Treatment of Soybean Processing Residues for Energy Recovery and Environmental Compliance: Technical and Economic Feasibility

Soybean molasses, a by-product of soy protein concentrate processing, presents environmental disposal issues and therefore researchers and industries have been studying its application. This study investigated methane production from soybean molasses anaerobic digestion at mesophilic conditions in an Anaerobic Sequencing Batch Biofilm Reactor (AnSBBR). The evaluation was focused on the impact of feeding strategy (batch or fedbatch mode), fill time, applied organic loading rate (OLRA), temperature, and co-processing with okara on system stability and performance. The best performance indicators were obtained at an OLRA of 14.9 kg-COD/ m3/d, fed-batch mode with 180 min of feed at 30 degrees C (86% COD removal, 140.9 mol-CH4/m3/d, 11.0 mol-CH4/ kg-COD, and 74% of methane in biogas). Crucial anaerobic digestion functional groups, such as hydrolytic and acetogens, were identified. Firmicutes and Proteobacteria were the most abundant phyla. In the inoculum, Euryarchaeota phylum exceeded Proteobacteria. Nine reactors of 3271 m3 each producing 72 GWh/year of energy would be needed to treat 216 tons-molasses/day according to the scale-up estimation using industrial data. A USD 61 M initial investment would be necessary. The net present value, internal rate of return and payback would be of USD 39 M, 14.0% and 13.8 years, respectively.