Sedimentation and biological phosphorus removal in a bottom-fed SBR

The purpose of this work was to examine the settling properties and the biological phosphorus removal of the sludge from a bottom-fed SBR (bf-SBR) and make a comparison with the sludges from a conventional SBR (c-SBR), a conventional activated sludge system (CAS) and a conventional activated sludge system with flotation (CAS-f). Bottom-fed SBR has the unique characteristics to be fed uniformly from the bottom through the sludge bed during the sedimentation phase while decantation and sedimentation occur simultaneously. Concentrations of the examined sludge varied between 3.5 and 5.0 kgSS m-3; analyses of the sludge comprised: sludge concentration, settling rate, sludge volume index, P release and uptake. The study revealed very good settling properties of the sludge of the bottom-fed SBR, superior to both conventional activated sludge as well as conventional SBR reactors. It presents lower SVI (70 – 100 ml g-1 SS) and higher settling rates for the whole considered period, and did not feature the typically low sludge settleability observed during the cold season in the CAS. Laboratory analyses showed that both SBRs have good ability to remove phosphorus operating at typical hydraulic retention times; batch experiments showed that this ability can be further improved with optimized operation condition, leaving the question open on whether bottom feeding does allow better bio-P performance compared to conventional SBR.