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Hydrolysis and acidogenesis of particulateorganic material in mesophilic andthermophilic anaerobic digestionM. Kim a , C Y. Gomec b , Y. Ahn c & R. E. Speece da Dept. of Civil & Environmental Engineering , Hanyang University , 1271Sa‐1 dong, Ansan, Kyunggi‐do, 425–791, Koreab Dept. of Environmental Engineering , Istanbul Technical University ,Maslak, Istanbul, 80626, Turkeyc Dept. of Civil Engineering , Yeungnam University , Kyungsan, 712–945,Koread Dept. of Civil & Environmental Engineering , Vanderbilt University , P.O.Box 1831, Station B, Nashville, TN, 37235, USAPublished online: 17 Dec 2008.

To cite this article: M. Kim , C Y. Gomec , Y. Ahn & R. E. Speece (2003) Hydrolysis and acidogenesis ofparticulate organic material in mesophilic and thermophilic anaerobic digestion, Environmental Technology,24:9, 1183-1190, DOI: 10.1080/09593330309385659

To link to this article: http://dx.doi.org/10.1080/09593330309385659

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Environmental Technology, Vol. 11. pp 1183-1190©Selper Ltd, 2003

HYDROLYSIS AND ACIDOGENESIS OF PARTICULATEORGANIC MATERIAL IN MESOPHILIC ANDTHERMOPHILIC ANAEROBIC DIGESTION

M. KIM*1, C Y. GOMEC2, Y. AHN3 AND R. E. SPEECE4

1Dept. of Civil & Environmental Engineering, Hanyang University, 1271 Sa-1 dong, Ansan, Kyunggi-do, 425-791, Korea2Dept. of Environmental Engineering, Istanbul Technical University, 80626, Maslak, Istanbul, Turkey

3Dept. of Civil Engineering, Yeungnam University, Kyungsan, 712-945, Korea4Dept. of Civil & Environmental Engineering, Vanderbilt University, P.O. Box 1831, Station B, Nashville, TN 37235, USA

(Received 24 February 2003; Accepted 11 June 2003)

ABSTRACT

The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for anaerobic hydrolysisand addogenesis of paniculate organic materials at both mesophilic (35 °C) and thermophilic (55 °C) temperatures.Hydrolysis and addogenesis of a synthetic sludge was observed in batch operation for the evaluation of the pH effect. pHwas uncontrolled in one reactor and controlled at 4.5, 5.5, and 6.5 in the other three reactors at both temperatures. Thegreatest degree of hydrolysis and addogenesis occurred when the pH was controlled at 6.5. The pH of the uncontrolledreactor dropped to 3.4 at both temperatures severely retarding hydrolysis and addogenesis. Concentrations of acetic and n-butyric adds predominated with lower concentrations of propionic add at both temperatures in all reactors. Lactic add wasproduced as the earliest intermediate but as the reaction proceeded, short chain VF As were produced as final end productswith a decrease in lactic add. The higher the pH, the earlier this trend was observed. For the controlled reactors at pH 6.5,the soluble COD production and the VSS reduction peaked in 4 days at 55 °C whereas it took about 11 days at 35 °C toobtain the same result. During the linear SCOD production period at a pH of 6.5 the hydrolysis rate of the thermophilicreactor was greater than that for mesophilic. Thermophilic conditions appeared to be more sensitive to pH than mesophilicones for both hydrolysis and addogenesis. Additional experiments were conducted to establish the effect of inorganicnutrient (Ca, Fe, Co, and Ni) supplementation on hydrolysis and addogenesis at both temperatures. It has, prior to this, beenassumed that only methanogenesis benefited from trace metal supplementation. However, the results demonstrated theimportance of inorganic nutrient supplementation to optimize hydrolysis and addogenesis at both temperatures.

