).

Methane production form anaerobic digestion of primary and biosludge

Both primary sludge and biosludge are suitable to produce methane via anaerobic digestiondue to their high VS content (Davidsson, 2007). Since the primary sludge contains moreeasily biodegradable fats and carbohydrates in comparison to biosludge, the anaerobicdegradability of primary sludge would be higher than for biosludge (Parkin and Owen, 1986).A long retention time in the activated sludge process increases the aerobic digestion of theorganic matter which leads to having a large fraction of non biodegradable and refractorycompounds in the waste activate sludge (WAS), consequently the biogas production decreaseswith increase of the activated sludge SRT (Bolzonella et al., 2005). As stated by Parkin andOwen (1986) the VS and COD reduction of the biosludge do not exceed 50% undermesophilic conditions and typical values in COD and VS reduction of the primary sludge are40-60 % and 40-70 % respectively.

Recently a great amount of attention has been paid to anaerobic digestion of sludge fromwaste water treatment plants to pursue two goals; handling of the sludge in a harmless way forthe environment and producing more methane. So the possible ways to increase the methaneproduction should be studied.

Particulateorganicmatter

Carbohydrates

Proteins

Lipids

Solubleorganicmatter

Sugars

Amino acids

Fatty acids

VFAs

Alcohols

HydrolysisAcidogenesis Acetogenesis

Methanogenesissesis

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There is a lack of knowledge about methane production from separate digestion of primaryand biosludge in previous studies. Therefore methane production from separate digestion ofprimary sludge, biosludge and also the mixture of the both should be investigated to find thepossible ways to increase the methane production at WWTPs. It would also be useful toevaluate the effects of the treatment processes on the methane production from the sludgesuch as influences of chemical addition, activated sludge or biological phosphorus andnitrogen removal.

1.2. Aim

The aim of the thesis work was to investigate the methane production from separate digestionof primary sludge, biosludge and the mixture of the both with emphasis on relationshipbetween the type of the sludge and the methane production. To evaluate the type of theprimary sludges and biosludges on methane production which lead to higher methaneproduction, sludges from two wastewater treatment plants were used.

One plant produced primary sludge with in situ hydrolysis and the biosludge from anextended activated sludge process including biological nitrogen and phosphorus removal(Öresundsverket WWTP), the other plant produces primary sludge through pre-precipitationand biosludge from a high loaded activated sludge process (Sjölunda WWTP).

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2. Sludge production at WWTPIn this study, the primary sludge and biosludge from two WWTPs which have differenttreatment processes were evaluated for methane production. The chosen WWTPs wereÖresundsverket WWTP with Biological phosphorus and nitrogen removal and SjölundaWWTP with a highly loaded activated sludge and pre-precipitation process.

Below follows the more detailed description of both WWTPs.

2.1. Öresundsverket WWTP

Öresundsverket wastewater treatment plant (WWTP) was built in 1974 and is located in thecity of Helsingborg in the south west of Sweden. The plant treats wastewater fromhouseholds, schools, hospitals, landfill sites and industry as well as handling rain and drainagewater which is led to the plant by combined sewer and storm drains (NSVA 2011). The loadin population equivalents (PE) to the plant is 200 000 and 1 PE is 70 (g BOD7/ person*day),(Miljörapport, NSVA Company, 2011).

The whole wastewater and sludge treatment process can be seen in Figure 2 with the markednumbers to show the described steps.

Figure 2. Öresundsverket WWTP (NSVA company catalog, with permission from NSVACompany).

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2

345

66 77

8

9

11

12

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1) Screening station 7) Post-sedimentation basins2) Pumping station 8) Filtration basins3) Grit chamber 9) Oultlet pipeline4) Equlization basins 10) Sludge thickener5) Pre-sedimentation basins 11) Digesters6) Biological treatment basins 12) Sludge dewatering unit

The treatment process can be divided into four separate stages: mechanical treatment,biological treatment, filtration and sludge treatment.

Mechanical treatment

In mechanical treatment stage, the solid impurities are removed by screens with 3 mm slits,then in the following grit chamber sands and heavy particles sink to the bottom. After the gritchamber, the wastewater is led to sedimentation basins where fats and oils are removed andtransport to the land fill site, and the pollutants that are heavier than the water sink to thebottom forming sludge (primary sludge). The condition at the inlet of the sedimentationbasins is anaerobic for hydrolyzing the sludge in order to produce readily biodegradableorganic matter to improve biological nitrogen and phosphorus removal. After thesedimentation basin wastewater contains mostly dissolved compounds and is pumped to thebiological treatment basins.

Biological treatment

The biological treatment step contains anaerobic, anoxic and aeration basins to provide thefavorable conditions for different types of bacteria performing the biological nitrogen andphosphorus removal (Bio-P process). The hydraulic retention time (HRT) is 12 hours for thewastewater in the biological treatment process. The majority of nitrogen in wastewater is inthe form of ammonium (NH4

+) or compounds easily converted to ammonium (KemiraKemwater, 2003). In the aeration basins, ammonium is converted into nitrate (nitrification)and later the produced nitrate in the absence of free oxygen (anoxic zone) is transformed intonitrogen gas which rises up to the atmosphere (denitrification). The biological phosphorusremoval process is based on alternating anaerobic and aerobic conditions for certain type ofbacteria, known as Bio-P bacteria, to use their special ability to store phosphate as a source ofenergy within the bacteria’s cell which is happening in the aerobic basins (Kemira Kemwater,2003). In the anaerobic basins the Bio-P bacteria release the stored phosphorus, to use theenergy to uptake the organic matter. The result of biological phosphorus removal would beless phosphorus in the effluent than the influent of the treatment plant as the most phosphorusis stored inside the Bio-P bacteria, forming the biosludge as well as other microorganism inthe following sedimentation basins.

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Most part of the biosludge is recycled to the inlet of the biological treatment process. In orderto keep the biomass concentration constant some part of the produced sludge is removed aswaste activated sludge (WAS). In the next step the water is polished using a sand filter.

Sludge treatment

The produced sludge from all the steps of treatment process is thickened in gravity thickeners.Then the thickened sludge is stabilized by anaerobic digestion in the digesters operating atmesophilic temperature (37 °C). The digesters are continuously stirred tank reactor (CSTR)with the HRT of three weeks. In the gas purification plant, the carbon dioxide and hydrogensulfide of the produces biogas are washed away using water and high pressure to upgrade themethane gas (NSVA 2011).

2.2. Sjölunda WWTP

Sjölunda wastewater treatment plant was built in 1963 and is located in the northern part ofthe Malmö harbor. The plant treats domestic wastewater from the greater part of Malmö andsome other smaller cities nearby. The load in population equivalents (PE) to the plant is300 000 which corresponds to an organic load of 40 tons BOD7/day (Miljörapport, VA SYDCompany, 2011). The whole treatment process can be seen in Figure 3 with the markednumbers to show the described steps.The wastewater treatment processes include mechanical, primary, biological and chemicaltreatments.

Mechanical treatment

Mechanical treatment includes screening separation with 3 mm meshes, and grit removal. Thefollowing primary treatment includes pre-precipitation and primary clarification. In theprimary treatment, a ferric-based precipitation chemical is added to the wastewater in order toremove phosphorus by precipitation.

Biological treatment

Biological treatment step at Sjölunda consists of an activated sludge process, nitrification intrickling filter and post denitrification with plastic carriers. In the activated sludge basins,microorganisms decompose organic matter in aerobic and anoxic conditions. In the followingsecondary clarifications, the sludge is removed through sedimentation (biosludge). But mostpart of this sludge is pumped back to the activated sludge basins to maintain a certain amountof microorganisms active in the basin. The waste activated sludge is removed and transportedto the sludge treatment. In the trickling filters which contain folded plastic material with alarge surface area, the ammonium in the wastewater is transformed to nitrate by aerobicmicroorganism growing on the plastic material. In the trickling filter, rotating spreadersdistribute the wastewater over the filter and the oxygen is supplied simultaneously. The basins

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for denitrification process are filled with plastic carrier material with microorganisms growingon them. These microorganisms transform the nitrate into nitrogen gas with the presence ofeasily degradable carbon source (methanol) in the anoxic condition.

1) Pumping station 7) Secondary clarification 13) Surplus sludge thickning2) Screening station 8) Nitrificatin in trikling filters 14) Anaerobic digestion3) Grit chamber 9) Post-denitrification 15) Gas holder4) Pre-precipitation 10) Flotation process 16) Sludge dewatering5) Primary clarification 11) outlet sewers 17) Sludge liquor treatment6) Activated sludge process 12) Thickening by gravity 18) Sludge disposal

Figure 3. Treatment processes at Sjölunda WWTP (VA SYD company catalog, with permission from VA SYDCompany).

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5 6 78 9 10 11

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Chemical treatment

In the flotation basins, air is blown into the wastewater and tiny air bubbles adhere to theparticles, and carry them to the surface where they are collected by scrapers and pumped tothe sludge treatment plant (tertiary sludge). Sometimes an aluminum-based coagulant is addedin the inlet section of the flotation basin to enhance the removal of phosphorus and particulatematerials.

Sludge treatment

The sludge treatment process includes thickening, anaerobic digestion of sludge anddewatering the sludge. The sludge from the primary clarifiers is thickened in the gravitythickeners where larger particles sink to the bottom and the water phase at the surface of thethickener is transferred to the inlet of the treatment plant. The sludge produced in thesecondary clarifiers and in the flotation process is thickened by transporting them on a filterbelt which allows the water to pass through the filter. The collected water is pumped back tothe inlet of the plant. Further the thickened sludge from gravity thickener and filter belt ispumped into digesters where the organic materials are degraded by microorganisms andconverted to biogas in the anaerobic environment. There would be the chance of having 0 to20% of biosludge in the thickened primary sludge due to mal functioning of the filter belt.The digesters are operating at the temperature of 35-37 °C and charged consecutively. Thedigested sludge is dewatered in centrifuges and dewatered sludge is transported to storages toutilize as soil fertilizers (VA SYD 2010).

