performance of uasb reactor treating leachate from acidogenic fermenter in the two-phase anaerobic...

Wat. Res. Vol. 35, No. 14, pp. 3441–3447, 2001# 2001 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0043-1354/01/$ - see front matterPII: S0043-1354(01)00041-0

PERFORMANCE OF UASB REACTOR TREATING

LEACHATE FROM ACIDOGENIC FERMENTER IN THE

TWO-PHASE ANAEROBIC DIGESTION OF FOOD WASTE

H. S. SHIN1*, S. K. HAN1, Y. C. SONG2 and C. Y. LEE3

1Department of Civil Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-Ku, Taejon, 305-701, South Korea; 2Division of Civil and Environmental Engineering,Korea Maritime University, Dongsam-Dong 1, Youngdo-Ku, Pusan, 606-791, South Korea and 3R&D

Team, Institute of Technology, Engineering & Construction Group, Samsung Corporation,428-5 Gongse-ri, Giheung-eup, Yongin-city, Kyunggi-Do, 449-900, South Korea

(First received 1 June 2000; accepted in revised form 9 January 2001)

Abstract}This study was conducted to investigate the performance of the upflow anaerobic sludgeblanket (UASB) reactor treating leachate from acidogenic fermenter in the two-phase anaerobic digestionof food waste. The chemical oxygen demand (COD) removal efficiency was consistently over 96% up tothe loading rates of 15.8 gCOD/l d. The methane production rate increased to 5.5 l/l d. Of all the CODremoved, 92% was converted to methane and the remaining presumably to biomass. At loading rates over18.7 gCOD/l d, the COD removal efficiency decreased due to sludge flotation and washout in the reactor,which resulted from short HRT of less than 10.6 h. The residual propionate concentration was the highestamong the volatile fatty acids (VFA) in the effluent. The specific methanogenic activity (SMA) analysisshowed that the VFA-degrading activity of granule was the highest for butyrate, and the lowest forpropionate. Typical granules were found to be mainly composed of microcolonies of Methanosaeta. Thesize distribution of sludge particles indicated that partially granulated sludge could maintain the originalstructure of granular sludge and continue to gain size in the UASB reactor treating leachate fromacidogenic fermenter. # 2001 Elsevier Science Ltd. All rights reserved

Key words}UASB reactor, leachate, acidogenic fermenter, food waste, COD removal efficiency,methanogenic activity

INTRODUCTION

The generation of municipal solid waste (MSW)amounts to 44,583 t/d in Korea, of which 26.5% is

food waste from restaurants, dinning halls, marketsand households (MOE, 2000). Food waste is themain source of decay, odor and leachate in collection

and transportation due to the high volatile solids(85–95%) and moisture content (75–85%). Most foodwaste has been landfilled together with other wastes,resulting in various problems such as emanating

odor, attracting vermin, emitting toxic gases, con-taminating groundwater and wasting landfill capa-city. Interest in anaerobic digestion has, therefore,

increased for the efficient management of food wastebecause it has advantages of volume reduction andmethane recovery as well as waste stabilization.

It is considered that two-phase anaerobic digestionis more effective in the anaerobic degradation of

particulate substrates, of which the rate-limiting step

is hydrolysis and liquefaction. Pohland and Ghosh(1971) suggested a two-phase anaerobic process usingtwo separate reactors, one for hydrolysis/acidifica-

tion and the other for acetogenesis/methanogenesis.The microorganisms were separated physically tomake use of the differences in their growth kinetics.

In order to accomplish phase separation, severaltechniques were developed such as kinetic control,leaching beds, membrane separation and pH control

(Fox and Pohland, 1994; Ince, 1998). The two-phaseprocess permits selection and enrichment of differentbacteria in each digester by independently controllingthe digester operating conditions.

In this study, the MUlti-step Sequential batchTwo-phase Anaerobic Composting (MUSTAC) pro-cess was newly devised as an ideal method for

treating food waste as shown in Fig. 1. TheMUSTAC process consists of two main parts: fiveleaching beds for hydrolysis, acidification and post-

treatment, and an upflow anaerobic sludge blanket(UASB) reactor for methane recovery. Feedstock forthis experiment was food waste collected from a

*Author to whom all correspondence should be addressed.

