design and operation of laboratory-scale anaerobic digesters: operating experience with poultry...

Agrttultura/ Waste~ 2 (1980) 119 133

DESIGN A N D OPERATION OF LABORATORY-SCALE ANAEROBIC DIGESTERS: OPERATING EXPERIENCE

WITH POULTRY LITTER

FREDA R. HAWKES 8/. B. V. YOUNG

Department of Science, The Polytechnic ~/~ Wales. Pontypridd, Mid-G/amorgan, CF37 IDL. Great Brilain

ABSTRACT

Small-scale anaerobic digesters may provide more stable rates of gas production than large-scale plant, since fluctuations in the feed waste may be minimised. A versatile, readily assembled 5-litre capacity digester and alternative gas-collecting systems are described, with some details of their operation on poultry litter which had been stored dry at 4°C. A continuously recording gas meter (Water Research Centre, UK) showed stable gas production rates were obtained with once-daily stirring and feeding. This rate dropped sharply over two days starvation, requiring 2-3 days consecutive feeding to return to a stable maximum. A minor component of the feed appeared to be utilised immediately. Continuous stirring had little effect on gas production," an immediate rise in gasproduction ratejbllowed the addition of glucose, corresponding to conversion of 60% of the added glucose to CO 2 + C H 4 in 24h.

INTRODUCTION

Tests using laboratory-scale digesters form an important part of work on anaerobic digestion. This is particularly true of studies on agricultural and food-processing wastes, since there are as yet relatively few full-scale plants in operation. This paper describes an easily constructed, flexible, type of digester assembly, currently operating on poultry litter, and presents data on changes in gas production rate after feeding, starvation, stirring and glucose addition, obtained using a continuously recording gas meter.

Small-scale digesters are obviously useful in preliminary investigations of the susceptibility of various wastes to digestion, and a number of examples of this type of work occur in the literature cited here; they have also been widely used in metabolic studies, particularly those involving the addition of radioactively labelled

119 Agricultural Wastes 0141-4607/80/0002/0119/$02-25 ~ Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

120 FREDA R. HAWKES, B. V. YOUNG

substances (e.g. Mountfort & Asher, 1978; Jeris & McCarty, 1965), and for testing the toxicity of various compounds (e.g. Mosey & Hughes, 1975). Bench-scale digesters have the advantage over larger digesters that fluctuations in the composition of the incoming waste can be minimised, since batches of feed may be stored refrigerated or frozen. Control over the composition of the incoming waste is rarely possible in large-scale operation so that fluctuations may mask complex relationships such as those between gas yield and loading rate (Horton & Hawkes, 1979), particularly in studies involving longer retention times. Knowledge of such relationships is necessary so that full-sized digesters may be operated to give a net energy output, and not a deficit.

However, there are problems in extrapolating results from laboratory-scale to pilot-plant or full-sized digesters. Parameters affected by scale-up include heating, stirring and gas recirculation patterns; but one significant difference is that while large-scale plants are fed semicontinuously or continuously, laboratory digesters are normally batch-fed daily by hand. Metabolic variations have been shown (Mountfort & Asher, 1978) to occur during the 24 hours following daily batch- feeding of a laboratory digester with bovine waste, and it is of interest to determine whether digesters fed daily show a declining gas production rate between feeds. If this were so, it might suggest that the performance of such laboratory-scale digesters would be poorer than that of full-scale, continuously fed plant. The necessity for daily manual feeding of small laboratory digesters arises from the lack of suitable pumps to deal with small volumes of non-homogeneous slurry. The simple syringe feeder suggested by Stander & Snyders (1950) could perhaps be modified, but at present it seems that only workers using a non-particulate substrate (see, for example, Pretorius, 1972) are able to feed continuously.

