co-digestion of grease trap sludge and sewage sludge
TRANSCRIPT
Available online at www.sciencedirect.com
www.elsevier.com/locate/wasman
Waste Management 28 (2008) 986–992
Co-digestion of grease trap sludge and sewage sludge
A. Davidsson a,*, C. Lovstedt b, J. la Cour Jansen a, C. Gruvberger c, H. Aspegren c
a Water and Environmental Engineering, Department of Chemical Engineering, Center for Chemistry and Chemical Engineering,
Lund University, P.O. Box 124, SE-221 00 Lund, Swedenb Department of Water Resources Engineering, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden
c Malmo Water and Sewage Works, SE-205 80 Malmo, Sweden
Accepted 19 March 2007Available online 22 May 2007
Abstract
Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interestin the Malmo-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps.Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measuredin batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from greasetraps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition ofgrease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps(on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge produc-tion. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methaneproduction in continuous digestion.� 2007 Elsevier Ltd. All rights reserved.
1. Introduction
The focus on handling and treatment of organic wastehas increased in the last few years. Biological treatmentinstead of the conventional treatment methods, landfillingand incineration, is considered in many places since itinvolves environmental benefits as recycling with recoveryof nutrients and energy. In addition, a general ban on land-filling of organic waste from the year 2005 in the EuropeanUnion has resulted in high fees for incineration. The EUpolicy promotes recycling of biodegradable waste as a pri-ority action (Gomes Palacios et al., 2002).
Redirection of organic waste in the Malmo-Copenhagenregion has been studied (Davidsson et al., 2007). An inven-tory of organic waste in the region was carried out. Wastecomposition, annual amounts generated and present
0956-053X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2007.03.024
* Corresponding author. Tel.: +46 46 222 89 98; fax: +46 46 222 45 26.E-mail address: [email protected] (A. Davidsson).
handling were studied to evaluate the possibilities for bio-logical treatment instead of landfilling or incineration.
One type of waste that was expected to be suitable forbiological treatment is sludge from grease traps. This wastecan be composted, as described in Coker (2006) or treatedby anaerobic digestion, which was studied by for exampleStoll and Gupta (1997) and by Deckena and Jannsen(1995). However, anaerobic digestion of fat containingwaste offers many advantages compared to composting(Beccari et al., 1996). Anaerobic treatment results in loweramounts of waste sludge and energy is produced in form ofbiogas, which can be used for heating and electricity pro-duction (Angelidaki and Ahring, 1997). In Malmo, greasetrap sludge has until now been separately collected anddeposited at a landfill. In Copenhagen, it was previouslyused for the production of meat and bone meal. Both oftheses methods of handling now pose problems becausedisposal of organic waste has been banned since 2005and because the directive on animal by-products hasalmost stopped the production of meat and bone meal.
Table 2Amount of inoculum, GS, SS and reference substrate (expressed as g VS)in the reactors at start-up
Reactor type Inoculum GS SS Reference
A. Davidsson et al. / Waste Management 28 (2008) 986–992 987
In addition, the wet sludge deteriorates the landfills. Fur-thermore, the wet sludge with an original TS-content of1–2% does not favour incineration even though the energycontent in the waste is very high, with a VS-content over95%. If thickened to a TS > 5%, it can be suitable as a sub-strate for anaerobic digestion. However, single-substratedigestion of sludge from grease traps could be unwise sincelong-chain fatty acids are known to inhibit anaerobicmicroorganisms (Angelidaki and Ahring, 1992; Rinzemaet al., 1994; Hwu and Lettinga, 1997). Thermophilic condi-tions in the digester led to inhibition at much lower concen-trations of oleate than did mesophilic conditions (Hwu andLettinga, 1997). One possibility for digestion of grease trapsludge is co-digestion with sewage sludge at a wastewatertreatment plant (WWTP). Mesophilic co-digestion ofsludge from grease traps and sludge from the largestWWTP in Malmo was evaluated, with the aim of simulat-ing future full-scale operation. Methane potential was mea-sured in batch laboratory testing, and continuous pilot-scale digestion was carried out to study the process behav-iour and the methane yield.
