Laboratory scale anaerobic digestion of fruit and vegetable solid waste
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Biomass 5 (1984) 245-259
Laboratory Scale Anaerobic Digestion of Fruit and Vegetable Solid Waste
CSIRO Division of Food Research, PO Box 52, North Ryde, New South Wales, 2113, Australia
(Received 11 October, 1983)
Anaerobic digestions that were fed waste apple, corn cobs, apple press cake, extracted sugarbeet pulp, pineapple pressings or asparagus waste were stable in trials lasting up to 226 days. Loading rates of 3.5-4.25 kg m-3 day -1 and conversions of 88-96% of the organic solids fed were obtained by ensuring adequate levels of alkalinity, nitrogen and other nutrients during digestion. Gas yields ranged from 0.429 to 0.568 litre (50-60% methaneJ per gram organic solids fed. For reasons not under- stood, gas yields from digestion of apricot waste declined after 63 days from 0.4 77 to 0.13 7 litre g-1 or feedstock.
Key words: anaerobic digestion, fruit, vegetables, methane yields, conver- sion, supplementation, recycle, alkalinity.
Disposal of the large quantities of wet, organic refuse generated by fruit and vegetable processing operations creates economic and environ- mental problems to which no fully satisfactory solutions have yet been found. At present, fruit and vegetable wastes are usually disposed of by dumping, spreading on land or by feeding to animals. Earlier reports presented results of laboratory and pilot scale trials carried out to evaluate the anaerobic digestion processes as a means of utilizing these materials, l'z These papers also discussed the potential benefits from commercial application of the process.
245 Biomass 0144-4565/84/$03.00- Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
246 A.G. Lane
This report presents results of further laboratory trials carried out to determine the long term stability of digestions fed a range of fruit and vegetable waste solids. Laboratory trials using orange peels as feedstock are the subject of a separate paper. 3
2. MATERIALS AND METHODS
Digestions were initiated using actively digesting sludge from a municipal sewage digester after screening through wire mesh having 5 X 5 mm openings. Digestions were carried out in 10 litre microbiological fer- menters (L.H. Fermentations, Stoke Poges, Bucks., UK), containing 8 litres of sludge which was stirred for 5 min each half hour at 200 rev min -1 and maintained at 36 -+ IC. The daily charge of feed was delivered to the digester over a 24 h period by means of a motor-driven syringe activated by a timer (Fig. 1). Gas was collected in a football bladder and the volume produced daily was measured by discharge through a gas meter.
~ ~i~ii~ ~
~ ~ ~i~ ~
Motor driven syringe (capacity 1 litre) used to deliver fruit and vegetable wastes to anaerobic digester.
Anaerobic digestion or fruit and vegetable refuse 247
A volume of mixed liquor equal to twice the volume of feed was removed from the digester each day and allowed to settle at IC in a conical 2 litre vessel (apex down). A volume of settled solids equal to the volume of feed was removed daily from the bottom of the vessel and returned to the digester.
Percentage conversion of feed volatile solids to gas was calculated from the average organic solids contents of feedstock and settled supernatant fluid, over a period of not less than 14 days.
Waste solids from fruit and vegetable processing were obtained from commercial processing plants. Pineapple pressings were the residue from the pressing of juice from peels and cores, apricot fibre was the residue from ,manufacture of pulp, and sugarbeet pulp was the residue after removal of solubles by countercurrent extraction. 4 Apple cake was the residue from the pressing of fruit for juice extraction, whereas apple waste consisted of peels, cores and rejected fruit. Asparagus waste consisted of the fibrous lower ends trimmed from fresh spears.
Wastes were hammermilled (12 mm screen) and stored at -20C. Before use, the wastes were diluted with water to 10% total solids then supplemented by addition of (NH4)2 HPO4 (2 g litre -1) and an elements solution (0.2 ml litre -1) containing (g litre -1 in 0.1 M HC1): NaC1, 250: K2SO4, 12.5; MgC12.6H20, 125; CaC12.2H20, 125; FeCI3, 25; ZnO, 1; MnC12.4H20, 0.25; CuSO4, 0.25; (NHa)6MoTO24.4H20, 0.25. All reagents were analytical grade.
Alkalinity (g litre -1) in digestion supernatant fluids was determined by titration to pH 3.7 using 0.05 M H2SO4, according to the American Public Health Association. s Alkalinity levels in digesting mixtures were adjusted by adding NaHCO3 to maintain pH values above 6.8.