The laboratory-scale digesters described in the literature so far quoted illustrate the range of digesters which is possible. The variety of heating, mixing and gas collecting systems which are in common use have been reviewed by Stafford et al. (In press). Since an important advantage of laboratory-scale work is that fluctuations in feed composition may be avoided, other variations in operating conditions between digesters used in the laboratory must also be minimised or the advantage of stability and reproducibility is lost. For example, a uniformly heated environment for all digesters such as a constant temperature room or a water bath, is preferable to the use of heating tapes (Varel et al., 1977) or hot plates, which may give rise to variations between digesters. The latter methods also require continuous mixing, so that any studies on variations in stirring rate will be complicated by heating effects. The use of a motor-driven rather than a magnetic stirrer also increases the versatility of the digester, since the revolutions per minute produced by a stirrer motor may be more readily measured and standardised between digesters than the revolutions of a magnetic bar; also more powerful mechanical stirrers allow waste of higher total solids contents to be investigated. However, if a mechanical stirrer is used, a leak- proof entry point for the stirrer shaft is required.

LAB-SCALE ANAEROBIC DIGESTER OPERATION 121

The design of the digester should be such that sampling and feeding do not introduce air, although many useful studies have ignored this point (e.g., Hart, 1963). The feeding method used is often dictated by the type of heating and gas collection system employed. A draw-and-fill system with a sample port at the base of the digester, as used, for example, by van Velsen (1977), is ideal for digesters in a constant-temperature room, but not for those in a simple, heated, water bath. There are four common methods of gas collection : balloons or expanding bags, as used, for example, by Varel et al., 1977; floating gasometers (e.g. Hart, 1963); liquid displacement systems (e.g. Jeris & McCarty, 1965; van Velsen, 1977), and gas meters. If the gas line enters the gas collecting system under liquid, as in some versions of the latter three methods, a gas reservoir with a siphon device will be necessary to avoid suck-back of liquid or entry of air during feeding and sampling.

Many methods of gas measurement involve collection over acidified water, brine, or water itself. It should be noted that the solubility of carbon dioxide in water is high. At 15°C, the solubility coefficient (ct15) in pure water is given as 1.014 (International Critical Tables, 1928). At 25 °C, this figure falls to 0.756 volumes per volume. In gas collecting systems where the digester gas is actually bubbled through a large volume of liquid, considerable errors in the gas volume and composition recorded can result until the solution becomes saturated with CO 2. In an attempt to overcome this problem, acidified water or salt solutions are commonly used. It can be seen that salt solutions are the more effective by comparing the carbon dioxide solubility coefficient at 25°C (~25) in IM HC1 (0.732) and in 1"195M NaC1 (0.583). Nevertheless, a considerable discrepancy could still result; van Velsen (1977) discusses this problem. Therefore, if choosing a water displacement system for gas collection, it seems desirable to avoid actually bubbling the gas through the liquid unless using an enclosed reservoir system where the same CO2-saturated solution is re-used constantly. Whether the accuracy of the experimental work warrants the correction of daily gas volumes to STP must also be considered. It seems unlikely that normal changes in room temperature or pressure in a temperate climate would individually give rise to errors greater than about 6 °/ /o '

A continuous recording of gas production is most useful in many studies. One small-scale design has been described which accomplishes this (van den Berg et al.,

1974). A gas recording device which deserves more general use has been developed by the Water Research Centre, UK (1975). Its ability to produce a continuous graphical record of gas production if connected to a chart recorder has not been stressed, and there appears to be no examples so far in the literature of its use in this manner, it consists of a dual-chamber gas collecting device (volume approximately 150 cm 3) immersed in acidified water. When filled with gas the chamber tips over, tripping a reed switch to produce a change in voltage. Thus, using a chart recorder, the length of time to collect this volume of gas can be continuously recorded.

The volume of digester chosen by different workers varies, e.g. from 0.75 to 9 litres in the representative literature quoted here. The limits on digester volume are that it


should provide sufficient gas or effluent samples for analysis and experimentation, use a large enough daily volume of feed to minimise error due to non-homogeneity, and produce sufficiently small volumes of waste and gas to make handling and disposal easy.

Bearing in mind the points so far raised, the following laboratory digester design was chosen, and operated on poultry litter.

MATERIALS AND METHODS

Apparatus Three 5-1itre capacity digesters were constructed out of standard Quickfit glassware (Corning Ltd, Stone, Staffordshire, UK) (Fig. 1). A 5-1itre, flat bottomed, wide-neck culture vessel was fitted with a multisocket flanged lid having four 19/26 ports and one 34/35 port. All joints were greased with silicone vacuum grease and the lid secured with a flange clip. A stirrer shaft of 3-mm stainless steel was fitted with two propellers separated by approximately 100mm and manufactured from 60-mm

s t i r r e ~ gland. ¢ gas exit

Feed ~or~ ..-...~..

