Laboratory scale anaerobic digestion of poultry litter: Gas yield-loading rate relationships

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  • Agricultural Wastes 13 (1985) 31-49

    Laboratory Scale Anaerobic Digestion of Poultry Litter: Gas Yield-Loading Rate Relationships

    A. R. Webb* & Freda R. Hawkest

    Department of Science, The Polytechnic of Wales, Pontypridd, Mid Glamorgan CF37 I DL, Wales, Great Britain

    ABSTRACT

    The mesophilic anaerobic digestion of poulto' litter (manure plus sawdust) was investigated at laboratory scale in dailyJed digesters over a range oJ retention times and influent concentrations. Digesters were operated at retention times between 29.2 and 11.7 days and influent Volatile Solids concentration was between 1 ')'o and 5 ')o. Biogas yield was shown to increase not only with retention time but also with increasing influent per cent Volatile Solids" (VS). Gas yields varied between 0"245 and 0.372m 3 biogas per kilogram oJ VS added, with mean methane

    ' o~ composition oJ 59 ~ o. NH~-N levels in the ejfluent didnot exceed 1500 mg per litre (64 mg per litre ojJi'ee NH3). A kinetic' model relating gas yield to influent concentration and retention time is proposed.

    INTRODUCTION

    Poultry litter, a mixture of poultry manure and a lignocellulosic base such as wood shavings or sawdust, arises in the production of laying hens or broiler chickens raised on deep litter. The litter base may remain on the floor of the hen-house for up to 12-14 months and is finally removed in dry form. Poultry litter might be more easily digested anaerobically over a wide range of influent Total Solids than poultry manure since the ammonium-N levels arising in digestion should be lower and might be below the limits reported to be toxic. As poultry litter arises in a dry form, high solids treatment with the addition of a minimal quantity of water * Present address: Satec Etd, PO Box 12, Weston Road, Crewe CW 1 1 DE, Great Britain. t To whom correspondence should be addressed.

    31 Agricultural Wastes 0141-4607/85/$03.30 ~C, Elsevier Applied Science Publishers Ltd, 1985. Printed in Great Britain

  • 32 A. R. Webb, Freda R. Hawkes

    should be most economic. However, there are no reports of successful high solids digestion in the literature, although there are a limited number of reports dealing with this waste as a slurry. Batch tests at laboratory scale at mesophilic temperatures (Farag et al., 1970; Hassan et al., 1975a,b) showed successful digestion, although batch thermophilic digestion is reported to give low gas yields (Shih & Huang, 1980). Previous work carried out at The Polytechnic of Wales with daily fed digesters at laboratory scale (Hawkes & Young, 1980) and pilot scale also showed successful mesophilic digestion of poultry litter.

    The objective of the study reported here was to investigate the way in which gas yield (GY, volume of biogas produced per weight of organic material added) varies with retention time (RT) and influent organic solids content. Gas yield may be expected to increase with increasing digestion time in the reactor, but a relationship apparent from a literature survey by Hawkes & Horton (1981) of the digestion of sewage sludge suggests that gas yield also increases with increasing inftuent Volatile Solids. Since small increases in gas yield may greatly improve the net energy balance of digestion, this effect of increasing influent solids, if verified, may be of practical importance. Poultry litter was chosen as a convenient source of dry, digestible waste able to be slurried to give influent of varying solids content; data obtained in the course of this investigation also supplement the sparse information in the literature on poultry litter digestion.

    METHODS

    Digester feedstock

    Poultry litter (manure plus sawdust bedding) was collected from a local smallholding raising laying birds (Aber Acres) and allowing litter to remain in the sheds for approximately 12 months. Each batch of litter was stored frozen. Feedstock was prepared weekly by homogenising the poultry litter plus tap water in an Atomix blender (MSE Crawley, Great Britain) at full speed for 1 min, followed by refrigeration.

