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Liquefaction and acidogenesis of tomatoes in an anaerobic two-phase solid-waste treatment system
Agricultural Wastes 11 (1984) 241 252
Liquefaction and Acidogenesis of Tomatoes in an Anaerobic Two-Phase Solid-Waste Treatment System
I. W. K o s t e r Agricultural University, Department of Water Pollution Control, De Dreijen 12, 6703 BC Wageningen, The Netherlands
ABSTRACT In The Netherlands approximateO' 13.5 x 106 kg of surplus tomatoes are disposed of each year. This solid waste can be treated in a two-step anaerobic system yielding methane and a ~compost-like" residue. In this paper, laboratoo, tests of the first phase (liqueJaction and acidogenesis) are described. Liquefaction of tomatoes appeared to be stimulated hy increasing the temperature to 32°C: the addition of tap water and continuous draining. The effluent o f batchwise liquefaction/acidogenesis of tomatoes has a COD: N ratio of 50.'l and can thus be used as an inlquent for a digester producing methane. In practice, the most appropriate liquefaction/acidogenesis reactor for the treatment of tomatoes would be a continuously drained reactor which also,functions as an eJ~uent-volume buffer.
INTRODUCTION In the period 1972-1978 the harvest of tomatoes in The Netherlands averaged 366 x 106kg year-1. Like almost all fruit and vegetables, tomatoes are sold at daily auctions. F o r reasons o f ' m a r k e t protection' the tomatoes that cannot be sold at a price above a weekly fixed minimum are destroyed. During the period 1972-1978 the a m o u n t of surplus tomatoes was 3.7 ~o o f the total harvest (13.5 x 106 kg y e a r - 1). The usual practice is that the surplus tomatoes are thrown in a waste-pit where they slowly rot. 241 Agricultural Wastes 0141-4607/84/$03.00 ~j Elsevier Applied Science Publishers Ltd,
England, 1984. Printed in Great Britain
242
I.W.
Koster
These waste-pits are often sources of environmental pollution. Therefore, controlled decomposition of surplus tomatoes becomes necessary. Operating a controlled system to dispose of the surplus tomatoes also has the advantage that the energy contained in the biomass of the tomatoes can be recovered (Versluys et al., 1982). The composition and energy value of tomatoes are shown in Table 1. This Table contains information derived from an information bulletin issued by the Sprenger Institute (Anon, 1979). TABLE 1
Composition and Energy Values of Tomatoes Water Protein Fat Carbohydrates Crude cellular tissue Ash Phosphorus Potassium Chloride Energy value
94.2* 0-95 0.21 33 0.75 0'61 0"03 0, 3 0,06 765 kJ/kg
* ~,,, by weight.
Organic solid wastes can be treated in a two-phase process consisting of a liquefaction/acidogenesis reactor and a reactor that converts the liquid product of the first reactor into methane (Rijkens, 1980, 1981a, b, 1982: Hofenk et al., 1982, 1983). The surplus tomatoes are only available during a relatively short period of time (the second half of July and the first half of August), but a slight modification of the system which was developed as a series of batchwise filled liquefaction/acidogenesis reactors, coupled with an upftow sludge blanket reactor for methanogenesis, could overcome this problem (Hofenk, 1983). This paper presents preliminary results of a test programme that should lead to design and operating parameters for a liquefaction/acidogenesis reactor for the treatment of surplus tomatoes. Since the aim of the proposed solid-waste treatment system is to liquefy biomass instead of preserving it, the agricultural practice of preserving cattle fodder as ensilage gives a few hints about conditions that should be avoided in the liquefaction/acidogenesis reactor (Wieringa & De Haan,
Lique[cwtion and acidogenesLs" o/ waste tomatoes
243
1961). Ensilage is only successful if the acidity of the piled biomass increases to a pH value below ca. 4. As long as the pH of the pile of biomass is at a value above ca. 4, the biomass can still be degraded by bacterial activity. Apart from the buffering capacity of the pile of biomass, the competition between butyric acid-producing bacteria and lactic acid-producing bacteria is the most important factor controlling the acidity. The development of a population of butyric acid-producing bacteria instead of a population of lactic acid-producing bacteria can be stimulated by increasing the temperature, by reducing the osmotic pressure of the medium, or by any measure that reduces the souring rate (e.g. the addition of lime). Hence, the following operation characteristics for the liquefaction/ acidogenesis reactor were tested: increasing the temperature to 32°C, adding tap water to the reactor contents, continuously removing the liquid resulting from bacterial activity in the reactor.
METHODS
Reactors All tests were performed with cylindrical reactors which were sealed after filling with tomatoes originating from the same grower. In practice surplus tomatoes are treated roughly, so the tomatoes were slightly damaged by squeezing them before they were put into a reactor. Reactors A and B were put in a room without any temperature control, so that the temperature varied daily in the range 15 25 °C. Reactor X was put in a temperature-controlled room at 22°C: reactors Y and Z were controlled at 32 °C. Reactors X, Y and Z were connected with a wet gas meter. The liquid produced in reactor Z was continuously removed through a U-shaped pipe at the bottom of the reactor. The liquid produced in the other reactors was removed batchwise. After every draining operation tap water was added to the contents of reactor B so that the remaining tomatoes were just below liquid level.
