Enzyme treatment to decrease solids and improve digestion of primary sewage sludge

Abstract

The aim of anaerobic digestion of primary sewage sludge is to convert the carbonaceous material contained in the solids into methane and carbon dioxide. The products of digestion are therefore gases, stabilised sludge solids which are subsequently dewatered and disposed of, and sludge liquor which is generally further treated. This investigation assessed the impact of addition of hydrolytic enzymes to anaerobic digesters. Cellulase and pronase E were added singly and in combination, and it was found that the mixture of the two enzymes resulted in an 80% reduction in solids (cf. 20% in the control), 93% removal of particulate chemical oxygen demand (COD) (59% in the control) and 97% total COD removal (vs. 63%). The total suspended solids (TSS) concentration was reduced by 80%, from 25 g/l to 5 g/l. Single enzymes had little or no impact on sludge solubilisation, and final COD and TSS, but all of the enzyme additions were seen to decrease the production of volatile fatty acids (VFAs). Since accumulation of VFAs can lead to digester failure, it was concluded that the enzyme additives enhanced digester performance in terms of degradation of COD, reduction in sludge solids remaining after digestion and improved digester stability owing to the stable prevailing pH. The results indicate that enzyme addition at full scale could be expected to lead to greater methane yields, lower strength sludge liquors and a significant reduction in the requirements for and costs of digested sludge dewatering and disposal.