Operating a full-scale poultry manure anaerobic digester

Biological Wastes 19 (1987) 79-90

Operating a Full-Scale Poultry Manure Anaerobic Digester* L. M. Safley, Jr Biological and Agricultural Engineering Department, North Carolina State University, Raleigh, North Carolina, USA

R. L. Vetter Director of Research, A. O. Smith Harvestore Products, Inc., Barrington, Illinois, USA

& Darrell Smith Rt 21 Box 45, Princeton, North Carolina 27569, USA (Received 3 January 1986; accepted 19 March 1986)

A BS TRA C T A full-scale, completely-mixeddigester, with a liquid capacity of 587 m 3, was constructed to process the manure from 70 000 caged layers. Biogas from the digester was used as fuel for an engine/generator set. The operating temperature was maintained at 35°C using waste heat from the engine. The digester was operated on a 22-24 day HRT. Digester influent averaged 5.90% TS, 5250ppm TKN, and 3790ppm NH3--N. Digester effluent averaged 3.11% TS, 5090ppm TKN, and 4060ppm NH3--N. Sustained operation of the digester was achieved during the period of study (8/83-4/85). During this period biogas production averaged 0.38 m 3 k g - 1 VS added (0"58 m 3 kg- 1 VS destroyed). The CH 4 content * Paper No. 9990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned. This research was partially supported by a gr~nt from A. O. Smith Harvestore (R) Products, Inc., Barrington, II, USA. 79 Biological Wastes 0269-7483/87/$03.50 O Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

80

L. M. Safley, Jr, R. L. Vetter, Darrell Smith

averaged 58"0%. The major operational problem encountered was grit accumulation in the digester. This problem was reduced by settling most of the grit from the manure prior to the digester. Biogas production was reduced when concentrated lagoon-liquid was used as make-up water. Approximately 22% of the electricity produced was required for operating the system.

INTRODUCTION Anaerobic digesters have been constructed on 85-100 farm sites in the US (Ashworth et al., 1984). Of this number, 8-10 have used poultry manure as the primary feed material. At the time this paper was prepared there were four operating poultry digesters in the country (Ashworth et al., 1984). Reasons that some of the originally constructed digesters are still not being operated include faulty system design and poor economics. Converse et al. (1981) reported on a 5 year study of a poultry digester for a laying flock of 15 000 birds. The mean loading rate was 1.95 kg volatile solids (VS) m - 3 day and the average biogas production was 0.44 m 3 kg-1 VS added, at 62% CH,~. They reported high values for ammonia, alkalinity, and volatile fatty acids (VFA); 7090 p p m mg liter- 1 N, 27 940 mg liter- 1 as CaCO3, and 8020 mg liter- 1 as acetic, respectively, in the digester content. The poultry manure was diluted from an average of 25% total solids (TS) to 11.4% TS before loading. The hydraulic retention time (HRT) ranged from 36 to 46 days and the digester was operated at 35°C. Using a kinetic model, Hill (1984) predicted the gas production from a poultry digester operated at 35°C and a H R T of 37 days to be 0.36m 3 kg-1 VS added (0"55 m 3 k g - 1 VS destroyed). SYSTEM D E S C R I P T I O N In the spring and early summer of 1983 an anerobic digester and appropriate manure processing facilities were installed on the Darrell Smith farm near Princeton, NC, USA. The primary motivation for constructing the digester was to utilize the manure to produce biogas which would be used to fuel an engine/generator. The owner, Mr Smith, received a federal grant to cover part of the construction cost. The local power utility offered a long-term contract to purchase on-peak electricity for $0.11 kWh-1. The digester/generator system was designed to produce onpeak electricity at the rate of 4800 k W h week- 1. At this generation rate the entire system would have an eight-year simple payback. Figure 1 identifies the facilities involved in the manure handling system. The manure from two 35 000 caged-layer houses is augered to a collection

Full-scale poultry waste digester

81

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Fig. 1. System layout. pit. At this point the manure is mixed with water to achieve a 6-7% TS mixture. Grit is allowed to settle in the first pit with the resulting supernatant being pumped into the second pit which is used to store the manure slurry prior to being fed into the digester. Some feather removal is practiced in the pits if necessary, by skimming the floating feathers into a channel adjacent to the pits. From the second pit the manure is automatically pumped (submersible pump) to a second pump in the mixpump building. The second pump mixes the 'feed' manure with digester liquid and pumps the slurry mixture either into the center (through heat exchanger), bottom (along floor), or top of the digester (at liquid level). The direction of flow is controlled by a series of valves located on a manifold on the discharge side of the mix-pump.

