Molecular Investigation of Aerobic Granule Sludge Formation

MOLECULAR INVESTIGATION OF AEROBIC GRANULAR SLUDGE FORMATION Abstract by Belinda Sue McSwain The efficiency of biological wastewater treatment depends upon the selection and growth of metabolically capable microorganisms and upon the efficient separation of those organisms from the treated effluent. Researchers have developed operational guidelines and microbial selection theories in order to design reactors that form fast-settling flocs and select against filamentous bulking sludge. In recent years, the Sequencing Batch Reactor (SBR) has been used to form even more compact sludge structures, in the form of aerobic granules. Granules can be described as a collection of self-immobilized cells into a spherical form. This special case of biofilm growth occurs without the addition of carrier material. Granular sludge have a wide range of beneficial properties compared to activated sludge flocs, most notably their strong structure and good settling property. A brief review of both anaerobic and aerobic granulation shows that a variety of microbial species are able to form granules, leading researchers to hypothesize that granulation is not a function of microbiological groups but of reactor operating conditions. To date, the operational factors controlling aerobic granulation are unclear, although granulation is mainly reported in SBRs. In this work, a series of completely aerobic SBRs were operated to form granules, and microbial investigations were conducted to correlate the reactor operation with the sludge structure at the macro- and microscale. The results are divided into 3 sections: kinetic selection, physico-chemical selection, and reproducibility and maintenance. First, by manipulating the SBR operation and substrate concentrations, differences in substrate removal are shown to influence microbial selection and the sludge structure. Second, settling times influence species selection and the composition of extracellular polymeric substances to aid granule formation, and high shear is important to provide a high dissolved oxygen in the reactor, which is necessary. Third, granule formation is reproducible using different inoculum sludges and stable over long times. In the discussion section, results are correlated with the kinetic and metabolic selection theories for bulking sludge and other investigations using anoxic and aerobic granules. Finally, an overall hypothesis for granule formation is presented. Kinetics, species selection, and short settling times are the determinant factors of aerobic granule formation.