Implementation of an excess sludge reduction step in an activated sludge process

In this study, excess sewage sludge reduction resulting from the modification of an activated sludge process by incorporation of an excess sludge digesting reactor (ESDR) was examined. The ESDR was coupled to the sludge return line, and enhanced the solubilization of cell mass under thermophilic aerobic conditions. The decrease in the level of total suspended solids (TSS) observed in the reference ESDR (without thermophilic microbial inoculation) was 13.76% whereas a TSS decrease of 32.09% was achieved by the test ESDR (with thermophiles), thus showing microbial enhancement of solubilization of 18.33% over a test period of 48 h. The average excess sludge solubilization ratios (beta values) of TSS were 51.17% and 41.56% in two distinct protocols varying in operative parameters. The calculated excess sludge reduction ratio was 49.60% with a sludge recirculation ratio of 2, but increased to 68.97% when the sludge recirculation ratio rose to 3. The sludge volume indexes (SVIs) for the control and test processes were 68.4 and 57.0 respectively, indicating the absence of any negative effect of the modification on sludge settling characteristics. Effluent water quality satisfied national legislative requirements.