aerobic digestion of septage

6
n. ( ELSEVIER PIl:S0960-8524(97)00170-3 Bioresource Technology 64 (1998) 219-224 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain (1960-8524/98 $19.00 AEROBIC DIGESTION OF SEPTAGE Chiu-Yue Lin* & Jen Chou Graduate Institute of Civil and Hydraulic Engineering, Feng Chia University, Taichung, Taiwan, R.O.C. (Received 11 February 1997; accepted 20 October 1997) Abstract This paper examines the operation parameters and treatment efficiency in septage processing. The septage originated from septic tanks serving non-sewered areas in Taichung City. An 8-I lab-scale digester was operated at ambient temperatures at solids retention times of 30, 20, 15, 10 and 5.3 days. The digester was fed once a day. The digester successfully operated at all the tested SRTs, but the highest efficiency was observed at SRT 10 days, organic loading 464g COD/m3-day (chemical oxygen demand), or volatile solids (VS) loading 1. 462 kg/m3-day, with a COD removal efficiency of 80.4% and 30% VS reduction. Under this optimal operating condition, the removal rates of COD, NH3-N and total phosphorus were 373, 26.1 and 0. 7 g/m3-day, respectively. The fraction of non-degradable VS to total VS was as high as 42.2% and the rate coefficient, K~, for degradable VS destruc- tion was O.132/day. © 1998 Elsevier Science Ltd. All rights reserved Key words: Aerobic digestion, septage, organic loading, nitrogen, phosphorus. INTRODUCTION Treating cesspool septage or night soil in non-sewered areas has been found to prevent environmental pollution (Hashimoto et al., 1982; Noike and Matsumoto, 1986; Sai Ram et al., 1993; Andreadakis et al., 1995). Only 3.5% of the popula- tion in Taiwan is served by sanitary sewer systems. More than 80% of the night soil is treated in septic tanks that produce thousands of tons of septage daily. This septage has different characteristics from night soil because some degree of biological conver- sion occurs in the septic tank. Most of the septage is not well treated and this causes pollution problems in the country. Aerobic digestion has been employed for several years to stabilize wastewater sludge. The aerobically digested night soil sludge has been *Author to whom correspondence should be addressed. 219 effectively utilized (Cho et al., 1991). This paper discusses the aerobic digestion of septage from Taichung City which has a population of 800 thousand people, and aims to investigate the treat- ment efficiency and the process operation param- eters for treating septage. METHODS Experiments were performed in a CSTR (completely stirred tank reactor) aerobic digester. The digester was an open top vessel of 8-1 working volume fitted with a porous diffuser aerator. The air was intro- duced for oxygen supply and agitation. The substrate feeding was in a fill-and-draw operation type and occurred once a day. The digester was operated at ambient temperatures (i.e., 28°C in the summer and 18°C in the winter). The seed sludge was obtained from the digested sludge of an aerobic digestion tank of the Taichung Night Soil Treatment Plant and was acclimated with the septage. The substrate septages originated from septic tanks in Taichung City. Table 1 lists the septage characteristics. No nutrient was added to the substrate. The solids retention times (SRT) equalled hydraulic retention times and were 30, 20, 15, 10 and 5.3 days. The experiments were started from SRT 30 days. After obtaining the data the retention time was then short- ened. Feeding was conducted once a day and the liquid lost through evaporation was compensated with distilled water before feeding. Throughout the experiments, the air flow was maintained at 0.70-0.901 air/liter liquor and the dissolved oxygen was more than 2 mg/l. No pH control was performed. The reactor was routinely monitored for pH, oxidation-reduction potential (ORP), chemical oxygen demand (COD), alkalinity, and solid concentrations. Under steady-state condi- tions, defined as the conditions during which the variations of the above parameter values were small (approx. 10%) during 2 weeks of operation, other water quality items, nitrogen, phosphorus, saccha- ride, protein, volatile fatty acids (VFAs) and capil-

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Page 1: Aerobic digestion of septage

n. (

ELSEVIER P I l : S 0 9 6 0 - 8 5 2 4 ( 9 7 ) 0 0 1 7 0 - 3

Bioresource Technology 64 (1998) 219-224 © 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain (1960-8524/98 $19.00

AEROBIC DIGESTION OF SEPTAGE

Chiu-Yue Lin* & Jen Chou

Graduate Institute of Civil and Hydraulic Engineering, Feng Chia University, Taichung, Taiwan, R.O.C.

