An evaluation of a mesophilic reactor for treating wastewater from a Zimbabwean potato-processing plant

Abstract

An evaluation of anaerobic treatment of potato-processing wastewater using an up flow Anaerobic Sludge Bed (UASB) reactor at 37°C was conducted. Wastewater from a potato-processing plant in Harare, with an average of 6.8 g COD/l, (COD = chemical oxygen demand) a high concentration of total solids (up to 6725 mg/l) and low pH of 4 - 4.9 was subjected to anaerobic treatment in an UASB reactor. The start up period, which was indicated by gas production and stabilization of COD reduction was 21 days. The organic loading rates (OLRs) investigated varied between 1.3 g COD/l/d and 13.1 g COD/l/d, the maximum methane yield was 0.3 l/g COD_removed at an organic loading rate of 6.6 g COD/l/d. An increase in OLRs to values above 13.1 g COD/ l/d resulted in digester failure. The maximum treatment efficiency (TE) in terms of COD reduction achieved was 90% at a hydraulic retention time (HRT) of 2.1 d and OLR of 3.3 g COD/l/d. The TE was reduced at higher reactor influent velocities, with a minimum TE (71%) recorded at HRT of 0.5 d. Total solids (TS) removal was inconsistent with an average reduction around 50% and phosphorus removal was poor and erratic. pH from the effluent oscillated around 7 with a notable decrease from 7.8 to 6.2 at HRT of 0.5 d. Based on these observations, the UASB process has potential to treat potato-processing wastewater as a pretreatment step before discharge into municipal sewerage system of Harare.

Key words: UASB reactor, potato wastewater, anaerobic digestion, methane yield, organic loading rate.