- Email: [email protected]
A pilot plant and anaerobic digestion process for raisin finishing wastewater treatment
Biological Wastes 22 (1987) 129-138
A Pilot Plant and Anaerobic Digestion Process for Raisin Finishing Wastewater Treatment
N. Athanasopoulos, A. A. Koutinas & A. Papadimitriou Department of Chemistry, University of Patras, Patras, Greece (Received 4 June 1986; revised version received 28 January 1987; accepted 5 February 1987)
ABSTRACT A process andpilot plant, utilizing raisin-finishing wastewater, are described. The wastewater contains invert sugar and SO~- (0"8-3"Oglitre -1) as pollutants. The process is a continuous anaerobic digestion of the wastewater. The bacteria were mainly immobilized in the reactor on a large surface area of plastic support media. For best performance of this upflow anaerobic biofilter, sulfites were removed with lime prior to fermentation. High sulfite concentrations in the initial wastewater did not then cause significant inhibition. Removal o f COD from lime-treated influent to the reactor was over 90% for loadings up to 13 kg COD per cubic metre of initial reactor volume per day. To determine the purification of wastewater at different flow rates and COD loadings, determinations of BOD 5, COD, SO~ -, SOZa-, S 2 - in the influent and effluent were made. Volatile Fatty Acids ( VFA ) in the reactor and the composition of biogas are also reported.
INTRODUCTION Raisins are a Greek national product. The annual exported quantities are
110000 tons. Pollution, caused by discharging the wastewater of the product-finishing process into the sea, is equivalent to 100 000 population. Raisin-finishing wastewater contains invert sugar and SO~- as pollutants. Sulfites are either free or b o u n d to the carbonyl group of reducing sugars. 129 Biological Wastes 0269-7483/87/$03.50 © Elsevier Applied SciencePublishers Ltd, England, 1987. Printed in Great Britain
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
130
vibrating screen vibrating setr e e n
) wastewa ter recycled water I ecycled w a t e r From final wash
% 11111 11121
V ,oooo , ')/7 7/' >
)
Fig. 1.
Raisin-finishing process.
Bound sulfites dissociate to free sulfites in alkaline solutions. The finishing process for raisins is shown in Fig. 1. The most common type of treatment of this effluent is biological oxidation (biofiltration/activated sludge), but the supply of the necessary air and nutrients for this aerobic process is very expensive. A possible alternative treatment for this wastewater is anaerobic digestion. Its key advantages are low energy consumption, and production of a useful fuel gas (methane) with a relatively low volume of sludge. The wastewater may contain such amounts of organic matter that the energy of the gas is greater than the energy needed to operate the reactor at its optimal temperature (35-3 7°C for mesophilic bacteria). Nutrient (nitrogen and phosphorous) requirements are related to sludge generation. Typical anaerobic systems need only 10-20% of the nutrients used in aerobic systems. Some sulfur compounds may cause problems. Reduction products often have a strong odor and contribute to the oxygen demand in the effluent. Sulfur compounds may affect the good performance of an anaerobic reactor either directly (S z-, SO32-), by inhibiting or killing methane-forming bacteria, or indirectly (SO42-, SOl-), because of the competition between methane-formers and sulfate-reducers for organic carbon. A number of publications discuss the inhibitory effect of sulfides and sulfites on anerobic digestion. The requirement for sulfide in a pure culture of Methanosarcina barkeri was determined (Mounford & Asher 1979). It was found that sulfide inhibited methanogenic activity when added in concentrations greater than 400 mg liter- 1, but because of loss of sulfide to the gas phase the actual level of soluble sulfide in the growth medium which caused inhibition was 240 mg liter-1. Other investigators have studied the effects of soluble sulfides on anaerobic digestion (Lawrence & Guerin, 1966).
