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JOURNAL OF BIOSCIENCE AND BIOENGINEERING © 2005, The Society for Biotechnology, Japan

Vol. 100, No. 2, 164–167. 2005

DOI: 10.1263/jbb.100.164

Upgrading of Anaerobic Digestion by IncorporatingTwo Different Hydrolysis Processes

Chulhwan Park,1,2* Chunyeon Lee,3 Sangyong Kim,2 Yu Chen,1 and Howard A. Chase1

Cambridge Unit for Bioscience Engineering, Department of Chemical Engineering, University of Cambridge,Pembroke Street, Cambridge CB2 3RA, UK,1 Industrial Ecology National Research Laboratory, Korea

Institute of Industrial Technology, ChonAn 330-825, Korea,2 and Department of Chemistry,Cleveland State University, 2399 Euclid Avenue, Cleveland, OH 44115, USA3

Received 18 February 2004/Accepted 19 March 2005

The purpose of this study was to increase the efficiency of anaerobic digestion of waste acti-vated sludge (WAS). Either thermochemical or biological hydrolysis was used as a pretreatmentand the effects of both were investigated and compared. Two different three-stage digestion sys-tems showed improved performance, although thermochemical hydrolysis showed better resultsthan biological hydrolysis in a bench-scale operation. After anaerobic digestion with thermo-chemical pretreatment, the total chemical oxygen demand (tCOD) reduction, volatile solid (VS)reduction, methane yield and methane biogas content were 88.9%, 77.5%, 0.52 m3/kg VS and79.5%, respectively. These results should help in determining the best hydrolysis pretreatmentprocess for anaerobic digestion and in improving the design and operation of the large-scale treat-ment of WAS by anaerobic digestion with hydrolysis systems.

[Key words: anaerobic digestion, hydrolysis, activated sludge, methane]

The main by-product of biological wastewater treatmentis waste activated sludge (WAS). The amount of WAS hasbeen increasing worldwide as a result of an increase in theamount of wastewater being treated. WAS produced withinthe process must be disposed of and this may account for60% of total plant operating costs (1). There is, therefore,considerable impetus to develop strategies for efficientlyreusing the WAS produced. Anaerobic digestion has beenemployed for activated sludge stabilization, resulting in areduction in the amount of sludge volatile solids (VS) withconcomitant biogas production. The anaerobic digestionprocess generally consists of four stages: hydrolysis, acido-genesis, acetogenesis and methanogenesis. These processesroutinely have some disadvantages such as long retentiontimes, low removal efficiencies of organics, and they maybe unstable. Biological hydrolysis is identified as the rate-limiting step in anaerobic digestion (2–4). To reduce theimpact of this rate-limiting step, pretreatment systems forWAS, such as thermal, alkaline, ultrasonic and mechanicaldisintegration systems, are required (2–12). These systemscan accelerate the solubilization of WAS and reduce theaverage particle size within the sludge, which subsequentlyimproves anaerobic digestion (2, 5). Our previous study con-ducted in batch experiments also emphasized the impor-tance of pretreatments and investigated the effects of vari-ous WAS pretreatments (thermal, chemical, ultrasonic andthermochemical pretreatments) on parameters such as chem-

ical oxygen demand (COD) solubilization, particle size re-duction and methane production enhancement (13).

As mentioned above, many studies have investigated pre-treatment by hydrolysis processes to achieve enhanced an-aerobic digestion and mostly involved a comparison of theresults of a system with a single pretreatment and those of asystem without a pretreatment. However, few reports havebeen published on the comparison of two different three-stage digestion systems. Accordingly, the objective of thisstudy was to demonstrate that enhanced anaerobic digestioncould be achieved by adopting two different hydrolysis pre-treatments and to investigate, in a continuous operation ofthe resultant three-stage system, the increase in particle sizereduction efficiency, the increase in soluble protein concen-tration, the increase in COD solubilization and the enhance-ment of methane production.

