Removal of Heavy Metals and Patho- Gens during Biphasic Fermentation of Solid Wastes

Research was conducted to study the effects of biphasic anaerobic fermentation upon an organic fraction of municipal solid waste, and to observe the fate of heavy metals and pathogenic organisms in the acidogenic and methanogenic environments of the biphasic process. The high-solids (16 wt % solids) substrate was packed in a bench-scale, anaerobic, solid-state fermenter with amounts of lead nitrate and cadmium chloride which corresponded to limits for these metals as per EPA 503 Regulations for biosolids (840 mg Pb/kg TS and 85 mg Cd/kg TS). Solid-state, hydrolytic-acidogenic fermentation was induced by an anaerobic digester innoculum; recirculation of the percolated culture (leachate) from bottom to the top of the solid bed accelerated the production of volatile fatty acids (VFAs) which reached concentrations of 18,000 mg/L at an ambient temperature of 23 C. Solid-state fermentation was characterized by a low culture pH of 4 – 5, production of soluble COD, and generation of high-CO2 head gases. Leachate VFAs were recycled to a methane fermenter where they were gasified to methane (75-80 mol%). Dissolved lead and cadmium concentrations were significantly reduced during two-phase operation. Destruction of fecal coliforms was observed in nine days of acid-phase operation, which suggests that digested solids may comply with EPA 503 Regulations for pathogens and metals.