ANAEROBIC CO-DIGESTION OF COFFEE PULP AND CATTLE MANURE: EVALUATION OF PROCESS STABILITY

Abstract

About 530 million dry tons of agricultural biomass are produced annually in the U.S. (Almomani et al., 2019). Coffee is the second largest agricultural crop produced globally (9.2 million tons of coffee cherries per year), generating nearly 42% of coffee pulp (CP) of the total waste produced during coffee processing. Furthermore, approximately 900 million tons of cattle manure (CM) are reported to be generated in the U.S. annually (U.S. Department of Agriculture, S.C.S., 2018). Anaerobic digestion (AD) is a widely adopted technology for treating various organic waste streams; however, mono-digestion of CP is challenging due to the presence of toxic compounds such as caffeine, free phenols, and tannins. Hence, co-digestion of CP waste with CM could be a strategy to enhance digestibility and process stability through nutrient balance and dilution of toxic compounds. Biochemical methane potential (BMP) tests were conducted to select the optimum inoculum source and feedstock ratio for co-digestion. Nine different kinetic models (modified Gompertz, Chen and Hashimoto, first-order, transfer, cone, superimposed, modified Gompertz with second order equation, two-phase exponential, and multi-stage models) were evaluated to find the best-fit for the BMP experimental data. In the next phase, the optimum co-digestion ratio was compared with mono-digestion of feedstocks in semi-continuous studies at mesophilic conditions. The biomethane production of CP alone and co-digestion with CM was almost similar, however in terms of process stability, co-digestion yielded better results. The close monitoring of the start-up period and the use of co-substrate with high buffering capacity are highly recommended for efficient AD of CP.