Optimization of a full-scale Unitank wastewater treatment plant for biological phosphorus removal

The Unitank process combines the advantages of traditional continuous-flow activated sludge processes and sequencing batch reactors, and has been extensively employed in many wastewater treatment plants (WWTPs) in China. Biological phosphorus removal (BPR) of a full-scale Unitank WWTP was optimized by increasing anaerobic time from 80 to 120 min in an operation cycle of 360 min and reducing solid retention time (SRT) from 21.3 to 13.1 d. The BPR efficiency of the full-scale Unitank system increased from 63.8% (SRT of 21.3 d) to 83.2% for a SRT of 13.1 d. When the anaerobic time increased from 80 to 120 min, the net anaerobic phosphorus release amount increased from 0.25 to 1.06 mg L ^-1, and sludge phosphorus content rose from 13.8 to 15.0 mgP·(gSS)^-1. During half an operation cycle, the average specific phosphorus release rate increased from 0.097 mgP·(gVSS·h) ^-1 in 0-40 min to 0.825 mgP·(gVSS·h)^-1 in 40-60 min. Reducing SRT and increasing anaerobic time account for 84.6% and 15.4% in the total increment of phosphorus removal of 1.15 mg L^-1.