Keywords: Addogenesis, hydrolysis, inorganic nutrient supplementations, mesophilic, pH, thermophilic

INTRODUCTION during the production of organic acids. Waste stabilizationoccurs during the methanogenic phase by conversion of the

The primary objective of anaerobic digestion of acetic acid and hydrogen into methane, which is essentiallywastewater sludges is to stabilize organic matter with a insoluble in water and readily separates from the sludge inconcurrent reduction in odors, pathogen concentration, and the gas which leaves the system [1,3].the volume of solid organic material still requiring further In order to increase the stability of anaerobic digestionprocessing as well as to produce a corresponding amount of the two-phase anaerobic system has been introduced andbiogas [1,2]. In general bacteria are unable to take up investigated [4-6]. The physical separation ofparticulate organic material because a breakdown into soluble hydrolysis/addogenesis and methanogenesis steps increasespolymers or monomers is required first [3]. Hydrolysis of process stability because methane reactor overload can beinsoluble organics is necessary to convert these materials to a prevented by proper control of the first step [5]. Advantagessize and form that can pass through bacterial cell walls for use of phase separation include increased stability with betteras energy or nutrient sources. Once complex organics are control of the add phase, higher organic loading rates,hydrolyzed, they can be fermented into long-chain organic increased specific activity of methanogens leading to anacids, sugars, and amino adds, and eventually into smaller increase in methane production rates, removal of compoundsorganic acids such as propionic-, butyric-, and valeric-add. toxic to methane bacteria, provision of a constant substrate forAcetic add, hydrogen, and carbon dioxide are also formed the methanogens. However, possible disadvantages of phase

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separation may also include hydrogen build-up in the first-phase reactor during the acid formation to levels inhibitory toacid-producing bacteria and elimination of possibleinterdependent nutritional requirements of acid and methaneformers 15,6].

Another noteworthy characteristic of phase separationis that sufficient levels of volatile fatty acids (VFAs) may beproduced to make possible biological nutrient removal (BNR)because municipal raw sewage is a potential feedstock ofreadily biodegradable carbon [5-7]. Domestic wastewateroften contains low concentrations of the readily biodegradablechemical oxygen demand (COD) which is needed forextensive BNR so the use of dissolved fermentation productsfrom anaerobic digestion results in comparable rates withother substrates such as methanol for BNR [8]. The VF As passreadily through the cytoplasmic membrane of heterotrophicorganisms present in the sewage flora of the BNR process,enabling metabolism internally as a carbon or energy source[7]. Therefore solubilized products from anaerobic digestionhave been assayed for use as the carbon source of BNR in awastewater treatment process itself.

The thermophilic regimes (>50 °C) have been adoptedfor anaerobic digestion providing several advantages, whencompared with mesophilic regimes (30 - 40 °C), such asincreased destruction rate of organic solids, improved solids-liquid separation, and increased destruction of pathogenicorganisms [9-13]. Class A biosolids, as termed by EPA, areessentially pathogen free and have no restrictions on cropchoice or human access to land application sites used fordisposal [13]. At relatively short sludge retention time (SRT)mesophilic anaerobic digestion cannot meet the criteria ofClass A biosolids because of insufficiently high temperaturebut thermophilic anaerobic digestion is one of the few provenmethods for reducing the pathogen content of domesticsludges to the point of ensuring disposal without restriction tofuture human contact.

New and improved facilities being built in the U.S. atthis time and many pilot and demonstration studies beingconducted reflect a U.S. trend toward upgrading theperformance of anaerobic digestion systems. The goal of mostof these projects is to meet the US EPA's 40 CFR 503regulation for reducing pathogens to produce Class Abiosolids [14]. The trend of pilot and full scale experimentsconducted around the world to improve anaerobic digestionwas summarized 115]. These new studies include stagedmesophilic, temperature-phased, acid-gas phased, and stagedthermophilic digestion.

However data regarding the effect of pH and nutrientsupplementation upon hydrolysis and addogenesis are rare.Better understanding of these two important steps may leadto the optimization of anaerobic digestion [5,8,16,17].

The purpose of this study was to evaluate the effect ofpH and inorganic nutrient supplementation on hydrolysisand addogenesis of suspended organic materials in terms ofvolatile suspended solid (VSS) solubilization, specific addproduction, and soluble COD production at both mesophilic

(35 °C) and thermophilic (55 °C) temperatures.