Digestion of raisin wastewater
131
It was reported that soluble sulfide levels up to 200 mg liter- 1 had little effect on digestion, while higher levels of sulfide caused severe toxicity. Later it was shown that soluble sulfide concentrations of up to 400 mg liter-1 did not have an inhibiting effect in a down-flow anaerobic filter (Szendrey, 1983). Sulfide concentration up to 250 mg liter-1 also caused no inhibition in a 5 m a down-flow anaerobic filter (Athanasopoulos, 1986). In a study concerning steady-state additions of sulfite and sulfide to a submerged media anaerobic reactor it was reported that sulfite or sulfide concentrations of 3500 mg liter- 1 and 400 mg liter- 1, respectively, could be tolerated after adaptation (Yang et al., 1979). In other anaerobic filters Eis et al. (1983) found that sulfite was not toxic at concentrations of 1200mgliter-1 and loadings of 6 kg COD m - 3 day-1. The presence of sulfate or s'ulfite may cause competition between methanogens and sulfate-reducing bacteria for the available hydrogen and acetate (Winfrey & Zeikus, 1977). The competitive inhibition was studied in purified cultures (Abram & Nedwell, 1978), It was found that methanogenic bacteria and sulfate-reducing bacteria can co-exist in the presence of sulfate, and the outcome of competition at any time is a function of the rate of hydrogen production, the relative populations and sulfate availability (Lorley et al., 1982). In the present work a pilot plant and a process for a sulfite-rich raisinfinishing wastewater treatment are presented and the applicability of an upflow fixed bed anaerobic reactor is examined.
METHODS BOD 5 and COD were determined as in S t a n d a r d M e t h o d s (APHA-AWWAWPCF, 1975) as were the sulfate, sulfide, sulfite and VFA. The composition of biogas was determined by gas chromatographic analysis in a Varian aerograph, series 1700, instrument with thermal conductivity detector. For the determination of methane and carbon dioxide a Poropac Q column was employed. During this study, which lasted one-and-a-half years, wastewater from 'A Kouniniotis' raisin-finishing plant was used. The main problem of this wastewater was its high sulfite concentration (up to 3000 mg liter-1). Sulfur dioxide is used to give raisins their golden color. Sulfur dioxide treatment takes place in a closed reactor to which continuously washed raisins and the gas are fed. At the exit of the reactor raisin washing is repeated. Some sulfur dioxide is removed by the water. This wastewater also contains invert sugar, expressed as a COD value of about 20 000 mg liter- 1. Concentrated wastewater (up to COD 80000 mg liter-1) was prepared by adding raisin juice.
N. ,4thanasopoulos, .4..4. Koutinas, A. Papadimitriou
132
Pilot plant Two types of upflow fixed-bed reactor were used. In the first phase of this study, a completely filled packed-bed reactor was used (Fig. 2 including section (a)). It consisted of a packed bed reactor of 80 liters total volume. The material used for the packed bed was PVC corrugated modular blocks with porosity greater than 95% and a specific surface area of 98 m 2 m -3. The height of the reactor was 2 m and the base 0.2 × 0.2 m. The temperature was kept constant at 36 + I°C by a water jacket around the tank. A variable flowrate peristaltic p u m p (Cole Parmer, USA) was used to feed the reactor, which was insulated with a 50 m m layer of glass wool. Biogas produced was passed through a zinc acetate and acetic acid solution trap of 5 liters capacity for H2S removal and collected and measured in two brine-sealed plastic gas holders of 100-liter capacity. During the second phase of the study, in order to overcome the problems of foaming and biomass washing out due to the design of the reactor, a settling tank of 25 liters was installed (Fig. 2 including Section (b)). This tank
>gas
TC
Fig. 2.
Process flowsheet for fixed-bed reactor. T.C., thermostatted water circulator.