Fig. 1. Five-litre capacity digester constructed from Quickfit glass-ware, showing entry of stirrer shaft through sleeve gland containing diethylene glycol as lubricant. Two other ports (not shown) are for gas

sampling, safety blow out point or port for gas recirculation.


diameter stainless steel discs. The shaft entered the digester through a stirrer guide and sleeve gland containing diethylene glycol as a lubricant and gas seal. Stirring was by means of a heavy-duty stirrer motor giving 100-600rpm and normally set at 300 rpm. The stirrers for the three digesters were linked to an electric timer normally operative for one or two 15-min intervals during 24 h.

The largest port of the lid was fitted with a polythene tube (30mm outside diameter, 4 mm wall, approximate length 160 mm) which dipped below the level of the liquid in the digester and was sealed in a gas-tight manner to the lid by firmly inserting the end of the heated, flexible tubing into the port from below. Sampling of contents and once-daily feeding were performed through this tube, and a 100-cm a plastic syringe with a modified end was used to remove the effluent.

Gas samples for analysis were removed via a 17-gauge syringe needle permanently inserted through a bung into the digester head space and closed externally by rubber tubing and a mohr clip. The two remaining ports in the lid accommodated the gas exit line and a greased Quickfit stopper, used as a pressure release point. This port was also used for gas recirculation, using a peristaltic pump, through a length of stainless steel tubing fitted with an aquarium aerator stone; under these circumstances the gas exit line fitment became the pressure release point.

The digesters were maintained at 35 °C + 0.1 by immersion in an aquarium tank heated by a Grant FHI5 flowheater (Grant Instruments Ltd, Cambridge, UK). An aquarium submersible thermostatic 125-watt heater and aerator were also used successfully. A sheet of plastic covering the top of the tank cut evaporation losses and caught spillages during feeding of the digesters.

Three types of gas collecting system were used during our investigations. These were a 0.25-1itre wet gas meter (Wright & Co., Tooting, London, UK), or a recording gas meter, manufactured by Bird & Tole Ltd, Bledlow Ridge, UK, to the Water Research Centre (1975) specification, both used with a siphon gas- reservoir system necessary to allow sample removal (Fig. 2(a)), and a simple, liquid- displacement siphon system involving 10-1itre aspirators of tap water marked with measuring tape (Fisons, Loughborough, UK) (Fig. 2(b)). Each method has a different sensitivity; the wet gas meter and aspirators measure gas volumes to the nearest 1 cm 3 and 40 cm 3, respectively, while the WRC meter records the passage of gas in 150-cm 3 batches.

Analytical methods Volatile fatty acids (VFA) as mg acetic acid litre-1 and volatile solids (VS)

recorded as percentage of TS were determined by standard methods (HMSO 1972). Total solids (TS) were determined by heating to constant weight (approximately 2 h) using a microwave oven set on defrost.

Gas samples removed daily from the digester head-space before or after feeding were analysed immediately for CO 2 and CH 4 using a 2-m Poropak T column in a


a

A

} {

Fig. 2. Gas collection systems preventing entry of air during feeding and sampling: (a) gas meter with siphon gas-reservoir. On feeding, clip A is closed and clip B opened; (b) liquid displacement using calibrated 10-1itre jar for gas measurement. Before feeding or recording gas volumes, water level in the

open aspirator is brought level with that of the closed jar.

Perkin-Elmer 452 gas chromatograph fitted with a gas sampling port and a thermistor detector, with nitrogen as carrier gas and an oven temperature of 60 °C.

Digester start-up One digester was seeded with approximately 4 litres of effluent from a 1-m 3

digester operating on the Polytechnic site on sewage sludge at a 10-day retention time (RT) with a 1.8 ~o TS feed.

A second digester was seeded with approximately 4 litres of the contents o fa 1 -m 3

digester, also on the site, which had been running on poultry litter to give the results reported by Hawkes et al. (1976). This digester had been inoperative for 20 months since the end of that experimental work. After 4 months of operation, half of the contents of the second digester were used to seed a third.


