Conversion of mixtures of dairy manure and soybean curd residue by black soldier fly larvae (Hermetia illucens L.)

Abstract

The increasing numbers of dairy farms cause massive manure production and its improper disposal poses threats to the environment. Larvae of the black soldier fly (BSF) have the potential to convert this animal manure into valuable biomass; however, the high fiber content of dairy manure (DM) makes conversion difficult. Therefore, the goal of this investigation was to evaluate the impact of co-digestion of DM with another ecologically problematic waste source: soybean curd residue (SCR). The waste sources were fed to BSF larvae using different mixing ratios. DM and SCR mixed in a ratio of 2:3 was found to result in the best outcome measures: development time (21 days), survival rate (98.4%), waste mass reduction (75.4% wet and 56.6% dry waste mass), bioconversion (11.6% on wet and 14.6% dry mass base), feed conversion ratio (6.4 wet and 4.0 dry), nutrient utilization (nitrogen, 62.1%; phosphorus, 52.9%; carbon, 66.4%), and fiber content reduction (cellulose, 64.9%; hemicellulose, 63.7%; lignin, 36.9%). Analysis of the structural changes of cellulose, hemicellulose, and lignin by scanning electron microscopy let us assume that black soldier fly larval activity did leads to structural modifications of the fibers. We assume that those alterations of surface and texture are in favour of associated bacteria and thus contribute to waste reduction and larval growth. Our results show that co-conversion of DM with soybean residues by Hermetia illucens could play a vital role in DM management.