    Digester construction

    Eight 5-1itre digesters constructed from Quickfit glassware (Corning Ltd, Stone, Great Britain), as previously described by Hawkes & Young

  • Digestion of poultry litter 33

    (1980), were maintained at 35 C in a water bath. The digesters were stirred with a motor (Citenco Ltd, Boreham Wood, Great Britain) operated at 600 rpm driving a stainless steel stirrer with 60 mm diameter vaned paddles. Gas volume was recorded using a 0.25 litre wet gas meter (Wright & Co., London, Great Britain) containing light transformer oil. A siphon gas trap system was utilised in an attempt to minimise the sucking in of air during feeding.

    Digester operation

    Over the 15 months of this investigation eight digesters were operated in pairs at each set of operating conditions in order to check the reproducibility of the results. The original start up of digesters operating on poultry litter has been previously described (Hawkes & Young, 1980). Digesters were fed daily for five days a week, being fed a double quantity on Fridays. During feeding, the digesters were stirred vigorously for 5 min, and a volume of effluent withdrawn. After the addition of feed, stirring continued for a further 5rain; thereafter, digesters were unstirred.

    The investigation involved two groups of experiments. In the first, digesters were operated at retention times (RT) of 29.2, 19.4, 16.7 and 14.6 days and influent Volatile Solids contents of approximately 2 ~o and 4 ~o. In the second, the range of operating conditions was widened and RT of 29.2 and 11.7 days and inftuent Volatile Solids contents of 1 ~o and 5 % were used.

    Between the two groups of experiments the contents of all of the digesters were completely inter-mixed and accumulated grit discarded. Digesters were considered stable when weekly measurements of gas volume, effluent Total Solids, Volatile Solids, NH4-N and total N all achieved stable values. Once stability was achieved, gas yields were calculated on a weekly basis. The retention times and loading rates quoted here were obtained by averaging the amount fed during the 5-day feeding cycle over the complete week.

    Analytical methods

    Methane and CO 2 contents of the digester gas were monitored by gas chromatography as described previously (Hawkes & Young, 1980), gas samples being removed before each feed. The reproducibility of the

  • 34 A. R. Webb, Freda R. Hawkes

    method was +2~o. Total Solids (TS) were determined by drying, to constant weight, approximately 20 ml samples in 250cm 3 beakers in a microwave oven set on 'defrost' (Hitachi MR 0050). This method takes approximately 2 h compared with the standard method (HMSO, 1972) of drying in a conventional oven at 105 C overnight. Volatile Solids (VS) were determined by the ignition of dry samples in an electric furnace at 500C for 30min (HMSO, 1972). Feed and effluent TS and VS were determined weekly.

    The pH of the effluent was determined as soon as possible after removing the samples in an attempt to minimize the effects of changes in dissolved CO 2. Alkalinity was determined by the standard method (HMSO, 1972) by titration to pH 4-5. Measurements ofpH and alkalinity were made fortnightly.

    NH~-N was assayed in samples spun in the bench centrifuge to clarify after dilution with an equal volume of 0.1 M HC1, followed by steam distillation in a Markham still and back titration. Total nitrogen was assayed by the distillation procedure following a microKjeldahl digestion with selenium dioxide catalyst.

    pS was measured using a sulphide electrode constructed and calibrated according to the method of Mosey & Hughes (1975). Holocellulose was determined by a delignification procedure (Allen, 1974) followed by alkaline hydrolysis to remove hemicellulose, leaving s-cellulose. BOD 5 was determined at room temperature using a Hach manometric apparatus to follow O 2 consumption. Volatile Fatty Acids were measured as milligrams acetic acid equivalent per litre using a colourimetric method (HMSO, 1972).

    Data handling

    Data were entered in a Data Base Management system, ADDSYS, developed by Dr B. L. Rosser at The Polytechnic of Wales. The system allows data retrieval, processing and graphical output.

