Greywater treatment in an aerobic SBR: sludge structure and kinetics

In order to have an efficient operation, sequencing batch reactors (SBR) must support granular biomass with high conversion rates, settling properties, and be able to deal with the inherent variability of the composition of real wastewaters. In this study, the effect of the influent composition and the specific organic loading rate (Bx) on the granulation process was evaluated in two SBRs, fed with greywater (GW) and a synthetic medium (SM). The feeding with SM led to the formation of compact granular biomass, with a sludge volume index (SVI) of 22.4 mL g^-1, and a zone settling velocity (ZSV) of 13.1 m h^-1. In contrast, feeding with GW induced the formation of filamentous granules, with lower settling properties (SVI = 165 mL g^-1 and ZSV = 10 m h^-1), when the system was operated at high Bx (4.4 kg COD kg VSS^-1 d^-1). However, the reduction of the average Bx to 2 kg COD kg VSS^-1 d^-1 induced an improvement in the morphology and properties of the granules (SVI = 98 mL g^-1 and ZSV = 13 m h^-1). Furthermore, the kinetic analysis indicated that granules cultivated with SM were formed by fast growing microorganisms with a high cell yield, whereas those cultivated in GW presented a much lower cell yield.