Effects of Feeding Conditions on Activated Sludge Characteristics and Anaerobic Digestion

The impact of reactor configuration on activated sludge properties and its subsequent influence on anaerobic digestion has been investigated in the laboratory. Activated sludge was generated in two types of reactors that mimic plug flow reactors (PFR) and continuously fed stirred tank reactors (CSTR). Two solids retention times (SRT), 4 days and 12 days, were used to generate sludge and the feed was primary effluent from a full-scale wastewater treatment plant. Various sludge properties and effluent parameters were monitored during the study. In addition, the generated activated sludges were digested under anaerobic conditions using both batch and semicontinuous modes. The settleability, dewaterability, and effluent quality of activated sludge were slightly better for PFR systems but the difference was marginal. However, anaerobic digestion of these sludges led to substantial difference between two systems. The results from both batch and semi-continuous digestions showed that sludge generated from a PFR system digests much better than a CSTR system under otherwise identical conditions. Moreover, old sludge (12 day SRT) from a PFR system showed better volatile solids reduction efficiency compared to young sludge (4 day SRT) from a CSTR system. Currently, a reactor configuration does not play a role in determining sludge digestibility. This study, however, indicated that a reactor configuration and the resultant feeding pattern have substantial influence on the digestibility of sludge under anaerobic conditions. Extraction and protein analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) displayed different protein profiles between two activated sludge systems. Furthermore, there was similarity and difference for bacterial community between PFR and CSTR systems. These results suggest that different reactor configurations provide microenvironments that resulted in development of different microflora at both diversity and phenotype levels and it is these changes that influence sludge digestibility under anaerobic conditions. A more detailed study should be followed up to verify a critical role the activated sludge reactor plays in generating sludge more amenable to anaerobic digestion.