Agricultural Wastes 2 (1980} 177 184

EFFECT OF CORN STOVER ADDITION ON THE ANAEROBIC DIGESTION OF SWINE MANURE

M. FUJITA, J. M. SCHA~R & M. MOO-YOuNG

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada

ABSTRACT

The addition of corn stayer to swine manure enhanced biogas production in both mesophilic and thermophilie anaerobic digestion. More than 50 % of the carbon in corn stayer was converted to gas. The addition of corn stayer had little effect on solids conversion efh'ciency. The digested solids were high in protein content and gave high digestibility in vitro.

INTRODUCTION

In anaerobic digestion, the optimal ratio of carbon to nitrogen (C/N ratio) has been reported to be 25:1 to 35:1 (Scharer & Moo-Young, 1979) but animal wastes usually contain excess nitrogen. We previously found that the C/N ratio of swine manure was 7-4:1 (Fujita et al., 1980). Thus, it may be advantageous to add an additional carbon source to the animal wastes in order to attain higher fuel gas production. As crop residues contain high amounts of carbon, they can serve as a supplementary carbon source.

In this study, we used corn stayer as a supplementary carbon source. Digested liquors with and without corn stayer (leaf and stem residues left after harvesting maize c o b s ) w e r e tested for dry matter disappearance as an indicator of digestibility.

MATERIALS AND METHODS

Dried swine manure and corn stover were ground to less than 2 mm by a laboratory mill and used as a substrate in both mesophilic and thermophilic digestion. An 8 ~,

177 Agricultural Wastes 0141-4607/80/0002-0177/$02"25 ~ Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

178 M. FUJITA, J. M. SCHARER, M. MOO-YOUNG

(w/v) slurry (6 % swine manure and 2 % corn stover) was prepared daily and allowed to stand overnight at room temperature. Volatile Solids and total Kjeldahl nitrogen (TKN) of the mixture were 76.8 % and 3.4 %, respectively. The pH of the slurry after overnight storage was, on average, 6.6.

The experimental anaerobic digester consisted of a conical, cylinder-type digester with an internal diameter of 23 cm, a circulation pump, a temperature controller, a wet gas meter, a thermometer and a scum breaker. The working volume of the digester was 30 litres. Steam was used as a controlled heating source. The volumetric retention time was 8 days in the thermophilic (55 °C) and 16 days in the mesophilic (39 °C) digestion process.

All analyses were carried out as described previously (Fujita et al., 1979). Dry matter disappearance (DMD) was determined by pepsin-cellulase assay (Goto & Minson, 1977).

RESULTS AND DISCUSSION

In previous studies by the writers, the digested liquor was recirculated intermittently. Since the substrate concentration was very high in this digestion, the scum breaker frequently became clogged when intermittent mixing was used and hence continuous recirculation was required.

Figure 1 shows the effect of recirculation on gas productivity in both mesophilic and thermophilic digestion. No difference is apparent between continuous and

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CORN STOVER ANAEROBIC DIGESTION 179

intermittent recirculation. This agrees with the observation of Schmid & Lipper (1969). In Fig. 1 the thermophilic digestion contained both swine manure and corn stover, but the mesophilic digestion contained swine manure only.

Figures 2 and 3 show the time course of gas productivity in thermophilic and mesophilic digestion, respectively. Corn stover enhanced gas productivity by more than 63% in thermophilic digestion and by 65% in mesophilic digestion in comparison with swine manure only. The increase is most probably due to the easily biodegradable carbohydrate content of corn stover. The corn stover/manure mixture yielded significantly more gas than the crop residue alone, as reported by Pfeffer (1978).

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Data for stationary states at thermophilic and mesophilic temperatures are summarised in Table 1. The loading rate for thermophilic digestion was twice as high as that for mesophilic operation. The reductions of Total Solids, total Kjeldahl nitrogen and cellulose in thermophilic and mesophilic digestion were similar despite the different loading rate. The total Kjeldahl nitrogen content was reduced by 10.3 °~i in the thermophilic digestion and 11.4 ~o in the mesophilic digestion. Due to the higher carbon content, the pH of the corn stover/manure mixture was consistently lower than that of a swine manure digestion at both digestion temperatures. The

180 M. FUJITA, J. M. SCHARER, M. MOO-YOUNG

cellulose reduction efficiency in both types of digestion was more than 50 ~ and was essentially identical to that of digesting swine manure.