Analysis Volatile Fatty Acids (VFA) were assayed gas-chromatographically using a Packard model 427 with a flame ionisation detector and a computer
244
I.w. Koster
integrator, model 602. Prior to injection 1 ml of the sample was mixed with 0.1 ml of 1N HC1 and 0.1 ml of a solution of iso-capronic acid of known strength. This mixture was centrifuged to remove any solids. The resulting chromatogram was interpreted and computed using the isocapronic acid as an internal standard. e-lactic acid was assayed by an enzymatic method (Boehringer, Mannheim). For reasons of convenience, the assays were performed after all samples had been collected. To preserve the lactic acid during storage, all samples were immediately diluted with an equal volume of a 5 ~ (wv) solution of ZnSO 4 and stored at - 2 0 °C. Chemical Oxygen Demand (COD) and Kjeldahl-nitrogen were determined according to Standard Methodsjbr the Examination of Water and Wastewater (1975). Phosphate was determined according to the method of Van Schouwenburg & Walinga (1967).
RESULTS Reactors A and B were drained four times. Each time the total liquefaction product was collected, measured, and sampled after thorough mixing. For each sample, pH, COD and VFA were determined. The results are presented in Table 2. Reactors X and Y were drained once. During the period between filling and draining, regular sampling took place from a tap at the bottom of the reactor. These samples were analyzed for VFA and e-lactic acid, and the pH of each sample was determined. Figure I shows the development of the concentrations of n-butyric acid and L-lactic acid, products of the main competing bacterial groups. The amount of/so-butyric acid was nil. For reasons of convenience, D-lactic acid was not determined. In contrast to reactors X and Y, reactor Z was drained continuously. In Fig. 2 the drained volume and the gas production rate are shown as functions of time. In order to be able to compare the tests performed at different temperatures (reactors X and Y) and also to be able to compare the tests which only differed in draining mode (reactors Y and Z), the gas production rate from each reactor as a function of time is shown in Fig. 3. The test conditions and main results of the tests with reactors X, Y and Z are summarized in Table 3.
29.3 32-1 34'8 37-5
COD
30 4.1 3.8 3'0
Volume
3.66 3'78 3"88 3"83
pH
2.9 3"5 4.1 6.0
Acetate
-<0.01 0"4
n-butyrate
COD value q/" VFA
Reactor A Characteristics of the drained liquid
* Added directly after previous draining operation. Not detectable. Concentrations in g litre-1. Volumes in litres.
14 26 54 76
Days .[?om start
2-9 3.5 4-6 6'9
Total
5"0 2.5 2.0
Tapwater* volume
29.4 29.8 27.3 28.8
COD
30 8"5 7.3 6.0
Volume
3-70 3.80 3.99 4.49
pH
2.8 3-2 3"4 7.8
Acetate
<0.01 <0.01 0.04 7.5
n-huo'rate
COD value o f I/FA
Reactor B Characteristics o f the drained liquid
TABLE 2 Liquefaction of Tomatoes in Batchwise Drained Anaerobic Reactors (65 litres) at Ambient Temperature
2.8 3"2 3'5 16.7
Total
246
1. W. Koster
COD(g/L) 8 [] expenmenf of 22°C
~o
o expernmenf of 32°C 6
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. . . .
50
. . . .
100 . . . .
I~0 . . . .
200 . . . .
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Fig. 1. Changes in the concentrations of n-butyric acid and L-lactic acid during batchwise liquefaction/acidogenesis of tomatoes in an anaerobic reactor. Reactors X, Y.
hquefochon producf ( I ) 10.
8
gos produchon rote(l/d) 6
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Fig. 2, Cumulative volume of liquefaction product (F-l), and gas production rate (O), for a reactor from which the liquid was removed immediately after being produced; temperature, 32°C. Reactor Z.
Liquefaction and acidogenesis of waste tomatoes
gels
produO,on
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( [/d
247
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100
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150
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200
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Fig. 3. Rate of gas production from anaerobic liquefaction/acidogenesis reactors treating tomatoes. /M Batchwise draining, 22°C. Reactor X. 0 , Batchwise draining, 32 °C. Reactor Y. O, Continuous draining, 32 °C. Reactor Z.
TABLE 3 Conditions and Results of Liquefaction Tests Performed in Identical Anaerobic Reactors
Reactor X Temperature (~C) 22 Draining mode Batch Weight of tomatoes at start (kg) 12.8 Period (h) First batch = 240 Liquefaction product (litres) 4-98 COD removed with product (g) 184 Kjeldahl-nitrogen in product (g litre- ') 0.71 Phosphate in product (g litre- ') 0.46 pH of product 3.6
Y 32 Batch 12.7 First batch 6.2 229 0.76 0.49 333
240
Z 32 Continuous 13.8 240 8.01 278 ND ND j 4-2 at start I 3-5 at end
Reactors X and Y were drained once at 240 h. Reactor Z was drained continuously for the 240h. ND, Not determined.