Digester feed Deep litter based on wood shavings and sawdust was collected from the floor of

huts housing laying hens on a local small-holding. A layer approximately 15 cm thick normally occurred on the floor of the hen houses, which were cleared approximately yearly. Samples dug from this layer were stored in polythene sacks at 4 °C and used as feed over a period of about six months. A batch of liquid feed was prepared every I to 2 weeks by macerating the litter plus tap water in an Atomix Blender (MSE, London) for 3 min at top speed, and stored at 4 °C. The VFA of this feed material was undetectably low.

RESULTS

There were no major problems during two years of continuous operation of the three digesters. After eighteen months operation a layer of grit which had accumulated in the digesters was removed by decanting the contents of each jar. Difficulties were experienced in removing the lid after this time, and the flanges of the lid and jar were subsequently left slightly overlapping so they could be prized apart. Small red patches, presumably of purple photosynthetic bacteria, growing on encrusted solid material, slowly appeared on the lids during the first few months, but did not appear to develop further. On two occasions, misalignment of the motor on the stirrer shaft led to fracture of the sleeve gland, allowing diethylene glycol to enter the digester and give a transient increase in VFA and gas production rate. If a non-metabolisable lubricant such as mineral oil had been used, its immiscibility and persistence in the digester might have created greater problems.

Changing the means of gas collection from either of the types of gas meter to the liquid displacement method gave rise to no measurable difference either in gas composition or the volume of gas recorded daily. This would suggest that the diffusion of CO 2 into the tap water used in the liquid displacement system was slow enough not to introduce significant error, even though the jars were topped up with fresh tap water several times weekly. Thus the alternative of using a thin layer of mineral oil over the liquid surface was considered to be unnecessary. The liquid displacement system also proved useful in detecting leaks which merely appeared as a drop in gas production when using a gas meter. By adjusting the liquid levels in the aspirators, a positive or negative pressure of up to 30 cm could be exerted on the digester+ and a leak under these conditions would be detectable since the levels in the two aspirators would equalise. The feeding system adopted appeared to exclude air from the digester head space; gas sampling either before or after feeding showed no variation in the percentage of CO+, and CH+ present.

Start-up The digesters seeded with sewage digester contents (VFA 100 mg l i t r e -1 1 ~o TS,

77 ?/o VS) or aged poultry digester contents (VFA 4500 mg litre- 1) showed different

126 FREDA R. H A W K E S . B. V. Y O U N G

operating characteristics over the first six months. The initial conditions of 100-day RT and 0.33kg VS m -3 d -1 were gradually changed to 35-day RT, 1.16 kg VS m - 3 d - 1 over a period of four months for the poultry-based and over a period of six months for the sewage-based digester. The VFA of the former fell gradually to 1000 mg litre- 1 over this period, while that of the latter remained below 500 mg litre- 1, though this digester had an unstable response to increases in loading rate. Once this phase was passed, digester stability was unaffected by decanting the contents or suddenly increasing the loading rate, and within 8 months of start-up the VFA of all three digesters operating under similar loading rates had stabilised at 800-1000mg litre-1. Results described here were obtained in the second year of operation.

Gas composition During operation on feed slurry prepared as described in the methodology, the

average gas composit ion at varying loading rates of all three digesters over the two- year period was 53~o CH4 + 3 ~ , 45~o CO2 + 3%. However, when slurry previously stored at room temperature to produce a high (6-13,000 mg litre- 1) VFA concentration was used as feed, the gas composit ion altered within 48 h of the change-over, giving an average composit ion of 64 % CH 4 + 3 ~ , 34 ~o CO2 --- 3 over the five-week period that this type of feed was used.

GAS PRODUCTION

2,

1.

0 : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 1 2 3 ~ 5 6 7

WEEKS

Fig. 3. Fluctuations in daily gas production recorded on a 5-day week feeding schedule. Digester fed 2.2 g VS litre- 1 on Mondays to Fridays, at 10 ~o TS, 35-day RT; gas volumes recorded before feeding.

Gas production shown for Mondays is average of gas volume produced over preceding 3 days.

