low-strength wastewater treatment by a uasb reactor

ELSEVIER 0 9 6 0 - 8 5 2 4 ( 9 5 ) 0 0 1 9 2 - 2

Bioresource Technology 55 (1996) 187-194 © 1996 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0960-8524/96 $15.00

LOW-STRENGTH WASTEWATER TREATMENT BY A UASB REACTOR

Kripa Shankar Singh, a* Hideki Harada b & T. Viraraghavan a

aFaculty of Engineering, University of Regina, Regina, Saskatchewan, Canada $4SOA2 bDepartment of Civil Engineering, Nagaoka University of Technology, Nagaoka, Nigata 940-21, Japan

(Received 30 May 1995; revised version received 20 November 1995; accepted 29 November 1995)

Abstract This study describes the feasibility of anaerobic treatment of a low-strength (500 rag-COD~l) synthetic wastewater using a semi-pilot-scale upflow anaerobic sludge blanket (UASB) reactor. Under ambient temperature conditions (20-35°C) with a hydraulic retention time (HRT) of 3 h and corresponding organic loading of 4 kg-COD/m3-day, 90-92% COD and 94-96% BOD reductions were achieved. Methane production was found to be about 141 l/kg-COD removed at standard temperature and pressure (STP). Because of the presence of a high concentration of active granular sludge in the lower portion of the reactor, 80% reduction of COD occurred within the bed level. Morphological examination of the sludge showed that the granules contained diverse groups of micro- organisms, where rod-type, Methanothrix-like, cells were dominant on the surface. Copyright © 1996 Else- vier Science Ltd.

Key words: UASB reactor, anaerobic treatment, organic loading, hydraulic retention time, low- strength wastewater, methanogenic bacteria, sulfate-reducing bacteria.

INTRODUCTION

The UASB reactor has been widely adopted for the treatment of various wastewaters (Lettinga et al., 1983; 1984; 1987; Hulshoff Pol et al., 1983; van den Berg, 1984; de Zeeuw, 1988; Fang et al., 1990) because of its enhanced treatment ability, with over 200 installations worldwide (Lettinga & Hulshoff Pol, 1991). One of the most important challenges is the adaptation of the UASB process to the treatment of low-strength wastewaters, such as municipal and domestic wastewaters.

*Author to whom correspondence should be addressed. 187

The objectives of this study were to investigate the process performance for low-strength wastewater at different HR T and organic loading-rates (OLR), and the feasibility of adopting the UASB system for low-strength wastewater treatment.

METHODS

UASB reactor A schematic diagram of the UASB reactor used is shown in Fig. 1. The reactor consisted of a poly- acrylic column with the following dimensions: 20 cm internal diameter and 4 m height with a conical- shaped bottom. A gas-solid-liquid three-phase separator (GSS device) was installed in the top portion of the column. An inclined plate-type settler was employed as the GSS device, to prevent granular sludge carry over, return up-rising granules to the reactor portion by gravity settling and allow the supernatant to overflow from the reactor. The total volume of the reactor was 140 1 (126 1 for column part and 14 1 for GSS part). Four perforated plates with an opening diameter of 3-8 mm and an opening area value of 13% were attached at an angle of 30 ° to a vertical shaft which was rotated at 1 rpm to prevent channelling of the feed solution within the sludge bed. A gas collection system was installed at the top portion of the reactor and the gas volume was measured by a wet gas meter. In addition, a scum breaker was also provided within the GSS device and controlled by a timer.

Seed material The UASB reactor was seeded with digested sludge from an anaerobic digester at Huay Kuan Sewage Treatment Plant, Bangkok. An amount of 20-25 kg sludge VSS/m 3 with MLSS concentration of 45.80 g/1 and VSS/SS ratio of 0.50 was added as inoculum.