248
1. W. Koster
DISCUSSION The results of the experiments with reactors A and B demonstrate that liquefaction is stimulated by the addition of tap water. Addition of water appears to give the butyric acid-producing bacteria an advantage over the lactic acid-producing bacteria leading to less acidification in the reactor to which water was added and resulting in an extension of the period during which bacterial activity can take place. Addition of water will also lead to a slower build up of an inhibitory osmotic pressure. These combined effects cause the drained liquid originating from the reactor to which water was added to have a total amount (volume x concentration) of COD considerably higher than the liquid originating from the reactor to which no extra water was added. If the last three batch drains are taken into account (the first batch was the same for both reactors) it appears that adding water resulted in the extraction of an extra 67 ~i of COD. Another method of improving liquefaction could be to increase the temperature (Wieringa et al., 1961). However, the results presented in Fig. i indicate that, at least during the first batch, a temperature increase from 22 °C to 32 °C did not give the butyric acid-producing bacteria an advantage in the competition with the lactic acid-producing bacteria. Increasing the temperature to a higher level would not be convenient in practical applications of the system. Figure 1 shows that, 50 h after the start of the experiment, butyric acid concentrations had reached the level at which they would remain during the rest of the experiment whereas, in both reactors, the lactic acid concentrations increased until the end. (During the first 30h of the experiment the amount of liquid in the reactors was not enough to permit proper sampling.) In both reactors the pH dropped continuously. At 41 h after the start of the experiment the pH in both reactors was 4.2; at the end of the experiment the pH was 3.6 in the reactor at 22 °C and 3.4 in the reactor at 32°C. It should be noted that these pH values are too low for optimum liquefaction followed by acidogenesis. Verstraete et al. ( 1981) mentioned that VFA production from a mixture of garbage and sewage sludge proceeded best at pH 5'8. Zoetemeyer et al. (1982) found that pH 6-0 was optimal for anaerobic acidogenic dissimilation of glucose. The fact that in reactors X and Y the butyric acid concentration did not increase indicates that the butyric acid-producing bacteria are not viable in a medium with a pH lower than 4.2. The lactic acid-producing bacteria in
Liquefitction and acidogenesis o[ waste tomatoes
249
the reactors tolerated a pH as low as 3.4. This is in accordance with observations by Wieringa & Beck (1964) on the ensilage of grass. As was expected, the bacterial activity was higher at 32°C than at 22 °C as, at 32 °C, the production of lactic acid started c a . 40 h earlier than at 22°C. The curves representing gas production rates from both reactors (Fig. 3) also reflect the higher bacterial activity at 32°C. The gas production rate from the reactor at 32°C was at its maximum at 25h after the start and decreased from that time on, while the rate for the reactor at 22°C reached a maxima at 50 h after the start and also at 150h. Between these maxima lies a minimum at 90 h. The difference in shape between the gas production rate curves may be explained by the lower bacterial activity at 22 °C. As long as both reactors differ only in temperature, more or less the same bacterial population will develop in both reactors. As the pH decreases the composition of the population will change. Apparently, at 32°C, the growth rates are fast enough to deal almost instantly with the changing environmental conditions whereas, at 22°C, the mortality rate of the initial population is faster than the growth of the successive population, thus resulting in a temporarily decreasing gas production rate. The most important result of increasing the temperature is the increase in the amount of COD that is extracted from the tomatoes with the first draining operation. At 32°C the amount of COD that could be extracted per unit tomatoes was 25 % more than at 22°C. Table 3 clearly shows the advantage of continuously removing the liquefaction product. In reactor Z (continuously drained) a considerably higher amount of decomposition occurred: per unit tomatoes 12 ~o more COD was removed and 19 0%more liquid was produced during the first 240 h of the experiments. A comparison between gas production rates (Fig. 3) indicates that the bacterial activity in the continuously drained reactor (Z) was considerably higher than in the batchwise drained reactor (Y). However, the fact that no gas production occurred after 235 h from the start did not mean that liquefaction had stopped. After gas production had stopped, another 2.15 litres of liquefaction product were produced containing a further 85 g of COD (Fig. 2). From the results shown in Table 3 the general C O D : N ratio for the effluent resulting from batchwise liquefaction/acidogenesis of tomatoes was calculated to be approximately 50: 1. This effluent can thus be used as an influent for a methane-producing reactor. It is often suggested that an
250
I. w. Koster
optimum C : N ratio for methane fermentation is between 20:1 and 30:1 (Meynell, 1982; Stafford e t al., 1980), which means a C O D : N ratio between 53:1 and 80:1, although, in many cases~ digestion of material having a much higher COD: N ratio has been reported (De Renzo, 1977). Based on an average composition of cell material of C 5H 7NO2 (Metcalf & Eddy, 1979), with a COD value of approximately 1.35 g C O D per gram of dry matter and a growth yield of 0.10-0.15 (w/w), the C O D : N ratio should be 100: 1 to 70:1. This means that, in an anaerobic digester treating the effluent of a liquefaction/acidogenesis reactor treating tomatoes, a surplus of nitrogen can be expected. This will not give rise to problems, although it is interesting to note that the specific methane production rate increases if the nitrogen content of the influent is lowered (Koster & Lettinga, 1984). If only the liquefaction step is taken into consideration then the most efficient system to operate would be one which could be drained continuously. In practice, however, the liquefaction/acidogenesis reactor will be coupled with a methane-producing reactor. Since about 70 °/o of the liquefaction product is produced during the first 4 days after the tomatoes are put in the reactor, and 80% of the tomatoes become available during a period of 5 weeks, continuous draining of this reactor would either require a large volume buffer between the liquefaction/acidogenesis reactor and the methane-producing reactor, or a methane-producing reactor that works below its capacity for most of the time. Therefore, the most appropriate system in practice will be a reactor that is drained continuously but which is operated at such a rate that the liquefaction/acidogenesis reactor itself is used as a volume buffer. Recirculation of effluent from the methane reactor or addition of water should be considered in order to stimulate liquefaction.