2.4. Gas analysis
The carbon dioxide content of digester gases was determined routinely using gas-analysis tubes, type CO2/A, range 5-60% v/v (Drager AG, Lubeck, West Germany). Gas chromatography was used periodically to
248 A. G. Lane
verify the ratio of methane : carbon dioxide. This was done using a Pye Series 104 Chromatograph fitted with a thermal conductivity detector and a glass column (1 m 4 mm) packed with Porapak Q (50-100mesh) and operating at 52C with helium carder gas.
2.5. Total dry solids (TS), organic solids (OS) and ash
The TS contents (% w/w) of digesting sludges and feedstocks, other than whole fruit wastes, were determined by drying samples for 24 h at 105 -+ 5C. Whole fruit wastes were dried at 70 -+ 2.5C for 48 h, then for a further 24 h under vacuum at 60 -+ 2.5C. Ash (% w/w of TS) and OS (% w/w) values were determined by weighing before and after the incineration of dried samples at 600 -+ 25C for 2 h. 'Organic solids' (OS) is thus equivalent to the term 'volatile solids' (VS) used by previous authors, but is used in preference as a more accurate descrip- tion of the material lost during incineration. TS, OS and ash contents of supernatant fluids and settled solids were determined either after settling as described under Digestions, or after centrifuging (1150g, 5 min), as specified in the text.
2.6. Volatile fatty acids (VFA)
VFA concentrations in centrifuged (1150g, 5 min) digestion super- natant liquors were estimated according to American Public Health Association 5 by absorption onto chromatographic silica gel (May & Baker, Dagenham, UK), elution with acidified butanol-chloroform and titration under nitrogen with 0.02 M NaOH. Acetic acid was used as the standard and results were expressed as g acetic acid litre -1.
2.7. Chemical oxygen demand (COD)
COD (g litre -1) of centrifuged (1150 g, 5 min) digestion supernatant fluid was determined according to the Department of the Environment, Great Britain, 6 except that the reaction mixtures in covered vessels were heated in an oil bath at 150C for 2 h and excess dichromate was titrated with ferrous ammonium sulphate (49.01 g litre -1).
Anaerobic digestion of fruit and vegetable refuse 249
2.8. Total and ammonia nitrogen
Ammonia nitrogen was determined by titration with H2SO4 (0.05 M) after steam distillation with MgO into boric acid solution (4% w/v). Total nitrogen was determined by distillation after Kjeldahl digestion in which selenium powder was used as catalyst. 7
Phosphorus content of fruit and vegetable materials was determined by the colorimetric, phosphomolybdate method of Fiske and Subbarow. 8
Results of analyses for total solids, ash, total nitrogen and phosphorus in corn cobs, apple press cake and wastes from processing apricots, apples, pineapples, asparagus and sugarbeet are given in Table 1.
These waste materials, after hammermilling and supplementation, were fed to digesters for periods of up to 226 days. Data obtained for maximum loading rates, gas yields and percentage conversion of volatile solids to gas for the various materials are given in Table 2. This table
TABLE 1 Total Solids, Ash, Total Nitrogen and Phosphorus Contents of Fruit and
Waste Total solids Ash Total N P (% w/w) (%)a (%)a (mg%)a
Apricot fibre 17.7 6.62 1.70 180 Corn cobs 20.9 2.31 0.90 190 Apple cake 16.5 2.82 0.63 110 Apple 12.0 2.08 0.42 72 Asparagus 9-5 1.91 3.51 910 Sugarbeet 7.0 3-36 2.11 130 Pineapple 19.6 3.36 0.72 110
a % w/w of total solids.
250 A. G. Lane
TABLE 2 Maximum Loading Rate, Gas Yield and CO2 Content, Average Ammonia Nitrogen Level in Settled Supernatant Fluids and % Conversion of OS During Anaerobic
Digestion of Fruit and Vegetable Waste Solids
Waste Duration Maximum Gas yieM C02 NHa-N Conversion of trial load (litre g -a (%) (mg litre -~) (% of OS) (days) (kg TS m -3 TS fed)
Apricot b 100 4.0 0.477 38-50 650 96.3 Corn cobs 91 4.0 0.465 36-52 500 95.7 Apple cake 61 4.0 0-454 39-54 510 93-4 Apple 226 3.5 c 0.437 42-56 300 88-1 Asparagus 27 4.25 0.460 48-55 1100 89.7 Sugarbeet 48 4-2 0.445 34-52 400 95-2 Pineapple 83 4.0 0-568 37-50 600 93-2
a Maximum loading rates tested. b Results to day 63. e Failed at loading of 4.0 kg m -a day -a.
also shows ammonia nitrogen levels in settled supernatant fluids during digestion of these materials.