MATERIALS AND METHODS

Three-stage digestion A three-stage digestion system formethane production was operated continuously. A schematic dia-gram of the three-stage digestion is shown in Fig. 1. In the firststage, either thermochemical or biological hydrolysis was adoptedas the hydrolysis process. For pretreatment by thermochemical hy-drolysis, in the first stage, WAS was thermally treated at 121°C for30 min. NaOH at 7 g per liter of WAS was added for chemical hy-drolysis. The optimal amount of NaOH per liter of WAS was deter-mined in our previous study (13). WAS treated thermochemicallywas cooled in a storage tank for about 2 h, the pH of the sludgewas adjusted to pH 6.7 by HCl and then it was transferred to thesecond stage. During pretreatment by biological hydrolysis, in thefirst stage, WAS was stirred in a 5-l reactor at 150 rpm and hydro-

* Corresponding author.e-mail: chpark@kitech.re.kr; biobank@hanmail.netphone: +82-41-589-8426 fax: +82-41-589-8580

PROCESS OPTIMIZATION FOR ZERO EMISSIONVOL. 100, 2005 165

lyzed by aerobic bacteria (Cellulomonas uda KCCM 12156 and C.biazotea KCCM 40760) at 30°C. In the second stage, Clostridiumbutyricum KCCM 35433 was used in an acidogenic process forvolatile fatty acid (VFA) production in a 10-l stirred reactor oper-ated at 100 rpm and 37 °C (14). In the third stage, rumen micro-organisms for methane production were obtained from cattle dung(13, 15) and used in a 20-l stirred reactor operated at 50 rpm and41°C. The retention times in each stage were 3, 6 and 12 d, respec-tively.

Analysis Standard methods were used for the estimation oftotal chemical oxygen demand (tCOD), soluble COD (SCOD), VSand pH (16). After the digestion of the samples in the COD reactor(model 45600; HACH, Loveland, CO, USA), the level of CODin a sample was measured by a colorimetric method. SCOD wasmeasured after the centrifugation of the sample at 10,000 rpm for10 min, whereas tCOD was measured without prior centrifugation.The degree of COD solubilization was calculated by the followingequation:

COD solubilization (%)= ×100 (%)

The amount of soluble protein was determined by the Bradfordmethod using bovine serum albumin (BSA) as the standard (17).The particle size of sludge samples was measured using a laserparticle size analyzer (MAF5001; Malvern, Worcestershire, UK).The biogas composition and VFAs were analyzed using a gas chro-matograph (6890N; Agilent, Palo Alto, CA, USA). The tCOD,SCOD, VS, soluble protein and pH of WAS obtained from a sew-age sludge treatment facility were measured to be 27,700 mg/l,2250 mg/l, 26 g/l, 30 mg/l and 6.7, respectively.

RESULTS AND DISCUSSION

In this work, two different hydrolysis processes for in-creasing the efficiency of anaerobic digestion (thermochem-ical or biological pretreatment) were compared using bench-scale experiments.

The effects of thermochemical and biological hydrolysison particle size distribution, soluble protein and COD solu-bilization under various conditions were compared, and theresults are shown in Fig. 2. The control sample comprisednonpretreated WAS. In the case of thermochemical hydroly-sis, the 10%, 50% and 90% accumulated values indicatethat 10%, 50% and 90% of particles were of sizes below 2,29 and 144 µm, respectively. In the case of biological hy-drolysis, 10%, 50% and 90% of the particles were of sizesbelow 15, 60 and 230 µm, respectively. After thermochemi-cal and biological hydrolysis, the soluble protein concentra-tion increased from 30 to 1983 mg/l and from 30 to 625

mg/l, respectively. These two different hydrolysis methodsdecreased the particle size and increased the level of solubleprotein. It is expected that pretreatment should be able toimprove the performance of anaerobic digestion substantial-ly. In addition, the control sample showed 8.1% COD solu-bilization (SCOD=2250 mg/l). On the other hand, 88%COD solubilization (SCOD=24,380 mg/l) and 24% CODsolubilization (SCOD=6620 mg/l) were achieved after ther-mochemical and biological hydrolysis, respectively. The tur-bidity of the supernatant obtained following settling also in-creased after hydrolysis, showing that the levels of WAScolloidal and dissolved solids increased after hydrolysis, re-mained in the supernatant, and were not removed by set-tling. As the levels of soluble protein and COD solubiliza-tion increased, the efficiency of anaerobic digestion wasalso expected to be improved.