    Solids digestion

    A preliminary batch experiment on the high solids digestion of poultry litter was conducted using a lO-litre capacity packed bed digester, constructed from perspex, in which a fixed volume of liquid was constantly recirculated through the litter retained in a nylon net and held

  • Digestion of poultry litter 35

    above the surface of the recirculating leachate by a perforated plate. Liquor was recirculated at a rate of 20mlmin -1. The digester, enclosed in a heated cabinet at 30 C, was initially filled with 2.9 kg wet weight of litter (TS content, 75.27~o, with 89.11~o of the TS being volatile). The liquid added (total volume, 5.3 litres) was a mixed effluent taken from laboratory scale digesters operated as described above on poultry manure at 2-6 ~o TS and diluted with an equal volume of water before use. The liquor was temperature equilibrated before measurement commenced and gas volume and composition, liquor TS, VS, alkalinity, VFA, pH and NH4-N levels were measured periodically over a 60-day period.

    RESULTS

    Daily fed digesters

    The composition of the batches of litter used during this investigation and the dates of their collection are shown in Table 1. It can be seen that the sample collected in the summer months (batch 4) was significantly drier.

    Table 2 shows the range of conditions at which the digester pairs were operated during the course of experiments 1 and 2. Table 3 shows the mean effluent characteristics for each digester pair at each operating condition averaged over the stable period. For digesters operating at equal influent VS, the effluent VS decreases with RT with the one exception of digesters operating at 1.98 ...../o influent concentration at 19-4

    TABLE 1 Composition of Poultry Litter used during this Investigation

    Batch TS VS Total N NH~-N No. ('!6) ( ~I~ q[ TS) ( "o dl3 weight) ( !',, dl 3

    weight)

    Holo~ellulose Date of ( ?~i do weight) collection

    l 51-79 60-63 4.62 0-881 --

    2 54.84 61.80 5-72 1.50 46.8

    3 50-25 57.88 5.08 1-47 46.5

    4 81-42 64.71 3-18 0.847 --

    20 March 1980 30 October 1980 12 February 1981 3 August

    1981

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  • 38 A. R. Webb, Freda R. Hawkes

    and 29-2 days RT. Effluent holocellulose also decreases slightly with increasing RT but BOD5 values do not, in most cases, decrease with increasing RT.

    Taking the overall figures for all digesters, the proportion of total N which is NH~-N increased from an average of 25.2 ~o in the influent to 42'8~o in the effluent. While the effluent NH2-N showed a variable relationship with the influent NH2-N levels, there was a definite relationship with influent total N such that digester NH~-N levels can be predicted from influent total N.

    Effluent pH also increased with effluent NH2-N. Table 3 gives levels of free ammonia calculated to occur in the effluent according to the relationship:

    conc. NH 3 = 1.13 x 10 9 (conc. NH4/conc. H +)

    given by McCarty & McKinney (1961). The highest value is 64.5mglitre -1 at 4.9~/o influent VS and the longest RT utilised-- conditions which also gave the highest pH value observed, 7.55.

    There was also a linear correlation between NH~-N and alkalinity, measured both in the feed and in the effluent. It would thus be possible for this particular waste, since this relationship is known, to predict NH~-N content from the alkalinity value.

    pS values in the digesters are lower than those in the influent (i.e. sulphide concentration is higher), presumably due to the reduction of sulphate or the release of organic sulphur and reduction to sulphide during digestion.

    The mean methane content of the gas throughout experiments 1 and 2 was 59.2 ~0. It can be seen from Table 3 that the mean methane content decreased very slightly with increasing retention time, except in the case of one combination (digester pairs 9 and 10). In digester pairs operated concurrently (1-4, 5 8, 9-12) increasing influent concentration decreases methane content very slightly.