Tel.: +82-42-869-3613; fax: +82-42-869-3610; e-mail:

[email protected]

3441

dining hall. The distribution of grains, vegetables and

meats in the waste was 40.5, 44.2 and 15.3%,respectively. The leaching beds were operated in arotation mode with a two-day interval betweendegradation stages as shown in Table 1. Acidified

products in leachate from four leaching beds wereconverted to methane in the UASB reactor. Eachleaching bed was operated in a sequential batch

mode. Rumen microorganisms (5% v/v) exhibitingenhanced cellulolytic activity were inoculated into thereactor to improve the low efficiency of acidogenic

fermentation (Gijzen, 1987; Song, 1995; Shin et al.,2000). After 6 h of acclimation, dilution water wasprovided to the leaching bed in order to transfer the

acidified products to the UASB reactor. Dilution rate(D rate; d�1) was defined as 1=y (hydraulic retentiontime; d), or Q (flowrate of dilution water; l=d)/V(effective reactor volume; l). The proper control of

dilution rate (2:1! 0:7 d�1), depending on the stateof the fermentation, could eliminate environmentalconstraints in the fermentation (Shin et al., 2000).

The different sizes of shaded portion in the leachingbeds (Fig. 1) indicated the volume reduction of foodwaste according to the degradation stages. Acido-

genic fermentation of 8 days was reasonable con-

sidering operation time and efficiency (Shin et al.,

2000), which was followed by the post-treatment. Theresidues were dewatered in the leaching bed for 6 hand then 15 l/min of air was introduced through thebottom of the reactor for 42 h.

This study was conducted to investigate theperformance of the UASB reactor treating leachatefrom acidogenic fermenter in the two-phase anaero-

bic digestion of food waste. The key operationalparameters were examined for the process develop-ment including the efficiency of chemical oxygen

demand (COD) removal at high loading rates and thecharacteristics of the granules, such as the specificmethane production rate (SMPR), the specific

methanogenic activity (SMA), the microstructureand the particle size distribution.

MATERIALS AND METHODS

Experimental set-up

The UASB reactor used in this study was 41.0 l inworking volume (lower part: 780mm high by 200mm ID;upper part: 400mm high by 280mm ID). Five evenlydistributed sampling ports were installed along the reactor

Table 1. Operating method of five leaching beds in the MUSTAC processa

Day Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 5

1–2 Stage 1 } } } }

3–4 Stage 2 Stage 1 } } }

5–6 Stage 3 Stage 2 Stage 1 } }

7–8 Stage 4 Stage 3 Stage 2 Stage 1 }

9–10 Stage 5 Stage 4 Stage 3 Stage 2 Stage 111–12 Stage 1 Stage 5 Stage 4 Stage 3 Stage 2

aStage 1: Acidification of grains (D rate=2.1 d�1), Stages 2 & 3: Acidification of vegetables & meats (D rate=0.7 d�1), Stage 4: Stabilization(D rate=0.7 d�1), and Stage 5: Post-treatment.

Fig. 1. Schematic diagram of the MUSTAC process for food waste.

H. S. Shin et al.3442

wall. The temperature of the reactor was maintained at 378Cby water-jacket.

Sludge inoculation and feed wastewater

Partially granulated sludge (8.5 l) from an anaerobicplant treating brewery wastewater was inoculated into theUASB reactor. The volatile suspended solids (VSS) con-centration of the seed sludge was 108.1 g/l. The biomass wasperiodically removed from the reactor to avoid excessbiomass accumulation, and also for the analyses of SMA,SEM and the particle size distribution. Leachate fromacidogenic fermenter treating food waste was continuouslyfed into the UASB reactor by a peristaltic pump. As shownin Table 2, the soluble COD of leachate was about 7000mg/l, in which the percentage of acetate, propionate, butyrate,valerate and caproate was 26, 18, 35, 17 and 4% on CODbasis, respectively. The pH was about 6.6, and the alkalinitywas about 3000mg/l as CaCO3.

Loading rate

The initial COD loading rate of the reactor was 1.8 g/l d,which corresponded to 3.9 d of hydraulic retention time(HRT). The loading rate was increased stepwise by reducingHRT, when the COD removal efficiency exceeded 95% andthe methane production rate was consistent (within 5%) forthree consecutive days. The COD removal efficiency wasdetermined based on the soluble COD after filtering thesample through glass–fiber filter paper (Whatman GF/C).Fang and Chui (1993a) reported that the COD removalefficiency of the UASB reactor was mainly dependent on theCOD loading rate and was not sensitive to either the HRTor the COD of the wastewater alone.

