Design considerations on primary sludge hydrolysis under psychrophilic conditions

Wastewater readily-biodegradable COD concentration is a critical parameter affecting biological nutrient removal efficiencies. Primary sludge hydrolysis has been recognized as a viable alternative to increase suspended solids degradability and provide biomasses with an extra load of available substrate. However, fermentation performance strongly depends on both feed sludge characteristics and process conditions. The aim of this paper is to evaluate the applicability of the process to real wastewater treatment plants operating under psychrophilic conditions. After a short review of previous research, some results of an experimental work are reported in order to explain some of the assumptions made. Then, a simple design procedure for primary fermentation systems is outlined and applied to a general situation. Results indicate that primary sludge hydrolysis at low temperatures (16-20°C) and recirculation of the separated liquid supernatant allows for little improvements in denitrification efficiencies. Maximum increase in nitrate reduction was estimated to be less than 3% as referred to the total influent nitrogen.