188 K. S. Singh, H. Harada, T. Viraraghavan

Wastewater The wastewater used as influent to the reactor was a synthetic wastewater with a total COD of 500 rag/1 for about 220 days and 300 rag/1 for 60 days. The wastewater was composed of 100 mg-COD/1 from cellulose (which is biodegradable) as solid compo- nent, 350 mg-COD/l from sucrose and 50 mg-COD/1 from peptone, as soluble components. Compositions of the buffer and trace elements used were as follows (in mg/1): NaHCO3, 326; NH4CI, 175; (NH4)2HPO4, 40; KH2PO4, 7.2; COC12.6H20, 1.2; NaaMoa.2H20, 1.0; FeC13, 5.0; CuSO4.5H20, 5.0; MgSOa.7H20, 39.0; MnSO4.4H20, 13.9; CaC12.2H20, 36.8; Na2SO4, 200.

Operating conditions The reactor was operated at a HRT of 4 h (corresponding organic loading rate of 3 kg-COD/m3-day), 3 h (4), 6 h (2), and 6 h (1.2) during the present study. The reactor was kept at the ambient temperature, which ranged from 20 to 35°C during the study period. Sufficient mixing was provided in the sub-

strate reservoir (kept at 4°C) to prevent cellulose particles in the influent from settling in the tank.

Chemical analysis Since sulfides cause interference in the COD determination, samples were acidified by 6 N sulfuric acid and bubbled with nitrogen gas in order to remove sulfide gas. COD was then determined by the standard dichromate method. For the determination of soluble COD, samples were filtered through a 0.45 micron membrane filter. BOD of the sample was measured by eliminating the sulfide interference and neutralizing the pH again by the addition of 6 N sodium hydroxide. Soluble COD of the sludge was determined by centrifuging the sludge sample at 3000 rpm for 15 min and filtering the supernatant through a 0.45 micron membrane filter.

Sulfate was determined by the turbidimetric method (Hach Turbidity Meter, 2100 A) as men- tioned in Standard Methods (APHA, 1985). Sulfide in the liquid was determined by the iodometric titration method.

9

8

2

12

Fig. 1. Schematic of experimental UASB reactor (GSS volume 14 1; reactor volume 126 1; total volume 140 1; 1. Feed pump; 2. Mixing motor; 3. Substrate reservoir; 4. Mixing device; 5. Sampling port; 6. Water jacket; 7. Inclined plate settler;

8. Gas trap; 9. Wet gas meter; 10. Effluent; 11. Skimmer; 12. Refrigerator).

Low-strength wastewater in a UASB reactor 189

Volatile fatty acids (VFA) concentration was determined using a gas chromotograph (GC Shi- madzu 14-A) with FID equipped with a glass column 2 m in length and an internal diameter of 2 mm, packed with 80/120 Carbopack B-DA/4% Carbowax 20 M. The chromotograph was operated in iso- thermal mode at a constant oven temperature of 175°C. Nitrogen was used as a carrier at 55 ml/min.

Gas analysis The composition of the biogas was determined by a gas chromotograph (GC Shimadzu 15-A) with TCD equipped with a steel column packed with Propak Q. Helium was used as the carrier gas at 30 ml/min and the oven temperature was kept at 55°C.

The concentration of hydrogen sulfide in the gas was determined by passing the gas through a solution of zinc acetate which absorbs hydrogen sulfide, and the solution was analyzed by an iodometric titration method.

120 t , 135

00 / V z0

60 3(4) 4(3) 2(6) 1.2(6) 15

o 4 0 OL(HR] 3 108=,

20 5

0 0 0 40 80 120 160 200 240 280

T i m e ( d a y s )

Fig. 2. Time course of COD removal and organic loading [OL (kg-COD/m3-day); HRT (h); -m- COD removal (%);

+ temperature (°C)].

a portion of the sample to avoid the collection of sludge accumulated near or inside the port.