To investigate the effects of pretreatment on the majorparameters of anaerobic digestion, namely, VFA reduction,tCOD reduction, SCOD reduction, VS reduction, methaneyield and methane content, experiments were performed us-ing a three-stage digestion process. In a comparison of theconcentrations of VFA and SCOD in the control and the twosystems incorporating pretreatment, the results showed thatboth pretreatment systems resulted in higher values than thecontrol (Fig. 3). In the case of the control sample, the total

FIG. 1. Flow diagram of the three-stage digestion system for treating waste activated sludge.

SCOD

tCOD-------------------

FIG. 2. Comparison of soluble protein, COD solubilization (A)and particle size distribution (B) after thermochemical or biologicalhydrolysis.

PARK ET AL. J. BIOSCI. BIOENG.,166

VFA concentration was 212 mg/l and comprised 133 mg/lacetic acid, 65 mg/l butyric acid and 14 mg/l propionic acid.For the system after thermochemical pretreatment, the VFAconcentrations in the pretreatment and acidogenic stageswere 1145 and 3053 mg/l, respectively. In the acidogenicprocess, the major components of VFA were 1572 mg/lacetic acid, 1051 mg/l butyric acid and 430 mg/l propionicacid. In the case of biological pretreatment, the VFA con-centrations in the pretreatment and acidogenic stages were762 and 1035 mg/l, respectively. The major components ofVFA in the acidogenic process were 406 mg/l acetic acid,420 mg/l butyric acid and 209 mg/l propionic acid. VFAconcentrations after the methanogenic process (stage III)decreased substantially from 3053 to 134 mg/l with ther-mochemical pretreatment and from 1035 to 38 mg/l withbiological pretreatment. These results indicated that VFAsare important substrates that are readily used by methano-genic microorganisms (3). The SCOD reduction was greatlyincreased by hydrolysis processes. In the control, two-stagedigestion process, the level of SCOD decreased from 2250to 1001 mg/l (55.5%) following the sequential acidogenicand methanogenic processes. When three-stage digestionwas performed by incorporating either thermochemical orbiological hydrolysis, there were increases in the reductionof SCOD: 91.6% (from 24,380 to 2055 mg/l) for thermo-

chemical hydrolysis and 73.2% (from 7870 to 2110 mg/l)for biological hydrolysis. Although the final levels of SCODwere only slightly different, the degree of reduction showeda considerable difference. In the three different processes,namely, two-stage digestion (control), three-stage digestionafter thermochemical hydrolysis, and three-stage digestionafter biological hydrolysis, the final levels of tCOD were16,564, 3074 and 9113 mg/l, respectively. The overall re-ductions in tCOD were 40.2%, 88.9% and 67.1%, respec-tively. These observations clearly indicate that the treatmentof WAS can be efficiently improved by incorporating eitherof the two hydrolysis processes studied here. Complex sub-stances in WAS were solubilized into readily biodegradablesubstances and these substances were easily converted intomethane together with significant increases in the amountsof VFA produced. Thus, the solubilization of WAS by hy-drolysis processes plays an important role in enhancing an-aerobic digestion and the efficiency of the destruction of or-ganics. Improved accessibility to soluble organic substancesresulted in higher, more extensive rates of VFA and meth-ane generation (3).

