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Monitoring anaerobic digesters on farms
J. agric. Engng Res. (1984) 29,357-365
Monitoring
Anaerobic
Digesters
on Farms
R. FRIMAN*
There are 15 anaerobic digesters installed on farms in the U.K. and these were largely stimulated by the opportunity to produce energy coupled with a reduction in slurry odour. A monitoring programme was developed in 1980 to evaluate the average performance of farm plants in terms of net energy production under practical operating conditions. The aim was also to determine the effects of digestion on slurry with regard to pollution reduction and fertilizer value. This paper reports data gathered from one pilot-scale and two full-scale digesters, treating pig slurry, and from one full-scale digester treating dairy cattle slurry. Monitoring showed on full-scale plants, that pig slurry achieved a specific gross biogas production of 0.5 (m3 biogas) (m3 digester volume)- ’d - I and for dairy cow slurry 0.8 (m” biogas) (m’ digester volume)) ’d-‘. A pilot plant operating on stored separated pig slurry at a retention time of8.3 d produced 1.8 (m3 biogas) (m3 digester volume)-’ d-l. The biogas yield from the total solids (TS) in pig slurry fed ranged from 028 to 0.37 m3 biogas/kg TS fed. The yield from dairy cow slurry was 0.12 m3 biogas/kg TS fed. Digester management was found to be important for the successful steady-state operation of digesters on farms. Digester feed solids concentration and feed age were found to influence specific gross biogas production.
1.
Introduction
Recent increases in energy costs have prompted interest in finding cheaper alternative energy sources for U.K. farms and at the same time intensification of livestock production has led to greater pollution risks. This has promoted the search for treatment systems, such as anaerobic digestion, capable of reducing the polluting power of animal wastes and with the additional bonus of producing energy. A range of prototype digester designs have been constructed on farms, primarily to produce energy but also, more recently, to reduce slurry odour. There are 15 anaerobic digesters installed on farms in the U.K.,1 14 of which were constructed since 1978. The majority of the plants (11) are full-scale and have capacities of greater than 70m3. Seven digesters operate on pig slurry, one plant treats a mixture of pig and poultry manure and seven treat dairy or beef cattle waste. Manufacturers in the U.K. have designed and constructed all the plants except two, one of which was farmer designed and constructed and the other of which was designed by a European company. Six full-scale plants have an above-ground continually stirred digester tank constructed from prefabricated vitreous coated steel panels, and one was a steel tank. Three full-scale plants (two operating on cattle slurry and one on pig slurry) are below-ground rectangular tanks constructed from concrete. In the below-ground tanks slurry is scraped into the digesters mechanically and overflows at the outlet. Three pilot-scale digesters are constructed from fibreglass, and one full-scale plant is constructed from prefabricated fibreglass panels and is available as a do-it-yourself package. All of the digesters except one have a facility for biogas recirculation in order to mix slurry, the other site has a mechanical mixing paddle which is powered by a windmill. All the digesters in the
*MAFF.
ADAS.
Received
IO November
Paper
presented
Farm Waste
IJmt. Coley Park,
19X3. accepted
at AG FNG
in revised
84. CambrIdge,
Reading.
Berkshire
form 5 March
U.K..
1984
I-5 April 1984
358
MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
U.K. operate in the mesophilic range 3%35°C and the digester contents are heated by internal plate or coil heat exchangers. The above-ground digesters are insulated to a thickness of 5-10 cm by expanded polyurethane coated with epoxy resin, which is usually applied internally. The belowground digesters rely on the surrounding soil for insulation. On plants which have a fixed roof, biogas is stored at lOC&200mm w.g. in a floating water-sealed gasometer. Two sites have a raised floating roof on the digester which acts as a biogas store. Storage is normally provided on farms for l-3 h of biogas production. The manufacturers of these prototype plants indicated a need for an independent assessment of the performance of farm-scale plants. A monitoring programme capable of assessing a number of sites was therefore developed in 1980 by the author and her colleagues.
2.
Monitoring methods
The monitoring programme was primarily aimed at evaluating the average performance of farm plants in terms of net energy production under practical operating conditions. Monitoring was also aimed at determining the effects of digestion on slurry with regard to pollution reduction and fertilizer value and at identifying indicators of digester performance which could be used to assist plant operators to manage plants more effectively. Monitoring was undertaken on a farm at the farmer’s or manufacturer’s request when the plant was considered to be operating under steadystate conditions, that is, when the operating parameters of hydraulic loading, feed quality and digester temperature had remained as constant as digester management would allow for a period of 2-3 months or at least four retention times. Monitoring equipment was housed in a mobile laboratory which was installed at each site for 6-8 weeks. Where feasible, equipment was commissioned to measure biogas production, biogas calorific value, electricity production, feed volume input and digester temperature. A data-logging system was employed to take hourly readings of all meters. Meters were also read manually, once daily, as a check on the logged readings. On full-scale plants, errors arising from unmeasured venting of biogas were avoided by ensuring its continuous use during monitoring. Samples representative of slurry entering and leaving the digesters were taken daily. Slurry samples were analysed for total solids (TS) and volatile solids (VS) on site and were deep frozen for more detailed analysis by regional ADAS laboratories. Full analysis included total solids, volatile solids, acetic and propionic acids, chemical oxygen demand, total nitrogen, ammonia-nitrogen, phosphorus, potassium, sodium, calcium, magnesium, zinc and copper, as previously described.2 Biogas was analysed on site for carbon dioxide and hydrogen sulphide. On one pig farm slurry was sampled before and after digestion in order to estimate odour reduction. Samples of untreated and digested slurry were presented to panellists who were asked to rate the offensiveness on a number scale of 1-5. Offensiveness scores for untreated and treated slurry were compared on each sampling occasion by the Kolmorgorov-Smirnov two sample test. 3 Panellists were also asked to provide descriptive terms for each sample. The gas chromatographic profiles of the slurry headspace gases were also determined. Monitoring was undertaken on four sites and Table 1 described the design features of the four prototype plants which were all completely different in design and management. The mobile laboratory returned to digester B for run 2 after extensive modifications were made to the plant and digester D was monitored during two different operating regimes. 3. Biogas production The gross biogas production is an important part of the economic returns of the digester.4 The gross production of biogas from animal slurries depends on the concentration of solids in the digester feed and their digestibility as expressed by the biogas yield, i.e. production of biogas