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3. Materials and methods3.1. Sludge characteristics and analyses

For analyzing the sludge characteristics, total solids (TS), volatile solids (VS), total COD(total chemical oxygen demand) and dissolved COD (chemical oxygen demand for readilybiodegradable material) was measured, in order to get an estimation of organic matter contentof the sludge which affects the methane production from the sludge.

TS (the measure of total dissolved and suspended solids) and VS (the portion of the totalsolids that has organic content) of sludges and inoculums were measured using Swedishstandard method, SS-EN 12880 and SS-EN 12879 respectively (SIS, 2000). For estimation ofthe total COD of all substrates, diluted samples (dilution factor of 5 to 100) were analyzedspectrophotometrically using the HACH LANGE test tube (LCK 114) and spectrophotometer(model DR 2800). For estimating the dissolved COD samples were centrifuged at 10 000 rpmfor 10 minutes followed by filtration through Munktell paper filter with the pore size of6~10μm. The dissolved COD of the prepared samples were determined in the same way as fortotal COD.

3.2. Bio-methane potential test (BMP)

The methane potential of the sludge at STP (standard temperature and pressure, 0°C and 1atm)was based on the method by Hansen et al. (2004). The method includes anaerobic digestion ofthe sludge followed by periodical measurements of the produced methane in the head space ofthe batch reactors (2-liter glass bottles). The methane potential is expressed as volume ofproduced methane per mass of substrate in terms of the initial VS (Volatile solids) content(Nml CH4/g VS). The methane potential is the accumulated volume of produced methaneduring the test period. The required period for determination of the total methane potential canvary between 30 to 50 days (Davidsson, 2007). The suitability of a substrate for anaerobicdigestion can be examined by this method before applying it in the digestion reactors atWWTPs in order to prevent the failure of the process at full-scale digesters.To evaluate the relation between the type of the sludge and methane production, two sets ofthe bio methane potential test (BMP test) were carried out between the 23th October- 28th

November 2012 and the 3rd December 2012- 25th January 2013 respectively.

3.3. Inoculum and substrate

Sludge from full scale anaerobic digesters (mesophilic digester operating at 35°C) at bothWWTPs was used as inoculum. The inoculum as well as primary and biosludge fromÖresundsverket and Sjölunda WWTPs were brought to the laboratory 4 days before startingthe experiment. Primary and biosludge were kept in cold room (5°C) and the inoculum waskept in incubator at 35°C till the start of the experiment to ensure the degradation of easydegradable organic matter present in the inoculum. The characteristics of the inoculum arepresented in Table 1:

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Table 1. Inoculum characteristics from Öresundsverket and Sjölunda WWTPs, TS and VSanalyses were done in triplicates and COD analyses in singles.

Inoculum TS(% of mass)

VS(% of mass)

CODTotal(mg/l)

CODdissolved

(mg/l)

Öresundsverket1st BMP experiment 3.2 2.1 33 800 7452nd BMP experiment 2.8 1.9 27 350 1 165

Sjölunda1st BMP experiment 2.1 1.3 22 800 5602nd BMP experiment 1.9 1.1 19 750 765

The characteristics of the primary sludge and biosludge from Öresundsverket and SjölundaWWTPs are presented in Table 2 and Table 3. All the measurements for inoculums, primarysludges and biosludges were carried out on the days of starting the tests.

Table 2. Primary sludge characteristics from Öresundsverket and Sjölunda WWTPs.

Primary sludge TS(% of mass)

VS(% of mass)

CODTotal(mg/l)

CODdissolved

(mg/l)

Öresundsverket1st BMP experiment 6.6 5.5 98 100 2 4002nd BMP experiment 6.8 5.6 104 600 3 320

Sjölunda1st BMP experiment 3.4 2.6 44 400 3 5002nd BMP experiment 3.8 3.0 50 200 2 860

Table 3. Biosludge characteristics from Öresundsverket and Sjölunda WWTPs.

Biosludge TS(% of mass)

VS(% of mass)

CODTotal(mg/l)

CODdissolved

(mg/l)

Öresundsverket1st BMP experiment 3.5 2.6 55 700 3982nd BMP experiment 2.9 2.1 34 900 392

Sjölunda1st BMP experiment 3.2 2.6 38 000 2 3302nd BMP experiment 4.9 3.5 70 300 4 120

3.4. Procedure of the BMP test

The cellulose, primary sludge, biosludge and the mixture of the both sludges from eachWWTP were digested by the inoculum taken from the same WWTP. Cellulose was used tocheck the quality of the inoculum due to the similar methane potential to sludge and its slowdegradation process (Davidsson, 2007).

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The maximum amount of initial VS in each bottle from both inoculum and sludge or cellulosewas 10 g and the ratio of 60:40 was applied between the VS from the inoculum and the VSfrom sludge or cellulose respectively. The volume of the bottle’s content was maximum 500ml. All the substrates were added at the beginning of the tests and anaerobic condition wasapplied by flushing nitrogen gas in the headspace of the bottle for 1 minute after adding thesubstrates. The bottles were sealed with rubber septum and screw cap and they were kept inincubator at mesophilic temperature (35°C) during the experiment. Also they were shakenonce per week to increase the contact between the microorganisms of the inoculum and thesludge. During the test period the pressure inside the bottle was increased due to the anaerobicdigestion of the substrates leading to biogas production (mainly contain methane and carbondioxide). Significant pressure buildup could be noticed from the shape of the rubber septumand therefore it should be released in order to keep the pressure below 2 bars according to themethod in Hansen et al. (2004).The methane potential test for each type of substrate was done in triplicates (Table 4) due tothe heterogeneity of the substrate. Since BMP tests are based on the activities ofmicroorganisms presented in the inoculum which can vary in abundance. Up to 10% ofstandard deviation between the accumulated methane productions of three bottles containingthe same substrate was considered reasonable.

Table 4. Description of the bottle’s content in the BMP test.

Bottle numberContents of the bottlesÖresundsverket

WWTPSjölundaWWTP

1-3 16-17 3 bottles containing primary sludge and inoculum

4-6 19-21 3 bottles containing biosludge and inoculum

7-9 22-24

3 bottles containing mixed of primary and biosludge

The ratio for Sjölunda WWTP was 75% primary sludgeand 25% biosludge*

The ratio for Öresundsverket WWTP was 60% primary sludgeand 40% biosludge

10-12 25-27 3 bottles containing inoculum

13-15 28-30 3 bottles containing cellulose and inoculum*There would be the chance of having 0 to 20% of biosludge in the primary sludge.

The ratios for the mixed sludges were according to the ratio between the primary sludge andthe biosludge in anaerobic digesters of Öresundsverket and Sjölunda WWTPs.

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3.4.1. Methane measurement

For measuring the produced methane in the batch reactors (Figure 4) a gas sample (0.2 ml)was taken with a gas syringe with pressure lock through the rubber septum of the bottle. Thepressure lock of the syringe made it possible to take a fixed amount of the gas from theheadspace at the actual pressure in a bottle. Then the gas sample was injected in a gaschromatograph (Agilent 6850 series GC system) as it can be seen in Figure 4. The gaschromatograph was equipped with FID (flame ionization detector) and a column with thedimensions of: 30 m length, 320 µm diameter and 0.25 µm film thickness.

The methane production in the headspace of the bottles was measured 1-3 times per weekdepending on the rate of gas production. During each measurement session, gas analyses foreach bottle were done in triplicates and the room temperature and atmospheric pressure wasmeasured by thermometer and barometer installed in the room. The rubber septum of thebottle with its elastic properties allowed the repeated measurements of the methane content, asthe small holes were closed after each set of gas sampling due to the pressure inside the bottle.

3.5. Calculation of the methane production

For calculation of the methane content in the headspace of a bottle, the signals from the GCmachine after injecting the fixed gas sample (0.2 ml) from the headspace and the sameamount of 100% methane gas reference was compared. The headspace of the bottle wascalculated from subtracting the mass of added inoculum and sludge from the whole volume ofthe bottle by assuming the density of 1 g/ml for inoculum and sludge. The whole volume ofeach bottle was determined by subtracting the weight of the empty bottle from the weight ofthe bottle full of water. By knowing the mass of the water inside the bottle and assumingdensity of water (1 g/ml) the volume of the bottle was determined. The calculated methanevolume is from certain amount of VS and related to the actual temperature and pressure (room

Figure 4. Left: batch reactors (2-liter glass bottles with rubber septum and screw lock), Right:gas chromatograph and gas syringe with pressure lock for measurement of produced methane.

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temperature and atmospheric pressure). The normalized methane production was calculatedusing the ideal gas law (Equation 1).

(Equation 1)

Where:

STPV is the produced methane in the head space of the bottle at standard pressure andtemperature (ml)

MX is the average of three signals from the GC for each bottle at room temperature andactual pressure

HeadspaceV is the gas volume in each bottle (ml)

%100X is the average of three signals from the GC for injection of 100% methane at roomtemperature and pressure

ST (standard temperature) is equal to 273.15 K

MP is the actual atmospheric pressure (room pressure) (mba)

MT is the room temperature (K)

SP (standard pressure) is 1 atmosphere that is equal to 1013.25 mba

If the methane content of the bottle was released during the test period to avoid the highpressure build-up in the bottle, the amount of released gas should be taken in to account incalculation of the total methane production. The difference between the methane content ofthe headspace of the bottle before and after the release was representative of the amount ofreleased gas.As the inoculum contains organic matter which would be degraded and produced methane,the amount of methane production from inoculum should be subtracted from the wholemethane content to calculate the methane production just from the substrate. The finalmethane potential is the accumulated volume of produced methane during the test period andtheoretically it could not decrease. For comparing the methane potentials, the maximumaccumulated methane production for each substrate was used. This value usually would becorresponded to the last measurement, but if the final value would become less, the maximumvalue which obtained at the end of the test period would be chosen for comparison of methanepotentials.