ACKNOWLEDGEMENTS The work described in this paper was performed at the Institute for Storage and Processing of Agricultural Produce--IBVL, PO Box 18, Wageningen, The Netherlands. I would like to thank B. A. Rijkens, J. W. Voetberg and G.W. Wieringa of IBVL for their help and inspiration. B.A. Rijkens can be contacted about a full-scale two-phase system treating tomatoes, which is being investigated by IBVL.
Liqu~[action and acidogenesis o[ waste tomatoes
251
REFERENCES Anon. (1979). Product il!/ormation." Vegetables and fruit. (In Dutch.) Communication No. 30, Sprenger Institute, Wageningen, Holland. A PHA (1975). Standard methods Jor the examination of water and wastewater (14th edn). American Public Health Association, Washington, DC. De Renzo, D. J. (1977). Energy from hioconversion of waste materials. Noyes Data Corporation, Park Ridge, NJ. Hofenk, G. (1983). Anaerobic digestion of solid organic wastes (process selection and pilot plant research). In: Anaerobic waste water treatment (van den Brink, W.J. (Ed.)). TNO Corporate Communication Department, The Hague, Holland. Hofenk, G., Rijkens, B. A. & Voetberg, J. W. (1982). Two-phase process for the anaerobic digestion of organic wastes yielding methane and compost. In: Solar energy R & D in the European Community. Series E, Vol. 3 (Grassi, G. & Palz, W. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Hofenk, G., Lips, S. J. J., Rijkens, B. A. & Voetberg, J. W. (1983). Two-phase process for the anaerobic digestion of organic wastes yielding methane and compost. In: Solar energy R & D in the European Community. Series E, Vol. 5 (Palz, W. & Pirrwitz, D. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Koster, I. W. & Lettinga, G. (1984). The influence of ammonium-nitrogen on the specific activity of pelletized methanogenic sludge. Agricultural Wastes, 9, 205 16. Metcalf & Eddy Inc. (1979). Wastewater engineering: Treatment, disposal, reuse (2nd edn). Tata McGraw-Hill Publishing Company Ltd, New Delhi. Meynell, P. J. (1982). Methane." Planning a digester (2nd edn). Prism Press, Chalmington, UK. Rijkens, B. A. (1980). A novel process for the anaerobic digestion of solid wastes leading to biogas and a compost-like material. In: EnergyJ?om biomass (1st EC Col!/krence) (Palz, W., Chartier, P. & Hill, D. O. (Eds)). Applied Science Publishers, London. Rijkens, B. A. (1981a). Two-phase process for the anaerobic digestion of'organic wastes yielding methane and compost. In: Solar energy R&D in the European CommuniO'. Series E, Vol. 1 (Chattier, P. & Palz, W. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Rijkens, B. A. ( 1981 b). A novel two-step process for the anaerobic digestion of solid wastes. Paper presented at the symposium ~Energy from Biomass and Wastes V', Lake Buena Vista, Fla, USA. Rijkens, B. A. (1982). Two-phase process for the anaerobic digestion of solid wastes. First results of a pilot scale experiment. In: Energy from biomass (2nd EC ConJerence) (Strub, A., Chartier, P. & Schleser, G. (Eds)). Applied Science Publishers, London. Schouwenburg, J. Ch. van & Walinga, I. (1967). The rapid determination of phosphorus in the presence of arsenic, silicon and germanium. Anal. Chim. Acta, 37, 271 4.
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Stafford, D. A., Hawkes, D. L. & Horton, R. (1980). Methane production from waste organic matter. CRC Press, Boca Raton, USA. Versluys, J., Martens, L. & Verstraete, W. (1982). Valorisation of intervention vegetables and fruit by anaerobic digestion. (In Dutch.) Landbouwtijdsehrift, 35, 1873-81. Verstraete, W., Baere, L. de & Rozzi, A. (1981). Phase separation in anaerobic digestion, motives and methods. In: BIOGAS, Proceedings of a Symposium Organised by the Technological Institute--KVIV, Antwerp, Belgium. Wieringa, G. W. & Beck, Th. (1964). Untersuchungen fiber die Verwendung von Milchs/iurebakterienkulturen dei der G~irfutterbereitung in Kleinbeh~ltern. Das Wirtschaftseigene Futter, 1, 34-54. Wieringa, G. W. & Haan, Sj. de (1961). Ensilage. (In Dutch.) IBVL, Wageningen, Holland. Wieringa, G. W., Schukking, S., Kappelle, D. & Haan, Sj. de (1961). The influence of heating on silage fermentation and quality. Neth. J. agrie. Sci., 9, 210-16. Zoetemeyer, R. J., Heuvel, J. C. van den & Cohen, A. (1982). pH influence on acidogenic dissimilation of glucose in an anaerobic digester. Water Res., 16, 303 1!.