Digestion of raisin wastewater
133
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
134
o o 0
.=_ 0
7
0 0 0 0
I ~ ° 0~
o 6 6 ~
~8 o e~
[j
0
© r~
©
Digestion of raisin wastewater
135
COD m - 3 d a y -1. In fluent COD was brought to 20 000 mg liter -1 by diluting concentrated wastewater. Sulfite concentration in the influent wastewater was up to 3000 mg liter- 1. Odor from the reactor was strong due to various volatile sulfur compounds which were formed and released with the biogas. This caused complaints in the laboratory building and could be a problem in a full-scale plant. In order to overcome this problem, it was decided to remove the sulfite from the feed wastewater. However, due to the design of the reactor, during change of load to 5 or 6 kg COD m 3 d a y - t a foam was formed and biomass was washed out by the biogas. Biomass was retained in a 5-liter capacity glass trap and returned to the reactor by a peristaltic pump (Cole Parmer, USA) but this was not efficient.
A modification of the pilot plant Raisin wastewater was treated with lime to pH = 12 and settled. Clear supernatant liquid was used for feeding the anaerobic reactor. Nutrients were added at the same ratio as before and the pH was controlled to 7.1-7.3 with NaOH and NaHCO 3. Since the production of biogas was quite high, it was impossible to continue the study with the same design of the pilot plant for higher loadings. It was decided to make some modifications in the exit of liquid and biogas. As shown in Fig. 2, a settling tank (b) was added. Up to the level of the overflow of effluent the volume was 25 liters. Including the tank the total volume of the anaerobic reactor was increased to 105 liters, with the packed bed volume (as before) 76% of this. Settled biomass was returned daily to the bottom of the filter, from the conical bottom of the settling tank, with a peristaltic pump.
Sulfite, BOD 5 and COD removal Table 2 shows BOD s and COD values of influent and effluent at different flow rates, when the time of treatment in each case was at least 15 days. At relatively low COD values (13 000-24 000 mg liter -1) of influent, COD removal was about 90% for all flow rates used. When COD of the influent was very high (near 62 000 mg liter- 1) and flow rate the highest used, COD removal was 95%, but the effluent COD was higher than that at low influent COD values. At small organic loads, digester VFA concentrations are smaller than 0"5 g liter -1 and the biogas contains 63% v/v CH4. With influent of high organic load, at the highest flow rate used, VFA concentrations are higher and methane concentration in the biogas is reduced. As is shown in Table 3, the concentrations of SOl -, SO~- and S 2 - in the
136
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
I
I
,~ ~
t~
e4a
r~
,d e~ I
© r~
© r~
0 .,a
°
I
Digestion of raisin wastewater
137
100
--0 o
~'
80 > 6(; 0 E 4O
8 20 I
i
I
I
I
i
2
4
6
8
10
1
loading
Fig. 3.
g COD./~'day
COD removal versus loadings in upflow filter: (©) For lime-precipitated feed. (A) For feed of up to 3000mgliter-i in sulfite concentration.
effluent are smaller than those in the influent. The SO~- and S O l - are apparently reduced during digestion to hydrogen sulfide or other sulfur compounds. Some of these pollutants are contained in the biogas and the rest is in the purified wastewater. The appearance of sulfites in the effluent is probably due to some sulfite being bound to glucose. From Table 3 it is concluded that a sum of sulfites, sulfates and sulfides concentrations equivalent to 1052 mg liter- 1 of sulfites does not cause any inhibition to COD removal. In Fig. 3 it is shown that COD removal for lime-treated feed was higher than with untreated feed by up to 17%. This difference may be characterized as inhibition, possibly due to the reduction of SO]- to sulfide being competitive with methane production. Some of the effluent COD may then be dissolved H2S. For lime-treated influent the COD removal slightly increased with the loading, while for untreated it increased only up to 4 kg COD m - 3 d a y - ~. Biogas production during this study ranged from 0"45 to 0"65 m 3 per kilogram of COD removed. F r o m the performance data of this study it is concluded that raisinfinishing wastewater is effectively treated by this anaerobic method. Sulfites remaining after lime treatment seem to be not toxic and not significantly inhibitory to the process. On removing the sulfites with lime, the COD removal efficiency increases. High levels of sulfite in the wastewater (up to 3000 mg liter-1) do not cause significant inhibition. V F A concentration in the effluent and the composition of the biogas seem to be dependent on the loading of the system.