Morphology of granular sludge Morphological observations of granular sludge were carried out by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Samples for scanning electron microscopy were pretreated by fixing the sample for 3 h with 2% glutaraldehyde in 0.1 M cacodylate buffer. After this, the postfixing was done by 1% osmium tetraoxide in 0.1 M cacodylate buffer for 2 h and the sample was washed twice, for 15 rain in each case, by distilled water, followed by dehydration in a graded ethanol series: 35%, 50%, 70%, 95% and, finally, absolute ethanol (twice in each concentration). Then the sample was dried in a critical point drier (Blazer Union; CPD 020). The dried sample was mounted on the stub by double-sided sticky tape and sputtered with gold (Blazer Union; SCD 040). Sec- tioning of the granules was done with a sharp knife prior to the gold sputtering. Finally, the sample was observed by SEM (JEOL JS-35 CF). Samples were pretreated for transmission electron microscopy by fixing in 2.5% glutaraldehyde in 0.1 M phosphate buffer at pH 7.4 for 24 h at 4°C and then rinsed in 0.1 M phosphate buffer three times for 15 min each time. After this, postfixing and dehydration were carried out in the same manner as in the SEM pre- treatment. Then the sample was infiltrated in Spurr's resin twice for 2 h. After this the Spurr's resin was polymerized in an oven at 70°C for 8 h, then it was cut by ultramicrotome and stained with uranyl acetate for 15 rain and lead acetate for 5 min before observing by TEM.

Profile sampling Sampling was done from the top to the bottom of the reactor for the determination of the substrate- removal profile and sludge hold-up along the height of the reactor. Samples were collected after wasting

RESULTS AND DISCUSSION

Reactor performance The continuous-flow experiment was conducted for more than 300 days at hydraulic retention times (HRT) of 4, 3, 6 and 6 h with loading rates of 3, 4, 2, and 1.2 kg COD m3/day, respectively.

Figure 2 shows the temporal variation of COD removal, along with COD loading rate and temperature, over the experimental period. The COD-removal efficiency achieved was fairly consist- ent in the range of 83-88% (based on total inf-COD and total eft-COD) and 90-92% (based on total inf- COD and soluble elf-COD). The BOD removal efficiency achieved was in the range of 90-92% (based on total inf-BOD and total eff-BOD) and 94-96% (based on total inf-BOD and soluble eff- BOD). The lower ratio of BOD/COD in the effluent suggested that most of the biodegradable components of the wastewater had been digested. After achieving steady-state conditions in each loading phase, it was observed that there was no significant difference between total COD removal (based on total inf-COD and total elf-COD) and soluble COD removal (total inf-COD and soluble elf-COD). The contribution of particulates to the total effluent COD was relatively small throughout all loading phases. VFA in the effluent was at all times less than 25 mg/l as acetic acid. Accordingly, the soluble- COD removal (based on total inf-COD and soluble eft-COD) achieved was as high as 88%, independent of loading rate up to 4 kg-COD/m3-day and influent concentration as low as 300 rag-COD/I, showing that this process is very promising for the anaerobic treatment of low-strength wastewater, such as municipal and domestic wastewaters. It also shows that the granular-sludge activity was sufficient to metabo- lize VFA at an appreciable rate. The time course of


140

.~ 120

I00 U) ' ~e

80

60

> 40

z0

4e~ i

2~

o

0 115 1 5 5 195 2 3 5 2 8 0

Time (days)

Fig. 3. Time course of VFA in the effluent [ - - ~ - - effluent VFA; --I- organic loading (OL)].

100 3 5

3 0 813

" 3 I-- 2 5 ~ "

o (. 6 0 o 2 0

4 0 ' 1 5 ~

loy. 2 0

5 _ . . -

0 0 4 0 8 0 1 3 0 1 6 0 2 0 0 2 4 0 2 6 0

T i m e ( d a y s )

Fig. 5. Time course of gas production and its variation with temperature (--z~-- methane; + gas; * temperature;

--n- loading).

total VFA in the effluent is illustrated in Fig. 3. VFA other than acetic acid were not detected in the effluent during the experimental period.

The temporal variation of influent and effluent SS and VSS is illustrated in Fig. 4. Fluctuations observed in the influent SS might be ascribed to insufficient mixing in the storage tank containing the feed. The average value of influent SS over the whole period was about 100 mg/1 (SD=38) with a VSS/SS ratio of 0.9 (SD=0.08). At a loading rate of 3 kg-COD/m3-day, the average effluent SS was found to be in the range of 15-20 rag/1 (80-85% SS removal). At a loading rate of 4 kg-COD/m3-day, effluent SS concentration increased to about 50 mg/1 due to the low HRT of 3 h and a high upflow velocity of 1.3 m/h. A severe granule washout took place during this loading rate, At other loading rates the SS concentration in the effluent was considerably lower ( < 3 0 mg/l). The relationship between total- and soluble-COD removal and the loading rate showed that the soluble-COD removal was mostly independent of loading rate and was approximately 88%. The total COD removal efficiency was also found to be linearly proportional to the removal efficiency based on the soluble COD.