MATERIALS AND METHODS

Experiments in this research were divided into twocategories to observe the effect of pH and nutrientsupplementation upon mesophilic and thermophilicprocessing. Experiment 1 utilized a 4 % and Experiment 2 a6 % total solids feed. The same seed source and temperatureswere used for both categories of experiments.

Substrates, Seed Biomass, and Temperatures

Dog food (ALPO®, Purina Co.) was selected for thesimulated primary sludge because it had a similar organiccomposition and a similar volatile suspended solids (VSS) tovolatile solids (VS) ratio to primary sludge [18,19]. The dogfood contained carbohydrate, protein, and fat contents of 45%,21 %, and 8 %, respectively. After blending with tap water toprepare the feed slurry, VS content comprised 90 % of totalsolids (TS) and VSS content comprised 84 % of VS. The feedalso contained all the macro and micronutrients, which wereC, N, P, Ca, K, Fe, Zn, Vitamin (A, D, E, Bl, B2), Niacin, andfolic acid, necessary for animal growth. Anaerobic,mesophilic, homogenized granules from an upflow anaerobicsequencing batch reactor (UASB) with solid retention time(SRT) over 50 days were used for the seed source andtemperatures of 55 °C and 35 °C were selected as thethermophilic and mesophilic temperatures, respectivelybecause these temperatures have been used the most widelyand 50 °C or higher temperatures are necessary to produceClass A biosolids.

Apparatus

Experiment 1

The experimental apparatus consisted of eightcompletely mixed anaerobic reactors at both temperatures. Atthe beginning of the experiment, the pH of three reactors wasfixed at 6.5, 5.5, and 4.5 respectively by HC1 subsequentlyautomatically controlled by a pH controller. The pH of thecontrol reactor was not artifidally manipulated. DilutedNaOH was automatically pumped into the pH-controlledreactors whenever the pH dropped below the designated pH.The pH in all reactors was measured daily. Reactors with twoliters volume were used with 1.8 1 of operating volumeselected in which 1.771 of sludges were seeded with 0.031 (1.9g V1 based on the reactor operating volume) of thehomogenized UASB granules at the start of the operation.

Experiment 2

Four upflow type plug flow reactors, being elutriatedby tap water, which contained 2 g I'1 of NaHCOj, were used atboth temperatures. The substrate was fed at the start ofoperation. Two of the four reactors were supplemented once aday by a cocktail of inorganic nutrients consisting

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, CoClj, and NiClz, at concentrations of 500, 10, 1, and 1mg I'1 of reactor volume respectively while the other tworeactors received no inorganic nutrients. Reactors with twoliters volume were used with 11 of operating volume selectedin which 0.98 1 of sludges were seeded with 0.02 1 (2.3 g 1'1

based on the reactor operating volume) of the homogenizedUASB granules at the start of operation. The reactors wereelutriated at the rate of one liter of tap water per day. Thesolubilized compounds were continuously elutriated by tapwater with the elutriate collected daily for analysis.

Analytical Methods

The pH in all reactors was measured daily using a pHmeter (Fisher Co. digital pH meter 107). Volatile fatty acids(VFAs) in all samples were measured using a gasChromatograph (Shimadzu Model GC-14A) equipped with aflame ionization detector and a 1.7 m glass column packedwith 0.3 % GP Carbopack C/0.3% Carbowax 20M/0.1%H3PO4 (Supelco, Inc.). The column temperature was 140 °Cand the injector/detector temperature was 200 °C. Nitrogenwas the carrier gas with a pressure of 1.75 kg cm'2. Dataintegration was accomplished using a Varian 4270 Integrator.Samples were prepared by centrifuging (BeckmanInstruments, Model GP) at 4000 rpm for 30 minutes andfiltering approximately 3 ml of the supernatant through 0.45micron filter paper (Micron Separations Inc.). The filteredsamples were then acidified with 10 % H3PO4 (volumetricbasis) to lower the pH below 3 and ensure that acids wereunionized and able to volatilize. The soluble COD (SCOD)was analyzed using the closed-reflux, spectrophotometermethod as described in Standard Methods [20] after filtering

the same as with VFAs. Total suspended solids werecentrifuged at a speed of 4000 rpm for 30 minutes and thepellet was washed into a dish using deionized water. Thetotal suspended solids (TSS) were dried at 103 - 105 °Caccording to section 2540 D and the volatile suspended solids(VSS) were burned at 500 °C according to section 2540 E inStandard Methods [20].

