Investigation of the effect of intermittent minimal mixing intensity on methane production during anaerobic digestion of dairy manure

Mixing plays an important role in maintaining the solids suspended and the fermenting liquid homogeneous throughout the digester, which helps to improve methane production. The pilot-scale digester with working volume of 1 m3 was mixed with a top-driven impeller. Three mixing intensities, 50, 100 and 150 rpm were minimally intermittent mixed once a day for 5 min under mesophilic temperature conditions (35 ± 0.3 °C) with average total solids content of 14.1% and hydraulic retention time of 30 days. The results show that 100 rpm mixing intensity outperformed the other two, indicating that mixing intensity threshold exists and beyond which methane production is negatively affected. However, 50 rpm was regarded as the economical mixing intensity even though dead zones were recorded. Minimally intermittent mixing once a day was enough to maintain anaerobic digestion process and performance efficiency for optimum methane production. Computational fluid dynamics (CFD) results agreed with the experiment, demonstrating that 100 rpm mixing intensity was sufficient to homogenize the digester content. Further, CFD was used to predict the mixing time in the digester.