Liquefaction and Acidogenesis of Tomatoes in an Anaerobic Two-Phase Solid-Waste Treatment System
I. W. K o s t e r Agricultural University, Department of Water Pollution Control, De Dreijen 12, 6703 BC Wageningen, The Netherlands
ABSTRACT In The Netherlands approximateO' 13.5 x 106 kg of surplus tomatoes are disposed of each year. This solid waste can be treated in a two-step anaerobic system yielding methane and a ~compost-like" residue. In this paper, laboratoo, tests of the first phase (liqueJaction and acidogenesis) are described. Liquefaction of tomatoes appeared to be stimulated hy increasing the temperature to 32°C: the addition of tap water and continuous draining. The effluent o f batchwise liquefaction/acidogenesis of tomatoes has a COD: N ratio of 50.'l and can thus be used as an inlquent for a digester producing methane. In practice, the most appropriate liquefaction/acidogenesis reactor for the treatment of tomatoes would be a continuously drained reactor which also,functions as an eJ~uent-volume buffer.
INTRODUCTION In the period 1972-1978 the harvest of tomatoes in The Netherlands averaged 366 x 106kg year-1. Like almost all fruit and vegetables, tomatoes are sold at daily auctions. F o r reasons o f ' m a r k e t protection' the tomatoes that cannot be sold at a price above a weekly fixed minimum are destroyed. During the period 1972-1978 the a m o u n t of surplus tomatoes was 3.7 ~o o f the total harvest (13.5 x 106 kg y e a r - 1). The usual practice is that the surplus tomatoes are thrown in a waste-pit where they slowly rot. 241 Agricultural Wastes 0141-4607/84/$03.00 ~j Elsevier Applied Science Publishers Ltd,
England, 1984. Printed in Great Britain
242
I.W.
Koster
These waste-pits are often sources of environmental pollution. Therefore, controlled decomposition of surplus tomatoes becomes necessary. Operating a controlled system to dispose of the surplus tomatoes also has the advantage that the energy contained in the biomass of the tomatoes can be recovered (Versluys et al., 1982). The composition and energy value of tomatoes are shown in Table 1. This Table contains information derived from an information bulletin issued by the Sprenger Institute (Anon, 1979). TABLE 1
Composition and Energy Values of Tomatoes Water Protein Fat Carbohydrates Crude cellular tissue Ash Phosphorus Potassium Chloride Energy value
94.2* 0-95 0.21 33 0.75 0'61 0"03 0, 3 0,06 765 kJ/kg
* ~,,, by weight.
Organic solid wastes can be treated in a two-phase process consisting of a liquefaction/acidogenesis reactor and a reactor that converts the liquid product of the first reactor into methane (Rijkens, 1980, 1981a, b, 1982: Hofenk et al., 1982, 1983). The surplus tomatoes are only available during a relatively short period of time (the second half of July and the first half of August), but a slight modification of the system which was developed as a series of batchwise filled liquefaction/acidogenesis reactors, coupled with an upftow sludge blanket reactor for methanogenesis, could overcome this problem (Hofenk, 1983). This paper presents preliminary results of a test programme that should lead to design and operating parameters for a liquefaction/acidogenesis reactor for the treatment of surplus tomatoes. Since the aim of the proposed solid-waste treatment system is to liquefy biomass instead of preserving it, the agricultural practice of preserving cattle fodder as ensilage gives a few hints about conditions that should be avoided in the liquefaction/acidogenesis reactor (Wieringa & De Haan,
Lique[cwtion and acidogenesLs" o/ waste tomatoes
243
1961). Ensilage is only successful if the acidity of the piled biomass increases to a pH value below ca. 4. As long as the pH of the pile of biomass is at a value above ca. 4, the biomass can still be degraded by bacterial activity. Apart from the buffering capacity of the pile of biomass, the competition between butyric acid-producing bacteria and lactic acid-producing bacteria is the most important factor controlling the acidity. The development of a population of butyric acid-producing bacteria instead of a population of lactic acid-producing bacteria can be stimulated by increasing the temperature, by reducing the osmotic pressure of the medium, or by any measure that reduces the souring rate (e.g. the addition of lime). Hence, the following operation characteristics for the liquefaction/ acidogenesis reactor were tested: increasing the temperature to 32°C, adding tap water to the reactor contents, continuously removing the liquid resulting from bacterial activity in the reactor.
METHODS
Reactors All tests were performed with cylindrical reactors which were sealed after filling with tomatoes originating from the same grower. In practice surplus tomatoes are treated roughly, so the tomatoes were slightly damaged by squeezing them before they were put into a reactor. Reactors A and B were put in a room without any temperature control, so that the temperature varied daily in the range 15 25 °C. Reactor X was put in a temperature-controlled room at 22°C: reactors Y and Z were controlled at 32 °C. Reactors X, Y and Z were connected with a wet gas meter. The liquid produced in reactor Z was continuously removed through a U-shaped pipe at the bottom of the reactor. The liquid produced in the other reactors was removed batchwise. After every draining operation tap water was added to the contents of reactor B so that the remaining tomatoes were just below liquid level.