cycle
Storage capacity. Utilization
Biogas Storage
Loading
Mixing
Heating
Insulation
Volume, m3 Feed Process Construction
Volume, m3
Digester design
m3
flexible neoprene roof on digester variable Ford SI6 converted diesel engine 42 kVA generator
400 separated pig slurry single stage above-ground glass-coated prefabricated steel external bitumencoated polyurethane foam external tubular heat exchanger recycling digester contents through heat exchanger 30 times per day
A
I
scraped
floating caps on digester 30 Ford S16 converted 37 kVA generator
continually mechanically
internal poly~ihylene panels recycling biogas
324 whole cow slurry single stage below-ground rectangular, concrete block walls none
B
Digesters monitored-plant
TABLF
i
Phnt C
biogas
coiled tubing
gasometer 10 Ford SI4 1600 cm3 converted petrol engine 20 kVA generator
water-sealed
48 times per day
recycling
internal
300 whole pig slurry single stage above-ground glass coated prefabricated steel internal waterproofed polyurethane foam
design
biogas
panel
gasometer 1 boiler for hot water
water-sealed
48 times per day
recycling
internal
(pilot-scale) 11 separated aged pig slurry single stage above-ground prefabricated fibreglass internal waterproofed polyurethane foam
D
360
MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
per unit of solids input (m3/kg TS). The biogas yield is also influenced by the operating parameters of the digester and the presence of inhibitors.5 Laboratory-scale digesters6 operating on pig slurry produced a specific gross biogas production of 1.8 (m3 biogas) (m” digester volume)) ’ d-r. The specific gross biogas production of digesters A and C was 05 (m3 biogas) (m3 digester volume)- 1 d- ‘, which was lower than has been found elsewhere. ‘JJ The lower value was attributable to the low feed solids concentration of 23 and 22 kg/m3, respectively, for digesters A and C. Digester D was operating on pig slurry from pigs fed with a mixture of skimmed milk, barley and soya. The slurry was separated by a belt press separator fitted with an 8 mm screen and the liquor was settled for approximately 30d. When the input feed to the digester consisted of the settled concentrate with a solids concentration of 59 kg/m3 from the base of the liquor tank a biogas yield of 0.33 m3/kg TS fed (0.49 m3/kg VS fed) was achieved at a 12.8 d retention time. This gave a gross biogas production rate of 1.6 (m3 biogas) (m3 digester volume)-’ d-l. The yield was further improved when the digester feed was changed to the fully-mixed separated liquor with a solids concentration of 40 kg/m3 and the retention time was reduced to 8.3 d. After a settling down period, the biogas yield was measured at 037 m3/kgTS fed (0.52 m3/kg VS fed) giving a specific gross biogas production of 1.8 (m3 biogas) (m3 digester volume)) ’ d- ‘. The digester was acting as the second stage of a two-stage process, the first stage being fermentation in the reception tank at water ambient temperatures yielding acetic and propionic acid levels of 68OOmg/l in the digester feed. Digester B monitored during run 1 produced a specific gross biogas production of 0% (m3 biogas) (m3 digester volume)) 1d- ’and a yield of01 1 m3/kg TS from slurry with a feed solids concentration of 105 kg/m3 at a retention time of 15.3 d. Biogas yield was approximately half that found experimentally with dairy cattle slurry%10and on other pilot scale plants.” With the aim of increasing biogas yield, the retention time was increased to 19.9 d by reducing the input feed volume. Excluding the parlour washings from the input feed increased the slurry feed solids concentration to 114 kg/m3, but the biogas yield only increased to 0.12 m3/‘kg TS fed and the specific gross biogas production was reduce to 0.7 (m3 biogas) (m3 digester volume)- ’d- ‘. Because no increase in biogas production occurred, and as other workers have suggested that retention times of only 8 d have allowed complete biogas production, 12the presence in the slurry feed of some inhibitory substance was suspected. Monitoring farm-scale plants pin-pointed a variety of reasons why plant performance in terms of biogas production did not match up to design criteria based on experimental data. These reasons are discussed in the following sections. 3.1.
Hydraulic retention time
Digesters A and C operated at hydraulic retention times of 12.3 d and 13.3 d, which is above the optimum 10d required for biogas production .6 Biogas production was reduced on one site by a variable hydraulic retention time. This was caused by farm staff treating the digester as a convenient disposal unit for washing water at weekends. Doubling normal hydraulic loading for 2 d reduced the biogas production by about 50%. Digester C had been operated continually for two years and during this time grit, amounting to a volume of 50 m3, had deposited in the base of the digestion tank. The retention time was reduced by 15%. 3.2.
Temperature
A stable digester temperature is necessary for optimum biogas production. A sudden drop of 5°C resulted in reduced biogas production in laboratory-scale anaerobic digesters?3 Fluctuating temperatures occurred in three of the plants monitored owing to inefficiencies in the heating and mixing systems. The maximum daily difference between daily maximum and minimum temperatures was 4.4”C in digester D. A maximum daily difference of 7~4°C was found between daily maximum and minimum temperatures in digester A. A temperature range of between 12.4”C and 348°C was recorded along the length of the rectangular digester B.
d-l
15.3 I05 5.6 37.0 252 0.1 I 0.14 0.8 20. I 1.0 257 29.5 33.0 34.0 96.0
12.3 23 1.1 35.6 215 0.28 0.50 0.5 25.0 0.6 339 21.0 29.0 41.0 88.0
Run 1
217 0.12 0.15 0.7 20.1 I.3 91 17.0 19.0 36.0 66.0
19.9 114 4.4 33.2
Run 2
163 0.32 0.47 0.5 25.8 0.5 215 34.0 43.0 40.0 88.0
13.3 22 1.1 33.8
Whole pig slurry
Dairy cow slurrl,
Separated pig slurry
Fwd
Digester operation Retention time, d Slurry total solids, kg/m3 Organic loading. kg VS (m-’ digester volume)-’ Temperature. “C Digester performance Daily biogas, m3 Biogas yield, m3/kg TS fed m3/kg VS fed Specific gross biogas production Biogas calorific value, MJ/m3 Hydrogen sulphide, % Electricity production, kWh/d Total solids reduction, % Volatile solids reduction. 2, COD reduction, % Acetic + propionic acid reduction. “/,
c
B
A
parameters and plant performance
Plant
Digesters monitored--operating
TAME 2
i
I7 0.33 0.49 I.6 23.7 0.5 N/A 33.0 37.0
12.8 59 3.2 36.1
Run I
slurry
Separated
D
56.0
20 0.37 0.52 I.8 26.2 0.7 N/A 13.0 15.5
8.3 40 3.4 33.8
Run 2
pig
362
MONITORING
3.3.