82

L. M. Safley, Jr, R. L. Vetter, Darrell Smith -

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The digester (A. O. Smith Harvestore Products, Inc., Methastore ~, Barrington, IL, USA) is 9-4 m in diameter and 8.5 m tall with a working volume of 587 m 3. The digester is constructed of two layers of glass-fusedto-steel metal sheets sandwiching a 10cm thickness of urethane foam insulation. Each day approximately 1/24th of the volume is exhausted to the lagoon via the overflow as a like amount of fresh manure is pumped into the digester. The processed manure is stored in the lagoon until needed for land application. The digester temperature is maintained at 35°C using waste heat from the engine/generator and a shell-in-tube heat exchanger. Gas pressure inside the digester ranges from 5 to 19cm H20 and is regulated by pressure switches that control a biogas compressor. The biogas is piped into the engine/generator control building (Fig. 2). First, the gas passes through a scrubbing unit filled with wood chips impregnated with iron filings to remove hydrogen sulfide (H2S). Next, the biogas is either directed to the engine or to a compressor. Compressed biogas is stored in a 45-m 3, 1724-kPa rated tank or used directly to fuel the engine. Scrubbed,

Full-scale poultry waste digester

83

pressurized biogas can also be used to fuel an auxiliary boiler to provide emergency heat for the digester. The 80 kW, single phase, 240 volt generator (Katolight, Inc., Mankato, MN, USA) is powered by a six cylinder, spark ignition, l l 6 k W engine (Minneapolis-Moline, Russelville, AR, USA). The engine is typically operated at 1212-1214 RPM to achieve the 80kW output. Hot water from the engine jacket is piped through a muffler/silencer unit where additional heat is collected. The water is then forced through a heat exchanger where heat is removed to heat the digester. The system typically provides surplus waste heat which is removed by the radiator cooling system to bring the water back into the engine at the desired temperature. A temperature/flow proportioning valve is used to help control the engine temperature. SYSTEM MONITORING Once the digester system was operational (8/83) weekly visits were made to collect data and gas and waste samples. On each visit one manure sample was taken from the mix pit prior to feeding and another was taken from the digester discharge pipe during an overflow event. Each of these samples was refrigerated and returned to the laboratory for analysis. Each sample was analyzed for TS, VS, total Kjeldahl nitrogen (TKN), NH3--N, conductivity alkalinity, VFA and pH. TS was determined by evaporation at 103°C (US EPA, 1979). VS was computed as the difference between TS and fixed solids (US EPA, 1979). TKN was analyzed using persulfate digestion modified for automated procedures (US EPA, 1979; Technicon, 1973). N H 3 - - N was determined using salicylate reaction modified for automated procedures (US EPA, 1979; Technicon, 1973). Conductivity was determined using the procedures outlined by US EPA (1979). Alkalinity was analyzed using Standard Methods procedures (APHA, 1981) and reported as ppm CaCO3. VFA was determined using a procedure developed by Taras (1979). The owner maintained a daily record of biogas production. Biogas measurement was determined using a diaphragm meter. No corrections were made to compensate for temperature, atmospheric pressure or gas moisture. During the weekly visit two samples of the stored biogas were collected for determination of methane content by a gas chromatograph. In addition onsite carbon dioxide (CO2) and H2S readings were made of the stored biogas using Draeger tubes (National Draeger Inc., 1984) and the raw biogas was analyzed for H2S by the same procedure. The owner maintained a record of the daily amount of sliarry fed to the digester. In addition a record was kept as to the daily kilowatt-hours produced and the amount of engine/generator operation time.

