Extracellular polymeric substances and bound water drastically affect bioleached sludge dewaterability at low temperature

Abstract

It is well documented that bioleaching with the co-inoculation of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans can drastically enhance sludge dewaterability under ambient temperature condition, but little information on low temperature effect on bioleached sludge dewaterability is available. In this study, the optimum sludge retention time of bioleaching treatment and the mechanisms responsible for the dewaterability enhancement of bioleached sludge were studied in two bioleaching systems conducted at a low temperature of 10°C and a normal temperature of 28°C, respectively. Results showed that Acidithiobacillus bacteria involved in the bioleaching were still active at 10°C, and sludge capillary suction time (CST) decreased in the first 3 days of bioleaching. Nevertheless, the degree of sludge dewaterability enhancement by bioleaching was much lower at 10°C than that at 28°C. The only slight decreases in the sludge pH value and relative high contents of extracellular polymeric substances (EPS) and bound water at a low temperature were responsible for the relatively lower dewaterability enhancement of bioleached sludge compared to that at the normal temperature. Moreover, statistical results showed that sludge CST had significant positive correlations with the sludge pH value, and EPS and bound water contents in the optimum bioleaching process (from day 0 to day 3). Therefore, reducing medium pH, and/or removing EPS from bioleaching bacteria and bound water of bioleached sludge through forced physical–chemical approaches prior to mechanical dewatering might be a good method to greatly improve sludge dewatering at a low temperature.