LAB-SCALE ANAEROBIC DIGESTER OPERATION ~')7

Effect of various feeding schedules on gas production A typical record o f daily gas volumes produced on a M o n d a y to Friday only

feeding schedule is shown in Fig. 3. The digesters were fed and the gas volumes recorded at the same time each morning on weekdays only. The start o f the eight- week period recorded in Fig. 3 corresponds to the change-over to a fresh batch of poultry litter (40 % TS, 55 % VS). Hence, since a gas yield can only be calculated after a retention time has been completed, a gas yield of 0.415 m 3 kg VS ~ is obtained from the data in Fig. 3. It can be seen that this type o f feeding schedule gives rise to a regular weekly pattern of fluctuations in the volume of gas produced per

t 200 ¸

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1 2 3 4 5 6 "7

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200

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. ' - . . - ,-. ~ , ~ . . : " ~ " - , ~-. 7-.- '- '-- . . : . . - ' - - - ' - ' : - - - - - ' - - " "

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.

~-,.. ,%_ . , , , ~ . : .

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Fig. 4. Fluctuations in gas production rate monitored using the continuously recording WRC meter. Prior to day l, digester operation was as described in Fig. 3, legend. T indicates addition of 11.0 g VS

(200cm s slurry, 10% TS). ~ indicates 15-min period of stirring.


day, with the lowest volume generated in the 24 h after the Monday feed, rising to a maximum value each Friday.

To provide more information on these fluctuations, the WRC continuously recording gas meter was used to monitor the events occurring over a similar period on both a five-day- and a seven-day-week feeding schedule, using the same batch of feed. The results are shown in Figs 4 and 5 which represent six consecutive weeks of digester operation. Figures 4(a), 5(a) and 5(b) show a steady decline in the hourly rate of gas production in the 72 h following feeding• In the absence of further feeding (Fig. 5(c)) this fall-offcontinues, stabilising after approximately 1 month at a rate of 50-150cm 3 gas per digester per day; a rate which continues for some 17 weeks.

T,:

Z 0

L .~

300.

200-

100

a)

300!

200

100

22 23 24 25 26 ~ 28

°

°

t t t 4 t

3oo I 200

I00

29 30 31 32 33 34 35

° ° ° ° ~ ° ° °o ~ ° ° o °

36 3"1 38 39 ~0 ~I 42 DAYS

Fig. 5 Fluctuations in gas production rate monitored using the continuously recording WRC meter. For operating conditions see Fig. 4.


Feeding after a short period of starvation produces an immediate but transient increase in gas production (Figs 4(a), 4(b) and 5(b)). This type of response to feeding persists for 2 to 3 days after starvation, but after this time consistent daily feeding produces a relatively steady hourly rate of gas production (Figs 4(b), 4(c) and 5(a)), demonstrating the stability of gas production which can be achieved with daily batch feeding. The gas yield obtained from the data given in Figs 4(c) and 5(a) corresponds to 0-403 m 3 kg VS- 1 at 10 ~o TS feed. However, since the change in feeding pattern produces a change in the retention time, the actual RT at which this gas yield is obtained can only be estimated as between 35 and 25 days.

It should be noted that using the WRC meter there is a short delay in recording gas production after feeding, since the system has to repressurise after being brought to atmospheric pressure. The length of this delay varies with the rate of gas production, but is usually 3~60 min.

Effect of stirring It could be argued that the effects of feeding on gas production rate shown in Figs

4(a), 4(b) and 5(b) were stirring effects only, since, as described in the methodology, stirring and feeding were coincident. The effects of stirring without feeding are shown in the latter part of Figs 4(a), 5(a) and 5(b). Stirring gives a rapid, transient increase in the volume of gas recorded, corresponding to the clearly visible release of trapped gas bubbles from the settled solids in the digester. It should be noted that the 15-min stirring period may occur at any point during the cycle of filling the WRC meter's dual chambers, so that it is possible, as for example occurs on day 18 in Fig. 4(c), that the filling time of both chambers consecutively may be influenced by one stirring period.

Figure 6 shows the effect of continuous rather than intermittent stirring on the gas production rate. No clear-cut change is observed.

Effect of addition of metabolisable substrates The increased gas production following the entry of diethylene glycol from a

fractured gland has been mentioned above. A digester producing approximately 2-2 litres of gas per day(loading rate 2.0 gVS litre-1 digester day- l , RT 29 days), into which approximately 8 g of diethylene glycol was spilled produced almost 7 litres of gas per day for 2 days, with the rate falling to the previous value within 5 days.