84

L. M. Safley, Jr, R. L. Vetter, Darrell Smith

The electrical consumption for processing the manure and biogas was monitored by kilowatt-demand meters connected to the individual motors. The five demand meters were for the following motors: radiator fan, digester hot-water circulation pump, gas compressor, digester mix-pump, and manure-pit pump.

RESULTS A N D DISCUSSION The digester system was monitored from August 1983 through April 1985. The typical HRT was 22-23 days. The mean digester operational characteristics are given in Tables 1 and 2. The values of biogas production, CH,~ content, and VS destruction are within the bounds reported by others. Figures 3 and 4 report biogas production and digester effluent characteristics over time. Gas production was reduced (6/84) when values on NH3--N, alkalinity, and conductivity were elevated (Fig. 3). Until this time all of the manure dilution water was taken from the adjacent anaerobic lagoon, with the digester effluent being recycled to the lagoon. During the month of June, 1984 the practice of diluting the manure with fresh water was initiated. The concentrations of N H 3 - - N , alkalinity, and conductivity immediately began to fall (Fig. 3) and the volumetric gas production (m 3 m -3 digester) began to increase (Fig. 4). In November, 1984 the owner was forced to return to using lagoon liquid for raw manure dilution due to an inadequate supply of fresh water. At this time the concentration of NH3--N, alkalinity, and conductivity in the digester effluent again began to increase (Fig. 3) with a corresponding loss of gas production (Fig. 4). In TABLE 1 M e a n Digester P e r f o r m a n c e

Parameter

VS destroyed, % CH 4, % Biogas p r o d u c t i o n (daily) m 3 m - 3 digester m 3 k g - 1 vs a d d e d m 3 k g - ~ vs destroyed m 3 bird- 1 Ratio o f effluent alkalinity to effluent V F A Feeding rate, kg VS m - 3 d a y - t ° Aug. 1983-Apr. 1985. b SD, S t a n d a r d deviation.

Mean a

54.5 58.0 0"80 0.38 0"58 0.007 7"16 1"63

Number o f observations 75 54 73 31 31 73 31 75

SD b

20.1 5.0 0-24 0-12 0.20 0.002 5.46 0"69

85

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TABLE 2 Mean Values of Digester Parameters Studied Parameter

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3 810

2 150

19 500

5 500

22 750

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3.10

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0"90

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6 140

27 450

4"4 5 480

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kW

k Wh day- l Mean

Radiator fan Digester hot-water pump Compressor Digester mix-pump Manure pit-pump

3'8 1,1 7.5 5-6 12-5

54-2 11"7 97.3 10.0 7.6

Total

Range

42'7-70.0 7.9-17'9 59-4-141.9 6'4-18-3 6.1-15.6

180.0

Time period 11/27/84-5/2/85.

February, 1985 a dedicated well was drilled to supply dilution water. At present the owner is using fresh water as one-third of the dilution liquid with the remainder coming from the lagoon. During January 1985 extreme cold was experienced at the digester site and the gas meter was damaged. In April 1985 the gas meter was replaced by a temperature compensating impellor meter. Consequently, the following biogas production was noted: 1.15-1.25 m 3 m - 3 digester (daily); 0-44-0-85 m 3 kg- t VS added;

L. M. Safley, Jr, R. L. Vetter, Darrell Smith

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0.75-1.00 m 3 k g - * VS destroyed; 0-0096-0.0100 m 3 bird- * (daily). The above numbers are for a thirty day period following installation of the new meter. Part of the increased gas production can be explained by the fact that the operator added undetermined quantities of pullet manure to the mix pits from February through April. This manure was brought in from another site. One difficulty experienced was the management of the solids content in

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the manure mix-pit or in other words to know how much water to add to the manure to achieve a desired % TS. This problem was minimized by developing a regression equation relating TS (%) to the calibrations on a soil hydrometer. This technique has been reported on previously by Tunney (1981), and Chescheir & Westerman (1984). The regression equation developed is as follows: TS -- 0.455 + 157(HYD-1)

