Anaerobic Digestion of Swine Manures with Different Types of Food Industry Waste

Abstract

Food industry by-products such as spent brewer’s yeast and whey from cheese production are being amended with nutrients and used as a feed source in the Croatian swine sector. However, as interest in energy production and anaerobic digestion of animal manures has increased, the co-digestion of these materials with manure could improve the economic viability of on-farm digesters. To determine if this is a feasible approach, the question of whether food industry by-products provide a better economic return as a low cost animal feed or as an anaerobic digester feedstock must be considered. This paper also considered the use of slaughter wastes as an additional co-digestion feedstock in combination with these materials and manure. The economic information and substrate selection presented in this paper is based on by-products available to a swine farm in Croatia and production data from the facility while feeding by-products. Biochemical methane potential (BMP) assays were used as an inexpensive and fast method to determine the potential methane production rates for the various substrates. Using BMPs, the potential methane production rates for various combinations of brewery yeast, whey, slaughter house waste, corn silage and swine manure were also determined. Results of the BMP assays were used to compare the potential economic return of using the food wastes to produce methane in the digester to the value of these materials as feed ingredients for swine production. Liquid feeding of food industry by-products were calculated to provide a $6.89 savings per finish pig produced over a 90 day period at the Croatia facility. Since the facility produce 14,000 finish pigs every 90 days, this represents a cost savings of $96,000 every turn, or over $307,000 per year considering that the facility finishes 3.2 turns of pigs per year. Using cheese whey or spent brewer’s yeast as a co-substrate in the proposed swine manure digester has the potential to provide an additional income via electricity generation of $26,000 and $34,000 over a 90 day period, or $83,000 and $109,000 annually, respectively. Based on the data, the better economic return is achieved when the by-products are used as a feed ingredient. However, if the swine digester is amended with all available co-substrates including whey, spent brewer’s yeast, slaughterhouse waste, and corn silage the potential additional income is $168,000 per 90 day turn or $538,000 per year. The data presented in the paper includes substrate characteristics, potential methane production normalized on the basis of mass of substrate volatile solids, estimated electricity generation potential, and economic data.