Table 3 shows the average quantities of sodium bicarbonate added daily to digesters to maintain pH values above 6.8 and ranges of alkalinity values observed during digestion at the maximum loading rates given in Table 1. Table 3 also shows the average VFA levels observed during these digestions.
The gas yield obtained during digestion of apricot waste was 0.477 litreg -l TS until day 63, when gas production fell sharply to 0.137 litre g-i (Fig. 2). In Table 4, dry matter and ash contents of mixed digesting liquor, settled supernatant fluid and settled solids before (day 23) and after (day 74) this change in gas production are compared with values for fresh digesting sludge from a municipal sewage digester. Average COD values for supernatant fluids from digestions are given in Table 5.
Anaerobic digestion or fruit and vegetable refuse 251
TABLE 3 Average VFA Concentrations, Average Quantities of Sodium Bicarbonate Added Daily and Alkalinity Ranges During Digestion of Fruit and Vegetable Wastes
(Loading Rates as in Table 2)
Waste Average VFA Average NattCOs added Alkalinity (g litre -1) (g litre -1 day -1) (g litre -x)
Apricot 1.46 0-0 4.4-6-7 Corn cobs 0.99 0-3 5.1-7-8 Apple cake 0.67 0.2 3.1-4.9 Apple 2.12 0.3 4.8-7.6 Asparagus 2.98 0-0 4.6-5.6 Sugarbeet 0-66 0.2 3.5-4.3 Pineapple 1.12 0.0 3.8-4-2
Q . . J 111
>- 400 113
lxl > I--- .< ._1
"~'~"n n a ! 0 200 tO0 600 800
CUMULATIVE WEIGHT FED
Grad ient 1 I ~ =0.137 l i t reg~. I .~
qv 1' ..7 / , /Y Day 63
2 /~'o'o'" Grad i ent
14." = 0 '477 l i t re g - I ./....
1000 1200 (g )
Cumulative gas yield vs cumulative weight of feedstock (TS) during digestion of apricot waste (loading 4.0 kg TS m -3 day-l).
252 A.G. Lane
TABLE 4 Total Solids (TS) and Ash (% of TS) Contents of Mixed Digester Liquor, Settled Supernatant Fluid and Settled Solids During Digestion of Municipal Sewage Sludge
and Apricot Waste
Day 23 Day 74
Sewage sludge digestions (~ w/w)
TS Ash TS Ash TS Ash
Mixed liquor 3.82 33.7 3.73 23.3 2.10 41.3 Supernatant fluid 0.23 48.6 0.72 50.0 0.17 46.3 Solids 11.5 41.0 11.4 18.6 12.8 42.6
TABLE 5 Average COD Values of Centrifuged Digestion Supernatant Fluids During Digestion
of Fruit and Vegetable Processing Wastes
Waste Loading (kg TS m-3 day -1) COD (g litre -1)
Apricot fibre 4.0 6.34 Corn cobs 4.0 4.97 Apple cake 4.0 2-26 Apple 3.5 4.51 Asparagus 4.25 10.11 Sugarbeet 4-2 3.05 Pineapple 4.0 5.35
4.1. Alkalinity levels
Loading rates and percentage conversions reported here for fruit and vegetable wastes are considerably higher than those obtained by previous authors. 9'1 Hills and Roberts 9 examined the anaerobic digestion of peach and melon wastes without nitrogen or phosphorus addition or alkalinity adjustment. Digestion of melon waste was balanced up to a loading rate of 3 kg m -3 day -1 and retention time of 15 days (alkalinity, 4.06 g litre -1 ; VFA, 2.20 g litre -1 ; pH, 8.0). However, peach digestion
Anaerobic digestion of fruit and vegetable refuse 253
was stable at a loading rate of only 1 kg m -3 day -x (retention time 15 days) and severe overloading occurred at 3 kg m -3 day -a, retention time 20 days (alkalinity, 1.08 g litre -1 ; VFA, 3.05 g litre -a ; pH, 6.2).
