Reasons for Ineffectiveness in Improving Dewaterability of Anaerobically Digested Sludge by Bioleaching

The use of bioleaching for anaerobically digested sludge (ADS) was found to be ineffective compared to using it for undigested sludge (UDS) for reasons elucidated in this study. Results showed that specific resistance to filtration of ADS increased during bioleaching. The pH value of ADS increased to 7.97 and remained unchanged during bioleaching, while it decreased to 2.98 for UDS. Added Fe2+ was not detected as the energy source for ADS. Higher alkalinity and unavailable Fe2+ in ADS prevented the growth of the Acidithiobacillus species. It was found that sludge pH increased to 8.40 and then stayed within an alkaline range, whereas slime EPS content rapidly increased to 8.13 mg DOC/g VSS. These results indicated that aeration seriously deteriorated the dewaterability of ADS through bioleaching due to the unexpected drastic increase of sludge pH and slime EPS content.