Agricultural Wastes 13 (1985) 207-216

Nutrient Recoveries from Plug-flow Anaerobic Digestion of Poultry Manure

J. A. Field, R. B. Reneau, Jr., W. Kroontje & J. S. Caldwell

Department of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24061, USA

ABSTRACT

Anaerobic digestion provides for the production of a biogas fuel while concurrently conserving the nutrient value of manures used. The nutrients placed in anaerobic digesters are, however, subject to possible transformations. The objectives of this study were to evaluate changes in the N composition following plug-flow digestion of poultry manure, and to determine the distribution of N, K, P, Ca and Mg between the outflowing eJfluent and the settled manure sludge which remained in the digester. Total Kjeldhal N ( TKN) was completely recovered in the pilot- scale digestion operated with 6 % influent TS, 35 °C and 30 day retention time. Mineralization of poultry manure organic N by the digestion process was 53"/o. Consequently, NH4-N+ in the digested manure accounted for the large majority (79.6~) of the TKN. The effluent accounted for the majority of the N and K present in the digested manure (91.2 and 95.9 %, respectively). The settled manure sludge, accounted for 26"3 % of the recovered TS and contained approximately 30 % of the P, Ca and Mg.

I N T R O D U C T I O N

Anaerobic digestion of organic matter is a process that evolves a CH 4- containing biogas t h a t c a n be utilized as fuel. An additional benefit of the process is that recovered nutrients can be utilized for animal feeds or crop fertilizers (Hashimoto et al., 1980). This has undoubtedly contributed to

207

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208 J. A. Field, R. B. Reneau, Jr, W. Kroontje, J. S. Caldwell

the growing use of anaerobic digesters in the developing nations where acute fuel shortages have led to the loss of nutrients through the direct combustion of agricultural residues such as manures (DaSilva, 1980).

Nitrogen, which is completely lost by direct combustion and subject to losses by conventional storage of animal manures, is essentially completely recovered following anaerobic digestion (Converse et al., 1977; Hashimoto et al., 1980). Traces o fNH a measured in biogas account for less than 0.01% of total Kjeldahl N (TKN) placed in digesters (Converse et al., 1981). Noticeable increases in NH2-N concentration in digested slurries of cattle, swine and poultry manure indicate that mineralization of organic N occurs during digestion (Fischer et al., 1979; Converse et al., 1981; Hashimoto, 1983). Consequently, the feeding values of manures are reduced during digestion since NH2-N is difficult to segregate with solids (Mattocks, 1981). Digested cattle manures, however, still contain significant quantities of organic N and have been useful as animal feeds (Hashimoto et al., 1980). Digested poultry manures have large NH2-N:TKN ratios and thus may be better utilized as crop fertilizers.

Other nutrients important to crop fertilization, such as K, P, Ca, Mg and micronutrients, theoretically should be 100% recovered during digestion. This is because these nutrients are not labile at digestion temperatures.

The objectives of this study were to evaluate changes in the N composition following plug-flow digestion of poultry manure and to determine the distribution of N, K, P, Ca and Mg between outflowing effluent and the settled manure sludge which remains in the digester.

METHODS

Pilot-scale study

A 3-8 m 3 plug-flow anaerobic digester, illustrated in Fig. 1 was loaded with 1 '9 m 3 of municipal sewage seed sludge. Poultry manure diluted to 6 % total solids (TS) was added to bring the contents to the final retention volume (2.7 m 3) at start-up. Feed of this concentration was added daily after start-up to correspond to a 30-day retention time. The influent was composed of litter-free droppings from ration-fed caged broilers. Hammer milled corn cobs were added as 10 % of the TS in the influent.

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210 J. A. Field, R. B. Reneau, Jr, W. Kroontje, J. S. Caldwell

The temperature was maintained at 35 °C ( _+ 1 °C) with a propane-fired hot-water heater and copper heat exchanger. An agitator bar with 300mm spokes was turned for 5min each day to prevent scum formation.

Six weeks into operation, collection of samples was initiated. Samples for TS, pH, extractable NH~-N and TKN were collected every 5 days from the influent and effluent hoppers. The term refers to the digested slurry displaced after each loading and does not include a fraction of the manure which settled and remained in the digester. This deposited layer is referred to as the settled manure sludge. The hypothetical mixture of the effluent and settled manure sludge is referred to as the digested manure.

