Effect of solids retention time (SRT) on pilot scale anaerobic thermophilic sludge digestion

Abstract

This paper describes anaerobic thermophilic sludge digestion (55 °C) in a continuously stirred tank reactor (CSTR) on a pilot-plant scale (150 L). The experimental protocol was defined to examine the effect of the increase in the organic loading rate on the efficiency of the digester and to report on its steady-state performance. The reactor was subjected to a programme of steady-state operation over a range of solids retention times (SRTs) of 75, 40, 27, 20 and 15 days and organic loading rates (OLR) in the range 0.4–2.2 kg VS/(m 3 day). The digester was fed with raw sludge (containing approximately 35 kg/m 3 volatile solids (VS)) once daily during the 75-day SRT period, twice daily during the 40-day SRT period and three times a day during the 27-, 20- and 15-day SRT periods. The reactor was initially operated with an organic loading rate of 0.4 kg VS/(m 3 day) and an SRT of 75 days. The volatile solids removal efficiency in the reactor was found to be 73%, while the volumetric methane production rate produced in the digester reached 0.02 m 3/(m 3 day). Over a 338-day operating period, an OLR of 2.2 kg VS/(m 3 day) was achieved with 49.1% VS removal efficiency in the pilot sludge digester, at which time the volumetric methane production rate content of biogas produced in the digester reached 0.4 m 3/(m 3 day). The chemical oxygen demand (COD) mass balance obtained indicated that COD used for methane generation increased when the SRT was decreased or when the influent organic loading rate was increased. This implies that the amount of COD used in the anabolism route decreased with SRT due the microbial population becoming adapted to new operational conditions and more COD being used to generate methane.