Specific methanogenic activity

The SMA analysis was performed in duplicates in serumvials (125ml) based on the reported method (Hwang andCheng, 1991). Sludge samples were taken for the SMAanalysis on day 150, when the reactor was operated at 6.0 gCOD/l d with over 96% COD removal and withoutdetectable volatile fatty acids (VFA) in the effluent. Themethanogenic activity of the granule (100mg VSS) wasmeasured for a specific substrate (2000mg COD/l). In thisstudy, acetate, propionate, butyrate, valerate, caproate andoriginal substrate (leachate) were used individually as thesubstrate.

Microbial examination and the particle size distribution

The microbial examination was conducted by usingscanning electron microscopy (SEM). The instruments andthe sample preparation procedures were as reportedpreviously (Fang and Chui, 1993b). At the end of thisstudy, the sludge particles were sequentially separated usingsix stainless-steel sieves (U.S. Mesh No. 10, 14, 18, 35, 60and 140 with corresponding sieve openings of 2.00, 1.41,1.00, 0.50, 0.25 and 0.105mm). The sludge samples for theparticle size distribution were taken from the second portfrom the bottom of the reactor.

Analytical procedures

VFA was measured using a gas chromatography (GC;Hewlett Packard model 5890A). The GC was equipped witha 10m� 0.53mm HP-FFAP fused-silica capillary columnand a flame ionization detector (FID), using helium as acarrier gas. The temperatures of injector and detector were

200 and 2508C, respectively. The initial temperature of thecolumn was 808C for 5min and increased gradually by108C/min, reaching the final temperature of 1308C. Gasproduction was determined using a wet gas meter. Forbiogas composition analyses, the GC (GowMac series 580)was equipped with a thermal conductivity detector (TCD)and a 2m� 2mm inside diameter (ID) stainless-steelcolumn packed with Porapak Q (80/100 mesh). Thetemperatures of injector, detector and column were keptat 80, 90 and 508C, respectively. COD and TKN weremeasured using the closed reflux, titrimetric method and theKjeldahl method, respectively (APHA, 1992). PO4-P wasanalyzed by ion chromatograph (Dionex DX-120). Theparameters such as total suspended solids (TSS), VSS, pHand alkalinity of the effluent were determined according toStandard Methods (APHA, 1992).

RESULTS AND DISCUSSION

COD removal efficiency

Figure 2 shows the efficiency of COD removal, thebiogas production rate and HRT throughout the

study. HRT was decreased stepwise from 3.89 to0.33 d. The COD removal efficiency was consistentlyover 96% up to HRT of 0.44 d, which correspondedto the loading rate of 15.8 g COD/l d and a food-to-

microorganism (F/M) ratio of 0.8 g COD/g VSS d.The pH in the effluent was maintained at a constantlevel of 7.5–7.6. The maximum biogas production

rate was 279.0 l/d at the COD loading rate of 15.8 g/l d. The experimented biogas production was com-pared with the theoretical value as shown in Fig. 2(b).

The theoretical biogas production rate was calculatedas

The maximum COD loading rate (15.8 g/l d) wascomparable to the maximum VFA converting

activity of 14.0 g COD/l d in the UASB reactortreating synthetic mixed VFA (acetate : propionate :butyrate=2 : 1 : 1 as COD) (Fang et al., 1995).

CODremoved�the conversion efficiency of CODremoved to CH4�0:3974 l CH4g COD

� CH4 content of biogas:

Table 2. Characteristics of leachate from acidogenic fermentertreating food waste

Parameter Units Value

Total COD mg/l 6600–8600Soluble COD mg/l 6000–8000Total suspended solids mg/l 350–620Volatile suspended solids mg/l 300–550Total VFA (as acetic acid) mg/l 5920–7910VFA composition (COD basis)Acetate % 26Propionate % 18Butyrate % 35Valerate % 17Caproate % 4

PO4-P mg/l 80–150TKN mg/l 150–250pH 6.4–6.8Alkalinity mg/l as CaCO3 2500–3500

Performance of UASB reactor 3443

Thereafter, the COD removal efficiency was drasti-cally reduced to about 70% at 0.37 d HRT (18.7 g

COD/l d), and to about 55% at 0.33 d HRT (21.4 gCOD/l d).Figure 3 shows the methane production rate,

residual VFA and solid concentration in the effluentat each COD loading rate. The methane content ofthe biogas was measured to be 77–81%. The methane

production rate increased linearly with COD loadingrates, until reaching the maximum value of 5.5 l/l d at

15.8 g COD/l d.Up to the loading rates of 15.8 g COD/l d, the

effluent had less than 50mg COD/l of acetate, 100mg

COD/l of propionate, 30mg COD/l of valerate andnegligible quantities of butyrate and caproate. VSSconcentration was maintained below 150mg/l. On

Fig. 2. Performance of UASB reactor: (a) COD removal efficiency; (b) biogas production rate; (c) HRT.