(Received 11 February 1997; accepted 20 October 1997)

Abstract This paper examines the operation parameters and treatment efficiency in septage processing. The septage originated from septic tanks serving non-sewered areas in Taichung City. An 8-I lab-scale digester was operated at ambient temperatures at solids retention times of 30, 20, 15, 10 and 5.3 days. The digester was fed once a day. The digester successfully operated at all the tested SRTs, but the highest efficiency was observed at SRT 10 days, organic loading 464g COD/m3-day (chemical oxygen demand), or volatile solids (VS) loading 1. 462 kg/m3-day, with a COD removal efficiency of 80.4% and 30% VS reduction. Under this optimal operating condition, the removal rates of COD, NH3-N and total phosphorus were 373, 26.1 and 0. 7 g/m3-day, respectively. The fraction of non-degradable VS to total VS was as high as 42.2% and the rate coefficient, K~, for degradable VS destruc- tion was O.132/day. © 1998 Elsevier Science Ltd. All rights reserved

Key words: Aerobic digestion, septage, organic loading, nitrogen, phosphorus.

INTRODUCTION

Treating cesspool septage or night soil in non-sewered areas has been found to prevent environmental pollution (Hashimoto et al., 1982; Noike and Matsumoto, 1986; Sai Ram et al., 1993; Andreadakis et al., 1995). Only 3.5% of the popula- tion in Taiwan is served by sanitary sewer systems. More than 80% of the night soil is treated in septic tanks that produce thousands of tons of septage daily. This septage has different characteristics from night soil because some degree of biological conver- sion occurs in the septic tank. Most of the septage is not well treated and this causes pollution problems in the country. Aerobic digestion has been employed for several years to stabilize wastewater sludge. The aerobically digested night soil sludge has been

*Author to whom correspondence should be addressed. 219

effectively utilized (Cho et al., 1991). This paper discusses the aerobic digestion of septage from Taichung City which has a population of 800 thousand people, and aims to investigate the treat- ment efficiency and the process operation param- eters for treating septage.

METHODS

Experiments were performed in a CSTR (completely stirred tank reactor) aerobic digester. The digester was an open top vessel of 8-1 working volume fitted with a porous diffuser aerator. The air was intro- duced for oxygen supply and agitation. The substrate feeding was in a fill-and-draw operation type and occurred once a day. The digester was operated at ambient temperatures (i.e., 28°C in the summer and 18°C in the winter). The seed sludge was obtained from the digested sludge of an aerobic digestion tank of the Taichung Night Soil Treatment Plant and was acclimated with the septage. The substrate septages originated from septic tanks in Taichung City. Table 1 lists the septage characteristics. No nutrient was added to the substrate. The solids retention times (SRT) equalled hydraulic retention times and were 30, 20, 15, 10 and 5.3 days. The experiments were started from SRT 30 days. After obtaining the data the retention time was then short- ened. Feeding was conducted once a day and the liquid lost through evaporation was compensated with distilled water before feeding.

Throughout the experiments, the air flow was maintained at 0.70-0.901 air/liter liquor and the dissolved oxygen was more than 2 mg/l. No pH control was performed. The reactor was routinely monitored for pH, oxidation-reduction potential (ORP), chemical oxygen demand (COD), alkalinity, and solid concentrations. Under steady-state condi- tions, defined as the conditions during which the variations of the above parameter values were small (approx. 10%) during 2 weeks of operation, other water quality items, nitrogen, phosphorus, saccha- ride, protein, volatile fatty acids (VFAs) and capil-

Page 2: Aerobic digestion of septage

220 C.-Y. Lin, J. Chou

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lary suction time (CST) were determined as well. The analytical procedures of Standard Methods (APHA, 1992) were employed to determine the general water quality. VFAs were analyzed with a gas chromatograph which had a flame ionization detector (glass column, 145°C; injection tempera- ture, 175°C). The Anthrone and Bradford methods were employed to measure carbohydrates and proteins, respectively. The Buchner funnel method was employed to measure CST with a CST Filter- ability Tester (Triton, Model 200, UK).

