Impact of Solid Accumulation on Anaerobic Treatment of High Strength Molasses Ethanol Wastewater using a Two Phase Semi-Continuously Stirred Tank Reactor

Anaerobic treatment of molasses stillage generated from an ethanol production plant was investigated in a two phase semi-continuously stirred tank reactor (semi-CSTR) operated under mesophilic conditions (37°C). Molasses stillage used in this study was a highly polluted wastewater having 150-200 g L-1chemical oxygen demand (COD), 146.48 g L-1 total solid (TS) and 110 g L-1 suspend solid (VSS). Furthermore, this wastewater contained a high amount of potassium, calcium and magnesium (9.6–9.9, 5.6–6.0 and 0.9–1.5 g L-1, respectively). The two phase semi-CSTR was operated at an organic loading rate (OLR) of 1.2–3.54 kg COD m−3 day-1 and a hydraulic retention time (HRT) of 140-40 days. The results showed that the average methane yield obtained was 0.3 m3 CH4 kg-1 CODremoved with 65% CH4 in biogas. The COD removal efficiency decreased from 97% to 68% with an increase in OLR. When the system had been operated for 90 days or at OLR of 3.71 kg COD m−3 day-1 and HRT of 38 days, the accumulation of solid was observed in the reactors. The concentration of SS in the first and second phase reactors was 20 and 27.55 g L-1, respectively. These concentrations increased by 2.49 and 3.43 times, respectively, compared to the initial solid concentration of each reactor. The ratios of VSS to SS of the second phase reactor decreased significantly while those of the first phase reactor were not significant. The ratios of VSS to SS of the first and second phase reactors decreased from 0.98 to 0.92 and 0.98 to 0.68, respectively. The solid in the first phase reactor consisted of mostly SS coming from influent feed whereas that in the second phase reactor consisted of inert material. The inert material was generated by the precipitation of calcium because of a high alkalinity inside the reactor. The average concentrations of calcium in the solid of the first and second phase reactors were 136.9 and 1,712 mg kg-1, respectively. The specific methanogenic activity of the sludge in the second phase reactor decreased from 0.155 to 0.004 g CODCH4 g-1 VSS day-1. The COD removal efficiency of the system dropped to 58%. The volatile acid to alkalinity ratio of the second phase reactor was higher than 0.4.