Converting digested residue eluted from dry anaerobic digestion of distilled grain waste into value-added fertilizer by aerobic composting

Biogas can be produced from distilled grain waste by dry anaerobic digestion, however, digested residue was eluted and needs further treatment. To develop a zero-emission process and achieve efficient resource recovery, the compostability of the digested residue was studied. Composting of the digested residue by adding a mature compost and bulking agent was performed using a lab-scale composting reactor. The composting process showed that the degradation of organic matters was mainly occurred during the first 30 days, and the final volatile solid (VS) degradation efficiency was about 15.2%. The digested residue was not suitable as a fertilizer as revealed by the lower germination index (GI) value. However, the composting process produced remarkable results; the physico-chemical parameters indicated that a highly mature end product was obtained 65 days after beginning the composting process. The final nitrogen content, NO3− concentration, and GI value in the final product were 1.7%, 11,128.8 mg·kg−1-TS, and 110.0%, respectively. A highly statically significant correlation was found among the GI, pH, soluble total organic carbon (S-TOC), inorganic carbon (IC), and concentrations of NH4+ and NO3−. The numbers of ammonia oxidizing bacteria (AOB) and archaea (AOA) decreased during the first 12 days, but significant increases occurred during the maturation phase, indicating that AOB and AOA were responsible for nitrification during the composting of digested residue.