SMMSHeadspacemSTP PTPTXVXV */**)/*( %100

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3.6. End of the BMP test

At the end of the BMP test, each bottle was weighed in order to evaluate the reduction of thecontent’s mass also the volume reduction of the bottle’s content. During empting the bottle’sgas content by inserting the hospital needle in the rubber septum of the bottle, the pressure inthe bottle was released. If the bottle didn’t have any pressure or low pressure (released gas),there had been a chance for gas leakage. For evaluation of the degradation in digestionprocess, samples were taken from each bottle to measure the TS and VS of the bottle’scontent. To compare the final results with initial measurements, the final VS and TS values ofthe sludge were calculated by subtracting the measurements of the blank bottles (containinginoculum) from the final values for each bottle according to Equation 2.

InoculumBottleSludge XXX (Equation 2)

Where:

SludgeX is the value of TS (g) and VS (g) for the primary sludge, biosludge and the mixed

sludge after the anaerobic digestion

BottleX is the final measurements of TS (g) and VS (g) for the bottle’s content at the end of theanaerobic digestion

InoculumX is the average final value of the TS (g) and VS (g) parameters in the blank bottles

At the end of the test, one bottle that had a good trend in methane production was chosen fromevery triplicate that could be a good representative of the substrate’s digestion for measuringthe total COD and dissolved COD. To compare the COD reduction of the bottle’s content,initial COD values of the bottle’s content were calculated according to Equation 3.

scontentBottleInoculumInoculumSludgeSludgeBottle VCODYCODYX '/)**( (Equation 3)

Where:

BottleX is the initial value of total COD (mg/l) and dissolved COD (mg/l) for the bottle’scontent

SludgeY is the initial volume of the sludge in the bottle (l)

SludgeCOD is the initial value of total COD and dissolved COD for the sludge in the bottle

(mg/l)

17

InoculumY is the initial volume of the inoculum in the bottle (l)

InoculumCOD is the initial values of total COD and dissolved COD for the inoculum in thebottle (mg/l)

scontentBottleV ' is the initial volume of the bottle’s content (l)

19

Tota

l CO

D(g

/l)4. Results and discussions4.1. COD versus VS analyses

The values of total COD (g/l) and VS (g/l) of all characterized primary sludges, biosludgesand inoculums of Öresundsverket (marked with Ö) and Sjölunda (marked with S) WWTPsare presented in Table 5. These values have been plotted in the Figure 5 to evaluate thecorrelation between COD and VS for the different sludges.

Table 5. Total COD and VS values of primary, bio and mixed sludge from Öresundsverketand Sjölunda WWTPs at two different times.

Primary sludge Biosludge InoculumTotal COD

(g/l) VS (g/l) Total COD(g/l) VS (g/l) Total COD

(g/l) VS (g/l)

44.4 S 26 34.9 Ö 21 19.7 S 1150.2 S 30 55.7 Ö 26 22.8 S 1398.1 Ö 55 38.0 S 26 27.3 Ö 19104.6 Ö 56 70.3 S 35 33.8 Ö 21

The R2 values of fitted line between the COD and VS values for the primary sludges and forthe inoculums are near 1, which shows a good fit to linearity between the total COD and VSvalues of primary sludge and inoculum. Therefore by increasing the VS value, the COD

y = 1.963x - 7.6314R² = 0.9958

y = 2.5422x - 18.913

R² = 0.8075

y = 1.2338x + 6.1838

R² = 0.92470

20

40

60

80

100

120

0 10 20 30 40 50 60

Primary sludge

Biosludge

Inoculum

VS (g/l)

Figure 5. Total COD (g/l) versus VS (g/l) for primary sludge, biosludge andinoculum from Öresundsverket and Sjölunda WWTPs.

20

increases close to linearly, accordingly there would be estimation for the unknown CODvalues from different plants when the VS values are measured or vice versa.

The values for COD and VS of the biosludge didn’t follow the linear correlation due to theirdifferent COD values even for the same amount of VS. The biosludge of the Öresundsverkethad the higher COD value for the same amount of VS than the biosludge from Sjölunda andthe higher slope of COD/VS. The reason is the differences between the biological treatmentprocesses at two WWTPs which results in having biosludges with different characteristicsespecially in COD values.

In the study by Schaum et al. (2012), independent of the type of the sludge the COD/VS ratioof 1.4-1.5 was stated. The ratio of COD/VS of primary sludges, biosludges and inoculums ofÖresundsverket and Sjölunda WWTPs was 1.9, 2.5 and 1.2, which are not in a range of statedvalues. But it should be considered that for having more precise ratio of COD/VS moremeasurements are needed.

4.2. Methane production from the first BMP experiment

In the first set of the experiment, the gas content of all the bottles (except bottles containinginoculum) started to decrease after some days. The possible reason could be gas leakage fromthe bottles. Hence the results of the first BMP test couldn’t be valid because of the declinedtrend of accumulative methane production graphs as it can be seen in Figures 6 and 7. Thedeclined trend couldn’t be as a result of subtraction of methane production of the inoculum asthe methane production from inoculum is much less than the methane production from thesludge and it cannot be the reason for the sharp decrease. Therefore the second BMPexperiment was carried out and the results of the first BMP test were discarded.

Figure 6. Accumulated methane production from cellulose and primary, bio, mixed sludgefrom Öresundsverket WWTP.

Acc

umul

ated

met

hane

pro

duct

ion

(Nm

l/g V

S)

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40

Primary sludge

Biosludge

Mixed sludge

Cellulose-reference

Time (day)

21

The methane potentials (the maximum methane production during the test period) forcellulose, primary, bio and mixed sludge according to the graphs are presented in Table 6.

Table 6. Maximum methane production and standard deviation of different substrates for twoWWTPs in the first BMP experiment.

First BMP testAccumulated

methane production(Nml/g VS)

SD (%)

ÖresundsverketWWTP

Digestion time36 days

Primary sludgeBio sludge

Mixed sludgeCellulose

318184196215

11%7%41%7%

SjölundaWWTP

Digestion time33 days


Mixed sludgeCellulose

386427375334

7%2%3%4%

Figure 7. Accumulated methane production from cellulose and primary, bio, mixed sludgefrom Sjölunda WWTP.

Acc

umul

ated

met

hane

pro

duct

ion

(Nm

l/gV

S)

Time (day)

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40

Primary sludge

Biosludge

Mixed sludge

Cellulose-reference

22

4.3. Methane production from the second BMP experiment

The second BMP experiment was carried out in order to measure the methane productionfrom primary, bio and mixed sludge of the Öresundsverket and Sjölunda WWTPs. The totaldigestion time was 53 days. Further the results from both WWTPs were compared to evaluatethe effect of sludge characteristics and treatment process on methane production. Thesummary of the methane potentials (the maximum methane production during the test period)for different substrates from Öresundsverket and Sjölunda WWTP is presented in Table 7.

Table 7. Maximum methane production and standard deviation of different substrates forÖresundsverket and Sjölunda WWTPs in the second BMP test after 53 days of digestion.

*Mixed sludge contains 60% primary sludge + 40% biosludge**Mixed sludge contains 75% primary sludge + 25% biosludge

The results from the second BMP test shows the invalid results of the first experiment as themeasured values for biosludge, mixed sludge and cellulose from Öresundsverket were muchlower than the second set’s results.

Further each value of the Table 7 is described and results for the same substrate of bothWWTPs are compared together. The graphs of methane potentials for average of everytriplicate for both WWTPs are presented in Figures 8 and 9. As it can be seen from thegraphs, for Öresundsverket WWTP, the rate of methane production as well as the maximummethane production from primary sludge was higher than the mixed sludge and biosludge.Therefore if there would be a choice to just digest one type of sludge, the primary sludgecould be the better option to maximize the methane production and minimize the retentiontime in the digesters to save costs of heating.

Second BMP testAccumulated

methane production(Nml/g VS)

SD (%)

ÖresundsverketWWTP


Mixed sludge*Cellulose

341256296320

1%3%14%3%

Sjölunda WWTP

Primary sludgeBiosludge

Mixed sludge**Cellulose

374390369370

5%3%10%2%

23

Acc

umul

ated

met

hane

pro

duct

ion

(Nm

l/g V

S)

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40 45 50 55

Primary sludge

Biosludge

Mixed sludge

Cellulose-reference

Figure 8. Accumulated methane production from cellulose and primary, bio, mixedsludge from Öresundsverket WWTP.

Time (day)

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40 45 50 55

Primary sludge

Biosludge

Mixed sludge

Cellulose-reference

Figure 9. Accumulated methane production from cellulose and primary, bio, mixed sludgefrom Sjölunda WWTP.

Time (day)

Acc

umul

ated

met

hane

pro

duct

ion

(Nm

l/g V

S)

24

4.3.1. Primary sludge

The maximum methane production from primary sludge of Öresundsverket WWTP was 341Nml/g VS (average of the measurements from three bottles) and the standard deviation formethane production of the three bottles was 1%. The standard deviation for each time ofmeasurements didn’t exceed 5% which is reasonable and shows that all the three bottles hadthe similar methane production during the test period.