RESULTS AND DISCUSSION

The Effect of pH at Both Temperatures

Three controlled-pH reactors with pH of 4.5, 5.5, and6.5 and one uncontrolled-pH reactor were operated for 20days using both mesophilic and thermophilic temperatures.The pH of the uncontrolled reactor dropped quickly to 3.4 atboth temperatures. As expected more NaOH was consumedto maintain the higher designated pH but compared withmesophilic reactors, thermophilic reactors consumed lessNaOH. For each respective pH set point, the reducedsolubility of CO2 at thermophilic temperatures, which isapproximately only half as much as at mesophilictemperature, probably required less alkalinity necessary forneutralizing the dissolved H2CO3 [21]. The result in thisexperiment supports the theory and implies an economicaladvantage with respect to alkalinity necessary forthermophilic temperature.

Figure 1 shows the variation of COD in the reactor withpH controlled at 6.5 in both temperatures because at this pHboth reactors showed the greatest solubilization in termsof VSS removal and SCOD production. For the reactors withpH controlled at 6.5, the soluble COD production and the VSS

B

0%i 1 4 11

Run days

D COD of 3 VFAsB SCODproduced except 3 VFAs• COD of VSSunsolubilized

1 4 11Run days

Ö COD of 3 VFAs0 SCODproduced except 3 VFAs• C O D of VSSunsolubilized

Figure 1. The variation of COD in the reactor with pH controlled at 6.5 for A) mesophilic and B) thermophilic.

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reduction peaked in 4 days at 55 °C while it took about 11days to peak at 35 °C. Solubilization of VSS at thermophilictemperature occurred slightly faster until day 4 comparedwith mesophilic manifested by COD of VSSunsolubiIize<j. Afterday 4, the thermophilic reactor stopped solubilization of VSSbut the mesophilic reactor continued until day 11. On theother hand, the mesophilic reactor produced over 60 % ofacetate, propionate, and n-butyrate of all COD in the reactorby day 4 while the thermophilic reactor produced only 45 %until day 20 as manifested by the COD of the three VFAs.These results imply that hydrolysis could be stimulated andmay not need as long running time at the higher temperaturebut acidogenesis could be retarded at the higher temperature.

The solubilization reaction is closely modeled as a firstorder reaction and the rate of reaction based on the SCOD wasinvestigated for 11 days of operation at mesophilic and 4 daysat thermophilic temperatures. As seen in Figure 2, at theoptimum pH of 6.5, the solubilization rate in the thermophilicreactor was greater than that in the mesophilic. However, theother rates in thermophilic reactors were lower than those inmesophilic reactors. In both temperatures, the rates decreasedas the pH decreased, but the thermophilic condition appearedto be more sensitive than the mesophilic to pH. Manyresearchers have reported that thermophilic bacteria have ahigher activity than mesophilic ones [22-25]. However, Figure2 shows that the thermophilic reactors did not show higheractivity than the mesophilic ones except in the reactors at apH of 6.5 implying the importance of optimum pH inthermophilic hydrolysis and acidogenesis.

Figure 3 shows the change of specific organic acidconcentrations in all reactors. Acetic and n-butyric acidspredominated with lesser amounts of propionic acid in thefinal products in all reactors. Lactic acid was produced as theearliest intermediate with short chain VFAs (SCVFAs)subsequently produced as final end products with acorresponding decrease in lactic acid. It was observed that thehigher the pH, the earlier this trend occurred. It was reportedthat the highest degree of beef extract acidification was foundat a pH of 7 and the acetic acid portion of the total VFAsproduced was decreased with decreasing pH [5]. The resultsin this study agree with those data.