    The variation in digestion efficiency with the operating parameters studied is given in Table 4. For influent of the same VS content, biogas yield increased with RT in every case, as expected. At common RT's, the biogas yield increased with increasing influent VS content in every case within experimental groups 1 and 2. Methane yield similarly increased with RT, and also increased with influent VS in all but one case, at 19.4 day RT and 2 }/o and 4 ~o influent VS content. The results for biogas yield from the two experimental groups are plotted in Figs l(a) and (b). It can

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  • 40 A. R. Webb, Freda R. Hawkes

    0.32

    A q0.36 W

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    ZZ

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    (a)

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    . . . . ' . . . . 't 'l . . . . . . . . . . o0 0 .50 .oo ' ' .5o ~.oo ~.E;o LORDIMG RATE (KG US ADDED/M3 ]D IGESTER/DAY)

    0. 325 .

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    0. 225 0 . . . . r 1 . o o . o o ~. oo ~. 0% ~ o~

    LORDTHG RRTE (KG US RDDED/M3 D IGESTER/DRY)

    Fig. 1. Relationship of gas yield to retention time and influent % Volatile Solids. (a) In experiment 1. (b) In experiment 2.

  • Digestion of poultry litter 41

    be seen that the data in Figs l(a) and (b) are not superimposable, gas yields being higher in experimental group 1, presumably due to differences in composition of the batches of litter used.

    However, although the % Volatile Solids reduction increases with increasing RT in every case, following the gas yield, it appears to decrease, not increase, with increasing influent % Volatile Solids in all but one case. The relatively large standard deviation for this data should be noted. From the values for Volatile Solids reduction, a biogas yield, calculated as cubic metres per kilogram of Volatile Solids destroyed, may be determined to be 1.15 (standard deviation, 0.24).

    Similarly, the per cent holocellulose reduction and per cent BOD reduction in most cases increase with increasing RT but, for the limited set of data available, show decreasing values with increasing influent %VS-- a relationship in the opposite sense to that shown by the gas yield. A holocellulose reduction of 50-69 ~o and a BOD reduction of 62 72 ~o can be obtained under the conditions used here.

    Batch digestion

    Some results from the first 20 days of the preliminary investigation of high solids batch digestion of poultry litter are summarised in Fig. 2. The rate of gas production was very high immediately after start up (55 litres in the first 45 h), but decreased sharply after approximately 7 days. The gas, however, was 3-16 ~o CH4, the remainder being CO 2. In the first 10 days of the experiment the VFA levels in the liquor rose from negligible values to 18 000 27 000 mg litre- 1, the pH of the liquor dropped sharply from 8.2 to 6.5--6.7 and the liquor alkalinity increased to 19 000 mg litre- 1. The highest NH,~-N concentration recorded was 6900 ppm. The TS content of the recirculating liquor increased from 0"86 ~o at start up to a constant higher level of approximately 4-2 O//o, of which approximately 56 ~o was volatile.

    In an attempt to restart methane gas production, the liquor was drained from the bed after 20 days and replaced with an equal volume of flesh inoculum (50% diluted poultry manure digester effluent). The methane composition rose to 20 25 ~ but the rate of gas production remained low. The liquor removed was fed at a 10-day RT to a 1.5-1itre capacity digester seeded with poultry manure digester effluent diluted to 50 ~o with water and operated at 35 C, but no methane production was recorded.

  • D,-,

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    LIQUOR UFA CONC. (MG/L)

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  • Digestion of poultry litter 43

    DISCUSSION

    Daily fed digesters

    Litter composition Poultry litter is a waste of very variable composition (Caswell et al., 1978) since the length of time hens are reared on the litter and the ambient temperature both influence composition. The nitrogen content of the litter will tend to increase with increasing manure deposition, but uric acid in the droppings breaks down to ammonia (Shefferle, 1965) which may volatilise and reduce the nitrogen content. While the manure collected at the height of the summer was dryer (Table 1), it also had a significantly lower total N content, although the NH~--N content, as a percentage of dry weight, was little different from batch 1. The total N content of the litter used here was of the order of that reported in the literature by Riley (1968), Caswell et al. (1978) and Shih & Huang (1980) and higher than that reported by Farag et al. (1970). The proportion of total nitrogen which is NH4-N found here is comparable with figures given by the last three authors.