ANAEROBIC
DIGESTERS
ON FARMS
Digester feed quality
The quality of slurry on pig farms has been found to be very variable due to the ingress of water from a number of sources. Rainwater gains access to slurry from any open yard areas which drain into the system, from groundwater leaking into unsealed or damaged channels and by falling on any open store areas such as flat-deck weaner houses. Leaking drinkers may deliver up to 1 l/pig each day into the slurry.14 The design of slurry collection systems on pig farms often incorporates storage in l-2m deep channels for periods of l-3 weeks. Sluice gates are opened manually as the channels become full, and the contents drain to a main storage area. Often solid material remains behind, which is periodically flushed or pushed out of the channels manually. This is another reason why pig slurry produced on farms is often heterogenous. The ideal total solids content required for anaerobic digestion9 of 60-80 kg/m’ is rarely achieved on pig farms and more typical values14 are between 20 and 30 kg/m31 At normal residence times of 15 d this results in low organic loading rates as illustrated by monitoring (Table 2). Slurry produced on the dairy farm had a high total solids content of 105 kg/m3, which allowed a high organic loading rate of 5.6 kg VS (m3 digester) d- ‘. The low biogas yield obtained was not improved by increasing the residence time and some inhibition by components in the slurry was considered. High hydrogen sulphide levels in the biogas of between l.oO/,and 1.3% indicated high sulphur levels in the slurry. Sulphite has been shown to inhibit methane fermentation?5 Hydrogen sulphide levels during an initial monitoring ranged from 0.12% to 0.44%. The following year, the introduction of apple pomace, preserved with sulphur dioxide into the feed was associated with an increase in hydrogen sulphide in the biogas. The replacement of pomace in the cattle diet by molasses and fishmeal in year 3 was also associated with high levels of hydrogen sulphide of 1.2% in the biogas. However, the link with low biogas production was not conclusive. 3.4.
Odour control
Anaerobic digestion is seen in the U.K. as an alternative treatment to aeration for controlling the odour from pig slurry. Work by van Velsen and Lettingale has shown that satisfactory removal of the malodorous compounds (e.g. indole, skatole) in slurry occurs with pig slurry of 6% TS at retention times of 15 d. A study was undertaken to assess the effect on pig slurry odour by plant C. Panel tests showed that offensiveness ratings were consistently lower for digested slurry than for the slurry feed to the digester. On four out of six occasions the differences were statistically significantly (P = 0.05)different. The odour quality ofslurry prior to digestion was described as “animal excreta” or “sulphide” compared with slurry after digestion which was described as “tarry” and “musty”. Work is continuing on the chemical assessment of slurry odour to improve separation of sample components and to manipulate the samples in order to look more closely at components present in low concentrations.17 3.5.
The efect of anaerobic digestion on slurry analyses
The total solids reduction for separated and whole pig slurry, digesters A and C, was 21% and 34% at retention times of 12.3d and 13.3d, respectively. In digester B, the shorter retention time in run 2 of 8.3 d compared with 12.8 d in run 1 resulted in lower total and volatile solids reductions. A mean reduction of 31% was noted in the concentration of total and volatile solids in run 1 compared with 14% in run 2. This effect has been observed in experiments with whole pig slurry.‘* The chemical oxygen demand (COD) was reduced by between 34% and 41% in all slurries. The COD of all feed stocks ranged between 13 400 and 72 000 mg/l, therefore reduction in the order of 40”/, was not significant for water pollution control. Storage of whole pig slurry before digestion in winter ambient conditions for periods of over 3 weeks produced mean acetic acid levels of 4700 mg/l and mean propionic acid levels of 1900 mg/l in the separated liquor feed on two sites. Reduction in these levels of acetic acid of 86% and propionic acid of 95% were noted at a 13.3d retention time, although at an 8.3 d retention time, reductions of 43% and 56%, respectively, were found.
363
R. FRIMAN
The concentration ofammonia-nitrogen in the digested slurry was increased by between 1lYOand 39% on all sites except one, although the concentration of total nitrogen remained essentially unchanged. The increase in ammonia-nitrogen may increase the availability of nitrogen if applied to growing crops. However, prolonged storage would produce greater losses of nitrogen from digested than from undigested slurry by volatilization. 4.
Digester design and operation 4.1.
Mixing
All the digester designs monitored were continually fed cylindrical mixed tanks except for digester B, which was rectangular in shape and operated on a semi-plug flow basis. Mixing was accomplished in digester A by recirculating digester contents through an external heat exchanger at a rate of 35 digester volumes per 24 h. This was not sufficient to prevent severe crust formation with whole pig slurry. Removal of the hairs and cereal husks from the slurry by mechanical separation prevented further crusting. On the other sites, mixing was by biogas recirculation, either delivered by individual pipes fitted with simple non-return valves positioned across the base of the tank or by a single dispersion plate positioned at the centre of the tank. The power provided for mixing using biogas recirculation by liquid ring gas pumps varied widely. Digester B was mixed for 7 min/d using a 1.64 kW motor, digester C was mixed in a 350 m3 tank for 8 h daily with a 2.0 kW motor and digester D was mixed in an 11 m3 tank for 5 h daily using a 0.75 kW motor. 4.2.
Heating
The operation of an external tube heat exchanger on digester A resulted in fluctuating digester temperatures. Heat was supplied by the engine cooling water and slurry from the digester continued to pass through the heat exchanger even when the engine was turned off. Digester C successfully maintained an even temperature throughout. The digester temperature was controlled by a thermistor sensor at the top of the tank, which automatically operated the flow of heating water through the internal coil heat exchangers. Digester B was heated using plate heat-exchangers positioned along the length of the rectangular tank supplied in parallel with water at 50°C. The high solids feed to digester B had poor mixing and heating abilities and a cold plug of slurry tended to form near the digester inlet on the floor of the digester. In digester D, a thermostat-controlled heat input to the digester. The combined effect of a relatively low volume of 11 m3 and an oversized boiler resulted in temperature fluctuations. The insulation of digesters C and D consisted of internally-applied expanded polyurethane foam sealed with waterproof paint. There was some evidence that the insulation was becoming detached from the tank surface. Another result of internally-applied insulating material was that any solar heating of the dark-coloured tanks was unable to dissipate and that the resulting uneven heating of the tank outer surface produced panel movement, and was therefore weakening the seams. The insulating effect of positioning digester B below ground on one site was nullified in winter when the groundwater rose to the ground surface. 4.3.