The immediate response to the addition of a known quantity of powdered glucose is shown in Fig. 7. The figure is slightly complicated by the effect of stirring in liberating the trapped gas bubbles, but it can be seen that the gas production rate reaches a maximum within 12 h, and returns to its original level within 24 h of the addition. The composition of the gas produced was 56 ~o CH4 +_ 3 ~o, 47 ~o CO2 +__ 3 '~%. By a simple carbon balance calculation, the 8.1 g of glucose added would be expected to give rise to 6-49 litres of gas at 20°; 5.90 litres were actually produced in the 24h following glucose addition. Allowing for the baseline production expected from the


BAS BROOUCTION cm 3 h -I

300

200

100

0

S T I R o 4

- . . . . - - - . . .

- : : . . . : . : . : . : . . . " . . . . . - : . .

t i t t t

Q I

. . ~ ; ',: .s -6 DAYS 7

Fig. 6. Effect of continuous stirring (shown by solid bar) on gas production. Crosses below axis indicate 15-min periods of stirring. Stirrer speed 390rpm; RT 25 days, 8% TS. Arrows indicate addition of

II.0gVS.

GAS PROOUCTION

cm3 ffl

600

400

• i ~ . , . . . * * . ° * ° ° . ° °

o 1' " ":~" " '~ ,:" " g " " 6 " DAYS

Fig. 7. Response to addition of glucose, recorded using WRC meter. 8.1 g glucose added at point shown to a digester starved for 2 days. RT29 days, 10 % TS, LR 2.16 g VS litre-1 day- t . Arrows marked F indicate addition of 12.6 g VS (200cm 3 slurry). Crosses below axis indicate 15-min periods of stirring.

F

. : . ' . - : . . : . . - . : - "


digester of approximately 2-0 litres during the 24-h period, approximately 3.9 litres of gas appears to have been derived from added glucose, corresponding to a conversion of 60 ~o of the added glucose to gas within 24 h of its addition.

DISCUSSION

The start-up period was longer when using sewage digester contents as a seed than when using aged poultry digester contents. A similar slow, low loading rate start-up procedure was described by Hobson & Shaw (1973)when adapting sewage digester seed to pig manure digestion.

The presence of purple photosynthetic bacteria in glass digesters has previously been reported (Toerien, 1967). Since the amount of growth here was extremely small, it is unlikely that these bacteria affected the overall process significantly.

As stated in the Introduction, it is frequently the case that large-scale anaerobic digesters are fed continuously or semicontinuously by an automated loading mechanism, while small-scale digesters are batch fed manually on a daily basis. The data presented in Figs 4(b), 4(c) and 5(a) show that a stable rate of gas production is perfectly feasible when feeding on a daily basis, but that failure to feed the digesters for 1 to 2 days drastically reduces the gas production rate, an effect which is only overcome by 2 to 3 days of consecutive feeding. It appears that if stable rates of gas production are required there is no substitute for feeding every 24 h, so that the use of some automatic device which could deliver variable volumes (e.g. 200-500 cm 3 ) of the high total solids feed material over weekend periods would be most desirable.

It is interesting to note from Figs 4, 5 and 6 that stirring appears to have only a transient effect on gas production as gas bubbles are dislodged from the settled sludge. The digester gas production rate is obviously more influenced by the frequency of feeding than that of stirring.

The digester appears capable of an immediate response to added substrate. suggesting that the metabolic rate of methanogenic bacteria does not limit the process. The response is obvious with diethylene glycol and glucose, but also occurs. if only transiently, in response to feeding poultry litter when this is preceded by a period of starvation (Figs 4(a), 4(b) and 5(b)). It appears that some component of the poultry litter is rapidly converted to gas, while the bulk of the material is broken down more slowly. Since the VFA concentration of the feed material, assayed both colorimetrically and by GLC analysis for C 2 C 5 acids, i s negligible, some component other than these acids is present in poultry litter and readily metabolised.