1 6 0 [ 1 . 2

x , 1 2 0 XXx ,00÷x x 80 0.6

I IV/ 40

L-A w 0 ~ - "" ~,f "

4 0 8 0 1 2 0 1 6 0 2 0 0 2 4 0 2 8 0

T i m e ( d a y s )

Fig. 4. Time course of influent and effluent SS and VSS ( + inf-SS; + inf-VSS; * eff-SS; -- l- eff-VSS; x eff-VSS/

eff-SS).

Methane production Temporal variations in gas and methane productions are illustrated in Fig. 5. Total methane production corresponded to the sum of methane present in biogas and methane lost in the effluent in dissolved form. The dissolved methane in the effluent was calculated by multiplying the methane content (par- tial pressure) of the biogas, on particular days, by Henry's constant. It was very likely that more methane would be present in solution because of over-saturation, but this could not be estimated.

The total methane production (l/day) and gas composition, with respect to organic loadings, are shown in Fig. 6. The average methane content of the biogas was approximately 70% (range 66-75%). The CO2 content was only 9% on average (range 6-11%) throughout the period, indicating that the major part of the CO2 formed might have been present in the dissolved form. It was also observed that the CO2 content of the gas was very low at lower temperatures. This was due to the increase in solubility of CO2 because of the decrease in temperature.

A considerably large fraction of the methane generated was lost in a dissolved form in the effluent

8o F

z O

£ ~u o

"~ 2q

O

8

o

3 4 2 1 .2

O r g a n i c l o a d i n g ( k g - - C O D / m 3 - d )

Fig. 6. The total methane production and gas composition with respect to organic loadings ( ! total methane; [] recovered methane in gas; I~ dissolved methane in

liquid; --E3-- methane (%); * CO2 (%).


(in this case 28-39%). Methane as off-gas was 65% of the total methane generated. This implied that the treatment of low-strength wastewater reduces the intrinsic advantage of an anaerobic process as an energy-recovery system. The amount of methane produced (liters at STP) for COD removed (kg- COD) for the whole experimental period was 141.20 I/kg-COD removed, based on the actual COD removal rate, in comparison to the theoretical value of methane production of 350 I/kg-COD removed; the lower value may be due to cellulose particles in the influent accumulating in the sludge-bed without complete hydrolysis, because of factors such as par- ticle size, pH and H R T not being optimal (Chyi & Dague, 1994). The relatively low amount of methane produced compared to COD removal could be attributed to the occurrence of sulfate reduction and methane over-saturation in the liquid.

COD balance and electron flow Figure 7 presents the COD mass-balance as a function of COD loading rate. The amount of COD that left the reactor consisted of: the COD of suspended solids in the effluent (eff-CODss); soluble COD in the effluent (eff-CODsol); recovered CH4-COD in the gas phase (CH4gas-COD); dissolved CH4-COD in the effluent (CH4aq-COD, calculated by Henry's Law); COD used for sulfate reduction (SO4-COD). The SO4-COD was estimated by multiplying the amount of SO4Z--S reduced by a conversion factor of 2 (g COD/g S).

In spite of receiving a rather low-strength wastewater and operating at low H R T at moderate temperatures, the reactor showed satisfactory levels of COD reduction. Solubility of CH4 in reactor water was taken into account because of the high COD value of CH4 on a weight basis and relatively low COD value of the wastewater. The amount of dissolved CH4 as COD in the aqueous phase ranged between 13 and 20% and was lost in the effluent. The percentages of COD recovered as CH4 in the gas phase and CH4 dissolved in the aqueous phase

of the effluent were quite high at all loading rates, but COD removal due to sulfate reduction was quite comparable (30-40%) to COD removal due to CH4 production. Soluble COD in the effluent comprised 10-15% of the total COD output, irrespective of loading rates.

Electron flow was calculated as follows (Harada et al., 1994):

Electron flow distributed to MPB=(CHagas - COD+CH4aq-COD)/(CH4gas-COD+CHaaq COD +SO4-COD).