2. Methods
2.1. Substrates and inoculum
The test substrates used were sludge from grease traps(GS), sewage sludge (SS) consisting of primary sludgeand waste activated sludge (50:50), and mixtures of bothGS and SS (see characteristics in Table 1). Sewage sludgeused in the continuous pilot-scale tests was sampled twicea week. Therefore, the characteristics varied during theexperimental period. The average values are given in Table1. Inoculum material used in both the laboratory tests andpilot-scale tests consisted of sewage sludge digested undermesophilic conditions at Sjolunda WWTP, Malmo,Sweden.
2.2. Laboratory batch test for methane potential
The methane potential of the sampled sludges was deter-mined in duplicate by laboratory-scale anaerobic batchtests described in Hansen et al. (2004). The tests were per-formed in 2 l reactors containing an amount of test sub-strate representing in most cases 40% of the total volatile
Table 1Characteristics of sludge types used in digestion tests
Greasetrap sludge(GS)a
Sewage sludge(SS) in batchtest
Sewage sludge(SS) in pilot-scale tests
Inoculum
pH 4.38 7.29 6.5 8.12Ammonium
(mg/l)136 55.3 45 948
VS (%) 17.0 4.6 3.1 1.2TS (%) 17.3 5.9 4.1 2.1
a The GS was thickened before analysis.
solids as well as 400 ml of inoculum (�5 g VS), see Table2. The reactors were kept at 35 �C, and methane produc-tion was monitored by a gas chromatograph. The experi-ment was terminated when the gas production ceased andthe accumulated gas production remained at a fixed level.The method provided an easy-to-operate and fast meansof measuring methane potentials in the different sludgesand mixtures of sludges. The size of the reactors allowssimultaneous tests of many reactors, although the volumeis larger compared with many other laboratory anaerobicdigestion methods (Hansen et al., 2004).
Three pure substrates (cellulose, oleic acid and stearicacid) were used to test the function of the inoculum. Anamount representing 3 g VS (40% of the total VS) wasadded to each reactor (see Table 2). Cellulose was chosenbecause it was expected to digest slowly and give aboutthe same potential as sewage sludge. The fatty acids (solidstearic acid and liquid oleic acid) were expected to givemethane potentials in the same range as the grease trapsludge. They were also partly chosen with the purpose ofstudying a possible inhibiting/overloading effect, whichwas also studied by Angelidaki and Ahring (1992), as wellas the difference in methane potential for almost the sameacid in solid and in liquid state. Moreover, oleic acid isthe most common long-chain fatty acid in municipal waste-water and sewage (Hwu et al., 1996, Quemeneur andMarty, 1994).
Combinations of grease trap sludge and sewage sludgewere digested in different concentrations. GS:SS on a VS-basis: 0:100, 10:90, 25:75 and 60:40 (see Table 2). The testsludges then represented about 40% of the VS in each reac-tor. The same amounts of GS were also digested separately,without sewage sludge. GS then represented 8–30% of theVS in each reactor.
2.3. Pilot-scale continuous digestion
Separate GS and SS, as well as different mixtures of sew-age sludge and sludge from grease traps, were digested inthe pilot-scale tests. The mixtures were, on a VS-basis:
(g VS) (g VS) (g VS) substrate (g VS)
Inoculum 4.9 0 0 0Cellulose 4.9 0 0 3Oleic acid 4.9 0 0 3Stearic acid 4.9 0 0 3100% SS 4.9 0 3.8 010% GS + 90% SS 4.9 0.4 3.4 025% GS + 75% SS 4.9 0.8 2.9 060% GS + 40% SS 4.9 1.9 1.5 010% GS 4.9 0.4 0 025% GS 4.9 0.8 0 060% GS 4.9 1.9 0 0
GS = sludge from grease traps, SS = sewage sludge.
