Effect of SRT on Nutrient Removal in SBR System

Abstract

The effect of solids retention time (SRT) on chemical oxygen demand (COD) degradation and nutrients removal was studied in a sequencing batch reactor (SBR). SRT of 10, 15, 20, 25, and 30 days was studied. A laboratory reactor of 6.6 L working volume with an 8-h-cycle time was used. In order to maintain relatively constant feed composition, a synthetic wastewater was employed. COD removal was unaffected by the variation of SRT (average removal of 83.6–94.4% for COD and total organic carbon (TOC). Removal of TKN and ammonium varied from 86.8 to 97.2% with SRT variation of 10–30 days. Phosphorus removal was strongly influenced by the SRT variation. Maximum phosphorus removal was observed between 20 and 25 days SRT (average of 90.6 and 97.2% Ptotal removal, respectively). P removal decreased to 68.7% at 10 and 15 days SRT and was 44–47.3% (ortho-P and Ptotal) at 30 days SRT. Maximum content of P in sludge solids was observed at 20 days SRT (6.2% g P/g VSS). In an SBR cycle, the P released in anaerobic phase was also maximum (62 mg/L) at 20 d SRT. The fraction of phosphorus in different phases (precipitated, adsorbed, P-organic, poly-P) was also determined for one cycle at each SRT studied.