(1)

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L. 3I. Safley, Jr, R. L. Vetter, Darrell Smith

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water, but TS is for the undiluted sample. The equation was developed using 45 observations covering the solids-content range of 2.89-10.74% TS. The rz value for the equation was 0.802. Table 3 reports the electricity consumption for the motors supporting the digester for the time interval 11/27/84-5/2/85. This interval was selected as being typical of sustained operation. During the same period of time for which these numbers were computed the engine/generator was operated on the average 9.6 h day- 1 (7 days week- 1). Weekday operation was typically longer than this since the generator was run to match the 12 daily onpeak hours. Average daily electricity generated was 821.6kWh (range: 500-1034 kWh day- 1) at 85.1 kWh h - 1. The energy consumed to operate

Full-scale poultry waste digester

89

the digester system represented 22.0% of the electrical energy produced. Figure 5 gives the average gas production, electricity generated, and electricity consumed by the digester system for this period. During the month of April 1985 a system was installed that allowed direct burning of scrubbed, uncompressed gas. This reduction in compressing the biogas resulted in only 17% of the produced electricity being required for system operation. Daily electrical production for this month was 931 kWh day-1 as compared to electrical use of 161 k W h d a y One of the major problems encountered at the Smith digester has been the grit in the manure. The grit is mixed with the chicken feed to aid digestion. However, most of this material (oyster shells) passes into the manure. At manure solids contents less than 8-9% TS the grit will settle in a short time. Accumulation of this material necessitated the cleaning of the digester on two occasions. To reduce the amount of grit reaching the digester a strategy of mixing the manure, letting the grit settle (2-10 minutes), and pumping the supernatant to an adjacent pit was developed. The degritted manure was then fed to the digester as needed. Laboratory tests determined that such a mixing and settling procedure could reduce the fixed solids reaching the digester by 55%. Of the remaining fixed-solid material approximately 33% or more is estimated to pass through the digester. Converse et al. (1981) reported a floating scum layer on their digester on each of the trial periods studied. A floating scum layer was also noted on the Smith digester. The thickness varied from 2 cm to 15 cm but typically was easily broken-up by feeding over-the-top.

ACKNOWLEDGEMENT This paper was originally published in the Proceedings of the 5th International Symposium on Agricultural Wastes, American Society of Agricultural Engineers, St Joseph, MI, USA.

REFERENCES APHA (1981). Standard methods for the examination of water and wastewater. 15th Edition. APHA, Washington, DC, USA. Ashworth, J. H., Bihun, Y. M. & Gee, L. E. (1984). The universe of US anaerobic digesters. ARD, Inc., Burlington, VT, USA. Chescheir III, G. M. & Westerman, P. W. (1984). Rapid methods for determining fertilizer value of livestock manures. Paper 84-4084, ASAE, St Joseph, MI, USA.

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L. M. Safley, Jr, R. L. Vetter, Darrell Smith

Converse, J. C., Evans, G. W., Robinson, K. L., Gibbons, W. & Gibbons, M. (1981). Methane production from a large-size on-farm digester for poultry manure. In: Livestock Waste: A Renewable Resource, ASAE, St Joseph, MI, USA, pp. 122-5. Hill, D. T. (1984). Methane productivity of the major animal waste types. Trans. of ASAE, 27(2), 530-4. National Draeger, Inc. (1984). Gas and vapor detection products. Pittsburgh, PA, USA. Taras, M. J., (Ed.) (1979). Standard methods for the examination of water and wastewater treatment. Technical Services Bulletin. Piscataway, N J, USA. Technicon. (1973). Technicon industrial methods. 321-74A, Technicon Industrial Systems, Tarrytown, NY, USA. Tunney, H. (1981). An overview of the fertilizer value of livestock manure. In: Livestock Waste: A Renewable Resource, ASAE, St Joseph, MI, USA, pp. 181--4. US EPA (1979). Methods for chemical anaysis of water and wastes. EPA 600-4-79-020. Environmental Monitoring and Support Lab., US EPA, Cincinnati, OH, USA.