0
200
400
600
800
1000
1200
0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
85085
0
time (hours)
Met
hane
pot
entia
l (N
ml C
H4/
g VS
in)
100% SS10% GS + 90% SS
25% GS + 75% SS60% GS + 40% SS
10% GS25% GS
60% GS
Fig. 1. Accumulated methane potentials for tested sludges during entiretest period (900 h). (GS = sludge from grease traps, SS = sewage sludge,Nml CH4 = normal ml of CH4 at 1 �C and 1 atm).
Table 3Average methane potentials and standard deviation for duplicate reactorsat the end of the experiment, 37 days
Nml CH4 per g VSin Standard deviation (%)
Cellulose 338 5Oleic acid 1018 3Stearic acid 1013 1100% SS 325 110% GS + 90% SS 425 325% GS + 75% SS 472 860% GS + 40% SS 681 410% GS 886 025% GS 928 160% GS 845 2
(Nml CH4 = normal ml of CH4 at 1 �C and 1 atm).
0
200
400
600
800
1000
1200
time (days)
Met
hane
pot
entia
l (N
ml C
H4/g
VS
in)
CelluloseOleic acid
Stearic acid100% SS
Theoretical methane potential for fat
0 50 100
150
200
250
300
350
400
450
500
550
600
650
700
750
85085
0
Fig. 2. Measured methane potentials for reference substrates. (NmlCH4 = normal ml of CH4 at 1 �C and 1 atm).
988 A. Davidsson et al. / Waste Management 28 (2008) 986–992
5% GS + 95% SS (only in pre-experiment), 10% GS + 90%SS, and 30% GS + 70% SS. Since the inoculum used forstarting up the reactors consisted of digested sewage sludgefrom the full-scale mesophilic digestion plant at the waste-water treatment plant, no start-up phase was required, thatis the reactors were fed with the proper amount from dayone.
The pilot-scale equipment resembles a full-scale biogasplant including heating, feeding once a day, stirring andgas collection. Each set of test equipment includes a cylin-drical 35 l digester connected to a 77 l gas collection tank(Jansen et al., 2004; Svard et al., 2002). The digesters werekept at mesophilic temperature, 35 �C. A top-mountedmechanical stirrer ensured a totally mixed tank. Feedingand residue removal was carried out manually once eachday. The hydraulic retention time was chosen to be 10–13days to simulate future full-scale conditions. The sewagesludge feed had a VS-content of �3% (see Table 1), whichresulted in an organic loading rate of �2.4–2.7 kg VS/m3 day. Analyses of produced gas (volume and content)and digested residues (temperature and pH) were carriedout every day.
2.4. Analyses
TS and VS were determined according to APHA (1995)standard methods. Ammonium (NH4) was determinedspectrophotometrically using DR Lange, Lasa 100, LCK303. Methane production was monitored by a gas chro-matograph (Agilent 6850 series equipped with a flame ioni-sation detector (FID) and a 30 m (length)/0.32 mm(diameter)/0.25 lm (film) column) in the batch experi-ments. Gas composition (CH4 and H2S) in the continuousdigestion experiments was analysed by a gas surveyor por-table gas detector, GMI Gas measurement InstrumentsLtd., Scotland, UK.
3. Results and discussion
3.1. Batch laboratory methane potentials
The methane potentials determined in the batch experi-ments are shown in Fig. 1. The methane production fromthe inoculum has been withdrawn, even though this effectwas rather low representing between 3% and 14% of thetotal methane potential measured in each reactor. Averagefinal methane potential values with standard deviation forthe duplicate reactors are shown in Table 3. Standard devi-ation was on average 3%.