Biogas
utilization
Biogas from all full-scale plants was utilized to produce electricity. Converted petrol or diesel spark-ignition engines were used with generators rated between 32 kVA and 42 kVA. Engine cooling water was used to provide heat to the digester only. Efficient utilization of all the biogas produced was found by a BABA/ADAS Economics Working Party4 to be essential for the successful economic performance of a digester on a farm. Matching supply to demand was found to be most difficult on the dairy farm where the majority of
364
MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
demand occurred between 04.00 and 19.00 h. On digester B, biogas was vented for 8 h during the night because the biogas storage capacity was sufficient for only one hour’s production. The unbalanced use of three-phase electricity on site A resulted in increased engine wear and reduced conversion efficiencies from biogas to electricity. The conversion of methane to electricity in kWh/m3 of methane ranged from I.8 to 2.3.
5.
Conclusions
Intensive monitoring of four on-farm anaerobic digesters yielded data on their performance essentially in terms of biogas production. An average specific gross biogas production of 0.5 (m3 biogas) (m3 digester volume)- ’ d- ’ was recorded for two full-scale digesters, one treating separated pig slurry and the other whole pig slurry. Corresponding values of 1.6 and 1.8 were recorded for a pilot-scale digester treating stored separated pig slurry. These values compare with a typical figure of 1.8 (m3 biogas) (m3 digester volume)) l d- 1 from previous laboratory experiments. Poor biogas production from pig farms was attributable to the high dilution of the slurry which had a typical total solids concentration of 22 kg/m3. The biogas yield from pig slurry was equivalent to, or better than, results produced experimentally.~~s.l3 An average specific gross biogas production rate of @8 (m’ biogas) (m’ digester volume)-’ d-’ was recorded for a full-scale digester treating whole dairy cattle slurry. This was approximately half that expected from laboratory-scale experiments. When the retention time was increased from 153 d to 19.9 d and the temperatures in the digestion tank were stabilized, the biogas yield was not increased and there is some evidence to suggest that biogas production was inhibited by high levels of sulphur in the slurry which was associated with the cow diet. The plants monitored were all continually fed, heated and mixed, digesters, although the individual designs and construction materials differed widely. Variable temperatures and loading rates were found on some sites which are being improved by better design and plant management. Management was seen to be of major importance for the successful operation of a biogas plant and monitoring undertaken elsewhere has produced a similar conclusion ? Successful plant operation was achieved when the plant was the responsibility of one trained person. A preliminary investigation into the effect of full-scale anaerobic digestion on the odour of pig slurry led to the conclusion that odour offensiveness was significantly reduced. There was potential for more efficient use of biogas on the farms monitored. Improved biogas storage to even out supply and demand was essential to prevent the biogas from being vented. All of the full-scale plants produced electricity for farm consumption and the engine heat was supplied to the digester. All the plants monitored were at the prototype, development stage. Alterations have since been made to improve reliability, provide better control ofmixing and heating in order to optimize biogas production and to improve biogas utilization. REFERENCES 1
* a 4 5
Anon. Agricultural digesters in the United Kingdom. Marlborough: BABA Ltd, The Trade Association for the British Biomass Industries, 1983 Friman, R. The ADAS mobile laboratory. Mtg on the Anerobic Digestion of Farm Wastes, National Institute for Research in Dairying, Shinfield, Reading, U.K., July 1983 (to be published) Campbell, R. C. Statisticsfor Biologists. Cambridge: Cambridge University Press, 1967 Anon. An economic assessment of anaerobic digestion. Discussion Document of an ADASBABA Working Party. London: MAFF, 1982 Hawkes, D. L. Factors affecting net energy production from mesophilic anaerobic digestion. In Anaerobic Digestion. Stafford, D. A.: Wheatley, B. I.: Hughes, D. E. (Eds). Barking: Applied Science Publishers, 1980 131-150.
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365
6 Summers, R.; Bousfield, S. A detailed study ofpiggery waste anaerobic digestion. Agric. Wastes, 1980 2 (1) 61-78 7 Fischer, J. R.; Meador, N. F.; Sievers, D. M.: Fulhage, C. D.; Iannotti, E. L. Design and operation ofa,furm anaerobic digesterfor swine manure. Trans. ASAE, 1979 2 (5) 1129-l 136, 1144 a Hill, D. T. Design parameters and operating characteristics of animal waste anaerobic digestion systemsswine and poultry. Agric. Wastes, 1983 5 (3) 147-158 s Summers, R.; Bousfield, S. Experimental experiences in digestion. In Proc. Sem. Anaerobic Digestion of Farm Wastes, Cardington, October 1978. Reading: MAFF Agricultural Development and Advisory Service, Farm Waste Unit 10 Singh,R.;Jain,M.K.;Tauro,P. Rateofanaerobicdigestionofcattlewaste. Agric.Wastes, 19824(4)267-272 11 Hill, D. T. Methane gas production from dairy manure at high solids concentration. ASAE Paper No. 80-2301. 1980 12 Hill, D. T. Design parameters and operating characteristics of animal waste anaerobic digestion systems. Dairy Cattle. Agric. Wastes. 1983 5 (4) 219-230 1s Hobson, P. N.; Shaw. B. G. The anaerobic digestion of waste from un intensive pig unit. Wat Res.. 1973 7 437449 14 Friman, R.; Nielsen, V. C. Study of slurryfrom large commercial pig units. Reading: MAFF Agricultural Development and Advisory Service, Farm Waste Unit, 1983 (unpubl.) 1s Hobma, S. W.; Maaskant, W. De invloed van sulfiet op anaerobe behandeling van af valwater. [The effect of sulphite on the anaerobic treatment of waste water.] HzO, 1981 14 (25) 596-598 1s van Velsen, A. F. M.; Lettinga, G. Digestion of animal manure. Stud. envir. Sci., 1981 9 55-64 17 Page, J. M. J.; Odam, E. M. Chemicalassessment ofodourfrom pig s/my. London: MAFF Pest Infestation Control Laboratory, 1982 (unpubl.) 1s van Velsen, A. F. M. Anaerobic digestion of piggery waste. 1. The influence of retention time und manure concentration. Neth. J. agric. Sci., 1977 25 151-169 1s Friman, R. Farm scale anaerobic digestion in Italy. Report of a study tour May 1983. Reading: MAFF Agricultural Development and Advisory Service (unpubl.)
