Improvement of anaerobic degradation of sludge using enzymatic treatment

A significant amount of sludge has been produced worldwide annually either from municipal and industrial wastewaters or drinking water by physical/chemical, biological treatment processes. Sludges should be treated in accordance with the environmental and health criteria before final disposal. Depending on the sources, sludge characteristics can widely vary. Although processing of the sludges depends on the characteristics and quantities, the core methods used for processing are thickening, stabilization, conditioning, dewatering, and final disposal- incineration, land application (Spinosa et al., 2001). Sludge stabilization is an important issue in sludge management field for effective reduction of organic matter, removal of pathogen and odor potential. For this purpose, alkaline stabilization, aerobic and anaerobic stabilization, aerobic thermophilic digestion, and composting are introduced. Among the methods, anaerobic digestion has been widely used with its many advantages including efficient degradation of biodegradable particulate organic matters in sludge, and efficient biogas production. Clear of the positive effects, bioprocessing of activated sludge either aerobically or anaerobically leads to some negative effects to the sludge like deprived dewaterability characteristics (Ayol et al., 2007a). The anaerobic sludge stabilization process is very sensitive to environmental conditions as well as waste characteristics since the processes undergo rich microbial consortia. Previous studies (e.g. Novak et al., 2003; Ayol, 2005) have examined the mechanisms of floc destruction during the bioprocessing of the sludge in laboratory scale studies. Concluding remark from the studies is that a flocculated matrix of extracellular polymeric substances (EPS) bridging with bacteria holds back the dewaterability properties of the bioprocessed sludges. The hydrolysis of complex organic structures in the degradation of biodegradable particulate organic matter heavily depends on the hydrolytic enzymes like glucosidases, lipases, and proteases (Ayol et al., 2007a). v Some selected hydrolytic enzymes act on a particular substrate ? biopolymer present in the sludge, break down it and release products with lower molecular weight into solution. Ultimately, depending on the floc structure break-up, important amounts of proteins, peptides and carbohydrates are released (Dey et al., 2006; Watson et al., 2004). Based on the recent findings, this research study has examined the effects of hydrolytic enzymes on dewaterability of sludges. Experimental studies have used three anaerobic reactors operated with activated sludge as feed material. Reactor performances with and without enzyme additions were regularly monitored in terms of organic matter reduction, methane and biogas productions, and dewaterability properties at two different operation time as 15 and 25 days. For enzyme additions, two hydrolytic enzymes were used. Initial part of the pilot?scale experiments, the reactors were batchly operated at 25 days at 37 ± 3 °C. Depending on the first part?s results, operation time was changed as 15 days. Experimental results showed that the enzyme additions improved floc disintegration. Enzyme added reactors indicated better performance in terms of organic matter stabilization and gas production when comparing with the control reactor. The protein concentrations in EPS solutions were significantly higher than the polysaccharide concentrations under anaerobic conditions. This thesis as part of a broader project examining the overall fate and effects of hydrolytic enzymes in biological sludge stabilization presents recent research results to determine the floc disintegration mechanisms by enzymes during anaerobic digestion of sludge. Keywords: Activated sludge, anaerobic digestion, sludge disintegration, biological hydrolysis, enzymatic treatment, extracellular polymeric substances, dewaterability.