The TS was determined by oven drying at 105 °C for 24h. The pH was determined with an Orion Research Ionanalyzer 901. Ammonium was extracted by shaking nine parts 2 y KC1 with one part sample for 1 h. The TKN digestion utilized followed the procedure of Bremner & Mulvaney (1982). Both NH~-N and TKN were determined via steam distillation (Keeny & Nelson, 1982). Extractable and total P, K, Ca and Mg were determined from single composite samples of the influent and effluent. Ground, oven-dried samples (0.2 g) were extracted with dilute acid (0.05 y HC1 and 0.025 y H2SO4) by shaking for 5 rain. Total P, K, Ca and Mg were determined from perchloric acid digests (Lim & Jackson, 1982). Determinations, except P, were conducted with a Perkin Elmer Model 503 atomic absorption spectrophotometer (Baker & Suhur, 1982). Phosphorus was determined colorimetrically as described by Olsen & Sommers (1982). Concentrations are reported on a wet basis assuming the average influent and effluent TS.

Samples of the settled manure sludge were collected after draining the effluent immediately following termination of digester operation. Care was taken to avoid mixing a discernible layer of the seed sludge with the settled manure sludge. Analyses were identical to those outlined previously. Mass distribution between the settled manure sludge and the effluent fraction of the digested manure was based on sludge volume and density.

Biogas yields were measured by a meter calibrated by direct volume measurements of gas collected in rubber bags. A water manometer was used for standardizing to atmospheric pressure.

Recoveries of TS and N following digestion were calculated as follows:

(C s x F s / C i + C e x F J C i ) ( M i -- M g / M i ) = R

Plug-flow digestion of poultry manure 211

where C S and C e = concentrations in sludge and effluent, F~ and F e = mass fraction of sludge and effluent, C~ = concentration in influent, M i and Mg = mass of influent and biogas, R --- recovery.

Since gas composition and mass were not directly measured, ideal gas laws and gas composition of 70 ~o CH4 and 30 % CO 2 were assumed for mass calculation. Recoveries of K, P, Ca and Mg are not reported since these nutrients were determined from single composite samples.

RESULTS A N D DISCUSSION

Biogas yield and TS

The daily biogas yields of the digester operation are shown in Fig. 2. The average daily biogas yield was 0.752 m 3 p e r m 3 retention volume which corresponds to 0.390m 3 biogas per kg TS loaded. The yield (per TS

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212 J. A. Field, R. B. Reneau, Jr, W. Kroontje, J. S. CaMwell

T A B L E 1

C o n c e n t r a t i o n s o f T S , N H 2 - N , T K N , E x t r a c t a b l e a n d T o t a l K , P, C a a n d M g

Units Influent SD ° Effluent SD Sludge Digested manure b

TS % 5.78 0.981 3.15 0.944 16.9 4 '00

p H ( m e d i a n ) - - 6.96 - - 7.91 - - - - - -

NH,~-N m g liter - t 2 240 336 3 690 194 2 690 3 630

T K N m g l i ter - ~ 3 940 519 4 420 222 6 700 4 560

N H ~ - - N / T K N ~ 56.9 10.0 83.5 5.60 40 79.6

Extrac table K m g liter - ~ 1 960 - - 2 300 - - 1 520 2 250

Tota l K mgl i t e r -~ 2070 - - 2620 - - 1 700 2560

Ext rac tab le /

total K ~ 94.7 - - 87.9 - - 89.4 88-0

Extrac table P m g l i ter - 1 832 - - 413 - - 4 070 638

To ta l P m g l i ter - ~ 1 720 - - 904 - - 5 340 I 180

Ex t rac tab le /

total P % 48.3 - - 45.7 - - 76-2 54-1

Extrac table Ca m g liter - ~ 4 470 - - 1 500 - - 19 200 2 590

Tota l Ca m g liter -1 5870 2670 - - 19300 3690

Extrac table /

total Ca % 76.1 - - 56.2 - - 99.5 70.1

Ext rac tab le Mg mgl i t e r ~ 419 - - 126 - - 1 690 222

To ta l Mg mg l i ter - ~ 427 - - 189 - - 1 550 273

Ext rac tab le /

total Mg % 98-1 - - 66.7 - - 100 c 81.3

a S tanda rd deviat ion.

b Hypothe t ica l mixture of effluent and sludge.

c A ssum ed 100%.

loaded) is between those reported by Converse et al. (1981) and Morrison et al. (1981).

