Effects of short-term temperature increases on the mesophilic anaerobic breakdown of sulfate containing synthetic wastewater

Abstract

A study was carried out to assess the effect of short-term temperature increases on the performance of a mesophilic (30°C) UASB reactor treating sulfate containing wastewater, with respect to process stability, process recovery and particularly the competition between sulfate-reducing and methanogenic and acetogenic bacteria. The reactor was fed with a synthetic medium consisting of acetate, propionate, butyrate and sulfate. Three temperature shocks in the range from 45 to 65°C during an 8–9 h period were applied. A temperature shock of 45°C did not show any detrimental effect. However, temperature shocks to 55 and 65°C gave a serious drop in the treatment efficiency. The exposure to 55°C is more detrimental for propionate and butyrate oxidation than for acetate oxidation. The exposure to 65°C is similarly detrimental for acetate, propionate and butyrate degradation. Recovery after both the 55 and 65°C temperature shock is relatively fast for propionate oxidation, butyrate oxidation and sulfate reduction, but slow for acetate oxidation and methanogenesis. Prior to imposing the temperature shocks all hydrogen was oxidized by sulfate-reducing bacteria whereas acetate was mainly oxidized by methanogens. After exposure to 65°C a shift was found with respect to acetate utilization by methanogens and sulfate reducers, leading to an apparent steady state situation in which 40 and 60% of the removed acetate was oxidized by respectively sulfate-reducing and methanogenic bacteria. In additional batch experiments conducted at 55°C, a decay rate of 2.22, 1.93, 1.75 and 1.33 h −1 for respectively propionate oxidizers, butyrate oxidizers, methanogenic and sulfate-reducing bacteria was found. At 65°C the decay rate exceeds 10 h −1 for all bacteria involved in the conversion reactions.