Fig. 3. Methane production rate, residual VFA and solid concentration in the effluent at various CODloading rates.


the other hand, the residual acetate, propionate,

butyrate, valerate and caproate increased sharply tothe levels of 450, 660, 60, 290 and 80mg COD/l at18.7 g COD/l d, and further to 700, 1,100, 80, 410 and110mg COD/l at 21.4 g COD/l d. The VSS concen-

tration in the effluent also increased sharply to241mg/l at 18.7 g COD/l d, and to 383mg/l at21.4 g COD/l d. This meant that the decrease of

COD removal efficiency was due to sludge flotationand washout in the reactor, which resulted fromshort HRT of less than 10.6 h.

The concentrations of residual butyrate and cap-roate were relatively low in the effluent. This indicatedthat even-numbered carbon fatty acids were de-

graded more easily than odd-numbered ones, andthat their degradation to acetate was not a rate-limi-ting step. The accumulation of residual acetate wascaused by the degradation of other fatty acids to acetic

acid. Residual acetate was not quickly converted tomethane due to excessive washout of methanogens.The propionate concentration in the effluent was

higher than any other acid. Among VFA, propionateis known to have the lowest tolerance level for theanaerobic bacteria. When an anaerobic treatment

system is overloaded, propionate tends to accumulatein the reactor and its removal is difficult duringrecovery. The degradation of propionate to acetate is

thermodynamically infeasible unless the by-producthydrogen is removed by the hydrogen-consumingbacteria (Boone and Xun, 1987). These indicated thatconverting propionate to acetate was the rate-limit-

ing step.

COD balance and sludge yield

Figure 4 shows that the specific methane produc-tion rate (SMPR) increased linearly with the specific

substrate utilization rate (SSUR) with a slope of 0.92,

until reaching the maximum of 0.70 g methane COD/g VSS d at the SSUR of 0.76 g COD/g VSSd. TheSMPR means the methanogenic activity of thegranules under the specific operating conditions of

the reactor. The SMPR for the biomass could beestimated from the methane production rate and thetotal biomass in the reactor as each gram of methane

was equivalent to 4 g of COD. The slope indicatedthat of all the COD removed, 92% was converted tomethane and the rest 8% was presumably converted

to biomass. Since the biomass in the reactor wasmeasured to have a COD/VSS ratio of 1.41, thesludge yield was estimated to be 0.057 g VSS/g COD

(=0.08/1.41), which was comparable to the yieldvalues on acetate (0.010–0.054 g VSS/g COD), short-chain fatty acids except acetate (0.025–0.047 g VSS/gCOD) and long-chain fatty acids (0.04–0.11 g VSS/g

COD) as reported by Pavlostathis and Giraldo-Gomez (1991).

Specific methanogenic activity

Table 3 summarizes the SMA of the granulesmeasured in serum vials using six different substrates,

i.e., acetate, propionate, butyrate, valerate, caproateand original substrate (leachate). The SMA is anindicator for evaluating the methanogenic activity of

the biomass under a condition in which the supply ofsubstrate is not a limiting factor. Correspondingstudies on the SMA of UASB granules treating

various types of wastewater were also shown forcomparison. Each gram of the VFA-degradinggranules was capable of producing 0.81, 0.69, 1.10,

0.87, 0.99 and 0.94 g of methane COD per day fromacetate, propionate, butyrate, valerate, caproate andoriginal substrate, respectively. This indicated that

Fig. 4. Specific methane production rates at various specific substrate utilization rates.


the VFA-degrading activity of granule was the

highest for butyrate, and the lowest for propionate.This is consistent with the observation that, atloading rates over 18.7 g COD/l d, high residualpropionate was found in the effluent while there was

little residual butyrate. The SMA in other studiesshowed that the VFA-degrading activity of granulewas the highest for acetate. However, in this study,

the SMA values for valerate and caproate as well asbutyrate were higher than that for acetate. Thismeant that the degradation of C4–C6 fatty acids was

dominant in the treatment of leachate from acido-genic fermenter for food waste.

