Croatian Food Industry By-Products: Co-Digestion with Swine Manure vs. Use as Liquid Animal Feed

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, co-digestion of these materials with manure could improve the economic viability of on-farm digesters. To determine the feasibility of this approach, consideration should be given to whether food industry by-products provide a better economic return as a low-cost animal feed or as an anaerobic digester feedstock. In addition, while slaughterhouse wastes are not used as an animal feed, this article also considers the use of this material as a co-digestion feedstock. The economic information and substrate selection presented in this article are based on by-products available to a swine farm in Croatia and production data from the facility while feeding with by-products. Biochemical methane potential (BMP) assays were used as a fast, inexpensive method to determine the potential methane production rates for the various substrates. Using BMPs, the potential methane production rates for various combinations of spent brewer's yeast, whey, slaughterhouse 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. Based on live production data, liquid feeding of food industry by-products was calculated to provide a $6.89 savings per finish pig produced over a 90-day period at the Croatian facility. Since the facility produces 14,000 finish pigs every 90 days, this represents a cost savings of $96,000 per 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. These values were based on the value of the substrate, assuming that either a digester or liquid feeding system already existed. Based on the data, the economic return is better 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 this article include substrate characteristics, potential methane production normalized on the basis of mass of substrate volatile solids, estimated electricity generation potential, and economic data.