Failure of peach digestion appears to have been due to inadequate levels of alkalinity to balance the levels of VFA in the digestion liquors. We have found that careful attention to alkalinity levels during digestion of fruit and vegetable materials is crucial to the success of the digestion. For stable digestions, it is imperative that a satisfactory ratio be main- tained between VFA and alkalinity levels. This ratio is given by the empirical relationship that for balanced digestion, alkalinity (mg litre -1) ---0.7 VFA (mg litre -a) should not be less than 1500. n'x2 On this basis, an alkalinity of 1080 mg litre -x in the presence of 3050 mg VFA litre -a (Hills and Roberts 9) is grossly inadequate and digestion failure would be anticipated.
Inadequate alkalinity levels appear to have also been the cause of digestion failure in experiments reported by Knol et al. xo Maximum loading rates for stable digestion reported by these authors for a variety of wastes were only 0.9-1.6 kg m -3 day -x, with retention time 32 days in all cases. Values for pH as low as 5.3 were reported during digestion of apple 'pulp' at higher loadings (3 kg m -3 day -a) and stable digestion was not achieved with this material even at loading rates as low as 1.02 kg m -3 day -~ (VFA, 1.5-4.5 g litre -x ; alkalinity not given). These authors also fed unsupplemented apple 'slurry' at 3 kg m -3 day -1 with- out alkalinity adjustment and found that pH began to fall after 10 weeks. This time corresponds to 2.2 volume changes, so the alkalinity originally present in the sewage digester sludge, typically 3 g litre -x in our experience, would have been diluted to inadequate levels.
Knol et al.X reported unbalanced digestion of asparagus waste at a loading of only 1.06 kg m -3 day -1, with only 40% conversion of VS to gas. Our results show digestion of asparagus waste was stable at loadings of 4.25 kg m -3 day -~ and 89.7% conversion of volatile solids to gas was obtained (Table 2). Addition of sodium bicarbonate was not required, because the digestion naturally maintained alkalinity levels (4.6-5.6 g litre -1) adequate to balance the levels of VFA. As observed by Knol et al., x VFA levels during asparagus digestion were somewhat higher than with other wastes (Table 3). Digestions that were fed apricot pulp and pineapple pressings also maintained adequate pH and alkalinity levels without the need for addition of bicarbonate. Small additions of bicarbonate were required to maintain correct alkalinity balance and
254 A. G. Lane
pH values during digestion of the other waste materials (Table 3), The maximum loading rate (3.5 kg m -a day -1) accepted for apple
waste (equivalent to the apple 'slurry' of Knol et al. ~0) in trials reported here was somewhat lower than rates accepted for the other wastes (4.0- 4.25 kg m -a day-l), as pH levels in apple waste digestions showed a marked tendency to fall even in the presence of high alkalinity levels (4.8-7.6 g litre-l). This has also been observed in digestions fed other whole fruits and fruit juices, but the explanation is not clear.
4.2. Solids recycling
Recovery and recycling of settled solids from discharged mixed liquors and their return to the digester is an important factor in the greatly improved conversions over those reported by previous authors. 9,1 Knol et at xo reported conversion as low as 40% OS for digestion of asparagus waste without solids recycling. Increasing the effective retention time of sludge solids by solids recycling has been reported to result in con- versions of 99% and approximately 100% for orange peel and spent coffee grounds respectively. 3,13 Recently, Callander and Barford demon- strated that solids recycling during pig manure digestion resulted in a 36% increase in gas yield and a 2-fold increase in fibre breakdown, x4 In the present trials, conversions of 89.7% OS and 93.2% OS were obtained for asparagus and pineapple wastes respectively (Table 2), which shows that even very fibrous materials can be degraded by anaerobic digestion, provided the effective sludge solids retention time is prolonged by solids recycling.
In operation, gas production in the settler caused poor settling of discharged liquors in hot weather during trials in a 25 m 3 reactor. These pilot scale trials will be reported in full elsewhere.