Microstructure and particle size distribution ofgranules

Although layered microstructure was reported for

granules treating wastewater of sucrose (MacLeod etal., 1990) and brewery (Fang et al., 1994a), granulesfrom this study did not exhibit any patterned

structure. The microorganisms were distributedthroughout the entire granule.Figure 5 shows the anaerobic microorganisms in

the sludge granules. Typical granules were found tobe mainly composed of microcolonies of Methano-saeta, which had a width of 0.7–0.8 mm and a lengthof 3.0–7.0 mm. Methanosaeta is an acetoclasticbacterium using acetate solely as substrate. With avery low half-rate constant of 30mg COD/l (Gujerand Zehnder, 1983), Methanosaeta outcompetes

other methanogenic bacteria when the acetate con-centration is low, as in the mixed liquor of the UASBreactor and in the interior of the granule. The

syntrophic association between acetogens (cocci)and methanogens (rods) allowed the rapid removalof hydrogen, which would otherwise hinder the

propionate degradation.The dry weight percent for different size ranges of

granular sludge is shown in Table 4. The original seedsludge had 64.3% of particles smaller than 1.4mm.

At the end of this study, 75.1% of the sludge particleswere larger than 1.4mm in the UASB reactor. Thisindicated that partially granulated sludge could

maintain the original structure of granular sludgeand continue to gain size in the UASB reactortreating leachate from acidogenic fermenter.

CONCLUSIONS

1. The UASB process showed that the CODremoval efficiency was consistently over 96% up tothe loading rates of 15.8 g COD/l d. The methane

Table 3. Specific methanogenic activity of granules

Original substrate fed to sludge SMA (gCH4 COD/gVSS d) using the following substrate Reference

Acetate Propionate Butyrate Valerate Caproate Original subtrate

Leachatea 0.81 0.69 1.10 0.87 0.99 0.94 This studyMixed VFAb 1.17 0.35 0.81 } } 1.03 Fang et al. (1995)Sucrose 1.20 0.52 0.61 } } 0.85 Fang et al. (1994a, b)Propionate 1.89 1.78 1.32 } } } Grotenhuis et al. (1991)Sugar 0.90 0.41 } } } } Dolfing and Mulder (1985)

aFrom acidogenic fermenter treating food waste.bAcetate : propionate : butyrate=2 : 1 : 1 as COD.

Fig. 5. SEM picture of typical sludge granule: (a) Metha-nosaeta; (b) Syntrophic microorganisms.

Table 4. Size distribution of sludge particles before and after thisstudy

Size (mm) % Dry weight in each size range

Granulesinitial Granulesfinal

>2.0 6.5 10.71.4–2.0 29.2 64.41.0–1.4 47.1 17.20.5–1.0 10.6 6.30.25–0.5 3.6 1.00.1–0.25 2.7 0.4

50.1 0.3 0.0


production rate increased to 5.5 l/l d. The COD

removal efficiency deteriorated at loading rates over18.7 g COD/l d due to sludge flotation and washoutin the reactor, which resulted from short HRT of lessthan 10.6 h. At these loading rates, high residual

propionate was found in the effluent, indicating thatpropionate degradation was a rate-limiting step.2. Each gram of VFA-degrading granules in the

reactor had a daily maximum capacity of converting0.70 g of COD to methane at the specific substrateutilization rate of 0.76 g COD/g VSS d. Of all the

COD removed, 92% was converted to methane andthe rest presumably to biomass with an averagesludge yield of 0.057 g VSS/g COD.

3. The SMA using acetate, propionate, butyrate,valerate and caproate, individually, as substrate were0.81, 0.69, 1.10, 0.87 and 0.99 g methane COD/gVSS d. This indicated that the VFA-degrading

activity of granule was the highest for butyrate, andthe lowest for propionate.4. Typical VFA-degrading granules were com-

posed of microcolonies of Methanosaeta. The sizedistribution of sludge particles indicated that par-tially granulated sludge could maintain the original

structure of granular sludge and continue to gain sizein the UASB reactor treating leachate from acido-genic fermenter.

Acknowledgements}This work was supported by grantNo. 95-1-13-01-01-3 from a research program of academicand industrial cooperation of the Korea Science andEngineering Foundation.

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