For the primary sludge from Sjölunda WWTP, the maximum methane production was 374Nml/g VS and the standard deviation for methane production of the three bottles was 5%. Thestandard deviation for each time of measurements didn’t exceed 5% till the 17th day of theexperiment which means that the methane production was similar in three bottles containingprimary sludge. Then it reached 19% at the end of the test showed the difference in methanecontent’s of the bottles. The total methane content of two bottles decreased which can beexplained by gas leaking from the bottles. At the end of the experiment the pressure of thesebottles were checked and as they didn’t have any pressure during emptying process, theleakage from the bottles was proved (bottles 16 and 17).

The methane content of the last bottle of the triplicate increased but the share of methaneproduction from the substrate slightly decreased due to subtraction of the methane productionfrom inoculum.

As the primary sludge of Sjölunda was taken after the pre-precipitation process, most of thedissolved organic matter has been converted to particulate material which results in a primarysludge with better organic matter regarding biodegradability for anaerobic digestion. Theprimary sludge of Öresundsverket was taken after hydrolysis of the sludge which results inpartly degradation of organic matter and consequently the remaining organic matter in theprimary sludge would not be as good as the primary sludge from Sjölunda WWTP. As aresult, the maximum methane production from primary sludge was higher in Sjölunda WWTPthan Öresundsverket WWTP.

4.3.2. Biosludge

The maximum methane production from biosludge of the Öresundsverket WWTP was 256Nml/g VS and the standard deviation for methane production of the three bottles was 3%. Thestandard deviation for each time of measurements didn’t exceed 9%, which is reasonable. Themethane production from biosludge was less than the methane production from primarysludge due to presenting less biodegradable organic matter in biosludge as it was undergonebiological treatment process.

For biosludge from Sjölunda WWTP, the maximum methane production was 390 Nml/g VSand the standard deviation for methane production of the three bottles was 3%. The standarddeviation for each time of measurements didn’t exceed 7%, which is reasonable.

As it can be seen from the results, the methane production from the biosludge of the SjölundaWWTP was much higher than from Öresundsverket WWTP. The reason could be thedifferent processes that biosludges had undergone. At Öresundsverket, after the hydrolysis,

25

the readily biodegradable organic matter and smaller organic particles with more surface areaswere utilized in the biological nitrogen and phosphorus removal, and with longer sludge agethe larger parts of the organic matter would be consumed by microorganisms, consequentlythe produced biosludge wouldn’t be good for methane production as the major part of easilybiodegradable material have already been consumed. The biosludge from the SjölundaWWTP was from a high loaded activated sludge process which means that with the shortersludge age and a lot of produced biomass, the biodegradability of the organic matter would behigher in the produced biosludge.

4.3.3. Mixed sludge

The maximum methane production from mixed sludge (60% primary sludge+40% biosludge)of Öresundsverket WWTP was 254 Nml/g VS and the standard deviation for methaneproduction of the three bottles was 31% which is quite high. The three bottles had the sametrend in methane production till approximately the 17th day of the test (the standard deviationfor these three bottles was less than 10%) then the standard deviation for the bottles increasedto 31% by the end of the experiment. The accumulated methane in one of the bottles (number7) became constant after the 17th day of the experiment. The bottle didn’t have any pressureduring empting the gas content of the bottle which indicates that a leakage from the bottle wasmost likely the cause and the results from the bottle should be discarded. The maximummethane production from the average of two other bottles was 296 Nml/g VS with thestandard deviation of 14%. The high standard deviation could be due to the non-homogeneityof the mixed sludge.

The calculated methane production for mixed sludge of the Öresundsverket based on the shareof each primary sludge and biosludge and their measured values of maximum methaneproduction gave a value of 307 Nml/ g VS, (0.6*341+0.4*256=307). The calculated value isnearly the same as the measured value (296 Nml/g VS) which shows that mixing the primarysludge and bio sludge doesn’t have a significant effect on methane production values, incomparison to the digestion of the same share of primary sludge and biosludge separately.

For mixed sludge (75% primary sludge+25% biosludge) from Sjölunda WWTP, themaximum methane production was 338 Nml/g VS and the standard deviation for methaneproduction of the three bottles was 10 %. Three bottles had the same trend in methaneproduction till approximately the 13th day of the experiment (the standard deviation for threebottles was less that 10%) then the methane content of one of the bottle (number 24) didn’tincrease and the decreased trend was observed due to the gas leakage as it didn’t have anypressure at the end of the test. The results from the methane calculation of this bottle gave ahigh standard deviation between the values of three bottles as it increased to 25% at the end ofthe test. Therefore the result from this bottle was discarded. The maximum methaneproduction from the average of the other two bottles of mixed sludge was 369 Nml/g VS withthe standard deviation of 10%. The standard deviation didn’t exceed 12%.

The calculated maximum methane production from mixed sludge of the Sjölunda based on theshare of each primary sludge and biosludge and their measured values of maximum methane

26

production gave a value of 378 Nml/g VS, (0.75*374+.25*390=378). The calculated value isnearly the same as the measured value (369 Nml/g VS) which shows that, mixing the primarysludge and biosludge would not result in higher or lower methane production in comparisonto the digestion of the same share of primary sludge and biosludge separately.

As it can be seen from the results, the methane production from mixed sludge of the SjölundaWWTP (369 Nml/g VS) was higher than the mixed sludge of Öresundsverket WWTP (296Nml/g VS) as a reason of having higher values of methane production for both primary sludgeand biosludge.

4.3.4. Cellulose as reference material

The maximum methane production from cellulose with the inoculum from ÖresundsverketWWTP was 297 Nml/g VS and the standard deviation for methane production of the threebottles was 10%. Until the 13th day of the BMP test, the triplicate bottles had the same trendin methane production and the standard deviation didn’t exceed 10%, then the standarddeviation increased up to 18% at the end of the test. The reason for the increased standarddeviation is coming from one of the triplicate bottles (number 13). The accumulated methaneproduction didn’t increase in the bottle after the 13th day of the experiment, which could bedue to gas leaking from the bottle as this bottle didn’t have any pressure during emptying thegas content of the bottle. Therefore this bottle should be disregarded from the experiment. Themaximum methane production from the other two bottles was 320 Nml/g VS with thestandard deviation of 3 % and the standard deviation didn’t exceed 5 % which is reasonableand shows that the methane production was similar in these two bottles. The measured value(320 Nml/g VS) was less than the values stated by Hansen et al., 2004 (377 Nml/g VS) andDavidsson, 2007 (353 Nml/g VS). The reason could be the activity of the inoculum, but as itcan be seen from the Figure 8, the methane production from cellulose had a good rate and theaccumulated methane content became constant after 20 days. The results from cellulosedigestion showed that the inoculum had a good quality as all the cellulose was digested. But itshould be mentioned that the inoculum of every treatment plant may differ from each other asthey origins from different digestion processes in full scale (HRT, temperature, loading rateetc). The reason for getting lower value from cellulose digestion could be that the inoculum ofÖresundsverket wasn’t adapted enough to produce more methane from cellulose digestion.The result can be trusted to ensure that the inoculum has a good enough quality to digest thesubstrates.

The maximum methane production from cellulose with the inoculum from Sjölunda WWTPwas 370 Nml/g VS and the standard deviation for methane production of the three bottles was2%. The standard deviation for each time of measurements didn’t exceed 6%, which isreasonable, showing that the methane content of three bottles was similar to each other. Thedecreasing trend at the end of the test is due to subtraction of the inoculums’ methaneproduction. The results from cellulose digestion showed that the inoculum had a good quality,because all the cellulose was digested and the measured methane production from cellulosewas close to the values stated by Hansen et al., 2004 (377 Nml/g VS) and Davidsson, 2007(353 Nml/g VS).

27

4.4. End of the BMP test

At the end of the BMP test, all bottles were weighed to evaluate the content’s mass reduction.Also the TS, VS of all the bottles and COD total, COD dissolved of just one of eachtriplicates were measured, since it was an important limitation to the analysis, to evaluate thedegradation of organic matter in the digestion process. The average of sludge mass reductionfor bottles containing primary sludge, biosludge and mixed sludge were 1.5%, 0.9% and 1.1%respectively for Öresundsverket WWTP and 1.3%, 1.2% and 1.2% for Sjölunda WWTP,which shows that the anaerobic digestion in a laboratory scale didn’t have that much effect toreduce the content’s mass, consequently the volume reduction was also less.

The data regarding the TS, VS reduction of each bottle’s substrate for both WWTPs arepresenting in Tables 8 and 9.

The data regarding the lost weight reduction and initial, final TS, VS values of each bottle’scontent for both WWTPs can be found in Appendix .

Table 8. TS reduction of each bottle’s substrate after the second BMP test for Öresundsverketand Sjölunda WWTPs.

* The results from these bottles were discarded.

Öresundsverket WWTP Sjölunda WWTP

Substrate Bottlenumber

TS % reductionof substrate

(withoutinoculum)

Bottlenumber

TS % reductionof substrate

(withoutinoculum)

Primary sludge1 67% 16 58%2 78% 17 53%3 76% 18 54%

Biosludge4 56% 19 50%5 58% 20 54%6 49% 21 53%

Mixed sludge7* 59% 22 56%8 60% 23 53%9 42% 24* 53%

Inoculum10 0% 25 0%11 0% 26 0%12 0% 27 0%

Cellulose13* 99% 28 100%14 97% 29 100%15 101% 30 102%

28

Table 9. VS reduction of each bottle’s substrate after the second BMP test for Öresundsverketand Sjölunda WWTPs.

* The results from these bottles were discarded.