Analysis Volatile Fatty Acids (VFA) were assayed gas-chromatographically using a Packard model 427 with a flame ionisation detector and a computer
244
I.w. Koster
integrator, model 602. Prior to injection 1 ml of the sample was mixed with 0.1 ml of 1N HC1 and 0.1 ml of a solution of iso-capronic acid of known strength. This mixture was centrifuged to remove any solids. The resulting chromatogram was interpreted and computed using the isocapronic acid as an internal standard. e-lactic acid was assayed by an enzymatic method (Boehringer, Mannheim). For reasons of convenience, the assays were performed after all samples had been collected. To preserve the lactic acid during storage, all samples were immediately diluted with an equal volume of a 5 ~ (wv) solution of ZnSO 4 and stored at - 2 0 °C. Chemical Oxygen Demand (COD) and Kjeldahl-nitrogen were determined according to Standard Methodsjbr the Examination of Water and Wastewater (1975). Phosphate was determined according to the method of Van Schouwenburg & Walinga (1967).
RESULTS Reactors A and B were drained four times. Each time the total liquefaction product was collected, measured, and sampled after thorough mixing. For each sample, pH, COD and VFA were determined. The results are presented in Table 2. Reactors X and Y were drained once. During the period between filling and draining, regular sampling took place from a tap at the bottom of the reactor. These samples were analyzed for VFA and e-lactic acid, and the pH of each sample was determined. Figure I shows the development of the concentrations of n-butyric acid and L-lactic acid, products of the main competing bacterial groups. The amount of/so-butyric acid was nil. For reasons of convenience, D-lactic acid was not determined. In contrast to reactors X and Y, reactor Z was drained continuously. In Fig. 2 the drained volume and the gas production rate are shown as functions of time. In order to be able to compare the tests performed at different temperatures (reactors X and Y) and also to be able to compare the tests which only differed in draining mode (reactors Y and Z), the gas production rate from each reactor as a function of time is shown in Fig. 3. The test conditions and main results of the tests with reactors X, Y and Z are summarized in Table 3.
29.3 32-1 34'8 37-5
COD
30 4.1 3.8 3'0
Volume
3.66 3'78 3"88 3"83
pH
2.9 3"5 4.1 6.0
Acetate
-<0.01 0"4
n-butyrate
COD value q/" VFA
Reactor A Characteristics of the drained liquid
* Added directly after previous draining operation. Not detectable. Concentrations in g litre-1. Volumes in litres.
14 26 54 76
Days .[?om start
2-9 3.5 4-6 6'9
Total
5"0 2.5 2.0
Tapwater* volume
29.4 29.8 27.3 28.8
COD
30 8"5 7.3 6.0
Volume
3-70 3.80 3.99 4.49
pH
2.8 3-2 3"4 7.8
Acetate
<0.01 <0.01 0.04 7.5
n-huo'rate
COD value o f I/FA
Reactor B Characteristics o f the drained liquid
TABLE 2 Liquefaction of Tomatoes in Batchwise Drained Anaerobic Reactors (65 litres) at Ambient Temperature
2.8 3"2 3'5 16.7
Total
246
1. W. Koster
COD(g/L) 8 [] expenmenf of 22°C
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Fig. 1. Changes in the concentrations of n-butyric acid and L-lactic acid during batchwise liquefaction/acidogenesis of tomatoes in an anaerobic reactor. Reactors X, Y.
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Fig. 2, Cumulative volume of liquefaction product (F-l), and gas production rate (O), for a reactor from which the liquid was removed immediately after being produced; temperature, 32°C. Reactor Z.
Liquefaction and acidogenesis of waste tomatoes
gels
produO,on
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247
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Fig. 3. Rate of gas production from anaerobic liquefaction/acidogenesis reactors treating tomatoes. /M Batchwise draining, 22°C. Reactor X. 0 , Batchwise draining, 32 °C. Reactor Y. O, Continuous draining, 32 °C. Reactor Z.
TABLE 3 Conditions and Results of Liquefaction Tests Performed in Identical Anaerobic Reactors
Reactor X Temperature (~C) 22 Draining mode Batch Weight of tomatoes at start (kg) 12.8 Period (h) First batch = 240 Liquefaction product (litres) 4-98 COD removed with product (g) 184 Kjeldahl-nitrogen in product (g litre- ') 0.71 Phosphate in product (g litre- ') 0.46 pH of product 3.6
Y 32 Batch 12.7 First batch 6.2 229 0.76 0.49 333
240
Z 32 Continuous 13.8 240 8.01 278 ND ND j 4-2 at start I 3-5 at end
Reactors X and Y were drained once at 240 h. Reactor Z was drained continuously for the 240h. ND, Not determined.
