Feasibility of using primary-sludge mesophilic hydrolysis for biological removal of nitrogen and phosphorus from wastewater

This work was suggested by the general lack of information regarding practical design aspects of a process widely studied at both theoretical and experimental levels, primary-sludge hydrolysis. The availability of sufficient amounts of readily biodegradable organic substrates is a crucial aspect of biological nutrient-removal processes. These substrates can be provided to biomasses either by dosing chemicals, or by exploiting internal carbon sources of the system. Hydrolysis allows for the conversion of particulate and soluble, slowly degradable fractions of primary sludge into readily biodegradable substrates, such as short-chain fatty acids. In order to evaluate the technical and economical feasibility of the process under mesophilic conditions, the general case of a medium-size wastewater treatment plant (100 000 PE) was considered. Given a typical domestic sewage, the production of readily biodegradable COD (RBCOD) via primary sludge hydrolysis was assessed under two different conditions. The effects of hydrolysate addition on nutrient removal efficiencies were also evaluated. Results show potential substrate productions of about 250–600 kgRBCOD day−1 depending on process conditions. Denitrification efficiency after addition of this internal carbon source was found to increase by 4–10% with respect to the influent total nitrogen. Cost is competitive with respect to current Italian costs for reagents.