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
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.
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.
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.
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.
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.
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 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.
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.
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 show