The results from bottles containing cellulose showed that the inoculum was of good qualitybecause all the cellulose was digested with both inoculum from Öresundsverket and SjölundaWWTPs. The variations in TS and VS values are likely due to errors like non homogeneity ofthe samples and errors in weighing the samples. For Öresundsverket WWTP, the primarysludge, biosludge and mixed sludge showed in average 80%, 57% and 59% reduction. ForSjölunda WWTP, the primary sludge, biosludge and mixed sludge showed in average 69%,66% and 68% reduction.

The results show that the organic matter (VS) of primary sludge in both WWTP had a higherdegradability and it can be explained by the fact that the primary sludge is raw sludge whichdidn’t get further biological and chemical treatment, thus the ratio of readily biodegradableorganic matter is higher in primary sludge.

The degradation of organic matter was higher in the biosludge of Sjölunda than forÖresundsverket. This coincides well with the expected results As the biosludge of Sjölundalikely had a higher ratio of easily biodegradable organic material than biosludge ofÖresundsverket, due to the biological treatment process at two WWTPs. Biological treatmentprocess of Öresundsverket consists of activated sludge process as well as nitrogen andphosphorus removal resulting in degradation of the higher ratio of easily biodegradable

Öresundsverket WWTP Sjölunda WWTP

Bottlenumber

VS % reductionof substrate

(withoutinoculum)

Bottlenumber

VS % reductionof substrate

(withoutinoculum)

Primary sludge1 75% 16 71%2 84% 17 68%3 81% 18 69%

Biosludge4 58% 19 65%5 64% 20 68%6 50% 21 66%

Mixed sludge7* 65% 22 69%8 63% 23 67%9 50% 24* 66%

Inoculum10 0% 25 0%11 0% 26 0%12 0% 27 0%

Cellulose13* 101% 28 99%14 96% 29 99%15 99% 30 102%

29

organic material, consequently this ratio would be less in the produced biosludge. ForSjölunda the biosludge was taken from activated sludge process resulting in presence of moreeasily biodegradable organic material in the produced biosludge.

The summary of the VS reduction and methane production for primary, bio and mixed sludgefrom both WWTPs are presented in Table 10.

Table 10. Average of VS reduction and methane production per gram of added VS forprimary, bio and mixed sludge from Öresundsverket and Sjölunda WWTPs.

ÖresundsverketWWTP

Average VSreduction of

substrate

methaneproduction(Nml/g VS)

SjölundaWWTP

Average VSreduction of

substrate

methaneproduction(Nml/g VS)

Primary sludge 80% 341 Primary sludge 69% 374

Biosludge 57% 256 Biosludge 66% 390

Mixed sludge 59% 296 Mixed sludge 68% 369

Calculatedmixed sludge 71% 307 Calculated

mixed sludge 68% 378

If the VS reduction of mixed sludge is calculated according to the share of primary sludge andbiosludge and the measured values of VS reduction for each of them, the value for theÖresundsverket would be 71% (0.6*80% + 0.4*57%= 71%) reduction which is higher thanthe measured value of VS reduction from mixed sludge (59%), which shows that mixing theprimary sludge and biosludge of Öresundsverket increases the potential VS degradation. Butit should consider the uncertainties of VS measurements as the measured methane productionfrom the mixed sludge was close to the calculated methane production.

For the mixed sludge of Sjölunda the calculated VS reduction would be 68% (0.75*69% +0.25* 66%= 68%) just like the measured value. It shows that the amount of VS reduction inmixed sludge of Sjölunda is according to the share of primary sludge and biosludge and theirseparate VS reduction. The results of methane production per gram of degraded VS for bothWWTPs are presented in Table 11.

Table 11. Methane production per gram of degraded VS for primary, bio and mixed sludgefrom Öresundsverket and Sjölunda WWTPs.

Methane production (Nml/g VS)

Substrate ÖresundsverketWWTP

SjölundaWWTP

Primary sludgeBiosludge

Mixed sludge

427448454

540588547

30

The calculation of methane production per gram of degraded VS shows the biodegradabilityof the organic matter and their potentials to produce methane in each type of sludge. Resultsfrom primary sludge for both WWTPs showed that addition of precipitation agent (Sjölunda)resulted in production of more methane per amount of degraded organic matter. The effect ofprecipitation agent is more to form flocs that are separable in the sedimentation basin.

The biosludge of the high loaded activated sludge process at Sjölunda would give moremethane production per gram of degraded VS as the remaining organic matter in this sludgecontains more easily biodegradable material, resulting in more methane production. But forbiosludge from Öresundsverket the major part of easily biodegradable material have alreadybeen consumed. Therefore the remaining biosludge has less potential to produce methane.

For Sjölunda WWTP, the methane production per gram of added VS as well as the methaneproduction per gram of degraded VS was higher for biosludge than for primary sludge.

For Öresundsverket WWTP, the methane production per gram of added VS was higher forprimary sludge than for biosludge while the methane production per gram of degraded VSwas less for primary sludge than for biosludge.

The initial and final COD values for the selected bottles containing primary, bio and mixedsludge from Öresundsverket WWTP are presented in Table 12.

Table 12. Initial and final COD values for the bottles containing primary, bio and mixedsludge from Öresundsverket WWTP.

ÖresundsverketWWTP

Initial CODdissolved

(mg/l)

Final CODdissolved

(mg/l)

CODdissolved

reduction %

Initial CODtotal(g/l)

FinalCOD total

(g/l)

CODTotal

reduction %

Primary sludge(bottle 3) 1 610 776 48% 43 200 25 700 59%

Biosludge(bottle 5)

890 848 95% 30 000 21 200 70%

Mixed sludge(bottle 8) 1 140 808 71% 34 700 19 000 55%

Inoculum(bottle 11) 1 165 1 148 98% 27 350 28 320 -

According to Table 3 in Appendix, the percent of VS reduction for the bottle’s content ofbottle 3, 5 and 8 was 45 %, 35 % and 37 %. As it can be seen there is a lot of differencebetween the values of VS reduction and total COD reduction.

31

There was an assumption for calculating the initial COD values of the bottle’s content, as itwas assumed that the initial COD would be according to the share of the primary sludge,biosludge and the inoculum and the measured COD values of them which cannot be preciseenough. Also all the final COD measurements of the bottle’s contents were carried out withthe diluted samples which introduce more uncertainties about the measured values. As thetotal COD for the inoculum increased after the digestion which could not be correct.

The initial and final COD values for the selected bottles containing primary, bio and mixedsludge from Sjölunda WWTP are presented in Table 13.

Table 13. Initial and final COD values for the bottles containing primary, bio and mixedsludge from Sjölunda WWTP.

SjölundaWWTP

Initial CODdissolved

(mg/l)

Final CODdissolved

(mg/l)

CODdissolved

reduction %

Initial CODtotal

(mg/l)

FinalCOD total

(g/l)

CODTotal

reduction %

Primary sludge(bottle 18) 1 320 450 34% 27 820 19 000 68%

Biosludge(bottle 21) 1 660 695 42% 33 200 18 400 55%

Mixed sludge(bottle 22) 1 400 512 37% 29 160 16 720 57%

Inoculum(bottle 26) 765 564 74% 19 750 16 880 85%

According to Table 6 in Appendix, the percent of VS reduction for the bottle’s content ofbottle 18, 21 and 22 was 42 %, 43 % and 37 %. As it can be seen the difference between thevalues of VS reduction and total COD reduction is considerably high. Also the final CODvalue for the bottle containing the mixed sludge could not be that much low, which is nearlyequal to the total COD of digested inoculum. In this case, it means that the whole mixedsludge was digested and the remaining inoculum has this value of COD and could not becorrect. As a result, due to incorrect values of COD and different values of COD reduction incomparison to VS reduction’s values the COD analyses are discarded.

33

Tota

l CO

D(g

/ l)

5. ConclusionsBy carrying out the laboratory-scale anaerobic digestion of the primary sludge, biosludge andthe mixture of the both from both WWTPs, the following conclusions can be drawn:

Digestion of primary sludge from the wastewater treatment plant with pre-precipitation(ferric-based precipitation agent) resulted in a higher methane production per gram ofadded VS (about 9%) and per gram of degraded VS (about 21%) than for digestion ofprimary sludge from the wastewater treatment plant with hydrolysis.

The biodegradability as well as the mass reduction was higher for primary sludge than forbio-sludge for both WWTPs which show that the ratio of biodegradable organic matterthat can be converted to biogas is higher in primary sludge.

Digestion of biosludge from the wastewater treatment plant with a high loaded activatedsludge process resulted in a higher methane production per gram of added VS (about35%) and per gram of degraded VS (about 24%) as well as higher biodegradability thanfor the biosludge taken at the plant with extended activated sludge process includingbiological nitrogen and phosphorus removal.

The methane production from mixed sludge corresponds to the share of the primarysludge and bio-sludge and their separate methane production. Thus no synergy was seen.

VS (g/ l)

35

6. Future workTo confirm the results of methane production from the primary sludges and biosludges andthe mixture of the both from both WWTPs, other sets of BMP tests were suggested. Alsodigestion of the sludges from Öresundsverket with the inoculum from Sjölunda WWTPshould be carried out to investigate the effect of different inoculum on methane production.

The measurements of the initial COD of the bottle’s content would be suggested to have atrustable value to compare with the final COD measurements to evaluate the COD reductionof the bottle’s content. Also the pH measurements of the bottle’s content at the beginning andend of the test should be performed to check the effect of anaerobic digestion on H+

production and the consequent effects on methane production.

For simulating the real condition at WWTPs, the digestion of the sludges in continuouslyoperated experiments would be suggested.

As the other approach, the sludge digestion from other WWTPs with different treatment stepsshould be investigated to have more comprehensive results about the effects of varioustreatments processes on methane production.

