An operational strategy for improved biomethanation of cattle-manure slurry in an unmixed, single-stage, digester

In the quiescent state, cattle-manure slurry stratifies into three discernible layers; a floating “scum” layer, a bottom “sludge” layer and a watery middle layer, with most of the biologically degradable component of the slurry being contributed by the particulate matter in these layers. The top layer has the highest biochemical methane potential and the middle layer the least. In a single-stage digester, an improved operational strategy to enhance methane production was determined. This strategy took advantage of the different rates of biomethanation and biochemical methane potentials of the different layers of the slurry. This was achieved by allowing the slurry to digest without mixing and discharging the digested material from the middle liquid layer rather than from the bottom or top layers.