The Effect of Mechanical Shear on Mesophilic Anaerobic Digestion

A combination of bench scale and full-scale studies were conducted to determine the effectiveness of high intensity mechanical shear in an internal recycle loop to enhance mesophilic anaerobic digestion and its implications for routine operations of a digestion system. During short-term batch studies (56-hours) a 46% increase in biogas production was observed. In long term batch studies (20-days) there was an observed 15% increase in biogas production and increased volatile solids destruction. The observed increase in digestion efficiency was supported by a full-scale demonstration, in which increased total and volatile solids destruction was observed, 22% and 21% for the primary digester and 17.2% and 11% for the secondary digester respectively. The data also suggests that protein degradation is one of the major mechanisms associated with the observed increases in volatile solids destruction. The full-scale demonstration also determined that shear enhanced digestion can be operated without process upset, based on volatile fatty acid profile and headspace biogas composition (methane and carbon dioxide). Dewatering properties as measured by polymer demand were deteriorated in the primary digester while there was an apparent improvement in the secondary digester. High intensity shear does not appear to enhance pathogen reduction based on indicator organisms, total and fecal coliforms.