Bioenergy production using high rate, low F:M anaerobic digestion

Abstract

Any scheme to create renewable energy from waste streams will undoubtedly utilize some degree of anaerobic conversion of organics to biogas (a mixture of methane and carbon dioxide). Traditionally, anaerobic treatment has been utilized as a pretreatment step (e.g., anaerobic lagoon) or as a sludge stabilization step at the tail end of a treatment scheme. The difficulty in using anaerobic digestion as the main treatment step is threefold: anaerobic bacteria are slow growers, anaerobic bacteria are difficult to separate from the non-productive solids, and toxic and/or inhibitory conditions can cause the digestion process to deteriorate to the point of process failure. The research presented here takes a fresh look at anaerobic digestion and provides an alternate strategy that reduces the required detention time and eliminates the necessity of operating at mesophilic or thermophilic temperatures. The anaerobic treatment of municipal wastewater was successfully performed with the Static Granular Bed Reactor (SGBR). Five-day, carbonaceous biochemical oxygen demand (CBOD5) and total suspended solids were reduced to less than 30 mg/L in the effluent at HRTs of 24 hours and above and at an HRT of eight hours. Suspended solids accumulated within the reactor on top of the granule bed, and were easily wasted from a valved port. Due to the limiting rate of hydrolysis, biodegradable suspended solids were not fully metabolized, and methane recovery of chemical oxygen demand (COD) removed by the reactor was incomplete.