RESULTS A N D D I S C U S S I O N

Septage characteristics The average concentrations of volatile solids (VS) and total COD (TCOD) in the septage were 16581 and 16770mg/1, respectively. The septic tank contents had not been periodically pumped and the septage was approximately 10% weaker than the night soils in the concentrations of total COD, ammonia nitrogen and total phosphorus (Jian, 1995). The fractions of VS to total solids (TS) and soluble COD (SCOD) to TCOD were 0.30 and 0.29, respectively.

Data under steady-state conditions Figure 1 presents the experimental data for the effluent pH, alkalinity, ORP, COD, TS and VS during the operation of the digester. The digester at the SRT of 15 days was operated in the winter period (18°C) and could be classified as psychro- philic digestion. In each run of a SRT, the digester was operated for a period twice the operating time of the SRT before reaching steady-state conditions.

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10000 -

0

8

6

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Ol~ration Time (days)

Fig. I. The daily variations of the digester contents: (a) pH, (b) Alkalinity, (c) ORP, (d) Solids, (e) COD.

Page 3: Aerobic digestion of septage

Aerobic digestion of septage 221

The experimental data under steady-state conditions are summarized in Table 2. Each data value in the table is an average of three to five readings obtained during the steady-state conditions. The coefficients of deviation were 1.4 to 15.3%. Table 2 shows that the digester successfully operated at the organic loadings of 178-1433 g COD/m3-day, or VS loadings of 0.633-3.769 kg/m3-day.

Digestion efficiency The SCOD concentration in the digester super- natant was 570-2187mg/1. The SRT affected the SCOD removal efficiencies (Fig. 2). Higher SCOD removal efficiencies (89.2%, 30 days) were obtained at longer SRT and the value was 71.1% for the shortest SRT, 5.3 days (organic loadings 1433 g COD/m3-day). The SCOD removal efficiency could be expressed as:

% SCOD removal =91.7(1- 1.21/SRT) (1)

1.21 denotes a value indicating the slope and is comparable with the values of an anaerobic filter (1.8) (Muller and Mancini, 1981). It is noted that a prompt decrease in SCOD removal efficiency occurred at SRT 5.3 days.

The reduction of VS is normally employed to indicate a digestion process efficiency. The VS reduction ranged from 17 to 43%, values which are comparable with the digestion of primary and secondary sludges (Eckenfelder and Santhanam, 1981; Metcalf and Eddy, 1991). The fractions of VS to TS were calculated to be 0.34, 0.40, 0.50, 0.51, and 0.60, for each SRT, respectively. Notably, a prompt increase in the VSfFS ratio occurred at SRT 5.3 days. The relationship relating VS reduction to SRT was:

% VS reduction = 1.68+0.792(SRT) (2)

Figure 2 also shows that the VS reduction increased with the increasing SRT. The relationship

Table 2. Data under steady-state conditions for each SRT

SRT (days) 30 20 15 10 5.3 Organic loading 178 309 422 464 1433

(g COD/m3-day) VS loading 0.633 0.839 1.159 1.462 3.769

(kg/m3-day) Temperature (°C) 28___ 2 23 + 2 18 ± 2 22 ± 2 28.2

Digester mixtures pH 7.2_0.2 a 7.0+0.2 ORP (mV) 295 ± 24 215 + 22 TS (mg/1) 31213 ± 658 25 800___ 2743 VS (mg/l) 10 787 ± 782 10 283 ___ 460 Alkalinity 2060 + 109 1288 ___ 98

(mg/l as CaCO3) TCOD (mg/1) 6840___ 330 8970 ± 280 CST (s) 19_+0.5 15±1

6.9±0.1 229 ± 19

23147 ± 1082 11583±515

1360+ 116

7310 + 180 12.5 + 0.5

6.9+0.2 218+15

20 330 ± 398 10 347 ± 460

1160___58

5920 ± 360 10.2±0.4

6.5 + 0.3 109+9

22670 + 1638 13 647 + 838

922 + 72

9983 ± 1524 33+6

Digester supernatants SCOD (mg/1) 570___ 30 860___ 22 1080 + 50 910_ 20 2187__+ 254 TVFA (mg COD/I) 241 ___ 17 310+ 14 479+50 566+37 891 + 104 NHrN (mg/1) 38___ 2 35 ___ 3 23 + 3 63 + 4 57___ 2 T-P (mg/1) 12_ 1 21 +2 7+0.5 10+ 1 11 ___ 1 Protein (mg/l) 67 ± 3 49 ___ 4 63 ± 2 64 + 3 57 + 2 Saccharide (mg/1) 82___ 3 82 ___ 5 67 + 5 50 + 4 30 ± 2