Hills and Roberts 9 reported a total nitrogen level of 1.70% (of TS) for peach wastes and 265 mg ammonia nitrogen litre -x for peach digestion fluids. Our own experience with digestion of similar materials has shown that an ammonia nitrogen level of 200 mg litre -x permits rapid and complete digestion. During digestion of spent coffee grounds without supplementation, ammonia nitrogen levels as low as 88 mg litre -1 were reported without apparent deficiency, since supplementa-
Anaerobic digestion or fruit and vegetable refuse 255
tion did not improve either the rate or extent of digestion, x3 Knol et al.l reported protein (N 6.25) data for a range of wastes, but in the absence of any data for ammonia nitrogen levels in the digestion fluids, no estimate can be made as to the proportion of this total nitrogen which was biologically available to the micro-organism involved in the anaerobic digestion process and there is no evidence that nitrogen levels were inadequate. It is therefore unclear from data presented by previous authors whether digestion failure and poor conversion ratios were due in part to inadequate nitrogen levels, as these authors suggest. 9,1 Poor digestion results reported by Knol et aL 1o and Hills and Roberts 9 were probably due mainly to inadequate levels of alkalinity, as discussed earlier.
The observed nitrogen and phosphorus contents of fruit and vegetable wastes (Table 1) suggest that digestion of these materials at a loading rate of 4 kg m -3 day -1 would produce nitrogen levels in the mixed liquors of 16.8-140 mg litre -1 and phosphorus levels of 2-9-36.4 mg litre -a. These levels are much lower than those generally regarded as necessary for digestion and, moreover, it was not known what propor- tion of the total nitrogen and phosphorus was biologically available to the micro-organisms involved in the digestion process.
Traditionally, the nitrogen and phosphorus requirements for digestion are calculated from the ratio C:N:P = 100:2.5:0.5. is On the basis of this ratio, the nitrogen level needed for fruit and vegetable materials (10% TS, 50% of which is carbon) would be 1250 mg litre -1. This approaches levels at which ammonia becomes toxic to the anaerobic digestion process 16 and would also be expensive to maintain, if chemical nitrogen sources were used.
A more rational basis for estimating nitrogen requirements is provided by McCarty, 12 who calculated that the daily nitrogen requirement is given by
aF 0.11A -
1 + b(SRT)
where A = biological solids produced per day (g), F = organic matter consumed per day (g), SRT = sludge solids retention time (days), a = dimensionless growth constant (range 0.054-0.240) and b = endogenous respiration rate (gg-a day-l) (range 0-014-0.038). In digestion with 100% solids recycling, SRT will be very long (say 100 days), so A approximates 200 mg litre -1 day -1 and the nitrogen requirement for
256 A.G. Lane
microbial growth will be only about 22 mg litre -1. Phosphorus require- ments for microbial growth are generally regarded as being 20% of that for nitrogen.17
From these considerations, it was decided to ensure adequate levels of nitrogen and phosphorus by supplementing feedstocks with (NH4)2 HPO4 (2 g litre -1). A solution containing other elements was added as described in Materials and Methods, to ensure an adequate supply of these trace nutrients.
It seems probable that the levels of phosphorus, nitrogen and elements used here could all be reduced substantially but the minimum levels required have not been established. The cost of supplementing a full scale reactor with chemical sources of nutrients would reduce the economic return from the process. An alternative would be to use nitrogen-rich manures from poultry or pigs as supplement. It is esti- mated that one charge (4 kg dry matter m -a) of poultry or pig manure each week would maintain ammonia nitrogen levels at about 200 mg litre -1, which, in our experience, is adequate to ensure efficient and complete digestion of fruit and vegetable wastes. A weekly charge of manure would also ensure adequate levels of phosphorus and other elements and assist towards maintaining adequate levels of alkalinity.
4.4. Inhibition of apricot digestion
Digestion of apricot fibre followed a similar pattern to that of other wastes for 63 days after start-up (Table 2). Conversion of solids to gas to that time was 96.3% of OS and the gas yield was 0.477 litre g-i TS fed (Fig. 2). The pH levels showed no tendency to fall and no sodium bicarbonate was required to maintain satisfactory alkalinity balance (Table 3). However, on about the sixty-third day, gas flows suddenly fell to 0.137 litre g-1 TS fed. Other symptoms of digestion overload or toxicity were absent - pH levels remained stable, VFA levels did not rise and the level of CO2 in the gas remained below 50%. This inhibition could not be explained in terms of elevated salts concentration, reduced sludge total solids levels or poor settling ability of the sludge (Table 4).