The reference substrates, which were added to test thefunction of the inoculum, gave methane potentials (seeFig. 2) close to the theoretical methane potential valuefor fat (1014 Nml CH4/g VS calculated by the Buswell for-mula (Buswell and Neave, 1930)). For oleic acid and stearicacid, 101% respective 99% of the theoretical value wasachieved but the methane production from digestion ofthe oleic acid was started much faster than for the solid
0
100
200
300
400
500
600
700
800Cellulose
Oleic acid
Stearic acid
100% SS
100% SS
10% GS + 90% SS
25% GS + 75% SS
60% GS + 40% SS
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
700
800
0 50 100 150 200
0 50 100 150 200
0 50 100 150 200
time (h)
time (h)
time (h)
Nm
l CH
4/g V
S in
Nm
l CH
4/g V
S in
Nm
l CH
4/g V
S in
10% GS
25% GS
60% GS
100% SS
Fig. 3. Methane potentials during the first 200 h of digestion for (a)reference substrates, (b) mixtures of GS and SS and (c) GS in differentconcentrations. (GS = sludge from grease traps, SS = sewage sludge, NmlCH4 = normal ml of CH4 at 1 �C and 1 atm).
A. Davidsson et al. / Waste Management 28 (2008) 986–992 989
stearic acid. The cellulose gave a methane potential corre-sponding to 81% of the theoretical value, which can beexplained by the slow degradation of this type of substrate.These results show that the inoculum was functioning wellboth with fatty acid and cellulose-containing substrates.
Reactors containing GS as a single substrate, but in dif-ferent concentrations, behaved similarly and all reached atotal methane potential of 845–928 Nml CH4/g VSin. Thisis close to the theoretical value for fat, which shows thatthe GS mainly consists of fat. It also indicates that the con-centrations of GS did not inhibit or overload the digestionprocess. The results are comparable with results fromDeckena and Jannsen (1995), where single substrate diges-tion of similar substrates was tested in one- and two-stagecontinuous digestion.
The results from the reactors with mixtures of GS andSS in different concentrations (10:90, 25:75 and 60:40GS:SS on VS-basis) show that the methane potential isincreasing with increasing amount of GS.
Most of the gas is produced in the beginning. About90% of the total methane produced in reactors with SS,GS or mixtures of GS and SS is produced the first 14 days(336 h) (Fig. 1). The reference substrate reactors are pro-ducing methane during a longer period. However, after600 h (25 days), 92–100% of the methane produced at theend of the experiment had been produced in all reactors(Fig. 2).
The methane potentials for the initiating 200 h of diges-tion are shown to illustrate the gas production rates in thestart-up phase in Fig. 3. The gas production rates in thestart-up phase for oleic acid and stearic acid are slowerthan for 100% sewage sludge (Fig. 3a). This lag-phasecould indicate overload of biodegradable organic matterto the anaerobic bacteria in the start-up phase. However,in Fig. 2, it can be seen that these substrates reached theirtheoretical methane potentials by the end of the test period,which means that the methanogens were able to recover.The reactors with three different mixtures of grease andsewage sludge give about the same methane amount asthe reactor with 100% SS during the first 80 h (Fig. 3b).Thereafter, the methane potentials differ. Higher potentialsare observed in the reactors with higher amounts of GS(Fig. 3b). The reactors with GS as single-substrate in threedifferent concentrations (Fig. 3c) have the fastest start-up.There are differences between the three concentrations dur-ing the first 150 h, which indicates that a lower grease con-centration gives a faster start-up. This can probably beexplained by an initial overload of reactors with the mostconcentrated GS. After 150 h, the methane potentials arein the same range, independent of GS concentration (seeFig. 1).
VS-content measured before and after digestion in allreactors is shown in Fig. 4. After digestion, reactors withreference substrates as well as reactors with GS as single-substrate contain the same amount of VS as the reactorsstarted up with inoculum alone. Since all reactors startedup with the same amount of inoculum, it can be concluded
that the added substrates (reference substrates and GS)were almost entirely degraded. In reactors with mixturesof GS and SS and with SS alone the VS-content after diges-tion is higher, showing that the added sewage sludge wasnot entirely degraded. It can also be seen that the reactorscontaining the highest amounts of sewage sludge have thehighest VS-contents after digestion.