Removal efficiency (%) VS reduction 43 39 33 30 17 COD 89 86 83 80 71 NH3-N 90 91 91 80 71 T-P 77 70 67 60 58 Saccharide 64 61 59 59 56 Protein 52 65 64 47 44

Removal rate (g/m3-day) b COD 159 266 350 373 1020 NH3-N 10.8 17.6 15.8 26.1 17 T-P 0.6 0.7 0.5 0.7 1.3 Saccharide 5 6.8 6.5 7.1 7.7 Protein 2.4 4.4 7.6 5.9 9.1

CST, capillary suction time. aMean _ SD (n = 3-5). bAverage (n = 3-5).

Page 4: Aerobic digestion of septage

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:COD Y=0.670*X+71.2

o :Observed VS destroyed Y=0.972*X+16.8 R-squared=0.861

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I I I I I

10 15 20 25 30

SRT (days) Fig. 2. The relationships between SRT, COD removal and VS reduction efficiency. Predicted VS destroyed was

calculated from eqn (5). VFA = 59+0.38(SCOD)

between VS reduction and product of temperature and digestion time has been utilized for aerobic digester design (Metcalf and Eddy, 1991). The plot in Fig. 3 relates the VS reduction and the product of the operating temperature and SRT. This plot agrees well with a reported result (Water Pollution Control Federation, 1985). Notably, the operating temperature and SRT were 18°C and 15 days, respectively, at the organic loading of 422g COD/m3-day. The temperature-SRT product value was calculated to be 270 degree-days, which is comparable with a reported result (Water Pollution Control Federation, 1985).

The CST assesses the digester mixture dewater- ability. The CST value was 121-185s for the substrate septage and 10.2-19s for the digested septage. An obvious improvement in the dewater- ability was observed.

Eff luent p H a n d a lkal in i ty The pH value was 6.5-7.2 for the influent. The effluent pH values were 6.5-7.2 and short SRT had low values. The alkalinity concentrations ranged from 922 to 2060 mg/1 as CaCO3. At shorter SRTs the digester mixtures were of lower alkalinity levels, which agreed with the pH variation. The reductions of pH and alkalinity might be caused by the occur- rence of nitrification (Qasim, 1994).

Eff luent volat i le fatty acids Short chain VFAs are often associated with waste treatment problems because of their offensive odors, but can be removed by aeration (Caunt and Hester, 1989). Figure 4 reveals that the VFA concentration in the supernatants decreased with increased reten- tion times. The relationship between effluent VFA and SCOD (both in units of mg COD/I) was regressed as follows, with a coefficient of relation (R) of R E = 0.89.

(3)

6 0 - -

The effluent VFA concentration ranged from 142 to 1157 mg COD/1.

The average concentrations of effluent VFAs were (mg COD/I): acetic acids (HAc) 150, propionic acid (HPr) 194, butyric acid (n-HBu) 68 and iso-butyric acid (i-HBu) 94. HPr was the major component of the VFAs (42%). Furthermore, normal forms of both valeric and caproic acids were present in moderate concentrations. Biodegradable VFAs produced from anaerobic digestion have been recently employed to promote biological nutrient removal of phosphorus or nitrogen (Abu-Gharahg and Randall, 1991; Pitman et al., 1992; Goncalves et al., 1994; Tam et al., 1994; Bernet et al., 1996; Hatzi- constantinou et al., 1996). In the present study, the effluent COD concentrations were far higher than the discharge standard of the country (200 mg/l) and further treatment seemed necessary. The VFAs in

1000 -

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I I I I

0 200 400 600 800 1000

Temperature * SRT (°c-days) Fig. 3. Volatile solids reduction as a function of operation

temperature and solids retention time.

800 -

>

600 -

400 -

200 I I I

222 C.-Y Lin, Z Chou

0 10 20 30

SRT (days)

Fig. 4. The relationships between SRT and effluent VFA concentration.

Page 5: Aerobic digestion of septage

Aerobic digestion of septage 223

the effluent might be useful for sequential biological nutrient removal.