    Ammonium-N levels From Tables 1 and 2 the digester NH,~-N content appears to be directly related not to feed NH 4-N, but to feed total-N. This is reasonable as NH 3 is produced by metabolism of nitrogen-containing compounds. The relationship is:

    [Effluent NH~-N (mglitre 1)] =

    [Influent total N (mg litre - 1)] 0.3167 + 199.914

    The effluent NH~-N is also related to pH; pH increases with increasing NH~-N concentration, thus increasing the proportion of NH,~-N which is as NH 3, reportedly the toxic species. Toxicity has been shown to occur in digestion at levels of greater than 80 mg of NH3-N per litre (Anderson et al., 1982). McCarty & McKinney (1961) give a toxicity threshold of 150 mg litre- 1, but levels of free ammonia in the effluent (Table 3) do not reach that figure.

    The relationship between N H 4 content and alkalinity in these results is:

    Alkalinity (mglitre 1) CaCO 3 = [NH~-N (mglitre 1)] 8.611 - 213.428

  • 44 A. R. Webb, Freda R. Hawkes

    If routinely operating on a waste for which the relationship between alkalinity and NH] -N is known, the rapid determination of alkalinity (which also involves a pH measurement) would allow prediction of the NH-N and free NH 3 levels in the sample without further measurements being necessary. Automatic titration of high-N wastes, such as poultry and piggery wastes, could thus be part of a control strategy to avoid toxic overload.

    Gas composition and biogas yields The trend for methane content of the gas to decrease marginally with increasing RT and increasing influent concentration is not easily explained. Pfeffer (1980) suggests that, at shorter RT, the methane content should be higher since the liquid throughput rate controls the amount of CO 2 removed from the digester as HCO3-.

    As expected, the biogas yield increases with increasing retention time (Table 4, Figs l(a) and (b)). These figures show that gas yield also increases with influent VS concentration, as proposed by Hawkes & Horton (1981).

    The gas yields obtained from digesters in experiment 2 are lower than those in experiment 1 so that results from Figs l(a) and (b) are non- superimposable. It should be noted that the holocellulose content, as a percentage of TS, is higher in the influent in experiment 2 than in experiment 1, and an increase in non-digestible volatile matter between these two experiments may account for the difference in gas yields obtained. Perhaps, for a similar reason, the mean biogas yields obtained here (0.245 to 0.372 m 3 per kilogram of VS added) were lower than those previously obtained by Hawkes & Young (1980) for the digestion of chicken litter (0-409 m 3 per kilogram of VS added). In batch tests at 30 C Farag et al. (1970) obtained gas yields of approximately 0.22m 3 per kilogram of VS added after 112 days.

    Kinetic model It is to be expected that the gas yield per gram of Volatile Solids added will increase with increasing retention time. A kinetic basis for the observation that gas yield, i.e. the degree of breakdown of organic material, increases with increasing influent concentration was sought using a model derived from the Monod equation.

    At steady state:

    Volume of gas produced = Kc(S o - S)

  • Digestion of poultry litter 45

    where K,. is a constant (litres of gas produced per gram of substrate destroyed), S o is the influent substrate concentration and S is the substrate concentration in the digester. All substrate concentrations are expressed as grams VS per litre. The gas yield can thus be expressed as:

    Gas yield per gram of (S~) Volatile Solids added = Kc 1 - (1)

    For the modified Monod equation at steady state:

    S = K~(D + kd) (2) Pmax - - D - k d

    where K s and Pmax are the constants in the Monod equation, D is the dilution rate (day - 1) and ka is the specific death rate (day - 1). Substituting for S in eqn (1), the expression:

