Effect of mixing on biomethanation of cattle-manure slurry

The benefits and extent of mixing required during biomethanation of cattle-manure slurry was studied by investigating the effect of: 1) continuous and intermittent mixing, 2) agitator impeller speed and position; 3) not providing assisted mixing; 4) mixing on production of extracellular polymeric substances; and 5) mixing on the ultimate anaerobic biodegradability. Biomethanation was not adversely affected: during intermittent mixing; or when only sufficient mixing was provided to maintain off-bottom suspension of digester contents; or by doubling impeller speed. In fact continuous digestion of cattle-manure slurry without mechanical stirring was superior in terms of gas production. This can be attributed to increased loss of active volatile solids during stirring. Moreover, long-term batch digestion studies showed that the rate of biomethanation in a continuously stirred digester was inferior to that of a non-stirred one. Mixing was found to decrease production of extracellular polymeric substances (EPS). The presence of an increased level of EPS during the quiescent state could indicate increased attachment of cells to each other, resulting in larger agglomerates with better settling properties thus increasing biomass retention time.