The main aim of the research was to evaluate the influence of the sludge retention time (SRT) and application rate (AR) on hydrolysis and acidogenesis of primary sludge from a sewage treatment plant. The study was carry out using a lab-scale hydrolytic sludge digestor (HSD) of 1.3 L useful volume, operated in sequencing batch mode, subjected to different sludge application rates (AR), expressed as the ratio between total COD and VSS concentration applied to each batch, along different sludge retention time (SRT). The values of applied AR expressed as kg COD/kg SSV.d were 1.7, 2.1, and 2.7. The reactor performance was evaluated in terms of filtered COD (and hydrolysis rate) and VFA (and acidogenesis rate) production, for each SRT tested. VFA composition was also determined. The results allowed concluding that the hydrolysis rate was only satisfactory for the AR of 2.1 and 2.7 gCODt/gVSS.d. The optimum acidogenesis rate was obtained at the SRT of two days for all the essays applied. VFA production was directly proportional to the applied AR only for the optimum acidogenesis rate period, at SRT of two days approximately, tending to decrease for the higher values of SRT. Acetic and propionic acids were the main VFA produced by the HSD reactor.
Domestic sewage; primary sludge; hydrolysis; acidogenesis; sludge retention time