The increase in methane potential, indicated in Fig. 1,where a higher amount of GS seemed to give a highermethane production per added VS, has been examined indetail. In Fig. 5, the enhancement is plotted for differentpercentages of GS. The relation can be fitted with a linearregression giving a R2 value of 0.974. The relation beinglinear shows that there is no tendency for long-term
0
2
4
6
8
10
Cellulo
se
Oleic a
cid
Stearic
acid
Inocu
lum
100%
SS
10% G
S + 90
%SS
25% G
S + 75
%SS
60% G
S + 40
% SS
10%GS
25% G
S
60%GS
VS-c
onte
nt (g
)
Before AD After AD
Inoculum
Fig. 4. VS-content in reactors before and after digestion. VS-content in the reactors with inoculum is shown as broken lines (black – before AD; grey –after AD).
0%
20%
40%
60%
80%
100%
120%
0% 10% 20% 30% 40% 50% 60%GS (% of total VS)
Met
hane
pot
entia
len
hanc
emen
t
Fig. 5. Enhancement of methane potential as a function of amount ofgrease trap sludge in fed VS.
990 A. Davidsson et al. / Waste Management 28 (2008) 986–992
methanogenic inhibition by the sludge from grease traps inthis test.
3.2. Continuous pilot-scale digestion results
Pilot-scale digestion tests with continuous operationwere performed. A pre-experiment with three parallel reac-tors was carried out with 100% SS, 5% GS + 95% SS and10% GS + 90% SS on a VS-basis. This test round lasted
Table 4Pilot-scale digestion results from a pre-experiment of 30 days (round 1) and a
Test round Digested sludge types Methane yieldNm3/ton VSin
CH4 (%)
1 SS 276 7195% SS + 5% GS 295 6990% SS + 10% GS 360 69
2 SS 271 6590% SS + 10% GSc 295 6690% SS + 10% GSc 308 6670% SS + 30% GS 344 69GS –a –a
Methane yield, methane content in the gas, pH, VS degradation, temperature,days within a stable operation period. (SS = sludge from WWTP; GS = greas
a No stable operation was achieved.b VS degradation was not calculated for the pre-experiment since the test pec Two parallel reactors with 90% SS + 10% GS.
only for 30 days with an evaluated period of seven daysat the end; this round should therefore be seen as a pre-experiment. The results, however, indicated that the addi-tion of sludge from grease removal traps (5–10% of thetotal VS) to sludge digestion at a wastewater treatmentplant can be viable and result in a significant increase inmethane production (see Table 4).
Therefore, a second test round was started, includingfive parallel anaerobic digestion reactors that were fedwith 100% SS, 10% GS + 90% SS (in two parallel reac-tors), 30% GS + 70% SS and 100% GS. This round lastedfor 4–6 mo, with an evaluated period of 10 days at theend of the experiment to determine the methane yield.The longer operation period was chosen to be able tosee long-term effects.
Results from the evaluated periods from the longer testrounds are shown in Table 4. All combinations of greasetrap sludge and sewage sludge (in different concentrations)showed higher yields and higher methane contents thansingle-substrate sludge digestion. Single-substrate digestionof sludge from grease traps failed to reach stable operation.After about 10 days of slow start-up with an organic load-ing of �1.7 kg VS/m3 d, the pH dropped and the methane
long-time experiment of 4–6 months (round 2)
pH VS deg. (%) Temp (�C) SRT (days) Org. load.kg VS/m3 d
7.07 –b 35 �10 �37.06 –b 35 �10 �37.06 –b 35 �10 �36.92 45 35 13 2.56.96 54 35 13 2.56.96 55 35 13 2.56.91 58 35 13 2.4–a –a 35 –a 0–2.3
SRT and organic loading rate are given for an evaluated period of 7 or 10e trap sludge; Nm3 CH4 = normal m3 of CH4 at 1 �C and 1 atm).
riod lasted only for 30 days.