    Gas yield per gram of ( Ks(D + kd) Volatile Solids added = K,. \1 So(~m,x-~kd) ] (3)

    is obtained. From the relationships between dilution rate, retention time and

    loading rate, substitution in eqn (3) allows expression of gas yield per gram of VS added in terms of loading rate and RT. These equations can be used to generate families of curves of constant RT and influent ~o VS in a plot of gas yield against loading rate (Fig. 3). This Figure shows qualitatively the same relationship between GY and RT and influent ~o VS as Figs l(a) and (b). To obtain this Figure, values for the kinetic constants, K s, /.tma x and ke, and also for the degree of biodegradability of the substrate, Kc, have been assumed on the following grounds: from a plot o rGY per VS added against 1/RT, the GY at infinite RT (that is, K(.) = 0.400 litres per gram of VS for the poultry litter used here; k~ is assumed to be a constant fraction of ]'/max: literature values for ke between 0.025 and 0.118 of Pm,x have been reported (Lawrence & McCarty, 1969; Kugelman & Chin, 1971 ; Anderson & Duarte, 1980) and a value of 0.1 Pmax has been assumed here following Hill (1983).

    Values in the literature for ,l/ma x at mesophilic temperatures vary between 0"28 day- 1 and 0"4032 day- 1. Hashimoto (1982) has shown that lama x is dependent on temperature; at 35C, according to Hashimoto, Pm~x = 0.326day 1; this is the value used to obtain Fig. 3.

    There is a wide variation in the values reported for Ks, suggesting that K s varies with waste type. Substituting the experimentally determined gas

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    E

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    GAS YIELD (M3/KGUS ADDED)

    ? i

    :,

    I

    sa~l,UVH "~I vpa~j 'qqaAl "~f "V 9~

  • Digestion of poultry litter 47

    yields for digesters in experimental groups 1 and 2 in eqn. (3), values of 8.933 and 6-667 g litre- 1 are obtained for the two groups, and a value of 8.933glitre-1 is assumed in Fig. 3.

    The gas yield-loading rate relationship may theoretically be extended. However, care must be taken in extrapolating and it is necessary to determine experimentally the limits beyond which toxic overload may occur.

    It should be noted that the Monod relationship from which the equation used here is derived applies to the degradation of soluble substrate only; such a relationship is compatible, for example, with the results of Switzenbaum & Jewell (1980) for soluble substrates. A portion of the biodegradable fraction of the influent is in a soluble form and it is presumably this fraction which is responsible for the similarity between the model and the experimentally determined results. Further work on the kinetics of insoluble substrate degradation is needed.

    Batch digestion

    The results from the high solids batch digestion (Fig. 2) indicate that hydrolysis and acidogenic fermentations occur rapidly under the initial conditions operative over the first 10 days of the experiment. The high VFA levels achieved, combined with the low pH, may serve to inhibit methanogenesis. A two-phase system, acidification followed by methano- genesis, would be possible if the liquid effluent from the reactor were digestible. As reported, preliminary tests have failed to achieve digestion of this leachate and dilution of the liquor is presumably required. The overall solids content in the packed bed reactor was 27 ~o. Greatly diluting the leachate may involve using the same amount of water--and hence energy--to raise this to 30 C. However, it is probable that the RT in the methanogenic reactor would be considerably shorter than that used for slurries and hence the size of the second reactor would be considerably reduced.

    Poultry litter proved to be a suitable material for an initial investigation of gas yield-loading rate relationships since levels of NH 4 involved were apparently below toxic levels. However, because of the high sawdust content, gas yields per VS added were low, and differences between gas yields at the various operating conditions were slight. To substantiate the proposed relationship further, using a waste in which larger differences in gas yield may be expected, and to investigate the effect of toxicity due to a

  • 48 A. R. Webb, Freda R. Hawkes

    component in the waste, a further investigation was undertaken on poultry manure, and will be reported upon in another paper.

    ACKNOWLEDGEMENTS

    This work forms part of the PhD thesis of ARW (Polytechnic of Wales/Council for National Academic Awards, 1984) and was financed by a research grant from the Science and Engineering Research Council, Great Britain. We thank B. L. Rosser and D. L. Hawkes of the Polytechnic of Wales for helpful discussions during the course of this project.

