Improving Anaerobic Digester Performance: Lessons Learnt from Digester Clean-out Operations

The increasingly higher costs of fossil fuel are pushing water companies to seek alternative sources of energy such as methane from anaerobic digestion of sludges. Anaerobic digestion can be hindered by several factors including accumulation of grit and rag inside the digester. Periodical clean-out of anaerobic digesters is a common practice in the Water Industry, but very little is known on the nature of the solid material in digester contents. This paper presents the composition of grit removed from a mesophilic anaerobic digester during clean-out operations. Grit and rag accounted for 40% of digester effective capacity but it was impossible to determine the nature of the constituents in the rag due to the large size of debris. However, grit analysis showed that 64% was of inorganic nature while 36 % was organic. It was found that fine grit particles (size between 425 and 1180 μm) represented the largest proportion, accounting for 67% of the total weight of grit. Accumulation of fine grit material could be attributed to inefficient degritting of digester feed, incomplete mixing of the digester content, as well as precipitation of heavy metals inside the digester. Fe₃O₄ was the main inorganic compound in the grit accounting for 66% of the inorganic fraction of the grit, unsurprisingly as this site uses iron salts for phosphorus removal. These results suggest that optimistaion of chemical dosage in secondary treatment and adequate screening of the digester feed could substantially improve the performance of mesophilic anaerobic digesters.