Insights into Rapid Volume Expansion in Anaerobic Digesters Associated with Gas Holdup

A research study was performed to evaluate the rapid rise/volume expansion concept proposed by Chapman and Krugel (2011). The concept relates the interplay of mixing, gas production and rheology of the solids and how they will impact volume expansion due to gas holdup. Gas holdup is defined as the production of gas bubbles that do not rise out of solution and instead contribute to the expansion of the solids volume. This research demonstrated that loss of mixing can increase the digester volume by 20% or more, and this was accompanied by a decrease in gas production rates measured by respirometer. The decrease in gas production was a good prediction of the subsequent gas holdup expansion. Volume expansion also occurred when mixing was decreased and not just halted completely. The results suggested a critical mixing or shear rate exists, below which the viscosity of the suspension increases rapidly, resulting in gas holdup and volume expansion. Additional research is needed to further evaluate the many variables that will impact volume expansion due to gas holdup and foaming.