The Effect of Inorganic Nutrient Supplementation at BothTemperatures

For the evaluation of the effect of inorganic nutrientssupplementation for hydrolysis and acidogenesis, anotherbatch experiment with and without nutrient supplementationwas conducted for 12 days using both temperatures.Inorganic nutrient supplementation did not affect the pHsignificantly. The pH in all four reactors dropped rapidly toabout 4 in the 1st day of operation and then recovered slowlyup to about 6.5. The thermophilic reactors showed a slightlyhigher pH than that of the mesophilic ones (data not shown).At the low pH range after day 3, lactic acid was produced andthen small amounts of VFAs were produced slowly in all fourreactors. Lactic acid should be further converted to SCVFAsduring acidogenesis, but since the elutriation rate in this

B

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y = 2500Ln(x) + 19000Ra = 0.995(uncontro!led)

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y = 2000Ln(x) + 16000R2 = 0.924(pH=4.5)

y=1900Ln(x) + 16000R2 = 0.999(uncontrolled)

0 0.1 1 10 100Run days

• uncontrolled BpH=6.5 ApH=5.5 «pH=4.5

Figure 2. Rates of solubilization for A) mesophilic for 11 days of operation and B) thermophilic for 4 days of operation.

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Figure 3. Organic acids change at various pH.

experiment was 1 day'1 (11 tap water per 1 day per 11 reactorvolume) and the produced lactic acid was wasted once a day,this interval was inadequate to further convert the producedlactic acid. Therefore in order to obtain acetate, propionate,and n-butyrate from the elutriation reactor, the elutriation rate

should probably be decreased. Further research is required toclarify this principle.

Figure 4 shows SCOD production for 12 daysof elutriation with and without supplementation of a cocktailof Ca, Fe, Co, and Ni for both temperatures. During the first

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30

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Figure 4. SCOD production for 12 days of elutriation with and without nutrients supplementation in mesophilic andthermophilic temperatures.

2 days, the thermophilic reactors produced more SCOD thanat mesophilic. After that, the mesophilic reactors producedmore total SCOD than the thermophilic. The reactorssupplemented by Ca, Fe, Co, and Ni produced more SCOD atboth temperatures. It has, prior to this, been assumed thatonly methanogenesis benefited from trace metal

supplementation. However, this result implies the importanceof inorganic nutrients supplementation for solubilization.

Figure 5 shows the various percentage of COD for 12days elutriation. After 12 days of elutriation, the percentagesof unsolubilized VSS as COD in the reactors with nutrientswere about 25 % and 20 % for mesophilic and thermophilic,

100%

80%

60%

40%

20%

mesowith mesow/o thermowith thermow/onutrients nutrients nutrients nutrients

Elutriation type

• C O D of VSSunsolubilized BSCODproduced except 8 Acids ECOD of 8 Acids

Figure 5. The various percentage of COD for 12 days elutriation.

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respectively, while they were about 50 % for both mesophilicand thermophilic reactors without nutrients. This implies thatsupplementation of these inorganic nutrients significantlystimulated hydrolysis of particulate organic compounds.Measured VFA compounds and organic acids were acetate,propionate, iso-butyrate, n-butyrate, 2-methylbutyrate, iso-valeric, n-valeric, and lactate. The percentages of 8 organicacids of the total COD in the reactors supplemented withnutrients were about 50 % and 25 % for mesophilic andthermophilic, respectively, while they were about 30 % and 10% for mesophilic and thermophilic reactors without nutrients,respectively. This implies that supplementation of theseinorganic nutrients also significantly stimulated acidogenesis.