Mueller et al. (1959) stated that gas productivity was inhibited by volatile fatty acids concentrations in excess of 2 kg m - 3. Since volatile fatty acids concentrations in both types of digestion were quite low, acid inhibition was not a factor in our experiments.

TABLE 1 SUMMARY OF ANAEROBIC DIGESTION OF SWINE MANURE WITH CORN STOVER

Parameters Type of digestion Thermophilic Mesophilic

(55 °C) (39 °C)

Retention time (days) 8 16

kgTSm 3 day- i 10.00 5.00 Loading rate kg VS m 3 day- ~ 7.68 3.84

inf. kg m -3 80 80 Total Solids eft. kg m - 3 45.06 + 1.90 46.98 + 4.30

reduction 43.7 41-3

inf. kg m -3 61.44 61.44 eft. kg m - 3 32-55 _+ 1.64 32'94 ___ 3.14

Volatile Solids ~o reduction 47-0 46.4 Volatile Solids, ~o 72.2 70.1

inf. kg m -3 12.32 12.32 Cellulose eft. kg m - 3 5.69 + 0.37 5.85 ___ 1.14

reduction 53.8 52-5

inf. kg m -3 2.71 2.71 TKN* eft. kg m -3 2.43+0.04 2.40+0-06

~o reduction 10.3 11.4

NHa-N eft. kg m -3 0.92+0.06 0.99+0.12

(1 per day) 2.064+0.104 1.171 +0.047 Gas production m 3 per kilogramme VS added 0.269 0.305

m 3 per kilogramme VS removed 0-572 0.657 methane content ~ 62.9 67.3

Volatile acid (as CHaCOOH ) kg m - 3 0.53 + 0.16 1.32 _ 0" 12

pH 7.47 7.13

* Total Kjeldahl nitrogen.

Gas production per kilogramme of Volatile Solids added in both cases was almost identical with that from swine manure alone, but gas production per unit of Volatile Solids removed was somewhat lower. This may have been due to the higher loading rate with the corn stover added.

Hassan et al. (1978) realised the effect of supplementary carbon on gas productivity in the low temperature digestion of poultry wastes. They added 2 ~ , 4 ~ and 8 ~ sawdust by weight to fresh poultry wastes. Sawdust improved gas

CORN STOVER ANAEROBIC DIGESTION 1 8 1

production, although gas productivity was not linearly dependent on the sawdust content.

The carbon balance with respect to corn stover is shown in Table 2. We assumed that the difference between gas productivity with and without corn stover depended on the supplementary carbon from corn stover. Fifty per cent of the carbon was converted to biogas. Biomass production was relatively high at both thermophilic and mesophilic temperatures. It appears that the additional carbon source reduced the ammonification of manure nitrogen.

TABLE 2 CARBON BALANCE 1N CORN STOVER/SW1NE MANURE DIGESTIONS

Type of digestion Thermophilic Mesophilic (55 °C) (39 °C)

Retention time (days) 8 16

Input Total Solids (g) 100 100 Carbon (g)* 35.01 35.01

Biogas generated (litres)t 31-88 37.05 Biomass produced (g):~ 11.25 23.75

Output Carbon in CH 4 (g) 10.74 13-35 Carbon in CO2 (g) 6.34 6-50 Carbon in biomass (g) 5-63 11.88

Carbon utilisation Biogas (~o) 48-8 56.7 Total (~o) 64.9 90-6

* Estimated from the data on total carbohydrate content. t Difference between gas productivity with and without corn stover. ~: Difference between biomass with and without corn stover.

The digested solids contained nearly 19~ protein. Hashimoto et al. (1978) reported the amino acid composition in digested cattle manure and compared it with alfalfa hay and soybean meal. They concluded that generally the protein from the anaerobic digestion is a good animal protein source. Favourable amino acid contents in swine manures were found by Day (1977).

