Applicability of Ultrasound Pre-Conditioning of WAS to Reduce Foaming Potential in Mesophilic Digesters

Many anaerobic digestion facilities processing waste activated solids (WAS) or combined primary and secondary solids with a high fraction of WAS have reported operational problems associated with excessive foaming of digester contents. It is widely recognized that in many instances these problems are related to the nature and amount of WAS in the feed. This study evaluated the use of ultrasound for improving disintegration of thickened waste activated solids (TWAS) and reducing foaming potential in conventional mesophilic digesters. The volatile solids reduction and biogas production from digesters receiving sonicated and unsonicated TWAS were compared to validate reported improvements in digester performance by using ultrasound. Findings from the study indicated that the energy input during sonication has to be optimized for effective foam control. Full-stream sonication of TWAS was found to be effective in controlling foam at lower energy inputs, but increased solubilization of feed at higher sonication energy inputs exacerbated foaming problems due to higher rates of substrate degradation. Contradictory to other reported studies, there was no noticeable difference in the performance of the three digesters with regard to volatile solids reduction (VSR). Consequently, digesters fed with unsonicated TWAS achieved comparable results as digesters that received sonicated TWAS, operating at a solids retention time of 15 days. Despite minimum improvement in the ultimate degree of degradation of the substrate, sonication did enhance the rate of substrate degradation in the digesters.