The effect of PO₄^3--P concentration on sludge settleability and nutrients removal performance

In order to explore the effect of influent PO₄^3--P concentration on the relationship of phosphorus remove performance and sludge settleability, three sequencing batch reactors (SBRs) operated as anaerobic/aerobic mode were used to treat wastewater with different PO₄^3--P concentrations (same COD), and the organic loading rate (OLR) was changed through adjusting the anaerobic (aerobic) duration. The sludge settleability, nutrients removal, and microorganism species were investigated. The results showed that when the influent PO₄^3--P were 4.3 mg·L^-1 and 8.6 mg·L^-1, increasing the OLR through decreasing aerobic duration could significantly improve sludge settleability, while decreasing anaerobic duration could not. It was found that increase the OLR could promote the denitrification compete for carbon sources with phosphorus release process by inhibiting the accumulation of NO₂^--N, leading to the decrease of phosphorus removal ability. When the influent PO₄^3--P was 17.2 mg·L^-1, sufficient nitrification was beneficial to enrich denitrifying phosphorus accumulating bacteria (DPAO), and the activities of Thauera and Flavobacterium (DPAO) were stronger. Therefore, increasing influent PO₄^3--P concentration and reducing aerobic duration could help phosphorus accumulating bacteria (PAO) compete with denitrification for COD and enrich DPAO, thus reducing carbon source consumption.