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1. W. Koster
DISCUSSION The results of the experiments with reactors A and B demonstrate that liquefaction is stimulated by the addition of tap water. Addition of water appears to give the butyric acid-producing bacteria an advantage over the lactic acid-producing bacteria leading to less acidification in the reactor to which water was added and resulting in an extension of the period during which bacterial activity can take place. Addition of water will also lead to a slower build up of an inhibitory osmotic pressure. These combined effects cause the drained liquid originating from the reactor to which water was added to have a total amount (volume x concentration) of COD considerably higher than the liquid originating from the reactor to which no extra water was added. If the last three batch drains are taken into account (the first batch was the same for both reactors) it appears that adding water resulted in the extraction of an extra 67 ~i of COD. Another method of improving liquefaction could be to increase the temperature (Wieringa et al., 1961). However, the results presented in Fig. i indicate that, at least during the first batch, a temperature increase from 22 °C to 32 °C did not give the butyric acid-producing bacteria an advantage in the competition with the lactic acid-producing bacteria. Increasing the temperature to a higher level would not be convenient in practical applications of the system. Figure 1 shows that, 50 h after the start of the experiment, butyric acid concentrations had reached the level at which they would remain during the rest of the experiment whereas, in both reactors, the lactic acid concentrations increased until the end. (During the first 30h of the experiment the amount of liquid in the reactors was not enough to permit proper sampling.) In both reactors the pH dropped continuously. At 41 h after the start of the experiment the pH in both reactors was 4.2; at the end of the experiment the pH was 3.6 in the reactor at 22 °C and 3.4 in the reactor at 32°C. It should be noted that these pH values are too low for optimum liquefaction followed by acidogenesis. Verstraete et al. ( 1981) mentioned that VFA production from a mixture of garbage and sewage sludge proceeded best at pH 5'8. Zoetemeyer et al. (1982) found that pH 6-0 was optimal for anaerobic acidogenic dissimilation of glucose. The fact that in reactors X and Y the butyric acid concentration did not increase indicates that the butyric acid-producing bacteria are not viable in a medium with a pH lower than 4.2. The lactic acid-producing bacteria in
Liquefitction and acidogenesis o[ waste tomatoes
249
the reactors tolerated a pH as low as 3.4. This is in accordance with observations by Wieringa & Beck (1964) on the ensilage of grass. As was expected, the bacterial activity was higher at 32°C than at 22 °C as, at 32 °C, the production of lactic acid started c a . 40 h earlier than at 22°C. The curves representing gas production rates from both reactors (Fig. 3) also reflect the higher bacterial activity at 32°C. The gas production rate from the reactor at 32°C was at its maximum at 25h after the start and decreased from that time on, while the rate for the reactor at 22°C reached a maxima at 50 h after the start and also at 150h. Between these maxima lies a minimum at 90 h. The difference in shape between the gas production rate curves may be explained by the lower bacterial activity at 22 °C. As long as both reactors differ only in temperature, more or less the same bacterial population will develop in both reactors. As the pH decreases the composition of the population will change. Apparently, at 32°C, the growth rates are fast enough to deal almost instantly with the changing environmental conditions whereas, at 22°C, the mortality rate of the initial population is faster than the growth of the successive population, thus resulting in a temporarily decreasing gas production rate. The most important result of increasing the temperature is the increase in the amount of COD that is extracted from the tomatoes with the first draining operation. At 32°C the amount of COD that could be extracted per unit tomatoes was 25 % more than at 22°C. Table 3 clearly shows the advantage of continuously removing the liquefaction product. In reactor Z (continuously drained) a considerably higher amount of decomposition occurred: per unit tomatoes 12 ~o more COD was removed and 19 0%more liquid was produced during the first 240 h of the experiments. A comparison between gas production rates (Fig. 3) indicates that the bacterial activity in the continuously drained reactor (Z) was considerably higher than in the batchwise drained reactor (Y). However, the fact that no gas production occurred after 235 h from the start did not mean that liquefaction had stopped. After gas production had stopped, another 2.15 litres of liquefaction product were produced containing a further 85 g of COD (Fig. 2). From the results shown in Table 3 the general C O D : N ratio for the effluent resulting from batchwise liquefaction/acidogenesis of tomatoes was calculated to be approximately 50: 1. This effluent can thus be used as an influent for a methane-producing reactor. It is often suggested that an
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optimum C : N ratio for methane fermentation is between 20:1 and 30:1 (Meynell, 1982; Stafford e t al., 1980), which means a C O D : N ratio between 53:1 and 80:1, although, in many cases~ digestion of material having a much higher COD: N ratio has been reported (De Renzo, 1977). Based on an average composition of cell material of C 5H 7NO2 (Metcalf & Eddy, 1979), with a COD value of approximately 1.35 g C O D per gram of dry matter and a growth yield of 0.10-0.15 (w/w), the C O D : N ratio should be 100: 1 to 70:1. This means that, in an anaerobic digester treating the effluent of a liquefaction/acidogenesis reactor treating tomatoes, a surplus of nitrogen can be expected. This will not give rise to problems, although it is interesting to note that the specific methane production rate increases if the nitrogen content of the influent is lowered (Koster & Lettinga, 1984). If only the liquefaction step is taken into consideration then the most efficient system to operate would be one which could be drained continuously. In practice, however, the liquefaction/acidogenesis reactor will be coupled with a methane-producing reactor. Since about 70 °/o of the liquefaction product is produced during the first 4 days after the tomatoes are put in the reactor, and 80% of the tomatoes become available during a period of 5 weeks, continuous draining of this reactor would either require a large volume buffer between the liquefaction/acidogenesis reactor and the methane-producing reactor, or a methane-producing reactor that works below its capacity for most of the time. Therefore, the most appropriate system in practice will be a reactor that is drained continuously but which is operated at such a rate that the liquefaction/acidogenesis reactor itself is used as a volume buffer. Recirculation of effluent from the methane reactor or addition of water should be considered in order to stimulate liquefaction.