Goto & Minson (1977) and Terry et al. (1978) reported that dry matter disappearance in the pepsin-cellulase assay could be used to estimate animal digestibility in vivo. Table 3 shows the dry matter disappearance (DMD) of digested swine manure, digested swine manure with corn stover, corn stover and dried swine manure. It includes barley straw as a control. Digested swine manure at an 8-day retention time and dried swine manure showed higher D MD than the others. Digested swine manure and corn stover showed a slightly lower D M D than swine manure digested without corn stover, but the difference was hardly significant. The Volatile Solids and cellulose contents have no correlation with the DMD.

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CORN STOVER ANAEROBIC DIGESTION 183

(1977) or from those of Terry et al. (1978). Terry et al. (1978) reported the following regression equations for grasses:

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Table 3 shows the calculated D M D by tureen liquor-pepsin as well as the calculated in vivo digestibility. From our experience, the experimental D M D with rumen liquor have usually been higher than the D M D found by the pepsin-cellulase analysis. Estimations by eqns (l) and (2) are reasonable. Digestibility in vivo is about 60 ~o and lower than the digestibility of grasses observed by Terry et al. (1978). Since swine manure with corn stover has a high protein content, it can be considered a very good potential animal feed.

Corn stover was proved to be a good supplementary carbon source in co-digestion with swine manure. If corn stover is mixed with swine manure solids in a 1:3 ratio, more than 50 ~o of the corn stover carbon is gasified. The gas productivity of the digesting slurry is substantially higher than that of digestions with either corn stover or swine manure alone. This complementary effect may be due to the more advantageous C :N ratio in the feed mixture.

The protein content of the digester effluent solids was near 19~o with corn stover/swine manure feed. In vitro digestibility studies gave encouraging results with respect to the potential use of the digested solids as an animal feed supplement.

ACKNOWLEDGEMENTS

Our thanks are due to the NSERC, O M A F and Imperial Oil Limited for financial support of this project.

REFERENCES

DAY, D. L. (1977). Utilization of livestock wastes as feed and other dietary products. In: Animal wastes (Taiganides, E. P. (Ed.)), Applied Science Publishers Ltd, London.

FUJITA, M,, SCHARER, J. M. & Moo-YOUNG, M. (1980). Comparative studies of swine manure digestions at mesophilic and thermophilic temperatures. Biotechnology & Bioengineering. In press.

GOTO, I. & MINSON, D. J. (1977). Prediction of the dry matter digestibility of tropical grasses using a pepsin-cellulase assay. Animal Feed Science and Technology, 2, pp. 247-53.

184 M. FUJITA, J. M. SCHARER, M. MOO-YOUNG

HASHIMOTO, A. G., PRIOR, P. L. & CHEN, Y. R. (I 978). Methane and biomass production systems for beef cattle manure. Paper presented at the Great Plains Extension Seminar on Methane Production from Livestock Manure, Liberal, Kansas, 15 February.

HAS,SAN, A. E., HASSAN, H. M. ~ SMITH, N. (1978). Energy recovery and feed production from poultry wastes. Personal Communication.

MUELLER, L. E., HINDIN, E., LUNSFORD, J. V. 8¢. DUNSTAN, G. H. (1959). Some characteristics of anaerobic sludge digestion I. Effects of loading. Sewage and Industrial Wastes, 31, pp. 669-77.

PFEFFER, J. T. (1978). Biological conversion of crop residue to methane. Paper presented at The Second Animal Symposium on Fuels from Biomass, Vol. II, pp. 759-85.

SCHARER, J. M. & Moo-YOUNG, M. (1979). Methane generation by anaerobic digestion of cellulose- containing wastes. In: Advances in Biochemical Engineering (Ghose, T. K., Fiechter, A. and Blakebrough, N. (Eds)), Vol. II, pp. 85-101.

SCHM1D, L. A. & LIPPER, R. I. (1969). Swine wastes: Characterization and anaerobic digestion. I n: Animal Waste Management. Proceedings of Cornell University Conference on Agricultural Waste Management, 13-15 January, pp. 50-7.

TERRY, R. A., MUr~DELL, D. C. & OSaOURN, D. F. (1978). Comparison of two in vitro procedures using rumen liquor-pepsin or pepsin-cellulase for prediction of forage digestibility. Journal o f the British Grassland Society, 33, pp. 13-18.