Since Figure 4 shows that the cumulative SCODproduction between mesophilic and thermophilic reactorswas reversed after 2 days, the rates of hydrolysis andacidogenesis were investigated for both the initial 2 days andremaining 10 days of operation as seen in Figure 6. Thehydrolysis rate was calculated by g COD of VSS solubilized/gCOD of VSS in feed /day. The acidogenesis rate wascalculated by the g COD of all measured organic acids/gSCOD total/day. Both hydrolysis and acidogenesis rates werecalculated for the period of the first 2 days and then for theperiod of the next 10 days. Overall, the rates of bothhydrolysis and acidogenesis for the initial 2 days were muchhigher than those for the remaining 10 days. For the initial 2days of operation, thermophilic reactors showed the higherhydrolysis rate but lower rate of acidogenesis than themesophilic reactors. Apparently hydrolysis was a rate-limiting step in the mesophilic reactors for 12 days ofoperation while thermophilic reactors showed a similar ratebetween hydrolysis and acidogenesis for the initial 2 days butacidogenesis was a limiting step for the remaining 10 days.Even though the rates of hydrolysis and acidogenesis for the

remaining 10 days were much lower than those of the initial 2days, nutrients supplementation still stimulated the ratesexcept for mesophilic acidogenesis. In the case of mesophilicaddogenesis, the lower rate of the reactor with nutrients wasprobably due to a small amount of remaining organiccompounds because the reactor already produced greateramounts of adds for the initial 2 days. Therefore, the resultsshowed that the supplementation by Ca, Fe, Co, and Nistimulated both hydrolysis and acidogenesis at bothtemperatures implying the importance of inorganic nutrientsupplementations for both hydrolysis and addogenesis.

Few studies have been done for the role of inorganicnutrient supplementation on hydrolysis and addogenesis.However, even though the dog food had adequate nutrientsfor animal growth, the results strongly imply the importanceof inorganic nutrient bioavailability for the microbialhydrolysis and addogenesis.

CONCLUSIONS

The overall conclusions from the results presented inthis study can be summarized as:

Experiment 1

• The greatest degree of solubilization was observed atthe pH of 6.5 at both temperatures in terms of VSSremoval and SCOD production. At this pH, thesolubilization rate of the thermophilic reactor wasgreater than for the mesophilic for 4 days of operation,while thermophilic conditions were more sensitive topH than mesophilic for both hydrolysis andaddogenesis implying the importance of optimum pHin thermophilic solubilization.

O Hydrolysis • Acidogenesis

mesowith mesow/o them» with thermow/onutrients nutrients nutrients nutrients

0.30

0.25

_-> 0.20

2- 0.15

01 0.10-

0.05-

0.00

B

n Hydrolysis • Acidogenesis

mesowith mesow/o them»with thermow/onutrients nutrients nutrients nutrients

Elutriation type Elutriation type

Figure 6. The rate of hydrolysis and addogenesis for A) initial 2 days and B) remaining 10 days of elutriation.

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• Acetic and n-butyric acids were primarily formed with Experiment 2lesser amounts of propionic acid at both temperatures. • The reactors supplemented with inorganic nutrientsLactic acid was produced the earliest as an intermediate showed more acids production and higher rates ofand as the reaction progressed, short chain VFAs were hydrolysis at both temperatures in terms of SCODproduced as final end products with a corresponding production, VSS removal, and organic acids productiondecrease in lactic acid. The higher the pH, the earlier implying the importance of inorganic nutrientthis trend happened. supplementations for hydrolysis and acidogenesis.

REFERECES

1. Parkin, G. and Owen, W. F., Fundamentals of anaerobic digestion of wastewater sludges, J. Environ. Eng., 112, 867-920(1986).

2. Dohanyos, M., Zabranska, J. and Jenicek, P., Enhancement of sludge anaerobic digestion by using of a special thickeningcentrifuge. Water Sci. Technol., 36, 145-153 (1997).