The TS concentrations of the influent, effluent and settled manure sludge are shown in Table 1. These concentrations indicate a 32.2 ~o destruction of solids during digestion (Table 2). The effluent accounted for the majority of the TS recovered after digestion (Table 2). The remaining TS (26.3 ~ of the recovered solids) is accounted for in solids which settle out of suspension and form a layer of manure sludge in the reactor. An eventual buildup of the settled solids would result in a less active sludge occupying the digester volume. Maintenance of the observed biogas yields and TS destruction would require intermittent

Plug-flow digestion of poultry manure

TABLE 2 The Recovery and Distribution of Mass, TS, NH~-N, TKN,

Extractable and Total Forms of K, P, Ca and Mg

213

Recover), Distribution in digestedmanure ("/o) (%)

Effluent Settled manure sludge

Mass" 97-7 93.9 6.15

TS 67.8 73.7 26.3

NH~--N 159 95-0 4.79 TKN 113 91.2 9.03

Extractable K - - 95.6 4.12 Total K - - 95.9 4.07

Extractable P - - 60.7 39-3 Total P - - 72.1 27.9

Extractable Ca - - 54.3 45-7 Total Ca - - 69.2 30.8

Extractable Mg 53.2 46.8 Total Mg - - 65-1 34.9

a Estimated from influent mass biogas mass.

r emova l of the accumula ted sludge. Such a m a n a g e m e n t system is c o m p a r a b l e with the semi-annual empty ing and reseeding o f biogas digesters in China (Lil jedahl et al. , 1983).

Nitrogen

A m m o n i u m concen t ra t ions were higher in the digested m a n u r e than in the influent (Table l). This increase o f NH,~-N concen t r a t i on indicates minera l iza t ion o f a large par t o f the pou l t ry m a n u r e organic N dur ing digest ion. Based on the differences in organic N ( T K N - N H 4 - N ) the minera l iza t ion o f the pou l t ry m a n u r e organic N was 53 ~o dur ing digest ion. This minera l iza t ion o f organic N resul ted in a digested m a n u r e T K N which was 79.6 ~o N H ~ - N .

The N mass balance o f the digester is presented in Table 2. Th e measured T K N recovery was 113 ~o. The larger than 100 ~ recovery is a t t r ibu ted to exper imenta l e r ror . However , a recovery o f close to 100 ~o can be assumed based on the previously cited observat ions tha t only small

214 J. A. Field, R. B. Reneau, Jr, W. Kroontje, J. S. Caldwell

quantities of N are recovered in the biogas from manure digestion. The eflluent fraction of the digested manure accounted for most (91.2 %) of the recovered TKN. This indicates that small amounts of N remained in the digester associated with the settled manure solids. The movement of large quantities of N out of the digester with the effluent is attributable to the presence of NH:-N in the liquid fraction of the digested manure.

The N recovered with the e&rent is suitable for use as an N fertilizer since most of the N (83.5 %) was in the NH:-N form. The predominance of inorganic N indicates a readily available source of N for crop utilization. However, NH:-N would be subject to NH, volatilization during eflluent storage and land application. This could lead to potentially significant losses of N if proper management practices to prevent NH, volatilization were not utilized.

K, P, Ca and Mg

The concentrations of K, P, Ca and Mg are shown in Table 1. K, Ca and Mg present in poultry manure should be considered highly available for plants since the extractable forms of these nutrients accounted for 94.7, 76.1 and 98.1 %, respectively, of the total nutrient concentrations. Extractable P accounted for approximately half of the total P.

The proportions of K, P, Ca and Mg distributed between fractions of the digested manure are shown in Table 2. The efhuent accounted for 95.9 % of the K present in the digested manure. This indicates that only limited quantities of K are associated with the manure sludge. However, the manure sludge accounted for about 30 % of the total P, Ca and Mg present in the digested manure. A similar proportion of the recovered TS was accounted for by the settled manure sludge. This suggests that P, Ca and Mg are associated with the solids in the digested manure fraction. Therefore, unstirred digestions of animal manures may have an important fraction of the manure P, Ca and Mg remaining in the digester due to the settling of manure solids.

ACKNOWLEDGEMENTS

The authors wish to express their appreciation to Mr Joe Wade for his help in the construction of the pilot-scale digester.

Results from this paper were from research funded by the US Department of Energy, Office of Appropriate Technology.

