Millions of wastewater treatment plants (WWTPs) based on the activated sludge process have been established worldwide to help to purify wastewater. However, a vast amount of sludge is inevitably generated, and the cost of sludge disposal could reach over half of the total operation cost of a WWTP. Various sludge reduction techniques have been developed, including physicochemical, biological, and combinational methods. Micro-organisms that could reduce sludge by cryptic growth are vital to the biological approach. Currently, only limited functional bacteria have been isolated, and the lack of knowledge on the underlying mechanism hinders the technique development. Therefore, the present study is aimed at isolating sludge-reducing bacteria and optimizing the sludge reduction process through response surface methodology. Nineteen strains were obtained from sludge. The mix-cultures did not show a higher sludge reduction rate than the pure culture, which may be ascribed to the complicated interactions, such as competition and antagonistic effects. In total, 21.2% and 13.9% of total suspended and volatile suspended solids were reduced within 48 h after optimization. The three-dimensional excitation-emission matrix fluorescence spectrum and hydrolases test results revealed that the sludge reduction might be promoted by the strain mainly through hydrolysis via proteinase and amylase. The results obtained from the study demonstrate the potential of using micro-organisms for sludge reduction through cryptic growth.
Keywords: cryptic growth; excess sludge; optimization; response surface methodology; sludge reduction.