Enhancement of the dewaterability of sludge during bioleaching mainly controlled by microbial quantity change and the decrease of slime extracellular polymeric substances content

Highlights

• Microbial quantity change, bound water and slime EPS contents are dominant factors.

• Contribution rate of microbial quantity change on sludge dewaterability is 32.50%.

• Contribution rate of bound water content on sludge dewaterability is 24.24%.

• Contribution rate of slime EPS content on sludge dewaterability is 22.37%.

Abstract

Contribution rates of factors controlling sludge dewaterability during bioleaching, such as sludge pH, microbial quantity, extracellular polymeric substances (EPS), etc., were investigated in this study. Results showed that the dewaterability of bioleached sludge was jointly enhanced by the growth of Acidithiobacillus sp., the increase of Fe³⁺ concentration, the decreases of sludge pH, heterotrophic microorganism quantity change, and the decreases of EPS and bound water contents. Ridge regression analysis further revealed that the contribution rates of microbial quantity change, bound water content and slime EPS content on sludge dewaterability enhancement were 32.50%, 24.24%, and 22.37%, respectively, all of which are dominant factors. Therefore, the enhancement of sludge dewaterability was mainly controlled by microbial quantity change and the decrease of bound water and slime EPS contents during bioleaching.

Introduction

The increasing demand for better water quality for human living, especially in Asian countries like China resulted in a large number of wastewater treatment plants being constructed in a short period of time, but it has also in the same time brought about an important shift in waste streams from the liquid phase to the semi-solid phase. As most municipal wastewater plants are operated by the activated sludge process, large amounts of waste activated sludge have been generated and require proper treatment before disposal. A subsequent dewatering step is usually needed to reduce the sludge volume for facilitating transport and handling, in which efficient sludge conditioning is crucial to improve sludge dewaterability and achieve a high solid content of sludge (Chen et al., 2001, Raynaud et al., 2012). However, the dewatered sludge still contains as high as 80% of moisture content when using conventional conditioning methods, such as adding organic or inorganic flocculants, followed by mechanical dewatering (Chen et al., 2001, Liu et al., 2012a). Therefore, more effective sewage sludge conditioning method should be developed to enhance the dewaterability of sewage sludge (Neyens et al., 2004, Liu et al., 2012b).

Bioleaching is reported recently as a microbial conditioning method which can improve sludge dewaterability by 4–10 times, and the moisture content of dewatered bioleached sludge cake is as low as 60% in commercial scale studies using bioleaching conditioning and diaphragm filter presses (Liu et al., 2012a, Liu et al., 2012b). During bioleaching, energy substances including Fe²⁺ and S⁰ are bio-oxidized by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, respectively, and as a result the concentration of Fe³⁺ in sludge increases rapidly and sludge pH decreases gradually (Tyagi et al., 1994, Liu et al., 2012b). In addition, it was found in previous studies that bioleaching process is effective in killing bacterial cells or lysing cells, as shown that the counts of neutrophilic bacteria decreased from an initial level of 4.33 × 10⁶ CFU/mL to below 1 × 10³ CFU/mL during the bioleaching of anaerobically digested sludge (Gu and Wong, 2007). However, the detailed mechanisms responsible for the sludge dewaterability improvement by bioleaching have not been clarified.

Previous studies have extensively studied factors influencing the dewaterability of sludge. For instances, Chen et al. (2001) and Neyens et al. (2003) reported that the dewaterability of activated sludge was improved when sludge pH decreased, and Fe³⁺ can improve the sludge dewaterability through altering small sludge particles to large flocs (Li et al., 2012). In addition, the decrease of sludge pH and the death or cell lysis of microorganisms in sludge potentially result in the change of extracellular polymeric substances (EPS) (Chen et al., 2001, Raynaud et al., 2012), which is widely considered as one of most important factors affecting sludge dewaterability (Chen et al., 2001, Liu and Fang, 2003, Neyens et al., 2004). Indeed, Houghton et al. (2001) and Bala Subramanian et al. (2010) have found that the decrease of sludge EPS content could make sludge to be more easily dewatered, and Yang and Li (2009) revealed that the EPS in sludge flocs determines the dewaterability of sludge and excessive EPS in the form of loosely bound EPS (LB-EPS) would deteriorate the sludge dewaterability and result in poor biosolid–water separation. In addition, bound water content is also an important factor influencing sludge dewatering (Vaxelaire and Cézac, 2004, Lee et al., 2006), and a lower bound water content in sludge usually means better sludge dewaterability (Lee et al., 2006).

Obviously, many factors including sludge pH, Fe³⁺ concentration, the death or cell lysis of sludge microorganisms or bioleaching bacteria, the contents of sludge EPS and bound water may jointly control sludge dewaterability during sludge bioleaching treatment, but previous studies have not studied the contribution of these factors on the sludge dewaterability enhancement during bioleaching treatment. Therefore, the objectives of the present study are to systematically investigate the changes of sludge pH, Fe³⁺ concentration, the counts of heterotrophic bacteria and the two Acidithiobacillus species, the contents of sludge EPS and bound water during sludge bioleaching treatment, and further find the dominant factors among these factors influencing the sludge dewaterability during sludge bioleaching. The outcome of the present study may be helpful in exploring the possibility of combining some physical and/or chemical techniques with bioleaching to optimize dominant factors found to further enhance sludge dewaterability in much shorter bioleaching periods.