Electron flow distributed to SRB = (SO4-COD)/ (CH4gas-COD + CH4aq-COD + SO4-COD).

The amounts of electron flow apportioned to MPB and SRB were almost the same, irrespective of loading rates (55% for MPB and 45% for SRB), as shown in Fig. 8.

Sulfur balance Figures 9 and 10 illustrate the temporal variation of sulfur compounds and their distribution as sulfur balance at each loading rate. It was found that approximately 90% of the total sulfate input was reduced at each loading rate. This result corre- sponds with a tendency for COD consumption by

1 0 0

l

8 0

Y 6 0

o

o~ * \ \ \ \ \

0 ~ \ \ \ \ \ ,'3

I !

! i

4 2 1 . 2

Organic Loading (kg-COD/m3-d)

Fig. 8. Electron flow distribution to MPB and SRB as a function of organic loading rate ([] MPB; [] SRB).

100

80

g

..o 40

0 0

2G

3 4 2 1.2 Organic loading (kg-COD/m3-d)

Fig. 7. Distribution of output COD as a function of organic loading rate (m eff-CODss; [] eff-CODsol; []

CH4-CODgas; [] CH4-CODaq; [] SO4-COD).

2 5 0

~ . 2 0 0

.~ 1 5 0

i00

so

1 2 0 1 4 0 1 6 0 i 7 3 2 1 0 2 3 2 2 5 2 2 7 7

T i m e ( d a y s )

Fig. 9. Time course of sulfur compounds in influent and effluent ( --O-- inf-sulfate; + eft-sulfate; * eft-sulfide;

- -m-- biogas sulfide).


I 00

8O

60

o 40

m 20

O r g a n i c L o a d i n g ( k g - C O D / m 3 - d)

Fig. 10. Distribution of output sulfur compounds as a function of organic loading rate (N eft-sulfide as S; • eft-

sulfate as S).

250

201)

- - - - b

3 4 2 1.2

150

I00 ~ _ _ . ~ o~

5O

0 --- lO 30 50 70 90 120 220 330 460 R e a c t o r h e i g h t ( c m )

Fig. 12. Profiles of sulfate along the reactor height [ --*-- OL (4 kg-COD/m3-day); - - v - - OL (2 kg-COD/m'-day);

--G-- OL (1.2 kg-COD/m3-day)].

SRB through sulfate reduction (in other words, a tendency for electron flow by SRB for sulfate reduction). Of the total sulfide production, the fraction present as H2S in the biogas was less than 1% at each loading rate. The major form of sulfur compounds (10% of total input sulfate as sulfur) that were accounted for as accumulating were organic sulfurous compounds or elemental sulfur. Sulfate levels were negligible in the effluent throughout the experimental period.

Profiles of COD, sulfate and MLSS Profiles of soluble COD, sulfate and MLSS were developed along the reactor height on the 178th (organic loading = 4 kg-COD/m3-day), 225th (organic l o a d i n g = 2 kg-COD/ma-day) and 279th days (organic loading = 1.2 kg-COD/ma-day). Pro- files of soluble COD, sulfate and MLSS are shown in Figs 11-13, respectively. More than 75% of the influent COD was reduced in the granular sludge bed zone of the reactor. This indicates the avail- ability of active sludge in this zone. At all loading rates, up to a height of 30 cm, the VFA concentration had a tendency to increase with height; for instance from 15 to 65 mg/1 (as acetic acid) at a

loading of 4 kg-COD/m3-day (Fig. 14). This may be ascribed to higher acidogenic activity than methanogenic activity in this zone. However, above 30 cm height the VFA concentration had a tendency to decrease, irrespective of loading rate. This observa- tion suggested that the liquefaction of entrapped

100

80

6o:

4o

~: e0

0 -- - -- "-- ---- 10 30 ,50 70 90 120 220 330

Reactor height (era)

4~0

Fig. 13. Profiles of MLSS along the reactor height I--A-- 3 J OL (4 kg-COD/m-day); - - v - - OL (2 kg-COD/m-day); OL (1.2 kg-COD/m3-day)}.