Monitoring
Anaerobic
Digesters
on Farms
R. FRIMAN*
There are 15 anaerobic digesters installed on farms in the U.K. and these were largely stimulated by the opportunity to produce energy coupled with a reduction in slurry odour. A monitoring programme was developed in 1980 to evaluate the average performance of farm plants in terms of net energy production under practical operating conditions. The aim was also to determine the effects of digestion on slurry with regard to pollution reduction and fertilizer value. This paper reports data gathered from one pilot-scale and two full-scale digesters, treating pig slurry, and from one full-scale digester treating dairy cattle slurry. Monitoring showed on full-scale plants, that pig slurry achieved a specific gross biogas production of 0.5 (m3 biogas) (m3 digester volume)- ’d - I and for dairy cow slurry 0.8 (m” biogas) (m’ digester volume)) ’d-‘. A pilot plant operating on stored separated pig slurry at a retention time of8.3 d produced 1.8 (m3 biogas) (m3 digester volume)-’ d-l. The biogas yield from the total solids (TS) in pig slurry fed ranged from 028 to 0.37 m3 biogas/kg TS fed. The yield from dairy cow slurry was 0.12 m3 biogas/kg TS fed. Digester management was found to be important for the successful steady-state operation of digesters on farms. Digester feed solids concentration and feed age were found to influence specific gross biogas production.
1.
Introduction
Recent increases in energy costs have prompted interest in finding cheaper alternative energy sources for U.K. farms and at the same time intensification of livestock production has led to greater pollution risks. This has promoted the search for treatment systems, such as anaerobic digestion, capable of reducing the polluting power of animal wastes and with the additional bonus of producing energy. A range of prototype digester designs have been constructed on farms, primarily to produce energy but also, more recently, to reduce slurry odour. There are 15 anaerobic digesters installed on farms in the U.K.,1 14 of which were constructed since 1978. The majority of the plants (11) are full-scale and have capacities of greater than 70m3. Seven digesters operate on pig slurry, one plant treats a mixture of pig and poultry manure and seven treat dairy or beef cattle waste. Manufacturers in the U.K. have designed and constructed all the plants except two, one of which was farmer designed and constructed and the other of which was designed by a European company. Six full-scale plants have an above-ground continually stirred digester tank constructed from prefabricated vitreous coated steel panels, and one was a steel tank. Three full-scale plants (two operating on cattle slurry and one on pig slurry) are below-ground rectangular tanks constructed from concrete. In the below-ground tanks slurry is scraped into the digesters mechanically and overflows at the outlet. Three pilot-scale digesters are constructed from fibreglass, and one full-scale plant is constructed from prefabricated fibreglass panels and is available as a do-it-yourself package. All of the digesters except one have a facility for biogas recirculation in order to mix slurry, the other site has a mechanical mixing paddle which is powered by a windmill. All the digesters in the
*MAFF.
ADAS.
Received
IO November
Paper
presented
Farm Waste
IJmt. Coley Park,
19X3. accepted
at AG FNG
in revised
84. CambrIdge,
Reading.
Berkshire
form 5 March
U.K..
1984
I-5 April 1984
358
MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
U.K. operate in the mesophilic range 3%35°C and the digester contents are heated by internal plate or coil heat exchangers. The above-ground digesters are insulated to a thickness of 5-10 cm by expanded polyurethane coated with epoxy resin, which is usually applied internally. The belowground digesters rely on the surrounding soil for insulation. On plants which have a fixed roof, biogas is stored at lOC&200mm w.g. in a floating water-sealed gasometer. Two sites have a raised floating roof on the digester which acts as a biogas store. Storage is normally provided on farms for l-3 h of biogas production. The manufacturers of these prototype plants indicated a need for an independent assessment of the performance of farm-scale plants. A monitoring programme capable of assessing a number of sites was therefore developed in 1980 by the author and her colleagues.
2.
Monitoring methods
The monitoring programme was primarily aimed at evaluating the average performance of farm plants in terms of net energy production under practical operating conditions. Monitoring was also aimed at determining the effects of digestion on slurry with regard to pollution reduction and fertilizer value and at identifying indicators of digester performance which could be used to assist plant operators to manage plants more effectively. Monitoring was undertaken on a farm at the farmer’s or manufacturer’s request when the plant was considered to be operating under steadystate conditions, that is, when the operating parameters of hydraulic loading, feed quality and digester temperature had remained as constant as digester management would allow for a period of 2-3 months or at least four retention times. Monitoring equipment was housed in a mobile laboratory which was installed at each site for 6-8 weeks. Where feasible, equipment was commissioned to measure biogas production, biogas calorific value, electricity production, feed volume input and digester temperature. A data-logging system was employed to take hourly readings of all meters. Meters were also read manually, once daily, as a check on the logged readings. On full-scale plants, errors arising from unmeasured venting of biogas were avoided by ensuring its continuous use during monitoring. Samples representative of slurry entering and leaving the digesters were taken daily. Slurry samples were analysed for total solids (TS) and volatile solids (VS) on site and were deep frozen for more detailed analysis by regional ADAS laboratories. Full analysis included total solids, volatile solids, acetic and propionic acids, chemical oxygen demand, total nitrogen, ammonia-nitrogen, phosphorus, potassium, sodium, calcium, magnesium, zinc and copper, as previously described.2 Biogas was analysed on site for carbon dioxide and hydrogen sulphide. On one pig farm slurry was sampled before and after digestion in order to estimate odour reduction. Samples of untreated and digested slurry were presented to panellists who were asked to rate the offensiveness on a number scale of 1-5. Offensiveness scores for untreated and treated slurry were compared on each sampling occasion by the Kolmorgorov-Smirnov two sample test. 3 Panellists were also asked to provide descriptive terms for each sample. The gas chromatographic profiles of the slurry headspace gases were also determined. Monitoring was undertaken on four sites and Table 1 described the design features of the four prototype plants which were all completely different in design and management. The mobile laboratory returned to digester B for run 2 after extensive modifications were made to the plant and digester D was monitored during two different operating regimes. 3. Biogas production The gross biogas production is an important part of the economic returns of the digester.4 The gross production of biogas from animal slurries depends on the concentration of solids in the digester feed and their digestibility as expressed by the biogas yield, i.e. production of biogas