R e m o v a l o f n i t r o g e n a n d p h o s p h o r u s The influent ammonia nitrogen (NH3-N) and phosphorus (T-P) concentrations were high, 123-380 and 2 2 - 5 1 m g / 1 , respectively. Nitrogen and phosphorus are employed as nutrients in biological treatment. Therefore, biological trans- formation in the chemical state of nitrogen and phosphorus occur. The concentrations of these two nutrients in the digester supernatants were 23-63 mg NH3-N/1 and 7-21 mg T-P/I. Those values were lower than values reported by Qasim (1994). Figure 5 shows that the removal efficiencies of nitrogen and phosphorus increased with increased retention time. T h e highest removal rates for ammonia nitrogen and total phosphorus were, respectively, calculated to be 26.1 and 0.7 g/m3-day at SRT 10 days and 17 and 1.3 g/m3-day at SRT 5.3 days. No determination of the various possible forms of nitrogen and phosphorus was done. However, the ammonia nitrogen and phosphorus would be biologically transformed into NO2 and NO3, and PO43-, respectively, during the aerobic digestion process (Matsuda et al., 1988).

R e m o v a l o f c a r b o h y d r a t e s a n d p r o t e i n The removal efficiencies of carbohydrates increased with the increased retention time (Fig. 6), but the protein removal efficiencies initially increased to 65% at SRT 20 days and then decreased to 52% at SRT 5.3 days. This indicated that protein degrada- tion was SRT dependent.

The highest efficiency was at SRT 10 days, or an organic loading of 464gCOD/m3-day. The VS loading was 1.462 kg/m3-day, the magnitude of which is comparable with design criteria (Qasim, 1994).

K i n e t i c s Several kinetic equations have been developed for aerobic sludge digestion (Benefield and Randall,

o

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100 -

80

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J

<5 Y = 0.691 *,.x + 74.9

Y = 0.790 * X + 38.3

T-P

20 I I I

0 10 20 30

SRT (days)

Fig. 5. The removal efficiency of ammonia nitrogen and total phosphorus at each SRT under steady-state

condition.

7 0 -

O "5

"d O

6 0 -

5 0 -

40

0

Carbohydrate

Protein

I I I I

10 20 30 40

SRT (days) Fig. 6. The removal efficiency of carbohydrate(s) and

protein at each SRT under steady-state condition.

1978, 1980; Mavinic and Koers, 1979). The kinetic equations utilizing a pseudo-second-order model were used here (Eckenfelder and Santhanam, 1981) [eqn (4)].

( X e - X n )

( Xo - Xn) =e -K"' (4)

where Xe = effluent total VS remaining at time t (rag/l), Xo = influent total VS at time 0 (rag/l), Xn = non-degradable portion of VS, assumed constant throughtout aeration period (mg/1), Kb = batch aeration rate for degradable VS destruc- tion (1/day), t = time of aeration (days).

For the septage studied herein, Fig. 7 plots the batch data and a value of 8020 mg VS/I was obtained for the non-degradable VS, Xo. The fraction of non-degradable VS to total VS was then calculated to be 42.2%. The rate coefficient, Kb, determined on a batch test (Fig. 8) was 0.132/day, which is lower than the value for aerobically digesting waste activated sludge (Eckenfelder and Santhanam, 1981). The lower Kb value in the present study was considered to be caused by the fact that the septage

20000 -

>

( 15000 -

10000 -

50000 1

0 I I I I I I I 5 10 15 20 25 30 35

Batch Aeration Time (days)

Fig. 7. Chronological destruction of VS in a batch reactor.

Page 6: Aerobic digestion of septage

224 C.-Y Lin, J. Chou

;>

10000 7

1 0 0 0 -

1 0 0 -

)

R - s q u a r e d = 0 . 9 8

1 0 I I I I I I I

0 5 10 15 20 25 30 35

B a t c h Aeration Time (days)

Fig. 8. Correlation between degradable VS and retention time.

had been biologically decomposed in the septic tanks. Applying the Kb value in eqn (5), which was derived from the mass balance of degradable VS, enabled the the effluent VS to be predicted. Figure 2 also lists the predicted values of VS destroyed. According to those plots, the observed values were in excellent agreement with the predicted values.