A. Davidsson et al. / Waste Management 28 (2008) 986–992 991
production decreased. In spite of repeated additions ofNaHCO3, the process could not be stabilised. This resultcontradicts the results achieved by Deckena and Jannsen(1995), where one-stage digestion of a similar substratecould be achieved at loading rates of 1.67–2.5 kg TS/m3 d. However, the retention time was longer, 30–46 days,in the work by Deckena and Jannsen.
Comparisons of methane yield for digestion of combina-tions of GS and SS with single-substrate digestion of SS areshown in Table 5. The results show that the addition ofgrease trap sludge seems to be a possible way of increasingthe gas production in digesters with WWTP sludge as basesubstrate. The combinations of sludge and GS (10% and30% of the total VS was tested) resulted in raised methaneyield of 9–27%.
A comparison of the measured methane yields andcalculated yields using methane potentials on GS frombatch experiments (890 Nml/gVS) or from theoreticalvalues for fat (1014 Nml/gVS) for different combinationsof GS and SS can be found in Fig. 6. It can be seen thatthe measured yields for 10% GS + 90% SS representaround 90% of the calculated potential from the batchexperiment and 85–89% of the calculated theoreticalpotential. The measured yield for the higher GS concen-tration (30% GS + 70% SS) only represents 75% respec-tive 70% of the calculated potential from the batchexperiment and the calculated theoretical potential. Thisindicates that the increase in methane production byadding grease trap sludge in different concentrations in
Table 5Increase in methane yield for combinations of GS and SS compared to100% SS (from test round 2 in Table 3)
Substrates Increase in methane yield (%)
10% GS + 90% SS 9–1430% GS + 70% SS 27
0
100
200
300
400
500
600
10% GS + 90%SS 10% GS + 90% SS 30% GS + 70%SS
Met
hane
yie
ld/p
oten
tial
(Nm
l CH
4/g V
S in)
Measured pilotCalculated batchCalculated theoret.
Fig. 6. Comparison of measured methane yield from pilot-scale experi-ments and calculated methane potentials using batch experimentalpotentials respective theoretical potentials on grease trap sludge. (NmlCH4 = normal ml of CH4 at 1 �C and 1 atm).
continuous operation is not linear as was seen in thebatch experiments.
4. Conclusions
Anaerobic co-digestion of sludge from grease traps andsewage sludge was successfully performed both in labora-tory batch tests and in continuous pilot-scale digestiontests. Single-substrate digestion of grease trap sludge gavehigh methane potentials in batch tests (845–928 Nml/gVSin), but could not reach stable methane production inthe continuous digestion tests.
Addition of grease trap sludge when digesting sewagesludge increases the methane potential and methane yield(amount of produced methane per added amount of VS).A relation between methane increase and GS-percentagecould be seen in batch laboratory digestion tests. In thepilot-scale tests, the increase in methane yield was 9–27%for GS-amounts corresponding to 10–30% of the total VSadded.
The addition of grease trap sludge to reactors digestingsewage sludge did not contribute much to the sludge pro-duction since it had a high VS-content (98%), which wascompletely degraded in the batch tests.
The reference substrates used in the batch laboratorytests, oleic acid (1018 Nml/g VSin), stearic acid(1013 Nml/g VSin) and cellulose (338 Nml/g VSin), gaveapproximately their theoretical methane potentials. Theywere suitable for testing the ability of the inoculum whendigesting fat and cellulose-containing substrates.
Acknowledgements
Part of this research has been carried out within anØresund cooperation project, Øforsk. It has been financedthrough Øresund Environment Academy with fundingfrom Oforsk, The Committee for Science and Research inThe Øresund Region together with Malmo municipalityand Copenhagen municipality. All involved are greatlyacknowledged.