37

7. ReferencesAppels, L., Baeyens, J., Degrève, J. and Dewil, R., (2008). Principles and potential of theanaerobic digestion of waste-activated sludge. Energy and Combustion Science, 34: pp. 755-781.

Bolzonella, D., Pavan, P., Battistoni, P. and Cecchi, F., (2005). Mesophilic anaerobicdigestion of waste activated sludge: influence of the solid retention time in the wastewatertreatment process. Process Biochemistry, Vol. 40, No. 3-4, pp. 1453-1460.

Brinch, P., Rindel, K. and Kalb, K., (1994). Upgrading to nutrient removal by means ofinternal carbon from sludge hydrolysis. Water Sci. Technol., Vol. 29, No. 12, pp. 31-40.

Davidsson, Å., (2007). Increase of Biogas Production at Wastewater Treatment PlantsAddition of urban organic waste and pre-treatment of sludge. Ph.D. Thesis, Dept. of Waterand Environmental Engineering, Lund University.

Gary, D., Morton, R., Tang, C.-C. and Horvath, R., (2007). The effect of the Microsludgetreatment process on anaerobic digestion performance. Water Environment Federation´sAnnual Technical Exhibition and Conference, San Diego USA 13-17 October 2007.

Gillberg, L., Hansen, B., Karlsson, I., Nordström Enkel, A. and Pålsson, A., (2003).Biological phosphorus removal. In: About water treatment. Helsinborg, Sweden: KemiraKemwater.

Gujer, W. and Zehnder, A.J.B., (1983). Conversion processes in anaerobic digestion. WaterSci. Technol., Vol. 15, No. 8-9, pp. 127-167.

Hansen, T. L. Hansen, T.L. Schmidt, J. E., Angelidaki, I., Marca, E., Jansen, J. la C.,Mosbæk, H. and Christensen, T. H., (2004). Method for determination of methane potentialsof solid organic waste. Waste Management, Volume 24, No 4, pp. 393-400.

Henze, M., Harremoës, P., Jansen, J. la C. and Arvin, E., (1997). Wastewater Treatment.Biological and Chemical Processes. Second Edition. Springer, Berlin. ISBN: 3-540-62702-2.

Mata-Alvarez, J., Macé, S. and Llabrés, P., (2000). Anaerobic digestion of organic solidwastes. An overview of research achievements and perspectives. Bioresource TechnologyVol. 74, No. 1, pp. 3-16.

Metcalf & Eddy. (1991). Wastewater Engineering: Treatment, disposal and reuse. 3rd ed.Mcgraw-Hill, New York. ISBN 0-07-041690-7.

NSVA (2011), Öresundsverket in Helsingborg, accessed 2013-04-02,<http://www.nsva.se/Global/Dokument/Broschyrer/%C3%96resundsverket%20%5BEnglish%5D.pdf?epslanguage=sv>.

http://www.nsva.se/Global/Dokument/Broschyrer/%C3%96resundsverket%20%5BEnglish

38

NSVA (2011), Öresundsverket Helsingborg Miljörapport 2011 (environmental report),accessed 2013-04-02,<http://www.nsva.se/Global/Dokument/MIlj%C3%B6rapporter/Milj%C3%B6rapport%202011%20%C3%96resundsverket.pdf?epslanguage=sv>.

Parkin, G. and Owen, W. F., (1986). Fundamentals of anaerobic digestion of wastewatersludges. Journal of Environmental Engineering, Vol. 112, No. 5, pp. 867-920.

Prescott, L.M., Harley, J.P. and Klein, D.A., (1999). Microbiology, 4th ed. WCB/McGraw-Hill. ISBN 0-697-35439-3.

Schaum, C., Lensch, D., Cornel, P., (2012). Energy resource sewage sludge: the relevance ofthe heating value and the impact of sludge treatment processes. Technische UniversitätDarmstadt, Institute IWAR, Germany.

SIS, Swedish Institute for Standards (2000). Characterization of sludge – determination of dryresidue and water content (SS-EN 12880) and determination of loss on ignition of dry mass(SSEN12879).

VA SYD (2010), The Sjölunda wastewater treatment plant, accessed 2013-04-02,<http://www.vasyd.se/SiteCollectionDocuments/Broschyrer/Vatten-%20och%20avloppsbroschyrer/Vatten%20och%20avlopp/Sjolunda_wastewater_treatment_plant.pdf>.

VA SYD (2010) SJÖLUNDA AVLOPPSRENINGSVERK MALMÖ MILJÖRAPPORTENLIGT MILJÖBALKEN FÖR ÅR 2010, accessed 2013-04-02,<http://www.vasyd.se/SiteCollectionDocuments/Vatten%20och%20avlopp/Avloppsvatten/Milj%C3%B6rapporter/Sj%C3%B6lunda_Milj%C3%B6rapport_2010.pdf>.

http://www.nsva.se/Global/Dokument/MIlj%C3%B6rapporter/Milj%C3%B6rapport%20201

http://www.vasyd.se/SiteCollectionDocuments/Broschyrer/Vatten-

http://www.vasyd.se/SiteCollectionDocuments/Vatten%20och%20avlopp/Avloppsvatten/Mi

39

Appendix

ÖresundsverketWWTP

Bottlenumber

Initial weightof the bottle’s

content(g)

Final weightof the bottle’s

content (g)

lost weightafter digestion

(g)

Lost weightreduction

%

Primary sludge1 363.77 358.55 5.22 1.43%2 360.40 354.79 5.61 1.56%3 362.88 357.42 5.46 1.50%

Biosludge4 445.52 441.75 3.77 0.85%5 443.57 440.40 3.17 0.71%6 444.52 439.80 4.72 1.06%

Mixed sludge7 413.61 408.70 4.91 1.19%8 412.38 407.80 4.58 1.11%9 414.39 410.71 3.68 0.89%

Inoculum10 286.25 284.18 2.07 0.72%11 286.48 285.63 0.85 0.30%12 286.58 284.66 1.92 0.67%

Cellulose13 290.62 283.39 7.23 2.49%14 290.15 283.98 6.17 2.13%15 290.00 283.97 6.03 2.08%

ÖresundsverketWWTP

Bottlenumber

Initial TSfrom

substrate(g)

Initial TSfrom

inoculum(g)

Initialtotal TS

(g)

Finaltotal TS

(g)

TotaldigestedTS (g)

Total TSreduction

%

Primary sludge1 5.30 8.00 13.31 8.92 4.39 33%2 5.05 8.01 13.06 8.31 4.75 36%3 5.07 8.07 13.14 8.50 4.65 35%

Biosludge4 4.58 8.06 12.63 9.27 3.37 27%5 4.55 8.02 12.58 9.15 3.43 27%6 4.59 8.01 12.60 9.56 3.04 24%

Mixed sludge7 4.82 8.06 12.88 9.21 3.66 28%8 4.83 8.01 12.84 9.12 3.72 29%9 4.90 8.04 12.94 10.06 2.88 22%

Inoculum10 0.00 8.02 8.02 7.21 0.81 10%11 0.00 8.02 8.02 7.25 0.78 10%12 0.00 8.02 8.02 7.16 0.86 11%

Cellulose13 4.00 8.03 12.03 7.24 4.78 40%14 4.00 8.01 12.01 7.31 4.70 39%15 4.00 8.01 12.01 7.15 4.86 40%

Table 2.TS values and total TS reduction of each bottle for Öresundsverket WWTP.

Table 1. Initial and final weight of the bottle’s content and their lost weight, for ÖresundsverketWWTP.

40

SjölundaWWTP

Bottlenumber

Initial weightof the bottle’s

content(g)

Final weightof the bottle’s

content (g)

lost weightafter digestion

(g)

Lost weightreduction

%

Primary sludge16 389.04 383.31 5.73 1.47%17 389.46 384.72 4.74 1.22%18 389.22 384.52 4.70 1.21%

Biosludge19 398.71 396.05 2.66 0.67%20 391.80 385.81 5.99 1.53%21 391.98 387.03 4.95 1.26%

Mixed sludge22 389.85 384.49 5.36 1.37%23 390.25 386.23 4.02 1.03%24 390.94 390.30 0.64 0.16%

Inoculum25 286.30 284.78 1.52 0.53%26 286.22 283.88 2.34 0.82%27 286.55 284.32 2.23 0.78%

Cellulose28 288.72 283.54 5.18 1.79%29 290.08 284.34 5.74 1.98%30 289.99 285.32 4.67 1.61%

ÖresundsverketWWTP

Bottlenumber

Initial VSfrom

substrate(g)

Initial VSfrom

inoculum(g)

Initialtotal VS

(g)

Finaltotal VS

(g)

TotaldigestedVS (g)

Total VSreduction

%

Primary sludge1 4.37 5.43 9.80 5.54 4.26 43%2 4.16 5.44 9.60 5.12 4.47 47%3 4.17 5.48 9.65 5.28 4.37 45%

Biosludge4 3.31 5.47 8.78 5.88 2.90 33%5 3.30 5.44 8.74 5.65 3.09 35%6 3.32 5.44 8.76 6.12 2.64 30%

Mixed sludge7 3.69 5.47 9.16 5.76 3.40 37%8 3.70 5.43 9.14 5.80 3.34 37%9 3.76 5.45 9.21 6.33 2.88 31%

Inoculum10 0.00 5.44 5.44 4.43 1.01 19%11 0.00 5.44 5.44 4.45 1.00 18%12 0.00 5.45 5.45 4.48 0.96 18%

Cellulose13 4.00 5.45 9.45 4.43 5.01 53%14 4.00 5.44 9.44 4.60 4.84 51%15 4.00 5.43 9.43 4.47 4.96 53%

Table 3.VS values and total VS reduction of each bottle for Öresundsverket WWTP.