138
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou ACKNOWLEDGEMENT
The study was supported by a grant from the Greek organization EOMMEX.
REFERENCES Abram, J. W. & Nedwell, D. B. (1978). Inhibition of methanogenesis by sulfate reducing bacteria competing for transferred hydrogen. Arch. Microbiol., 117, 89-92. APHA-AWWA-WPCF (1975). Standard methods for the examination of water and waste-water, 439-41,443-6, 448-52, 469-70, 483-92. Athanasopoulos, N. (1986), Anaerobic treatment of beet molasses alcoholic fermentation wastewater in a down flow filter. Resources and Conservation, Elsevier Science Publishers. Eis, B. J., Ferguson, J. F. & Benzamin, M. M. (1983). The fate and effect of bisulfate in anaerobic treatment. J. Water Pollut. Control Fed., 55, 1355. Lawrence, A. W. and Guerin, F. J. A. (1966). The effects of sulfides on anaerobic treatment. Int. Jour. Air and Water Poll., 202, March. Lorley, D, R., Dwyer, D. F. & Klug, M. J. (1982). Kinetic analysis of competition between sulfate reducers and methanogens in sediments. Appl. Environ. MicroboL, 43, 1373. Mounford, D. O. & Asher, R. A. (1979). Effects of inorganic sulfide on the growth and metabolism of Methanosarcina barkeri Strain D.M., Appl. and Environ. MicrobioL, 43, 1373. Szendrey, L. M. (Jan. 1983). The Barcardi Corporation Digestion process for stabilizing run distillery wastes and producing methane. Proceedings 7th symposium. Energy from Biomass and Wastes, Orlando, Fla. Winfrey, M. R. & Zeikus, J. G. (1977). Effect of sulfate on carbon and electron flow during microbial methanogenesis in fresh water sediments. Appl. Environ. Microbiol., Feb. Yang, J., Parkin, G. F. & Speece, R. E. (1979). Recovery of anaerobic digestion after exposure to toxicants. Final Report of contract No. EC-77-S-02-4391, Drexel University, Pa 19104.
A Pilot Plant and Anaerobic Digestion Process for Raisin Finishing Wastewater Treatment
N. Athanasopoulos, A. A. Koutinas & A. Papadimitriou Department of Chemistry, University of Patras, Patras, Greece (Received 4 June 1986; revised version received 28 January 1987; accepted 5 February 1987)
ABSTRACT A process andpilot plant, utilizing raisin-finishing wastewater, are described. The wastewater contains invert sugar and SO~- (0"8-3"Oglitre -1) as pollutants. The process is a continuous anaerobic digestion of the wastewater. The bacteria were mainly immobilized in the reactor on a large surface area of plastic support media. For best performance of this upflow anaerobic biofilter, sulfites were removed with lime prior to fermentation. High sulfite concentrations in the initial wastewater did not then cause significant inhibition. Removal o f COD from lime-treated influent to the reactor was over 90% for loadings up to 13 kg COD per cubic metre of initial reactor volume per day. To determine the purification of wastewater at different flow rates and COD loadings, determinations of BOD 5, COD, SO~ -, SOZa-, S 2 - in the influent and effluent were made. Volatile Fatty Acids ( VFA ) in the reactor and the composition of biogas are also reported.
INTRODUCTION Raisins are a Greek national product. The annual exported quantities are
110000 tons. Pollution, caused by discharging the wastewater of the product-finishing process into the sea, is equivalent to 100 000 population. Raisin-finishing wastewater contains invert sugar and SO~- as pollutants. Sulfites are either free or b o u n d to the carbonyl group of reducing sugars. 129 Biological Wastes 0269-7483/87/$03.50 © Elsevier Applied SciencePublishers Ltd, England, 1987. Printed in Great Britain
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
130
vibrating screen vibrating setr e e n
) wastewa ter recycled water I ecycled w a t e r From final wash
% 11111 11121
V ,oooo , ')/7 7/' >
)
Fig. 1.
Raisin-finishing process.