References
Angelidaki, I., Ahring, B.K., 1992. Effects of free long-chain fatty acids on
thermophilic anaerobic digestion. Applied Microbiology and Biotech-
nology 37, 808–812.
Angelidaki, I., Ahring, B.K., 1997. Codigestion of olive oil mill wastewa-
ters with manure, household waste or sewage sludge. Biodegradation
8, 221–226.
APHA, 1995. Standard Methods for the Examination of Water and
Wastewater, nineteenth ed., American Public Health Association/
American Water Works Association/Water Environment Federation,
Washington DC, USA.
Beccari, M., Bonemazzi, F., Majone, M., Riccardi, C., 1996. Interaction
between acidogenesis and methanogenesis in the anaerobic treatment
of olive oil mill effluents. Water Research 30 (1), 183–189.
Buswell, A.M., Neave, S.L., 1930. Laboratory studies of sludge digestion.
Illinois Division of State Water Survey, Bulletin No. 30.
Coker, C., 2006. Composting grease trap wastes. BioCycle 47 (8), 27–30.
992 A. Davidsson et al. / Waste Management 28 (2008) 986–992
Davidsson, A., Appelqvist, B., Gruvberger, C., Hallmer, M., Jansen, J. La
Cour., 2007. Anaerobic Digestion Potential of Urban Organic Waste:
A Case Study in Malmo. Waste Management and Research 25, 162–
169.
Deckena, S., Jannsen, S., 1995. Anaerobe Behandlung von Fettschlammen
aus Kommunen und der Nahringsmittelindustrie in ein- und zweistuf-
igen Anlagen (Anaerobic Treatment of Grease Sludges from Local
Communities and the Food industry in Single-stage and Two-stage
Treatment Plants). Korrespondenz Abwasser 43 (3), 426–432, in
German.
Gomes Palacios, J.M., Ruiz de Apodaca, A., Rebello, C., Azcarete, J.,
2002. European policy on biodegradable waste: A management
perspective. Water Science and Technology 46 (10), 311–318.
Hansen, T.L., Schmidt, J.E., Angelidaki, I., Marca, E., Jansen, J.la C.,
Mosbæk, H., Christensen, T.H., 2004. Method for determination of
methane potentials of solid organic waste. Waste Management 24,
393–400.
Hwu, C., Donlon, B., Lettinga, G., 1996. Comparative toxicity of long-
chain fatty acid to anaerobic sludges from various origins. Water
Science and Technology 34 (5-6), 351–358.
Hwu, C., Lettinga, G., 1997. Acute toxicity of oleate to acetate-utilizing
methanogens in mesophilic and thermophilic anaerobic sludges.
Enzyme and Microbial Technology 21, 297–301.
Jansen, J.la C., Gruvberger, C., Hanner, N., Aspegren, H., Svard, A.,
2004. Digestion of sludge and organic waste in the sustainability
concept for Malmo, Sweden. Water Science and Technology 49 (10),
163–169.
Quemeneur, M., Marty, Y., 1994. Fatty acids and sterols in domestic
wastewaters. Water Research 28, 1217–1226.
Rinzema, A., Boone, M., van Knippenberg, K., Lettinga, G., 1994.
Bactericidal effect of long chain fatty acids in anaerobic digestion.
Water Environmental Research 66, 40–49.
Stoll, U., Gupta, H., 1997. Management strategies for Oil and Grease
residues. Waste Management and Research 15, 23–32.
Svard, A., Gruvberger, C., Aspegren, H., Jansen, J. la C., Christen-
sen, T.H., 2002. Pilot-scale digestion of source-sorted household
waste as a tool for evaluation of different pre-sorting and pre-
treatment strategies. In: Third International Symposium on
Anaerobic Digestion of Solid Wastes, 18–20 September 2002,
Munich, Germany.