Table 4. Initial and final weight of the bottle’s content and their lost weight for SjölundaWWTP.

41

SjölundaWWTP

Bottlenumber

Initial TSfrom

substrate(g)

Initial TSfrom

inoculum(g)

Initialtotal TS

(g)

Finaltotal TS

(g)

TotaldigestedTS (g)

Total TSreduction

%

Primary sludge16 3.91 5.44 9.34 6.49 2.86 31%17 3.92 5.44 9.36 6.68 2.68 29%18 3.92 5.44 9.35 6.62 2.73 29%

Biosludge19 5.28 5.53 10.81 7.55 3.26 30%20 5.13 5.45 10.59 7.23 3.36 32%21 5.11 5.47 10.57 7.29 3.29 31%

Mixed sludge22 4.21 5.44 9.66 6.69 2.96 31%23 4.20 5.46 9.66 6.83 2.83 29%24 4.22 5.46 9.68 6.85 2.84 29%

Inoculum25 0.00 5.44 5.44 4.87 0.57 11%26 0.00 5.44 5.44 4.80 0.63 12%27 0.00 5.44 5.44 4.85 0.60 11%

Cellulose28 2.66 5.44 8.10 4.85 3.25 40%29 2.66 5.46 8.12 4.87 3.25 40%30 2.66 5.46 8.12 4.80 3.32 41%

SjölundaWWTP

Bottlenumber

Initial VSfrom

substrate(g)

Initial VSfrom

inoculum(g)

Initialtotal VS

(g)

Finaltotal VS

(g)

TotaldigestedVS (g)

Total VSreduction

%

Primary sludge16 3.08 3.15 6.23 3.56 2.68 43%17 3.09 3.15 6.24 3.65 2.59 42%18 3.09 3.15 6.24 3.63 2.62 42%

Biosludge19 3.77 3.20 6.97 4.03 2.94 42%20 3.66 3.16 6.82 3.84 2.98 44%21 3.65 3.17 6.81 3.92 2.89 42%

Mixed sludge22 3.23 3.15 6.38 3.67 2.72 43%23 3.22 3.16 6.38 3.74 2.64 41%24 3.24 3.16 6.40 3.76 2.64 41%

Inoculum25 0.00 3.15 3.15 2.68 0.47 15%26 0.00 3.15 3.15 2.65 0.50 16%27 0.00 3.15 3.15 2.66 0.49 16%

Cellulose28 2.66 3.15 5.81 2.67 3.13 54%29 2.66 3.16 5.82 2.71 3.12 54%30 2.66 3.16 5.82 2.62 3.20 55%

Table 5. TS values and total TS reduction of each bottle for Sjölunda WWTP.

Table 6.VS values and total VS reduction of each bottle for Sjölunda WWTP.

1

Methane production from separatedigestion of primary and biological sludge


Water and Environmental Engineering, Department of Chemical Engineering, Lund University,Sweden

June 2013

Abstract

Recently a great amount of attention has been paid to anaerobic digestion of sludge fromwastewater treatment plants to pursue two goals; handling of the sludge in a harmless way for theenvironment and producing more biogas. The produced biogas is a renewable energy source that canbe used as a substitute for fossil vehicle fuels, also for production of heat and electricity. Toinvestigate possible ways to increase the methane production from sludge digestion at wastewatertreatment plants the digestion of primary sludge, bio-sludge and the mixture of the both wasinvestigated. To evaluate the relationship between the type of sludge and methane production, theprimary sludges and bio-sludges of two municipal Swedish wastewater treatment plants were used.The results showed that digestion of primary sludge from the wastewater treatment plant with pre-precipitation (ferric-based precipitation agent) resulted in a 9% higher methane production than fordigestion of primary sludge from the wastewater treatment plant with hydrolysis. Digestion of bio-sludge from the wastewater treatment plant with a high loaded activated sludge process resulted in a35% higher methane production than for the bio-sludge taken at the plant with extended activatedsludge process including biological nitrogen and phosphorus removal.

Keywords: Anaerobic digestion, Primary sludge, Bio-sludge, Separate sludge digestion, Methaneproduction

Introduction

Nowadays in an energy demandinglifestyle, biogas from anaerobic digestion canbe used as a substitute for fossil vehicle fuels,also for production of heat and electricity as arenewable and eco-friendly energy source.

Anaerobic digestion (AD) consists ofseveral stages in which microorganisms breakdown biodegradable material in the absence ofoxygen, to produce biogas. Recently a greatamount of attention has been paid to anaerobicdigestion of sludge from wastewater treatmentplants to pursue two goals; handling of the

sludge in a harmless way for the environmentand producing more biogas.

Both primary sludge and bio-sludge aresuitable to produce methane via anaerobicdigestion due to their high VS content [1].Since the primary sludge contains more easilybiodegradable fats and carbohydrates incomparison to bio-sludge, the anaerobicdegradability of primary sludge should behigher than that for bio-sludge [2].

In this study, the methane production fromseparate digestion of primary sludge, bio-sludge and digestion of the mixture of the bothwas investigated with the emphasis onrelationship between the type of the sludge andthe methane production. To evaluate the type

2

of the primary sludges and bio-sludges onmethane production which lead to highermethane production, sludges from twowastewater treatment plants were used. Oneplant produced primary sludge through pre-precipitation and bio-sludge from a highloaded activated sludge process (SjölundaWWTP), the other plant produces primarysludge with in situ hydrolysis and the bio-sludge from an extended activated sludgeprocess including biological nitrogen andphosphorus removal (Öresundsverket WWTP).

Materials and methods

For analyzing the sludge characteristics,TS (total solids) and VS (volatile solids) weremeasured using Swedish standard method, SS-EN 12880 and SS-EN 12879 respectively.Total COD (total chemical oxygen demand)and dissolved COD (chemical oxygen demandfor readily biodegradable material) weremeasured spectrophotometrically using theHACH LANGE test tube (LCK 114) andspectrophotometer (model DR 2800). Forestimation of the dissolved COD samples werecentrifuged at 10 000 rpm for 10 minutesfollowed by filtration through Munktell paperfilter with the pore size of 6~10 μm beforeanalysis.

Bio-methane potential test (BMP)

The methane potential of the sludge atSTP (standard temperature and pressure, 0°Cand 1atm) was based on the method by Hansenet al. (2004) [4]. The method includes batchanaerobic digestion of the sludge in 2 literglass bottles with periodical measurements ofthe produced methane in the head space of thebatch reactors. The methane potential isexpressed as volume of produced methane permass of substrate in terms of the initial VScontent of the substrate (Nml CH4/g VS). Themethane potential is calculated using theaccumulated volume of produced methaneduring the test period. The required period for

determination of the total methane potentialvaries between 30 to 50 days [1].

Sludge from full scale anaerobic digesters(mesophilic digester operating at 35°C) at bothWWTPs was used as inoculum. The inoculumas well as primary and bio-sludge fromÖresundsverket and Sjölunda WWTPs werebrought to the laboratory 4 days before startingthe experiment. Primary and bio-sludge werekept in cold room (5°C) and the inoculum waskept in incubator at 35°C till the start of theexperiment to ensure the degradation of easydegradable organic matter present in theinoculum. Cellulose was used to check thequality of the inoculum due to the similarmethane potential to sludge and its slowdegradation process [1].

The maximum amount of initial VS ineach bottle from both inoculum and sludge orcellulose was 10 g and the ratio of 60:40 wasapplied between the VS from the inoculum andthe VS from sludge or cellulose respectively.The volume of the bottle’s liquid content wasmaximum 500 ml. All the substrates wereadded at the beginning of the tests andanaerobic condition was applied by flushingnitrogen gas in the headspace of the bottle for1 minute after adding the substrates. Thebottles were sealed with rubber septum andscrew cap and they were kept in incubator atmesophilic temperature (35°C) during theexperiment. Also they were shaken once perweek to increase the contact between themicroorganisms in the inoculum and thesludge. The methane potential test for eachtype of substrate was done in triplicates due tothe heterogeneity of the substrate. Since BMPtests are based on the activities ofmicroorganisms presented in the inoculumwhich can vary in abundance. Up to 10% ofstandard deviation between the accumulatedmethane productions of three bottlescontaining the same substrate was consideredreasonable. The ratio of the mixed sludge forSjölunda WWTP was 75% primary sludge and25% bio-sludge and for ÖresundsverketWWTP was 60% primary sludge and 40% bio-sludge based on VS amounts.

3

Methane measurement and its calculation

The methane production in the headspaceof the bottles was measured 1-3 times per weekdepending on the rate of gas production. Formeasuring the produced methane in the batchreactors a gas sample (0.2 ml) was taken with agas syringe with pressure lock. Analysis wasdone in a gas chromatograph (Agilent 6850)equipped with FID (flame ionization detector)and a column with the dimensions of: 30 mlength, 320 µm diameter and 0.25 µm filmthickness. Gas analysis for each bottle wasdone in triplicates and the room temperatureand atmospheric pressure was measured bythermometer and barometer installed in theroom.

For calculation of the methane content theresults were compared to a 100% methane gasreference. The headspace of the bottle wascalculated from subtracting the mass of addedinoculum and sludge from the whole volumeof the bottle by assuming the density of 1 g/mlfor inoculum and sludge. The normalizedmethane production was calculated using theideal gas law.

Where:

STPV is the produced methane in the head spaceof the bottle at standard pressure andtemperature (ml), MX is the average of threesignals from the GC for each bottle at roomtemperature and actual pressure,

HeadspaceV isthe gas volume in each bottle (ml),

%100X is theaverage of three signals from the GC forinjection of 100% methane at roomtemperature and pressure, ST (standardtemperature) is equal to 273.15 K, MP is theactual atmospheric pressure (room pressure)(mba), MT is the room temperature (K), SP(standard pressure) is 1 atmosphere that isequal to 1013.25 mba.