cycle
Storage capacity. Utilization
Biogas Storage
Loading
Mixing
Heating
Insulation
Volume, m3 Feed Process Construction
Volume, m3
Digester design
m3
flexible neoprene roof on digester variable Ford SI6 converted diesel engine 42 kVA generator
400 separated pig slurry single stage above-ground glass-coated prefabricated steel external bitumencoated polyurethane foam external tubular heat exchanger recycling digester contents through heat exchanger 30 times per day
A
I
scraped
floating caps on digester 30 Ford S16 converted 37 kVA generator
continually mechanically
internal poly~ihylene panels recycling biogas
324 whole cow slurry single stage below-ground rectangular, concrete block walls none
B
Digesters monitored-plant
TABLF
i
Phnt C
biogas
coiled tubing
gasometer 10 Ford SI4 1600 cm3 converted petrol engine 20 kVA generator
water-sealed
48 times per day
recycling
internal
300 whole pig slurry single stage above-ground glass coated prefabricated steel internal waterproofed polyurethane foam
design
biogas
panel
gasometer 1 boiler for hot water
water-sealed
48 times per day
recycling
internal
(pilot-scale) 11 separated aged pig slurry single stage above-ground prefabricated fibreglass internal waterproofed polyurethane foam
D
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MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
per unit of solids input (m3/kg TS). The biogas yield is also influenced by the operating parameters of the digester and the presence of inhibitors.5 Laboratory-scale digesters6 operating on pig slurry produced a specific gross biogas production of 1.8 (m3 biogas) (m” digester volume)) ’ d-r. The specific gross biogas production of digesters A and C was 05 (m3 biogas) (m3 digester volume)- 1 d- ‘, which was lower than has been found elsewhere. ‘JJ The lower value was attributable to the low feed solids concentration of 23 and 22 kg/m3, respectively, for digesters A and C. Digester D was operating on pig slurry from pigs fed with a mixture of skimmed milk, barley and soya. The slurry was separated by a belt press separator fitted with an 8 mm screen and the liquor was settled for approximately 30d. When the input feed to the digester consisted of the settled concentrate with a solids concentration of 59 kg/m3 from the base of the liquor tank a biogas yield of 0.33 m3/kg TS fed (0.49 m3/kg VS fed) was achieved at a 12.8 d retention time. This gave a gross biogas production rate of 1.6 (m3 biogas) (m3 digester volume)-’ d-l. The yield was further improved when the digester feed was changed to the fully-mixed separated liquor with a solids concentration of 40 kg/m3 and the retention time was reduced to 8.3 d. After a settling down period, the biogas yield was measured at 037 m3/kgTS fed (0.52 m3/kg VS fed) giving a specific gross biogas production of 1.8 (m3 biogas) (m3 digester volume)) ’ d- ‘. The digester was acting as the second stage of a two-stage process, the first stage being fermentation in the reception tank at water ambient temperatures yielding acetic and propionic acid levels of 68OOmg/l in the digester feed. Digester B monitored during run 1 produced a specific gross biogas production of 0% (m3 biogas) (m3 digester volume)) 1d- ’and a yield of01 1 m3/kg TS from slurry with a feed solids concentration of 105 kg/m3 at a retention time of 15.3 d. Biogas yield was approximately half that found experimentally with dairy cattle slurry%10and on other pilot scale plants.” With the aim of increasing biogas yield, the retention time was increased to 19.9 d by reducing the input feed volume. Excluding the parlour washings from the input feed increased the slurry feed solids concentration to 114 kg/m3, but the biogas yield only increased to 0.12 m3/‘kg TS fed and the specific gross biogas production was reduce to 0.7 (m3 biogas) (m3 digester volume)- ’d- ‘. Because no increase in biogas production occurred, and as other workers have suggested that retention times of only 8 d have allowed complete biogas production, 12the presence in the slurry feed of some inhibitory substance was suspected. Monitoring farm-scale plants pin-pointed a variety of reasons why plant performance in terms of biogas production did not match up to design criteria based on experimental data. These reasons are discussed in the following sections. 3.1.
Hydraulic retention time
Digesters A and C operated at hydraulic retention times of 12.3 d and 13.3 d, which is above the optimum 10d required for biogas production .6 Biogas production was reduced on one site by a variable hydraulic retention time. This was caused by farm staff treating the digester as a convenient disposal unit for washing water at weekends. Doubling normal hydraulic loading for 2 d reduced the biogas production by about 50%. Digester C had been operated continually for two years and during this time grit, amounting to a volume of 50 m3, had deposited in the base of the digestion tank. The retention time was reduced by 15%. 3.2.
Temperature
A stable digester temperature is necessary for optimum biogas production. A sudden drop of 5°C resulted in reduced biogas production in laboratory-scale anaerobic digesters?3 Fluctuating temperatures occurred in three of the plants monitored owing to inefficiencies in the heating and mixing systems. The maximum daily difference between daily maximum and minimum temperatures was 4.4”C in digester D. A maximum daily difference of 7~4°C was found between daily maximum and minimum temperatures in digester A. A temperature range of between 12.4”C and 348°C was recorded along the length of the rectangular digester B.
d-l
15.3 I05 5.6 37.0 252 0.1 I 0.14 0.8 20. I 1.0 257 29.5 33.0 34.0 96.0
12.3 23 1.1 35.6 215 0.28 0.50 0.5 25.0 0.6 339 21.0 29.0 41.0 88.0
Run 1
217 0.12 0.15 0.7 20.1 I.3 91 17.0 19.0 36.0 66.0
19.9 114 4.4 33.2
Run 2
163 0.32 0.47 0.5 25.8 0.5 215 34.0 43.0 40.0 88.0
13.3 22 1.1 33.8
Whole pig slurry
Dairy cow slurrl,
Separated pig slurry
Fwd
Digester operation Retention time, d Slurry total solids, kg/m3 Organic loading. kg VS (m-’ digester volume)-’ Temperature. “C Digester performance Daily biogas, m3 Biogas yield, m3/kg TS fed m3/kg VS fed Specific gross biogas production Biogas calorific value, MJ/m3 Hydrogen sulphide, % Electricity production, kWh/d Total solids reduction, % Volatile solids reduction. 2, COD reduction, % Acetic + propionic acid reduction. “/,
c
B
A
parameters and plant performance
Plant
Digesters monitored--operating
TAME 2
i
I7 0.33 0.49 I.6 23.7 0.5 N/A 33.0 37.0
12.8 59 3.2 36.1
Run I
slurry
Separated
D
56.0
20 0.37 0.52 I.8 26.2 0.7 N/A 13.0 15.5
8.3 40 3.4 33.8
Run 2
pig
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MONITORING
3.3.
