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

Abstract

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.

Impacts of feed dilution and lower solids retention time on performance of thermal hydrolysis/anaerobic digestion

“…The reduced viscosity at lower SRTs has several advantages, including easier mixing and pumping, and a reduction in the potential for volume expansion due to gas holdup (Higgins, Beightol, Nielsen, et al., ). Higgins, Beightol, Nielsen et al.…”

“…Higgins, Beightol, Nielsen et al. () showed that as the viscosity of digestate increased, greater volumes of gas bubbles can be entrapped in the digestate, leading to an expansion in the digestate volumes. This volume expansion is thought to be the cause of numerous adverse events that were originally described as “foaming” events (Chapman & Krugel, ; Higgins, Beightol, Nielsen, et al., ).…”

“…Volume expansion due to gas holdup and foaming potential. Volume expansion potential was evaluated by turning off the mixers in the digesters and measuring the change in volume of the digestate as a function of time, typically for about 2 hours (Higgins, Beightol, Nielsen, & Murthy, 2014). The tests provide insights into the potential for volume expansion due to gas holdup.…”

“…The reduced viscosity at lower SRTs has several advantages, including easier mixing and pumping, and a reduction in the potential for volume expansion due to gas holdup (Higgins, Beightol, Nielsen, et al, 2014). Higgins, Beightol, Nielsen et al (2014) showed that as the viscosity of digestate increased, greater volumes of gas bubbles can be entrapped in the digestate, leading to an expansion in the digestate volumes. This volume expansion is thought to be the cause of numerous adverse events that were originally described as "foaming" events (Chapman & Krugel, 2011;Higgins, Beightol, Nielsen, et al, 2014).…”

“…Higgins, Beightol, Nielsen et al (2014) showed that as the viscosity of digestate increased, greater volumes of gas bubbles can be entrapped in the digestate, leading to an expansion in the digestate volumes. This volume expansion is thought to be the cause of numerous adverse events that were originally described as "foaming" events (Chapman & Krugel, 2011;Higgins, Beightol, Nielsen, et al, 2014). The worst-case scenarios for volume expansion are associated with a decrease or a loss in mixing since this increases viscosity leading to greater gas holdup (Higgins, Beightol, Nielsen, et al, 2014).…”