Abstract
Extracellular polymeric substances (EPS) play important roles in regulating the dewaterability of sludge. This study sought to elucidate the influence of EPS on the dewaterability of sludge during bioleaching process. Results showed that, in bioleaching system with the co-inoculation of Acidithiobacillus thiooxidans TS6 and Acidithiobacillus ferrooxidans LX5 (A. t+A. f system), the capillary suction time (CST) of sludge reduced from 255.9 s to 25.45 s within 48 h, which was obviously better than the controls. The correlation analysis between sludge CST and sludge EPS revealed that the sludge EPS significantly impacted the dewaterability of sludge. Sludge CST had correlation with protein content in slime and both protein and polysaccharide contents in TB-EPS and Slime+LB+TB layers, and the decrease of protein content in slime and decreases of both protein and polysaccharide contents in TB-EPS and Slime+LB+TB layers improved sludge dewaterability during sludge bioleaching process. Moreover, the low sludge pH (2.92) and the increasing distribution of Fe in the solid phase were another two factors responsible for the improvement of sludge dewaterability during bioleaching. This study suggested that during sludge bioleaching the growth of Acidithiobacillus species resulted in the decrease of sludge pH, the increasing distribution of Fe in the solid phase, and the decrease of EPS content (mainly including protein and/or polysaccharide) in the slime, TB-EPS, and Slime+LB+TB layers, all of which are helpful for sludge dewaterability enhancement.
Highlights
Activated sludge method is an effective way of treating wastewater, but it has a serious drawback of producing huge amounts of excess waste sludge
A concept of sludge flocs stratification was introduced by Yu et al, who found that extracellular polymeric substances (EPS) in sludge flocs were consisted of slime, loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) [18]
Sludge dewaterability during sludge bioleaching It can be seen from Fig. 1a that, in the treatment inoculated with A. thiooxidans TS6 and A. ferrooxidans LX5 (A. t+A. f), the pH value of sludge decreased from 6.92 to 2.92 in the 72 h bioleaching period
Summary
Activated sludge method is an effective way of treating wastewater, but it has a serious drawback of producing huge amounts of excess waste sludge. Effective and economical methods should be developed for treating the huge amount of sludge with the aim of enhancing sludge dewaterability, which usually refers to the degree of difficulty in removing water from sludges. Some researchers found that proteins had a high waterholding capacity, and reducing protein fraction or increasing the content of polysaccharide fraction in sludge EPS could improve sludge dewaterability [15,19]. The increase of protein content or the content ratio of protein to polysaccharide (PN/PS) in slime had a negative effect on the sludge dewaterability improvement [12,13]. There are still some controversies in the relationship between sludge EPS and its dewaterability
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