ANAEROBIC
DIGESTERS
ON FARMS
Digester feed quality
The quality of slurry on pig farms has been found to be very variable due to the ingress of water from a number of sources. Rainwater gains access to slurry from any open yard areas which drain into the system, from groundwater leaking into unsealed or damaged channels and by falling on any open store areas such as flat-deck weaner houses. Leaking drinkers may deliver up to 1 l/pig each day into the slurry.14 The design of slurry collection systems on pig farms often incorporates storage in l-2m deep channels for periods of l-3 weeks. Sluice gates are opened manually as the channels become full, and the contents drain to a main storage area. Often solid material remains behind, which is periodically flushed or pushed out of the channels manually. This is another reason why pig slurry produced on farms is often heterogenous. The ideal total solids content required for anaerobic digestion9 of 60-80 kg/m’ is rarely achieved on pig farms and more typical values14 are between 20 and 30 kg/m31 At normal residence times of 15 d this results in low organic loading rates as illustrated by monitoring (Table 2). Slurry produced on the dairy farm had a high total solids content of 105 kg/m3, which allowed a high organic loading rate of 5.6 kg VS (m3 digester) d- ‘. The low biogas yield obtained was not improved by increasing the residence time and some inhibition by components in the slurry was considered. High hydrogen sulphide levels in the biogas of between l.oO/,and 1.3% indicated high sulphur levels in the slurry. Sulphite has been shown to inhibit methane fermentation?5 Hydrogen sulphide levels during an initial monitoring ranged from 0.12% to 0.44%. The following year, the introduction of apple pomace, preserved with sulphur dioxide into the feed was associated with an increase in hydrogen sulphide in the biogas. The replacement of pomace in the cattle diet by molasses and fishmeal in year 3 was also associated with high levels of hydrogen sulphide of 1.2% in the biogas. However, the link with low biogas production was not conclusive. 3.4.
Odour control
Anaerobic digestion is seen in the U.K. as an alternative treatment to aeration for controlling the odour from pig slurry. Work by van Velsen and Lettingale has shown that satisfactory removal of the malodorous compounds (e.g. indole, skatole) in slurry occurs with pig slurry of 6% TS at retention times of 15 d. A study was undertaken to assess the effect on pig slurry odour by plant C. Panel tests showed that offensiveness ratings were consistently lower for digested slurry than for the slurry feed to the digester. On four out of six occasions the differences were statistically significantly (P = 0.05)different. The odour quality ofslurry prior to digestion was described as “animal excreta” or “sulphide” compared with slurry after digestion which was described as “tarry” and “musty”. Work is continuing on the chemical assessment of slurry odour to improve separation of sample components and to manipulate the samples in order to look more closely at components present in low concentrations.17 3.5.
The efect of anaerobic digestion on slurry analyses
The total solids reduction for separated and whole pig slurry, digesters A and C, was 21% and 34% at retention times of 12.3d and 13.3d, respectively. In digester B, the shorter retention time in run 2 of 8.3 d compared with 12.8 d in run 1 resulted in lower total and volatile solids reductions. A mean reduction of 31% was noted in the concentration of total and volatile solids in run 1 compared with 14% in run 2. This effect has been observed in experiments with whole pig slurry.‘* The chemical oxygen demand (COD) was reduced by between 34% and 41% in all slurries. The COD of all feed stocks ranged between 13 400 and 72 000 mg/l, therefore reduction in the order of 40”/, was not significant for water pollution control. Storage of whole pig slurry before digestion in winter ambient conditions for periods of over 3 weeks produced mean acetic acid levels of 4700 mg/l and mean propionic acid levels of 1900 mg/l in the separated liquor feed on two sites. Reduction in these levels of acetic acid of 86% and propionic acid of 95% were noted at a 13.3d retention time, although at an 8.3 d retention time, reductions of 43% and 56%, respectively, were found.
363
R. FRIMAN
The concentration ofammonia-nitrogen in the digested slurry was increased by between 1lYOand 39% on all sites except one, although the concentration of total nitrogen remained essentially unchanged. The increase in ammonia-nitrogen may increase the availability of nitrogen if applied to growing crops. However, prolonged storage would produce greater losses of nitrogen from digested than from undigested slurry by volatilization. 4.
Digester design and operation 4.1.
Mixing
All the digester designs monitored were continually fed cylindrical mixed tanks except for digester B, which was rectangular in shape and operated on a semi-plug flow basis. Mixing was accomplished in digester A by recirculating digester contents through an external heat exchanger at a rate of 35 digester volumes per 24 h. This was not sufficient to prevent severe crust formation with whole pig slurry. Removal of the hairs and cereal husks from the slurry by mechanical separation prevented further crusting. On the other sites, mixing was by biogas recirculation, either delivered by individual pipes fitted with simple non-return valves positioned across the base of the tank or by a single dispersion plate positioned at the centre of the tank. The power provided for mixing using biogas recirculation by liquid ring gas pumps varied widely. Digester B was mixed for 7 min/d using a 1.64 kW motor, digester C was mixed in a 350 m3 tank for 8 h daily with a 2.0 kW motor and digester D was mixed in an 11 m3 tank for 5 h daily using a 0.75 kW motor. 4.2.
Heating
The operation of an external tube heat exchanger on digester A resulted in fluctuating digester temperatures. Heat was supplied by the engine cooling water and slurry from the digester continued to pass through the heat exchanger even when the engine was turned off. Digester C successfully maintained an even temperature throughout. The digester temperature was controlled by a thermistor sensor at the top of the tank, which automatically operated the flow of heating water through the internal coil heat exchangers. Digester B was heated using plate heat-exchangers positioned along the length of the rectangular tank supplied in parallel with water at 50°C. The high solids feed to digester B had poor mixing and heating abilities and a cold plug of slurry tended to form near the digester inlet on the floor of the digester. In digester D, a thermostat-controlled heat input to the digester. The combined effect of a relatively low volume of 11 m3 and an oversized boiler resulted in temperature fluctuations. The insulation of digesters C and D consisted of internally-applied expanded polyurethane foam sealed with waterproof paint. There was some evidence that the insulation was becoming detached from the tank surface. Another result of internally-applied insulating material was that any solar heating of the dark-coloured tanks was unable to dissipate and that the resulting uneven heating of the tank outer surface produced panel movement, and was therefore weakening the seams. The insulating effect of positioning digester B below ground on one site was nullified in winter when the groundwater rose to the ground surface. 4.3.