    REFERENCES

    Allen, S. E. (1974). Chemical analysis of ecological materials. Blackwell Scientific Publications, London.

    Anderson, G. K. & Duarte, A. C. (1980). Research and application of anaerobic processes. Environ. Technol. Letts, 1, 484-93.

    Anderson, G. K., Donnelly, T. & McKeon, M. K. (1982). Identification and control of inhibition in the anaerobic treatment of industrial waste waters. Process Biochemistry, 17(4), 28-32; 41.

    Caswell, L. F., Fontenot, J. P. & Webb, K. E. (1978). Fermentation and utilization of broiler litter ensiled at different moisture levels. J. Anim. Sci., 46(2), 547-61.

    Farag, F. A., Bedaiwi, E. H. & EI-Fadl, M. E. (1970). Production of methane gas from poultry droppings. UAR Agric. Res. Rev., 48(2), 98-106.

    Hashimoto, A. G. (1982). Methane from cattle waste: Effects of temperature, hydraulic retention time and influent substrate concentration on kinetic parameter (K). Biotechnol. Bioeng., 24, 2039-52.

    .Hassan, H. M., Belya, D. A. & Hassan, A. E. (1975a). Characterization of methane production from poultry manure. Proc. 3rd Int. Syrup. on Livestock Wastes, University oJ Illinois, 244 51.

    Hassan, A. E., Hassan, H. M. & Smith, N. (1975b). Energy recovery and feed production from poultry wastes. In: Energy, Agriculture and Waste Management, Proc. Cornell Agricultural Waste Management Conjerence, Ann Arbor Science, Michigan, USA, 289-305.

    Hawkes, D. L. & Horton, H. R. (1981). Optimization of anaerobic digesters for maximum energy production. Studies in Environ. Sci., 9, 131-42.

    Hawkes, F. R. & Young, B. V. (1980). Design and operation of laboratory-scale anaerobic digesters: Operating experience with poultry litter. Agricultural Wastes, 2, 119--33.

  • Digestion of poultry litter 49

    Hill, D. T. (1983). Simplified Monod kinetics of methane fermentation of animal wastes. Agricultural Wastes, 5, 1-16.

    HMSO (1972). The analysis of raw, potable and waste waters. Her Majesty's Stationery Office, London.

    Kugelman, I. J. & Chin, K. K. (1971). Toxicity, synergism and antagonism in anaerobic waste treatment processes. Adv. Chem. Ser., 105. (Gould, R. F. (Ed.)), American Chemical Society, New York, 55-90.

    Lawrence, A. W. & McCarty, P. L. (1969). Kinetics of methane fermentation in anaerobic treatment. J. Water Pollut. Control Fed., 41, R1-R15.

    McCarty, P. L. & McKinney, R. E. (1961). Volatile acid toxicity in anaerobic digestion. J. Water Pollut. Control Fed., 33, 223-32.

    Mosey, F. E. & Hughes, D. A. (1975). The toxicity of heavy metal ions to anaerobic digestion. Water Pollut. Control, 74, 18-39.

    Pfeffer, J. T. (1980). Anaerobic digestion processes. In: Anaerobic digestion. (Stafford, D. A., Wheatley, B. I. & Hughes, D. E.), Applied Science Publishers, London.

    Riley, C. T. (1968). A review of poultry waste disposal possibilities. Water Pollut. Control, 67, 627 31.

    Schefferle, H. E. (1965). The decomposition of uric acid in built-up poultry litter. J. Appl. Bact., 28(3), 412-20.

    Shih, J. C. H. & Huang, J. J. H. (1980). A laboratory study of methane production from broiler chicken litter. Biotechnol. Bioeng. Symp., 10, 317 23.

    Switzenbaum, M. S. & Jewell, W. J. (1980). Anaerobic attached-film expanded- bed reactor treatment. J. Water Pollut. Control Fed., 52, (7), 1953 65.

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