3. Gujer, W. and Zehnder, A. J. B., Conversion process in anaerobic digestion. Water Sci. Technol., 15, 127-167 (1983).4. Ghosh, S., Conrad, R., and Mass, D. L., Anaerobic acidogenesis of wastewater sludge J. Water Pollut. Control Fed., 54, 30-45

(1975).5. Dinopoulou, G., Rudd, T., and Lester, J. N., Anaerobic acidogenesis of complex wastewater: I. The influence of operational

parameters on reactor performance. Biotechnol. Bioeng., 31, 958-968 (1988).6. Bhattacharya, S. K., Madura R. L., Walling D. A., and Farrell, J. B., Volatile solids reduction in two-phase and conventional

anaerobic sludge digestion. Water Res., 30, 1041-1048 (1996).7. Banister, S. S. and Pretorius W. A., Optimization of primary sludge acidogenic fermentation for biological nutrient

removal. Water SA, 24, 35-41 (1998).8. Moser-Engeler, R., Kuhni, M., Bernhard, C, and Siegrist, H., Fermentation of raw sludge on an industrial scale and

applications for elutriating its dissolved products and non-sedimentable solids. Water Res., 33, 3503-3511 (1999).9. Kim, M. and Speece, R. E., Reactor configuration-Part II Comparative process stability and efficiency of thermophilic

anaerobic digestion, Environ. Technol., 23, 643-654 (2002).10. Buhr, H. O. and Andrews, J. F., Review paper, the thermophilic anaerobic digestion process. Water Res., 11, 129-143 (1977).11. Rimkus, R. R., Ryan, J. M., and Cook, E. J., Full-scale thermophilic digestion at the West-Southwest Sewage Treatment

Works, Chicago, Illinois. J. Water Pollut. Control Fed., 54, 1447-1457 (1982).12. Krugel, S., Nemeth, L., and Peddle, C, Extended Thermophilic Anaerobic Digestion for Producing Class A Biosolids at the

Greater Vancouver Regional Distreict"s Annacis Island Wasterwater Treatment Plant. Water Sci. Technol., 38, 409-416(1998).

13. US EPA, Environmental Regulations and Technology: Control of Pathogens and Vector Attraction in Sewage SludgeUnder 40 CFR Part 503, Environmental Protection Agency, Washington. D.C. USA. (1999).

14. Shimp, G. F., Stukenberg, J. R., and Sandino, J., The future of solids treatment? Water Environ. Technol., 12, 35-39 (2000).15. Schafer, P. L. and Farrell, J. B., Turn up the heat. Water Environ. Technol, 12, 27-32 (2000).16. Eastman, J. A. and Ferguson, J. F., Solubilization of particulate organic carbon during the acid phase of anaerobic

digestion. J. Water Pollut. Control Fed., 53, 352-366 (1981).17. Suzuki, H., Yoneyama, Y., and Tanaka, T., Acidification during anaerobic treatment of brewery wastewater. Water Sci.

Technol., 35, 265-274 (1997).18. Elefsiniotis, P. and Oldham, W. K., Acidogenesis of primary sludge: The role of solids retention time. Biotechnol Bioeng.,

44, 7-13 (1994).19. Miron, Y., Zeeman, G., van Lier, J., and Lettinga G., The role of sludge retention time in the hydrolysis and acidification of

Iipids, carbohydrates, and proteins during digestion of primary sludge in CSTR systems. Water Res., 34, 1705-1713 (2000).20. APHA, Standard Methods for the Analysis of Water and Wastewater, 18th Edition, American Public Health Association,

Washington, USA. (1992).21. Speece, R. E., Anaerobic Biotechnology for Industrial Wastewaters. Archae Press, Nashville, TN, USA (1996).22. Zinder, S. H., Anguish, T., and Cardwell, S. C, Effects of temperature on methanogenesis in a thermophilic (58°C)

anaerobic digester. Appl. Environ. Microbiol., 47, 808-813 (1984).23. Wiegant, W. M., Hennink, M., and Lettinga, G., Separation of the propionate degradation to improve the efficiency of

thermophilic anaerobic treatment of acidified wastewater. Water Res., 20, 517-524 (1986).24. Harris, W. L. and Dague, R. R., Comparative performance of anaerobic filters at mesophilic and thermophilic

temperatures.Wafer Environ. Res., 65, 764-771 (1993).25. Kim, M., Ahn, Y. H., and Speece, R. E., Comparative process stability and efficiency of anaerobic digestion; mesophilic vs.

thermophilic. Water Res., 36, 4369-4385 (2002).

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