Biogas
utilization
Biogas from all full-scale plants was utilized to produce electricity. Converted petrol or diesel spark-ignition engines were used with generators rated between 32 kVA and 42 kVA. Engine cooling water was used to provide heat to the digester only. Efficient utilization of all the biogas produced was found by a BABA/ADAS Economics Working Party4 to be essential for the successful economic performance of a digester on a farm. Matching supply to demand was found to be most difficult on the dairy farm where the majority of
364
MONITORING
ANAEROBIC
DIGESTERS
ON FARMS
demand occurred between 04.00 and 19.00 h. On digester B, biogas was vented for 8 h during the night because the biogas storage capacity was sufficient for only one hour’s production. The unbalanced use of three-phase electricity on site A resulted in increased engine wear and reduced conversion efficiencies from biogas to electricity. The conversion of methane to electricity in kWh/m3 of methane ranged from I.8 to 2.3.
5.
Conclusions
Intensive monitoring of four on-farm anaerobic digesters yielded data on their performance essentially in terms of biogas production. An average specific gross biogas production of 0.5 (m3 biogas) (m3 digester volume)- ’ d- ’ was recorded for two full-scale digesters, one treating separated pig slurry and the other whole pig slurry. Corresponding values of 1.6 and 1.8 were recorded for a pilot-scale digester treating stored separated pig slurry. These values compare with a typical figure of 1.8 (m3 biogas) (m3 digester volume)) l d- 1 from previous laboratory experiments. Poor biogas production from pig farms was attributable to the high dilution of the slurry which had a typical total solids concentration of 22 kg/m3. The biogas yield from pig slurry was equivalent to, or better than, results produced experimentally.~~s.l3 An average specific gross biogas production rate of @8 (m’ biogas) (m’ digester volume)-’ d-’ was recorded for a full-scale digester treating whole dairy cattle slurry. This was approximately half that expected from laboratory-scale experiments. When the retention time was increased from 153 d to 19.9 d and the temperatures in the digestion tank were stabilized, the biogas yield was not increased and there is some evidence to suggest that biogas production was inhibited by high levels of sulphur in the slurry which was associated with the cow diet. The plants monitored were all continually fed, heated and mixed, digesters, although the individual designs and construction materials differed widely. Variable temperatures and loading rates were found on some sites which are being improved by better design and plant management. Management was seen to be of major importance for the successful operation of a biogas plant and monitoring undertaken elsewhere has produced a similar conclusion ? Successful plant operation was achieved when the plant was the responsibility of one trained person. A preliminary investigation into the effect of full-scale anaerobic digestion on the odour of pig slurry led to the conclusion that odour offensiveness was significantly reduced. There was potential for more efficient use of biogas on the farms monitored. Improved biogas storage to even out supply and demand was essential to prevent the biogas from being vented. All of the full-scale plants produced electricity for farm consumption and the engine heat was supplied to the digester. All the plants monitored were at the prototype, development stage. Alterations have since been made to improve reliability, provide better control ofmixing and heating in order to optimize biogas production and to improve biogas utilization. REFERENCES 1
* a 4 5
Anon. Agricultural digesters in the United Kingdom. Marlborough: BABA Ltd, The Trade Association for the British Biomass Industries, 1983 Friman, R. The ADAS mobile laboratory. Mtg on the Anerobic Digestion of Farm Wastes, National Institute for Research in Dairying, Shinfield, Reading, U.K., July 1983 (to be published) Campbell, R. C. Statisticsfor Biologists. Cambridge: Cambridge University Press, 1967 Anon. An economic assessment of anaerobic digestion. Discussion Document of an ADASBABA Working Party. London: MAFF, 1982 Hawkes, D. L. Factors affecting net energy production from mesophilic anaerobic digestion. In Anaerobic Digestion. Stafford, D. A.: Wheatley, B. I.: Hughes, D. E. (Eds). Barking: Applied Science Publishers, 1980 131-150.
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6 Summers, R.; Bousfield, S. A detailed study ofpiggery waste anaerobic digestion. Agric. Wastes, 1980 2 (1) 61-78 7 Fischer, J. R.; Meador, N. F.; Sievers, D. M.: Fulhage, C. D.; Iannotti, E. L. Design and operation ofa,furm anaerobic digesterfor swine manure. Trans. ASAE, 1979 2 (5) 1129-l 136, 1144 a Hill, D. T. Design parameters and operating characteristics of animal waste anaerobic digestion systemsswine and poultry. Agric. Wastes, 1983 5 (3) 147-158 s Summers, R.; Bousfield, S. Experimental experiences in digestion. In Proc. Sem. Anaerobic Digestion of Farm Wastes, Cardington, October 1978. Reading: MAFF Agricultural Development and Advisory Service, Farm Waste Unit 10 Singh,R.;Jain,M.K.;Tauro,P. Rateofanaerobicdigestionofcattlewaste. Agric.Wastes, 19824(4)267-272 11 Hill, D. T. Methane gas production from dairy manure at high solids concentration. ASAE Paper No. 80-2301. 1980 12 Hill, D. T. Design parameters and operating characteristics of animal waste anaerobic digestion systems. Dairy Cattle. Agric. Wastes. 1983 5 (4) 219-230 1s Hobson, P. N.; Shaw. B. G. The anaerobic digestion of waste from un intensive pig unit. Wat Res.. 1973 7 437449 14 Friman, R.; Nielsen, V. C. Study of slurryfrom large commercial pig units. Reading: MAFF Agricultural Development and Advisory Service, Farm Waste Unit, 1983 (unpubl.) 1s Hobma, S. W.; Maaskant, W. De invloed van sulfiet op anaerobe behandeling van af valwater. [The effect of sulphite on the anaerobic treatment of waste water.] HzO, 1981 14 (25) 596-598 1s van Velsen, A. F. M.; Lettinga, G. Digestion of animal manure. Stud. envir. Sci., 1981 9 55-64 17 Page, J. M. J.; Odam, E. M. Chemicalassessment ofodourfrom pig s/my. London: MAFF Pest Infestation Control Laboratory, 1982 (unpubl.) 1s van Velsen, A. F. M. Anaerobic digestion of piggery waste. 1. The influence of retention time und manure concentration. Neth. J. agric. Sci., 1977 25 151-169 1s Friman, R. Farm scale anaerobic digestion in Italy. Report of a study tour May 1983. Reading: MAFF Agricultural Development and Advisory Service (unpubl.)