400

30•

.~ 200

100

0 0 I0 30 50 70 90 lEO 220 330 460

R e a c t o r h e i g h t ( e m )

Fig. 11. Profiles of soluble COD along the reactor height I--A-- OL (4 kg-COD/m3-day); - - v - - OL (2 kg-COD/

ma-day); -o - OL (1.2 kg-COD/m3-day)].

v

7O

I

6O

5o ]

4 0

301

2O !

10

10 30 50 70 90 120 220 330 460

R e a c t o r h e i g h t ( c m )

Fig. 14. Profiles of VFA along the reactor height [--A-- OL (4 kg-COD/m3-day); - - v - - OL (2 kg-COD/ma-day);

- - e - - OL (1.2 kg-COD/m3-day)].


solid cellulose and the methanization took place concomitantly in the lower part of the bed.

Sulfate reduction had almost the same trend as COD along the height of the reactor at all loading rates. About 90% reduction occurred at 10 cm height, suggesting the highest activity of SRB about this height in the reactor.

The sludge concentration in terms of MLSS at the end of the phase of organic loading of 4 kg-COD/ m3-day was only 29 g/l at a height of 10 cm, which was due to washout of the sludge. At the end of the loading rate of 2 kg-COD/m3-day the sludge concentration was in the range of 60-80 g/1 from the bottom to 70 cm height of the reactor. In the period of loading at 1.2 kg-COD/m3-day, 30-50 g/1 concentration of MLSS was obtained from the bottom to 70 cm height of the reactor. The settling characteristics of the granules were quantified by SVI (sludge volume index). This index tended to increase from the bottom to the top of the reactor. It was approximately 16 ml/g at the loading rates of 2 and 1.2 kg-COD/m3-day, whereas at 4 kg-COD/m3-day a higher SVI of about 20 ml/g was observed; so, better settling characteristics were present at 2 and 1.2 kg- COD/m3-day than at 4 kg-COD/m3-day.

The sludge layer as high as, or above, 70 cm and regarded as the sludge-blanket portion, exhibited a good retention of a high concentration of active sludge. This capability for retention of sludge seems to be a prerequisite for anaerobic treatment of low- strength wastewaters at ambient temperature and at an HRT comparable to conventional aerobic processes.

Kinetic rate constants Effluent soluble COD and removals in steady-state at all loading rates were used to estimate the half- velocity constant and the maximum rate of substrate utilization. It was assumed that UASB reactor kinetics could be approximated by the Monod relationship given by the following equation (Grant & Lin, 1995):

60

50

~4o

b~ 30

2o

I0

0 0 0.01 0.02 0 .03 0.04 0 .05 0 .06 0 .07 0 . 0 8

l / S e (L/mg)

Fig. 15. Determination of kinetic constants Ks and k [--t t-- X(HRT)/(Si-Se); R 2= 0.73; X(HRT)/(Si-Se) =

(572.8/Se) + 3.85].

of 236, 393 and 675 mg/1 for temperatures of 10, 20 and 30°C, respectively, determined by Grant and Lin (1995). The differences in values could be attributed to the different wastewater characteristics and operational conditions in the two studies, but because of the few data points the values of Ks and k determined from the present slope and intercept are only approximate.

Morphology of granular sludge High densities of rod-type, filamentous-type and coccus-type anaerobic cells were observed on the surface of granules (Fig. 16). Polymeric substances excreted by cells, which might have conferred integrity to the granules, were also observed. Large cavities were seen in cut-sections; these were indica- tive of vigorous gas production (Fig. 17). The size of granules was estimated to be in the range of 1.5-2 mm mean diameter, and the granules were mostly spherical in shape. On decreasing the strength of the feed from 500 mg-COD/1 to 300 rag-COD/I, compar- atively smaller size (1.0-1.5 mm mean diameter) granules were observed, which might have been produced by the disintegration of granules by starvation.

X(HRT) Ks 1 1 - - - + (1)

(Si-Se) k Se k

where k is the maximum specific substrate utilization rate (1/day); Ks is the half-velocity constant; Si is the substrate concentration in the influent (mg/l); Se is the substrate concentration in the effluent (rag/l); and X is the biomass concentration (rag/l) (Monod, 1949).