The gas production from the inoculumswas subtracted from the measured values toobtain the gas production from the substrate

alone. This value usually would becorresponded to the last measurement, but ifthe final value would become less, themaximum value which obtained at the end ofthe test period would be chosen for comparisonof methane potentials.

Results and discussions

The total digestion time of the BMP testwas 53 days. The summary of the methanepotentials (the maximum methane productionduring the test period) for different substratesfrom Öresundsverket and Sjölunda WWTP ispresented in Table 1.

Table 1. Maximum methane production(Nml/g VS)and standard deviation of different substrates forÖresundsverket and Sjölunda WWTPs in the BMPtest after 53 days of digestion.


SjölundaWWTP

Primarysludge 341 (1%) 374 (5%)

Bio-sludge 256 (3%) 390 (3%)

Mixed*sludge 296 (14%) 369 (10%)

Cellulose 320 (3%) 370 (2%)

* Mixed sludge of Öresundsverket WWTP contains60% primary sludge + 40% bio-sludge and mixedsludge of Sjölunda WWTP contains 75% primarysludge + 25% bio-sludge.

The graphs of methane potentials foraverage of every triplicate for both WWTPsare presented in Figures 1 and 2.

SMMSHeadspacemSTP PTPTXVXV */**)/*( %100

Time (days)

4

As the primary sludge of Sjölunda wastaken after the pre-precipitation process, mostof the dissolved organic matter has beenconverted to particulate material which resultsin a primary sludge with better organic matterregarding biodegradability for anaerobicdigestion. The primary sludge ofÖresundsverket was taken after hydrolysis ofthe sludge which results in partly degradationof organic matter and consequently theremaining organic matter in the primary sludgewould not be as good as the primary sludgefrom Sjölunda WWTP. As a result, the

maximum methane production from primarysludge was higher in Sjölunda WWTP thanÖresundsverket WWTP.

As it can be seen from the Table 1, themethane production from the bio-sludge of theSjölunda WWTP was much higher than fromÖresundsverket WWTP. The reason could bethe different processes that bio-sludges hadundergone. At Öresundsverket, after thehydrolysis, the readily biodegradable organicmatter and smaller organic particles with moresurface areas were utilized in the biologicalnitrogen and phosphorus removal, and withlonger sludge age the larger parts of theorganic matter would be consumed bymicroorganisms, consequently the producedbio-sludge wouldn’t be good for methaneproduction as the major part of easilybiodegradable material have already beenconsumed. The bio-sludge from the SjölundaWWTP was from a high loaded activatedsludge process which means that with theshorter sludge age and a lot of producedbiomass, the biodegradability of the organicmatter would be higher in the produced bio-sludge.

The calculated methane production formixed sludge of the Öresundsverket based onthe share of each primary sludge and bio-sludge and their measured values of maximummethane production gave a value of 307 Nml/gVS, (0.6*341+0.4*256=307). The calculatedvalue is nearly the same as the measured value(296 Nml/g VS) which shows that mixing theprimary sludge and bio-sludge doesn’t have asignificant effect on methane productionvalues in comparison to the digestion of thesame share of primary sludge and bio-sludgeseparately. The calculated maximum methaneproduction from mixed sludge of the Sjölundagave a value of 378 Nml/g VS,(0.75*374+.25*390=378). The calculatedvalue is nearly the same as the measured value(369 Nml/g VS) which shows that, mixing theprimary sludge and bio-sludge would not resultin higher or lower methane production incomparison to the digestion of the same shareof primary sludge and bio-sludge separately.

Time (days)

Acc

umul

ated

met

hane

pro

duct

ion

Nm

l/g V

S

Figure 1. Accumulated methane productions fromcellulose and primary, bio, mixed sludge ofÖresundsverket WWTP.

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40 45 50 55

Primary sludge Biosludge

Mixed sludge Cellulose-referenceAcc

umul

ated

met

hane

pro

duct

ion

Nm

l/g V

S

0

50

100

150

200

250

300

350

400

450

0 5 10 15 20 25 30 35 40 45 50 55

Primary sludge Biosludge

Mixed sludge Cellulose-reference

Time (days)

Figure 2. Accumulated methane productions fromcellulose and primary, bio, mixed sludge ofSjölunda WWTP.

5

The maximum methane production fromcellulose with the inoculum fromÖresundsverket WWTP was 320 Nml/g VSthat was less than the values stated by Hansenet al., 2004 (377 Nml/g VS) [4] andDavidsson, 2007 (353 Nml/g VS) [1]. Thereason could be the activity of the inoculum,but as it can be seen from the Figure 1, themethane production from cellulose had a goodrate and the accumulated methane contentbecame constant after 20 days. The resultsfrom cellulose digestion showed that theinoculum had a good quality as all thecellulose was digested. But it should bementioned that the inoculum of everytreatment plant may differ from each other asthey origins from different digestion processesin full scale (HRT, temperature, loading rateetc). The reason for getting lower value fromcellulose digestion could be that the inoculumof Öresundsverket wasn’t adapted enough toproduce more methane from cellulosedigestion. The result can be trusted to ensurethat the inoculum has a good enough quality todigest the substrates.

The maximum methane production fromcellulose with the inoculum from SjölundaWWTP was 370 Nml/g VS. The decreasingtrend (Figure 2) at the end of the test is due tosubtraction of the inoculums’ methaneproduction. The results from cellulosedigestion showed that the inoculum had a goodquality, because all the cellulose was digestedand the measured methane production fromcellulose was close to the values stated byHansen et al., 2004 (377 Nml/g VS) [4] andDavidsson, 2007 (353 Nml/g VS) [1].

At the end of the BMP test, all bottleswere weighed to evaluate the content’s massreduction. Also the TS, VS of all the bottleswere measured. The summary of the VSreduction for different substrates fromÖresundsverket and Sjölunda WWTPs ispresented in Table 2.

Table 2. VS reduction of different substrates forÖresundsverket and Sjölunda WWTPs in the BMPtest after 53 days of digestion.


SjölundaWWTP

Primarysludge 80% 69%

Bio-sludge 57% 66%

Mixedsludge 59% 68%

The results show that the organic matter(VS) of primary sludge in both WWTP had ahigher degradability and it can be explained bythe fact that the primary sludge is raw sludgewhich didn’t get further biological andchemical treatment, thus the ratio of readilybiodegradable organic matter is higher inprimary sludge.

The degradation of organic matter washigher in the bio-sludge of Sjölunda than forÖresundsverket. This coincides well with theexpected results as the bio-sludge of Sjölundalikely had a higher ratio of easilybiodegradable organic material than bio-sludgeof Öresundsverket, due to the biologicaltreatment process at two WWTPs. Biologicaltreatment process of Öresundsverket consistsof activated sludge process as well as nitrogenand phosphorus removal resulting indegradation of the higher ratio of easilybiodegradable organic material, consequentlythis ratio would be less in the produced bio-sludge. For Sjölunda the bio-sludge was takenfrom activated sludge process resulting inpresence of more easily biodegradable organicmaterial in the produced bio-sludge. Insummary, the hydrolysis of primary sludge andlonger sludge age of bio-sludge at activatedsludge process lead to less biogas production.

Time (days)

6

Conclusions

By carrying out the laboratory-scaleanaerobic digestion of the primary sludge, bio-sludge and the mixture of the both from bothWWTPs, the following conclusions can bedrawn:

Digestion of primary sludge from thewastewater treatment plant with pre-precipitation (ferric-based precipitationagent) resulted in a 9% higher methaneproduction than for digestion of primarysludge from the wastewater treatment plantwith hydrolysis.

The biodegradability as well as the massreduction was higher for primary sludgethan for bio-sludge for both WWTPswhich show that the ratio of biodegradableorganic matter that can be converted tobiogas is higher in primary sludge.

Digestion of bio-sludge from thewastewater treatment plant with a highloaded activated sludge process resulted ina 35% higher methane production as wellas higher biodegradability than for the bio-sludge taken at the plant with extendedactivated sludge process includingbiological nitrogen and phosphorusremoval.

The methane production from mixedsludge corresponds to the share of theprimary sludge and bio-sludge and theirseparate methane production. Thus nosynergy was seen.

AcknowledgmentsThis article is a part of a master thesis at

Water and Environmental Engineering,Department of Chemical Engineering, LundUniversity. I would like to thank mysupervisors, Åsa Davidsson and HamseKjerstadius and my examiner Jes la CourJansen for all the help, constructive commentsand suggestions during this work.

My thanks also go to Gertrud Persson,Disa Sandström, Max Grankvist, MarinetteHagman and Beata Matulaniec.

References

[1] Davidsson, Å., (2007). Increase of BiogasProduction at Wastewater Treatment PlantsAddition of urban organic waste and pre-treatment of sludge. Ph.D. Thesis, Dept. ofWater and Environmental Engineering, LundUniversity.

[2] Parkin, G. and Owen, W. F., (1986).Fundamentals of anaerobic digestion ofwastewater sludges. Journal of EnvironmentalEngineering, Vol. 112, No. 5, pp. 867-920.

[3] SIS, Swedish Institute for Standards(2000). Characterization of sludge –determination of dry residue and water content(SS-EN 12880) and determination of loss onignition of dry mass (SSEN12879).

[4] Hansen, T. L. Hansen, T.L. Schmidt, J. E.,Angelidaki, I., Marca, E., Jansen, J. la C.,Mosbæk, H. and Christensen, T. H., (2004).Method for determination of methanepotentials of solid organic waste. WasteManagement, Volume 24, No 4, pp. 393-400.