Bound sulfites dissociate to free sulfites in alkaline solutions. The finishing process for raisins is shown in Fig. 1. The most common type of treatment of this effluent is biological oxidation (biofiltration/activated sludge), but the supply of the necessary air and nutrients for this aerobic process is very expensive. A possible alternative treatment for this wastewater is anaerobic digestion. Its key advantages are low energy consumption, and production of a useful fuel gas (methane) with a relatively low volume of sludge. The wastewater may contain such amounts of organic matter that the energy of the gas is greater than the energy needed to operate the reactor at its optimal temperature (35-3 7°C for mesophilic bacteria). Nutrient (nitrogen and phosphorous) requirements are related to sludge generation. Typical anaerobic systems need only 10-20% of the nutrients used in aerobic systems. Some sulfur compounds may cause problems. Reduction products often have a strong odor and contribute to the oxygen demand in the effluent. Sulfur compounds may affect the good performance of an anaerobic reactor either directly (S z-, SO32-), by inhibiting or killing methane-forming bacteria, or indirectly (SO42-, SOl-), because of the competition between methane-formers and sulfate-reducers for organic carbon. A number of publications discuss the inhibitory effect of sulfides and sulfites on anerobic digestion. The requirement for sulfide in a pure culture of Methanosarcina barkeri was determined (Mounford & Asher 1979). It was found that sulfide inhibited methanogenic activity when added in concentrations greater than 400 mg liter- 1, but because of loss of sulfide to the gas phase the actual level of soluble sulfide in the growth medium which caused inhibition was 240 mg liter-1. Other investigators have studied the effects of soluble sulfides on anaerobic digestion (Lawrence & Guerin, 1966).
Digestion of raisin wastewater
131
It was reported that soluble sulfide levels up to 200 mg liter- 1 had little effect on digestion, while higher levels of sulfide caused severe toxicity. Later it was shown that soluble sulfide concentrations of up to 400 mg liter-1 did not have an inhibiting effect in a down-flow anaerobic filter (Szendrey, 1983). Sulfide concentration up to 250 mg liter-1 also caused no inhibition in a 5 m a down-flow anaerobic filter (Athanasopoulos, 1986). In a study concerning steady-state additions of sulfite and sulfide to a submerged media anaerobic reactor it was reported that sulfite or sulfide concentrations of 3500 mg liter- 1 and 400 mg liter- 1, respectively, could be tolerated after adaptation (Yang et al., 1979). In other anaerobic filters Eis et al. (1983) found that sulfite was not toxic at concentrations of 1200mgliter-1 and loadings of 6 kg COD m - 3 day-1. The presence of sulfate or s'ulfite may cause competition between methanogens and sulfate-reducing bacteria for the available hydrogen and acetate (Winfrey & Zeikus, 1977). The competitive inhibition was studied in purified cultures (Abram & Nedwell, 1978), It was found that methanogenic bacteria and sulfate-reducing bacteria can co-exist in the presence of sulfate, and the outcome of competition at any time is a function of the rate of hydrogen production, the relative populations and sulfate availability (Lorley et al., 1982). In the present work a pilot plant and a process for a sulfite-rich raisinfinishing wastewater treatment are presented and the applicability of an upflow fixed bed anaerobic reactor is examined.
METHODS BOD 5 and COD were determined as in S t a n d a r d M e t h o d s (APHA-AWWAWPCF, 1975) as were the sulfate, sulfide, sulfite and VFA. The composition of biogas was determined by gas chromatographic analysis in a Varian aerograph, series 1700, instrument with thermal conductivity detector. For the determination of methane and carbon dioxide a Poropac Q column was employed. During this study, which lasted one-and-a-half years, wastewater from 'A Kouniniotis' raisin-finishing plant was used. The main problem of this wastewater was its high sulfite concentration (up to 3000 mg liter-1). Sulfur dioxide is used to give raisins their golden color. Sulfur dioxide treatment takes place in a closed reactor to which continuously washed raisins and the gas are fed. At the exit of the reactor raisin washing is repeated. Some sulfur dioxide is removed by the water. This wastewater also contains invert sugar, expressed as a COD value of about 20 000 mg liter- 1. Concentrated wastewater (up to COD 80000 mg liter-1) was prepared by adding raisin juice.