The three-stage digestion system involving pretreatmentusing thermochemical hydrolysis showed that the tCOD re-duction, SCOD reduction, VS reduction, methane yield andmethane content were 88.9%, 91.6%, 77.5%, 0.52 m3/kg VSand 79.5%, respectively. Alternatively, the adoption of bio-logical hydrolysis as the pretreatment process showed thatthe tCOD reduction, SCOD reduction, VS reduction, meth-ane yield and methane content were 67.1%, 73.2%, 75.0%,0.43 m3/kg VS and 75.3%, respectively. Anaerobic diges-tion including either of the two hydrolysis pretreatment pro-cesses showed improved performance, although the levelsof improvement in the performance of three-stage digestionincorporating thermochemical hydrolysis were more pro-nounced than those achieved by incorporating biologicalhydrolysis in a bench-scale operation. In the three-stage di-gester, the residual levels of substrates from the second- andthird-stage digesters could be reduced by using a longer re-tention time. For readily fermentable wastes, a three-stagereactor can have a lower overall retention than a single-stage system (18).

The three-stage digestion system developed in this studywas compared with those described in other studies (Table 1).One three-stage operation described previously comprisedbiological hydrolysis, an acidogenic process and a methano-genic process (14), and was similar to our three-stage di-gestion processes. Four different two-stage systems (2, 11,19), namely, an acidogenic process-methanogenic process,ultrasonic disintegration-anaerobic digestion, mechanicalpretreatment-anaerobic digestion and a continuously stirredtank reactor-upflow anaerobic filter, showed relatively lowefficiencies in comparison with three-stage digestion sys-tems, although four different two-stage systems showedgood performances. In addition, the use of mechanical pre-treatment-anaerobic digestion resulted in higher methaneyield (11), but this system showed a relatively lower tCODreduction, VS reduction and methane content in comparisonwith our systems. A single-stage system resulted in a verylow performance and it was considered that the develop-ment of the process should be required (13).

FIG. 3. Changes in levels of VFA and SCOD in the three-stage di-gester incorporating either thermochemical (A) or biological hydroly-sis (B), and in the two-stage digester (control) (C).

PROCESS OPTIMIZATION FOR ZERO EMISSIONVOL. 100, 2005 167

It is concluded that the three-stage digestion describedhere is competitive in comparison with other similar sys-tems for enhanced anaerobic digestion. Thermochemicalhydrolysis provides faster treatment and a higher efficiencythan biological hydrolysis, but it will be expensive to applyit in an actual process because of its additional costs interms of adding large quantities of chemical reagents andmaintaining a high temperature. The rate of hydrolysis isrelatively low in biological hydrolysis, but this method in-volves relatively cheaper maintenance costs. For this rea-son, thermochemical hydrolysis would be preferred whenrapid treatment of a small quantity of WAS is required,whereas biological hydrolysis would be preferred when costis of importance and large-scale treatment of WAS is re-quired. The above results show that the use of a three-stagedigester including hydrolysis pretreatment is a good methodfor increasing the process performance in anaerobic diges-tion and the digestion efficiency of the WAS.

ACKNOWLEDGMENTS

This work was supported by the post-doctoral fellowship pro-gram of the Korea Science Engineering Foundation (KOSEF), andby the national research laboratory program of the Korea Ministryof Science and Technology (MOST).

REFERENCES

1. Horan, N. J.: Biological wastewater treatment systems. Wiley,Chichester (1990).

2. Tiehm, A., Nickel, K., Zellhorn, M., and Neis, U.: Ultra-sonic waste activated sludge disintegration for improving an-aerobic stabilization. Water Res., 35, 2003–2009 (2001).

3. Wang, Q., Kuninobu, M., Kakimoto, K., Ogawa, H. I., andKato, Y.: Upgrading of anaerobic digestion of waste activatedsludge by ultrasonic pretreatment. Bioresour. Technol., 68,309–313 (1999).

4. Li, Y. Y. and Noike, T.: Upgrading of anaerobic digestion ofwaste activated sludge by thermal pretreatment. Water Sci.Technol., 26, 857–866 (1992).

5. Tanaka, S., Kobayashi, T., Kamiyama, K. I., and Bildan,M. L. N. S.: Effects of thermochemical pretreatment on theanaerobic digestion of waste activated sludge. Water Sci.

Technol., 35, 209–215 (1997).6. Sawayama, S., Inoue, S., Tsukahara, K., and Ogi, T.: Ther-

mochemical liquidization of anaerobically digested and de-watered sludge and anaerobic retreatment. Bioresour. Tech-nol., 55, 141–144 (1996).