4.5. Reactor design
The high capital cost of a large, fully-mixed, 'high rate' digester based on the design of a Western sewage digester may well be prohibitive for
Anaerobic digestion of fruit and vegetable refuse 257
many companies. However, a number of low cost designs have been developed in recent years which could well provide an economic solution to the problem. In particular, the plug flow 'Taiwanese' digester, based on a cylindrical rubber bag which serves as both digester and gas holder (Union Industrial Research Labs, 1021 Kuang Fu Road, Hsinchu, Taiwan 3000) is technically the simplest digester design to appear so far. No reports on the performance of these reactors have appeared from Taiwan, but trials carried out with feeding livestock manures to similar reactors in other countries have demonstrated the efficacy of this design. 1a-2
In view of the potential benefits, it would be of interest for pilot scale versions of the 'Taiwanese' and other low-cost designs to be established at factories and used to determine optimum conditions for commercial utilization of fruit and vegetable processing wastes.
4.6. Final treatment
In Australia, many sewage treatment authorities will not accept effluents with COD levels above 600 mg litre -1, or charge heavy penalty rates for treating such effluents. Direct discharge to watercourses of effluents with COD levels above 100 mg litre -1 is often prohibited. The COD levels in supernatant fluids from fruit waste digestion ranged from 2.62 to 10.11 g litre -~ (Table 5), so that further treatment would be required before final discharge.
Possible means of low cost final treatment which may yield effluents of acceptable quality include stabilization (eutrophication) lagoons, oxidation ditches and anaerobic ponds or tanks. Alternatively, the digestion fluids could be used on pastures or in trenches or ponds for cultivation of grasses, forage crops or water weeds, 21, 22 taking advantage of the fertilizer value of the nitrogen and phosphorus that the fluids contain.
Waste solids from processing of many fruit and vegetables can be utilized as feedstock for anaerobic digestion, with gas (50-65% methane) yields of 0.429-0.568 (average 0.463) litre g-~ TS fed. Recovery of settled solids from discharged mixed liquors and their return to the digester enables 88-96% conversion of organic solids to be obtained.
258 A. G. Lane
Provided adequate alkalinity levels are maintained, a loading rate of 4 kg TS m -3 day -1 can be maintained for many fruit and vegetable wastes (pineapple pressings, apple press cake, extracted sugarbeet pulp, corn cobs, asparagus waste); for whole fruit waste the maximum accepted loading rate is 3.5 kg TS m -3 day -1. The reason for inhibition during apricot digestion is not known.
On this basis, a factory generating 10 000 tonnes (fresh weight) of waste per year ( 15% TS) could anticipate generating 690 000 m 3 of gas per year, equivalent in calorific value to about 410 000 litres of diesel fuel. At a loading rate of 4 kg TS m -3 day -1, a digester capacity of 250 m 3 per tonne TS fed per day would be required.
Further study is required to determine the minimum required levels of added nitrogen, phosphorus and other elements and to examine the use of pig and poultry manure as a source of these nutrients.
COD levels in digestion supernatant fluids are too high to permit direct discharge to sewers or watercourses. Simple, low cost processes for reducing the COD to discharge standards should be examined.
A range of low cost reactor designs should be tested on a pilot scale to enable design of the most efficient and economical reactor for utilization of fruit and vegetable processing wastes.
I wish to acknowledge the co-operation and support of the Letona Cooperative Cannery Ltd, Leeton, NSW, the Golden Circle Cannery, Brisbane, Queensland and the skilled technical assistance of B. Crowley. I thank D. Mugford (Bread Research Institute of Australia, North Ryde, NSW, 2113, Australia) for kindly carrying out analyses for nitrogen and phosphorus.
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Anaerobic digestion o f fruit and vegetable refuse 259
5. American Public Health Association (1976). Standard methods for the exami- nation of water and wastewater, 14th edition, APHA, Washington, DC.
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2nd edition. University of Tokyo Press, Tokyo. 18. Gron, G. (1980). In: Anaerobic digestion, D. A. Stafford, B. I. Wheatley &
D. E. Hughes (eds.). Applied Science Publishers Ltd, London, pp. 377-93. 19. Hayes, T. D., Jewell, W. J., Dell'Orto, S., Fanfoni, K. J., Leuschner, A. P. &
Sherman, D. F. (1980). In: Anaerobic digestion, D. A. Stafford, B. I. Wheatley & D. E. Hughes (eds.). Applied Science Publishers Ltd, London, pp. 255-88.
20. Pound, B., Bordas, F. &Preston, T. R. (1981). Trop. Anim. Prod., 6, 146-53. 21. Anon (1982). Process Eng., 10 (1), 7. 22. Finlayson, M. (1983).Pacific ScL Congr. Proc., 15 (1-2), 72-6.