N. ,4thanasopoulos, .4..4. Koutinas, A. Papadimitriou
132
Pilot plant Two types of upflow fixed-bed reactor were used. In the first phase of this study, a completely filled packed-bed reactor was used (Fig. 2 including section (a)). It consisted of a packed bed reactor of 80 liters total volume. The material used for the packed bed was PVC corrugated modular blocks with porosity greater than 95% and a specific surface area of 98 m 2 m -3. The height of the reactor was 2 m and the base 0.2 × 0.2 m. The temperature was kept constant at 36 + I°C by a water jacket around the tank. A variable flowrate peristaltic p u m p (Cole Parmer, USA) was used to feed the reactor, which was insulated with a 50 m m layer of glass wool. Biogas produced was passed through a zinc acetate and acetic acid solution trap of 5 liters capacity for H2S removal and collected and measured in two brine-sealed plastic gas holders of 100-liter capacity. During the second phase of the study, in order to overcome the problems of foaming and biomass washing out due to the design of the reactor, a settling tank of 25 liters was installed (Fig. 2 including Section (b)). This tank
>gas
TC
Fig. 2.
Process flowsheet for fixed-bed reactor. T.C., thermostatted water circulator.
Digestion of raisin wastewater
133
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
134
o o 0
.=_ 0
7
0 0 0 0
I ~ ° 0~
o 6 6 ~
~8 o e~
[j
0
© r~
©
Digestion of raisin wastewater
135
COD m - 3 d a y -1. In fluent COD was brought to 20 000 mg liter -1 by diluting concentrated wastewater. Sulfite concentration in the influent wastewater was up to 3000 mg liter- 1. Odor from the reactor was strong due to various volatile sulfur compounds which were formed and released with the biogas. This caused complaints in the laboratory building and could be a problem in a full-scale plant. In order to overcome this problem, it was decided to remove the sulfite from the feed wastewater. However, due to the design of the reactor, during change of load to 5 or 6 kg COD m 3 d a y - t a foam was formed and biomass was washed out by the biogas. Biomass was retained in a 5-liter capacity glass trap and returned to the reactor by a peristaltic pump (Cole Parmer, USA) but this was not efficient.
A modification of the pilot plant Raisin wastewater was treated with lime to pH = 12 and settled. Clear supernatant liquid was used for feeding the anaerobic reactor. Nutrients were added at the same ratio as before and the pH was controlled to 7.1-7.3 with NaOH and NaHCO 3. Since the production of biogas was quite high, it was impossible to continue the study with the same design of the pilot plant for higher loadings. It was decided to make some modifications in the exit of liquid and biogas. As shown in Fig. 2, a settling tank (b) was added. Up to the level of the overflow of effluent the volume was 25 liters. Including the tank the total volume of the anaerobic reactor was increased to 105 liters, with the packed bed volume (as before) 76% of this. Settled biomass was returned daily to the bottom of the filter, from the conical bottom of the settling tank, with a peristaltic pump.
Sulfite, BOD 5 and COD removal Table 2 shows BOD s and COD values of influent and effluent at different flow rates, when the time of treatment in each case was at least 15 days. At relatively low COD values (13 000-24 000 mg liter -1) of influent, COD removal was about 90% for all flow rates used. When COD of the influent was very high (near 62 000 mg liter- 1) and flow rate the highest used, COD removal was 95%, but the effluent COD was higher than that at low influent COD values. At small organic loads, digester VFA concentrations are smaller than 0"5 g liter -1 and the biogas contains 63% v/v CH4. With influent of high organic load, at the highest flow rate used, VFA concentrations are higher and methane concentration in the biogas is reduced. As is shown in Table 3, the concentrations of SOl -, SO~- and S 2 - in the
136
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou
I
I
,~ ~
t~
e4a
r~
,d e~ I
© r~
© r~
0 .,a
°
I
Digestion of raisin wastewater
137
100
--0 o
~'
80 > 6(; 0 E 4O
8 20 I
i
I
I
I
i
2
4
6
8
10
1
loading
Fig. 3.