Xo+ Kbt Xn x e - (5 )

l+Kbt

A C K N O W L E D G E M E N T S

The authour would like to thank the Environmental Protection Agency, Taiwan, Republic of China for financially supporting this work under Contract No. EPA-85-E3G1-09-02.

R E F E R E N C E S

Abu-Gharahg, Z. H. & Randall, C. W. (1991). The effect of organic compound on biological phosphorus removal. Wat. Sci. Tech., 23, 585-594.

Andreadakis, A. D., Kondili, G., Mamais, D. & Noussi, A. (1995). Treatment of septage using single and two stage activated sludge batch reactors system~ Wat. Sci. Tech., 32, 95-104.

APHA, AWWA, WPCF (1992). Standard Methods for the Examination of Water and Wastewater, 18 edn. American Public Health Association. Washington, D.C., USA.

Benefield, L. D. & Randall, C. W. (1978). Design relationships for aerobic digestion. J. Water Pollut. Control Fed., 50, 518-523.

Benefield, L. D. and Randall, C. W. (1980). Biological Process Design for Wastewater Treatment. Prentice-Hall, New Jersey, pp. 479-513•

Bernet, N., Habouzit, F. & Moletta, R. (1996). Use of an industrial effluent as a carbon source for denitrification of a high-strength wastewatet Appl. Microbiol. Biotechnol., 46, 92-97.

Caunt, P. & Hester, K. W. (1989). A kinetic model for volatile fatty acid biodegradation during aerobic treat- ment of piggery wastes Biotech. Bioeng., 34, 126-130.

Cho, K.-S., Hirai, M. & Shoda, M. (1991). Removal of dimethyl disulfide by the peat seeded with night soil sludge• J. Ferm. Bioeng., 71, 289-291.

Eckenfelder, Jr., W. W. and Santhanam, C. J. (1981)• Sludge Treatment. Marcel Dekker, New York.

Goncalves, R. F., Charlier, A. C. & Sammut, F. (1994). Primary fermentation of soluble and particulate organic matter for wastewater treatment. Wat. Sci. Tech., 30, 53-62.

Hashimoto, S., Fujita, M. & Baccay, R. A. (1982). Biomass determination in the anaerobic digestion of night soit J. Ferment. Technol., 60, 51-54•

Hatziconstantinou, G. J., Yannakopoulos, P. & Andrea- dakis, A. (1996). Primary sludge hydrolysis for biological nutrient removat Wat. ScL Tech., 34, 417-423.

Jian, H. C. (1995). The treatment and disposal of septage produced from septic tanks of buildings. An EPA Report, Taiwan, ROC (EPA-84-G205-09-08) (in Chinese).

Matsuda, A., Ide, T. & Fujii, S. (1988). Behavior of nitrogen and phosphorus during batch aerobic digestion of waste activated sludge - - continuous aeration and intermittent aeration by control of DO. Wat. Res., 22, 1495-1501.

Mavinic, D. S. & Koers, D. A. (1979). Performance and kinetics of low-temperature aerobic sludge digestion. J. Water Pollut. Control Fed., 51, 2088-2097.

Metcalf & Eddy Inc. (1991). Wastewater Engineering, 3rd edn. McGraw Hill, Inc., Singapore. pp. 835-842.

Muller, J. A. and Mancini, J. L. (1981). Anaerobic filter kinetics and application. Proc. 36th Ind. Waste Conf., Purdue University.

Noike, T. & Matsumoto, J. (1986). Upgrading of anaerobic digestion processes for night soit Wat. Sci. Tech., 18, 249-256•

Pitman, A.R., Lotter, L.H., Alexander, W.V. & Deacon, S.L. (1992). Fermentation of raw sludge and elutriation of resultant acid to promote excess phosphorus removat Wat. Sci. Tech., 25, 185-194.

Qasim, S. R. (1994)• Wastewater Treatment Plants. Technomic Publishing Company, Inc. Lancaster, Pennsylvania, USA. pp. 451-488.

Sai Ram, M., Singh, L. & Alam, S. I. (1993). Effect of sulfate and nitrate on anaerobic degradation of night soit Biores. Tech., 45, 229-232.

Tam, N., Leung, G. & Wong, Y. S. (1994). The effects of external carbon loading on nitrogen removal in sequencing batch reactors Wat. Sci. Tech., 30, 73-81.

Water Pollution Control Federation (1985). Sludge Stabili- zation, Manual of Practice FD-9.