Plug-flow digestion of poultry manure 215

R E F E R E N C E S

Baker, D. E. & Suhur, N. H. (1982). Atomic absorption and flame spectrometry. In Methods of soil analysis Part 2." Chemical and microbiological properties (Page, A. L., Miller, R. H. & Keeney, D. R. (Eds)). Am. Soc. Agrn., Inc., Soil Sci. Soc. Am., Inc., Madison, Wisconsin, pp. 13-27.

Bremner, J. M. & Mulvaney, C. S. (1982). Total nitrogen. In Methods of soil analysis Part 2." Chemical and microbiological properties (Page, A. L., Miller, R. H. & Keeney, D. R. (Eds)). Am. Soc. Agrn., Inc., Soil Sci. Soc. Am., Inc., Madison, Wisconsin, pp. 595-624.

Converse, J. C., Graves, R. E. & Evans, G. W. (1977). Anaerobic degradation of dairy manure under mesophilic and thermophilic temperatures. Trans. Am. Soc. Agric. Eng., 20, 336-40.

Converse, J. C., Evans, G. W., Robinson, K. L., Gibbons, W. and Gibbons, M. (1981). Methane production from a large size on-farm digester for poultry manure. In Livestock wastes: a renewable resource, Proceedings of the 4th international symposium on livestock wastes--1980. Am. Soc. Agric. Eng., St Joseph, Michigan, pp. 122-5.

DaSilva, E. J. (1980). Biogas: fuel of the future? Ambio, 9, 2-9. Fischer, J. R., Iannotti, E. L., Porter, J. H. & Garcia, A. (1979). Producing

methane gas from swine manure in a pilot-size digester. Trans. Am. Soc. Agric. Eng., 22, 370 4.

Hashimoto, a,. G. (1983). Conversion of straw-manure mixtures to methane at mesophilic and thermophilic temperatures. Biotechnol. Bioeng., 25, 185-200.

Hashimoto, A. G., Chen, Y. R., Varel, V. H. & Prior, R. L. (1980). Anaerobic fermentation of agricultural residues. In Utilization and recycle of agricultural wastes and residues (Shuler, M. (Ed.)). CRC Press, Boca Raton, Florida, pp. 135-96.

Keeney, D. R. & Nelson, D. W. (1982). Nitrogen--inorganic forms. In Methods of soil analysis Part 2: Chemical and microbiological properties (Page, A. L., Miller, R. H. & Keeney, D. R. (Eds)). Am. Soc. Agrn., Inc., Soil Sci. Soc. Am., Inc., Madison, Wisconsin.

Liljedahl, J. B., Tyner, W. E., Butter, J. & Caldwell, J. S. (1983). Biogas digesters in the People's Republic o/China, paper 83-4060. Am. Soc. Agric. Eng., St Joseph, Michigan.

Lim, C. H. and Jackson, M. L. (1982). Dissolution for total elements analysis. In Methods" of soil analysis Part 2: Chemical and microbiological properties (Page, A. L., Miller, R. H. & Keeney, D. R. (Eds)). Am. Soc. Agrn., Inc., Soil Sci. Soc. Am., Inc., Madison, Wisconsin, pp. 1--12.

Mclnerney, M. J. & Bryant, M. P. (1981). Basic principles of bioconversion in anaerobic digestion and methanogenesis. In Biomass conversion processes jor energy and Juels (Sofer, S. S. & Zarborsky, O. R. (Eds)). Plenum Press, New York, pp. 277-96.

Mattocks, R. P. (1981). Nutrient recovery from swine waste, Masters Thesis. Virginia Polytechnic Institute andState University, Blacksburg, Virginia.

216 J. A. Field, R. B. Reneau, Jr, W. Kroontje, J. S. Caldwell

Morrison, S. R., Vohra, P., Shupe, W. L. & Hills, D. J. (1981). Biogas from poultry manure: volatile solids loading rate and hydraulic detention time. In Livestock wastes: A renewable resource, Proceedings of the 4th international symposium on livestock wastes--1980. Am. Soc. Agric. Eng., St Joseph, Michigan, pp. 96-8.

Olsen, S. R. & Sommers, L. E. (1982). Phosphorus. In Methods o f soil analysis Part 2: chemical and microbiologicalproperties (Page, A. L., Miller, R. H. & Keeney, D. R. (Eds)). Am. Soc. Agrn., Inc., Soil Sci. Soc. Am., Inc., Madison, pp. 403-30.