g COD./~'day
COD removal versus loadings in upflow filter: (©) For lime-precipitated feed. (A) For feed of up to 3000mgliter-i in sulfite concentration.
effluent are smaller than those in the influent. The SO~- and S O l - are apparently reduced during digestion to hydrogen sulfide or other sulfur compounds. Some of these pollutants are contained in the biogas and the rest is in the purified wastewater. The appearance of sulfites in the effluent is probably due to some sulfite being bound to glucose. From Table 3 it is concluded that a sum of sulfites, sulfates and sulfides concentrations equivalent to 1052 mg liter- 1 of sulfites does not cause any inhibition to COD removal. In Fig. 3 it is shown that COD removal for lime-treated feed was higher than with untreated feed by up to 17%. This difference may be characterized as inhibition, possibly due to the reduction of SO]- to sulfide being competitive with methane production. Some of the effluent COD may then be dissolved H2S. For lime-treated influent the COD removal slightly increased with the loading, while for untreated it increased only up to 4 kg COD m - 3 d a y - ~. Biogas production during this study ranged from 0"45 to 0"65 m 3 per kilogram of COD removed. F r o m the performance data of this study it is concluded that raisinfinishing wastewater is effectively treated by this anaerobic method. Sulfites remaining after lime treatment seem to be not toxic and not significantly inhibitory to the process. On removing the sulfites with lime, the COD removal efficiency increases. High levels of sulfite in the wastewater (up to 3000 mg liter-1) do not cause significant inhibition. V F A concentration in the effluent and the composition of the biogas seem to be dependent on the loading of the system.
138
N. Athanasopoulos, A. A. Koutinas, A. Papadimitriou ACKNOWLEDGEMENT
The study was supported by a grant from the Greek organization EOMMEX.
REFERENCES Abram, J. W. & Nedwell, D. B. (1978). Inhibition of methanogenesis by sulfate reducing bacteria competing for transferred hydrogen. Arch. Microbiol., 117, 89-92. APHA-AWWA-WPCF (1975). Standard methods for the examination of water and waste-water, 439-41,443-6, 448-52, 469-70, 483-92. Athanasopoulos, N. (1986), Anaerobic treatment of beet molasses alcoholic fermentation wastewater in a down flow filter. Resources and Conservation, Elsevier Science Publishers. Eis, B. J., Ferguson, J. F. & Benzamin, M. M. (1983). The fate and effect of bisulfate in anaerobic treatment. J. Water Pollut. Control Fed., 55, 1355. Lawrence, A. W. and Guerin, F. J. A. (1966). The effects of sulfides on anaerobic treatment. Int. Jour. Air and Water Poll., 202, March. Lorley, D, R., Dwyer, D. F. & Klug, M. J. (1982). Kinetic analysis of competition between sulfate reducers and methanogens in sediments. Appl. Environ. MicroboL, 43, 1373. Mounford, D. O. & Asher, R. A. (1979). Effects of inorganic sulfide on the growth and metabolism of Methanosarcina barkeri Strain D.M., Appl. and Environ. MicrobioL, 43, 1373. Szendrey, L. M. (Jan. 1983). The Barcardi Corporation Digestion process for stabilizing run distillery wastes and producing methane. Proceedings 7th symposium. Energy from Biomass and Wastes, Orlando, Fla. Winfrey, M. R. & Zeikus, J. G. (1977). Effect of sulfate on carbon and electron flow during microbial methanogenesis in fresh water sediments. Appl. Environ. Microbiol., Feb. Yang, J., Parkin, G. F. & Speece, R. E. (1979). Recovery of anaerobic digestion after exposure to toxicants. Final Report of contract No. EC-77-S-02-4391, Drexel University, Pa 19104.