ACKNOWLEDGEMENTS The work described in this paper was performed at the Institute for Storage and Processing of Agricultural Produce--IBVL, PO Box 18, Wageningen, The Netherlands. I would like to thank B. A. Rijkens, J. W. Voetberg and G.W. Wieringa of IBVL for their help and inspiration. B.A. Rijkens can be contacted about a full-scale two-phase system treating tomatoes, which is being investigated by IBVL.
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REFERENCES Anon. (1979). Product il!/ormation." Vegetables and fruit. (In Dutch.) Communication No. 30, Sprenger Institute, Wageningen, Holland. A PHA (1975). Standard methods Jor the examination of water and wastewater (14th edn). American Public Health Association, Washington, DC. De Renzo, D. J. (1977). Energy from hioconversion of waste materials. Noyes Data Corporation, Park Ridge, NJ. Hofenk, G. (1983). Anaerobic digestion of solid organic wastes (process selection and pilot plant research). In: Anaerobic waste water treatment (van den Brink, W.J. (Ed.)). TNO Corporate Communication Department, The Hague, Holland. Hofenk, G., Rijkens, B. A. & Voetberg, J. W. (1982). Two-phase process for the anaerobic digestion of organic wastes yielding methane and compost. In: Solar energy R & D in the European Community. Series E, Vol. 3 (Grassi, G. & Palz, W. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Hofenk, G., Lips, S. J. J., Rijkens, B. A. & Voetberg, J. W. (1983). Two-phase process for the anaerobic digestion of organic wastes yielding methane and compost. In: Solar energy R & D in the European Community. Series E, Vol. 5 (Palz, W. & Pirrwitz, D. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Koster, I. W. & Lettinga, G. (1984). The influence of ammonium-nitrogen on the specific activity of pelletized methanogenic sludge. Agricultural Wastes, 9, 205 16. Metcalf & Eddy Inc. (1979). Wastewater engineering: Treatment, disposal, reuse (2nd edn). Tata McGraw-Hill Publishing Company Ltd, New Delhi. Meynell, P. J. (1982). Methane." Planning a digester (2nd edn). Prism Press, Chalmington, UK. Rijkens, B. A. (1980). A novel process for the anaerobic digestion of solid wastes leading to biogas and a compost-like material. In: EnergyJ?om biomass (1st EC Col!/krence) (Palz, W., Chartier, P. & Hill, D. O. (Eds)). Applied Science Publishers, London. Rijkens, B. A. (1981a). Two-phase process for the anaerobic digestion of'organic wastes yielding methane and compost. In: Solar energy R&D in the European CommuniO'. Series E, Vol. 1 (Chattier, P. & Palz, W. (Eds)). Reidel Publishing Company, Dordrecht, Holland. Rijkens, B. A. ( 1981 b). A novel two-step process for the anaerobic digestion of solid wastes. Paper presented at the symposium ~Energy from Biomass and Wastes V', Lake Buena Vista, Fla, USA. Rijkens, B. A. (1982). Two-phase process for the anaerobic digestion of solid wastes. First results of a pilot scale experiment. In: Energy from biomass (2nd EC ConJerence) (Strub, A., Chartier, P. & Schleser, G. (Eds)). Applied Science Publishers, London. Schouwenburg, J. Ch. van & Walinga, I. (1967). The rapid determination of phosphorus in the presence of arsenic, silicon and germanium. Anal. Chim. Acta, 37, 271 4.
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Stafford, D. A., Hawkes, D. L. & Horton, R. (1980). Methane production from waste organic matter. CRC Press, Boca Raton, USA. Versluys, J., Martens, L. & Verstraete, W. (1982). Valorisation of intervention vegetables and fruit by anaerobic digestion. (In Dutch.) Landbouwtijdsehrift, 35, 1873-81. Verstraete, W., Baere, L. de & Rozzi, A. (1981). Phase separation in anaerobic digestion, motives and methods. In: BIOGAS, Proceedings of a Symposium Organised by the Technological Institute--KVIV, Antwerp, Belgium. Wieringa, G. W. & Beck, Th. (1964). Untersuchungen fiber die Verwendung von Milchs/iurebakterienkulturen dei der G~irfutterbereitung in Kleinbeh~ltern. Das Wirtschaftseigene Futter, 1, 34-54. Wieringa, G. W. & Haan, Sj. de (1961). Ensilage. (In Dutch.) IBVL, Wageningen, Holland. Wieringa, G. W., Schukking, S., Kappelle, D. & Haan, Sj. de (1961). The influence of heating on silage fermentation and quality. Neth. J. agrie. Sci., 9, 210-16. Zoetemeyer, R. J., Heuvel, J. C. van den & Cohen, A. (1982). pH influence on acidogenic dissimilation of glucose in an anaerobic digester. Water Res., 16, 303 1!.