Effect of solid retention time on primary sludge prefermentation in the up-flow settler/prefermenter

The study examined prefermentation of primary sludge in an up-flow settler/prefermenter model to enrich wastewater with biodegradable substrate for denitrification. The experiment, preceded by batch tests, was conducted under real conditions in a municipal wastewater treatment plant. The effect of solid retention time (SRT) was the focus while other parameters, such as hydraulic retention time and up-flow velocity, remained constant. The efficiency of chemical oxygen demand (COD) solubilisation and improvement of denitrification potential increased as the SRT increased from 1 to 5 d. At 5 d SRT, dissolved COD and denitrification potential increased by 56 mg/L and 7.6 mg.N-NOx/L, respectively, corresponding to 35% and 36% of the influent values. However, the onset of intensive biogas production occurred at SRT of 7 d, hindering sludge sedimentation and preventing achievement of a longer SRT.