7. Penaud, V., Delgenès, J. P., and Moletta, R.: Thermo-chem-ical pretreatment of a microbial biomass: influence of sodiumhydroxide addition on solubilization and anaerobic biode-gradability. Enzyme Microb. Technol., 25, 258–263 (1999).

8. Rajan, R. V., Lin, J. G., and Ray, B. T.: Low-level chemicalpretreatment for enhanced sludge solubilization. Res. J. WaterPollut. Control Fed., 61, 1678–1683 (1989).

9. Ray, B. T., Rajan, R. V., and Lin, J. G.: Low-level alkalinesolubilization for enhanced anaerobic digestion. Res. J. WaterPollut. Control Fed., 62, 81–87 (1990).

10. Lin, J.-G., Chang, C.-N., and Chang, S.-C.: Enhancementof anaerobic digestion of waste activated sludge by alkalinesolubilization. Bioresour. Technol., 62, 85–90 (1997).

11. Nah, I. W., Kang, Y. W., Hwang, K. Y., and Song, W. K.:Mechanical pretreatment of waste activated sludge for anaer-obic digestion process. Water Res., 34, 2362–2368 (2000).

12. Choi, H. B., Hwang, K. Y., and Shin, E. B.: Effect on an-aerobic digestion of sewage sludge pretreatment. Water Sci.Technol., 35, 207–211 (1997).

13. Kim, J., Park, C., Kim, T.-H., Lee, M., Kim, S., Kim, S.-W.,

and Lee, J.: Effects of various pretreatments for enhancedanaerobic digestion with waste activated sludge. J. Biosci.Bioeng., 95, 271–275 (2003).

14. Kim, S. W., Park, J. Y., Kim, J. K., Cho, J. H., Chun, Y. N.,Lee, I. H., Lee, J. S., Park, J. S., and Park, D.-H.: Develop-ment of a modified three-stage methane production processusing food wastes. Appl. Biochem. Biotechnol., 84, 731–741(2000).

15. Shapton, D. A. and Board, R. G.: Isolation of anaerobes.Academic Press, London, New York (1971).

16. APHA, AWWA, and WEF: Standard methods for the exam-ination of water and wastewater, 19th ed. American PublicHealth Association, Washington, D.C. (1995).

17. Bradford, M. M.: A rapid sensitive method for the quantita-tion of microgram quantities of protein utilizing the principleof protein-dye binding. Anal. Biochem. Tech., 72, 248–254(1976).

18. Gunaseelan, V. N.: Anaerobic digestion of biomass for methaneproduction: a review. Biomass Bioenerg., 13, 83–114 (1997).

19. Held, C., Wellacher, M., Robra, K. H., and Gübitz, G. M.:Two-stage anaerobic fermentation of organic waste in CSTRand UFAF-reactors. Bioresour. Technol., 81, 19–24 (2002).

TABLE 1. Comparison of the performances of digestion processes

Stage FeedtCOD

reduction(%)

VSreduction

(%)

CH4

yield(m3/kg VS)

CH4 contentin off-gas

(%)Ref.

Three-stage a WAS 88.9 77.5 0.52 79.5 This studyThree-stage b WAS 67.1 75.0 0.43 75.3 This studyThree-stage b Food wastes 94.7 38.0 0.48 72.0 14Two-stage c WAS 40.2 32.3 0.29 70.2 This studyTwo-stage d WAS – 33.7 0.30 68.9 2Two-stage e WAS – 29.0 0.85 70.0 11Two-stage f Organic wastes 79.8 79.6 0.31 61.0 19Single-stage g WAS 15.2 20.5 0.07 68.6 13a Thermochemical hydrolysis process–acidogenic process–methanogenic process.b Biological hydrolysis process–acidogenic process–methanogenic process.c Acidogenic process–methanogenic process.d Ultrasonic disintegration–anaerobic digestion.e Mechanical pretreatment–anaerobic digestion.f Continuously stirred tank reactor–upflow anaerobic filter.g Anaerobic digestion.