The digester gas collection assemblies described here have proved versatile and trouble-free. The WRC meter deserves wider recognition as a means of continuously recording gas production. The meter is not only useful in monitoring the low flo~' rates produced in laboratory-scale work, since in tests with pilot-plant digesters on


the Polytechnic of Wales campus it has successfully operated at gas production rates > 1 m 3 day- 1.

Poultry litter has proved an amenable material for these laboratory-scale trials, as it is easily stored in the form of a solid, unobjectionable to handle, and can easily be made up into feed slurries of varying total solids concentrations. The proportion of methane in the digester gas reported here (an average of 53 9/0 with the remainder CO2) is similar to that reported by Gramms et al. (1971) for digesters operating on fresh poultry manure. It appears likely that storing the poultry litter as a slurry under conditions conducive to the development of high VFA concentrations at least partially contributes to the higher methane composition reported by other workers, e.g. Hawkes et al. (1976). Investigations on the operation of laboratory-scale digesters on poultry litter are continuing.

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the consistent technical help of Mrs Alvine Jones and Mr H. Hopkins.

REFERENCES

GRAMMS, L. C., POLKOWSKI, L. B. & WITZEL, S. A. (1971). Anaerobic digestion of farm animal wastes, (dairy bull, swine and poultry). Trans. Amer. Soc. Agric. Eng., 14, pp. 7-11.

HART, S. A. (1963). Digestion tests of livestock wastes. Journal WPCF, 35, pp. 748-57. HAWKES, D. L., HORTON, R. & STAFFORD, D. A. (1976). The application of anaerobic digestion to

producing methane gas and fertilizer from farm wastes. Process Biochemistry, 12(2), pp. 32-6. HORTON, R. & HAWKES, D. L. (1979). Anaerobic digester design fundamentals Part If. Process

Biochemistry, 14(9), pp. 12-16. HMSO (1972). Analysis of raw, potable and waste waters. HMSO, London. HOBSON, P. N. & SHAW, B. G. (1973). The anaerobic digestion of waste from an intensive pig unit. Water

Research, l, pp. 437-49. International Critical Tables of Numerical Data, Physics Chemistry and Technology, Vol. 3 (1928).

McGraw Hill, New York, p. 279. JERIS, J. S. & MCCARTY, P. L. (1965). The biochemistry of methane fermentations using C t4 tracers.

Journal WPCF, 37, pp. 178-92. MOSEY, F. E. & HUGHES, D. A. (1975). The toxicity of heavy metal ions to anaerobic digestion. Water

Pollution Control, 74, pp. 18-39. MOUNTFORT, D. O. & ASHER, R. A. (1978). Changes in proportions of acetate and carbon dioxide used as

methane precursors during the anaerobic digestion of bovine waste. Appl. Environ. Microbiol., 35, pp. 648-54.

PRETOR1US, W. A. (1972). The effect of formate on the growth of acetate-utilising methanogenic bacteria. Water Research, 6, pp. 1213-17.

STAFFORD, D. A., HAWKES, n . L. • HORTON, R. (In press). Methane Production from Waste Organic Matter. CRC Press Inc., Florida.

STANDER, G. J. d~£ SNYDERS, R. (1950). Re-inoculation as an integral part of the anaerobic digestion method of purification of fermentation effluents. Inst. Sew. Purific. Journal and Proceedings, Part 4, pp. 447-58.

TOERIF •, D. F. (1967). Enrichment culture studies on aerobic and facultative anaerobic bacteria found in anaerobic digesters. Water Research, 1, pp. 147-55.


VAN DEN BERG, L., LENTZ, C. P., ATHEY, R. J. & ROOKE, E. A. (1974). Assessment of methanogenic activity in anaerobic digestion: apparatus and method. Biotech. Bioeng., XVI, pp. 1459-69.

VAN VELSEN, F. M. (I 977). Anaerobic digestion of piggery waste. 1. The influence of detention time and manure concentration. Neth. J. Agric. Sci., 25, pp. 151-89.

V^REL, V. H., IS^ACSON, H. R. & BRVXNT, M. P. (1977). Thermophilic methane production from cattle waste. Appl. Environ. MicrobioL, 33(2), pp. 298-307.

WATER RESEARCH CENTRE (1975). Equipment for measurement of gas production at low rates of flow. Technical Memorandum TM 104. 'ArRC, Stevenage, Herts.

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