From a linear plot of X(HRT)/(Si - Se) versus USe for different loadings, k and Ks were determined. The slope of such a plot is equal to KJk and the Y intercept is equal to 1/k (Fig. 15). The values of k and K~ determined were 0.26 per day and 149 mg/1, respectively, which were quite comparable to the values of k of 0.22, 0.80 and 1.38 per day and Ks

Fig. 16. Microbial types on the surface of a granule (scale b a r : - 1/~m).


Thai Government for the financial support of this study at the Asian Institute of Technology, Bangkok, Thailand.

Fig. 17. The cut-section view of a granule (scale bar: - - 100 #m).

GENERAL DISCUSSION

The role of sulfate reduction is relevant to anaerobic treatment of low-strength wastewaters because the amount of sulfate present in wastewaters ranges, in general, from 50 to 200 mg-SO4-2]l (Yoda et al., 1987). Since SRB are capable of oxidizing various forms of organic matter present in wastewaters while utilizing sulfate as an electron acceptor, the role of SRB in anaerobic digestion of wastewater cannot be ignored. The amount of electron flow imparted was 55% to MPB and 45% to SRB, and remained constant at all loading rates. This might be attributed to the high sulfate to organic matter ratio in the wastewater at all loading rates. This showed that both sulfate reduction and methanogenesis can be the final step in the anaerobic removal pathway of COD from wastewater. SRB as well as MPB are capable of using acetate and hydrogen as substrate. More- over, in the presence of sulfate, SRB may outcompete MPB for hydrogen and acetate.

ACKNOWLEDGEMENTS

The writers wish to thank the Japan International Cooperation Agency, ARRA (Japan) and The Royal

REFERENCES

APHA (1985). Standard Methods for the Examination of Water and Wastewater, 16th edn. American Public Health Association, Washington, DC.

Chyi, Y. T. & Dague, R. R. (1994). Effects of particulate size in anaerobic acidogenesis using cellulose as a sole carbon source. Water Environ. Res., 66 (5), 670.

de Zeeuw, W. (1988). Granular sludge in UASB reactors. Proc. GASMAT Workshop, ed. G. Lettinga & A. J. B. Zehnder.

Fang, H. H. P., Liu, G., Zhu, J., Cai, B. & Gu, G. (1990). Treatment of brewery effluent by UASB processes. J. Environ. Engng. Div., ASCE, 116 (3), 454.

Grant, S. & Lin, K.-C. (1995). Effect of temperature and organic loading on the performance of upflow anaerobic sludge blanket reactors. Can. J. Civ. Engng, 22, 143.

Harada, H., Uemura, S. & Mamonoi, K. (1994). Inter- action between sulfate-reducing bacteria and methane-producing bacteria in UASB reactors fed with low-strength wastes containing different levels of sulfate. Water Res., 28 (2), 355.

Hulshoff Pol, L. W., de Zeeuw, W. J. & Lettinga, G. (1983). Granulation in UASB reactor. Water Sci. Tech- nol., 15, 291.

Lettinga, G., Roersma, R. & Grin, P. (1983). Anaerobic treatment of raw domestic sewage at ambient temperatures using a granular bed UASB reactor. Biotechnol. Bioengng, 25, 1701.

Lettinga, G., de Zeeuw, W., Wiegant, W. & Hulshoff Pol, L. W. (1987). High-rate anaerobic granular sludge UASB reactors for wastewater treatment. In Bioenvir- onmental Systems, Vol. I, ed. D. L. Wise. CRC Press, Boca Raton, p. 131.

Lettinga, G. & Hulshoff Pol, L. W. (1991). UASB-process design for various types of wastewaters. Water Sci. Tech- nol., 24 (8), 87.

Monod, J. (1949). The growth of bacterial cultures. Ann. Rev. Microbiol., 3, 371.

van den Berg, L. (1984). Developments in methanogenesis from industrial wastewater. Can. J. Microbiol., 30, 975.

Yoda, M., Kitagawa, M. & Miyaji, Y. (1987). Long-term competition between sulfate-reducing bacteria and methane producing bacteria in anaerobic biofilm. Water Res., 21, 1547.

low-strength wastewater treatment by a uasb reactor

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