The Devil is in the Details: Full-Scale Optimization of the EBPR Process at the City of Las Vegas WPCF

To optimize the enhanced biological phosphorus (P) removal (EBPR) at the City of Las Vegas' Water Pollution Control Facility, the effects of dissolved oxygen (DO) level, hydraulic retention time in aeration basin and the operation mode of UCT versus A2O on phosphorus removal performance were investigated. DO levels in the range of 0.5 to 3.5 mg/L in the aeration basin did not have significant impact on effluent ortho-P concentration in a completely mixed basin within the EBPR process. Hydraulic retention time in the aeration basin was shown to affect effluent ortho-P concentration. Extending the aeration time beyond the aeration basin further reduced the ortho-P level; however, secondary P release occurred with excessive aeration timelength. Therefore, optimization of aeration time was beneficial to achieve the lowest P possible and eliminate secondary release. The UCT mode had better P removal performance than the A2O mode at LVWPCF. Bench-scale P release and uptake tests with sludge samples from the UCT process mode showed higher phosphorus removal rates and volatile fatty acids (VFAs) utilization efficiency than that from the A2O process. Measurement of P profiles in different zones within the EBPR process showed higher P release and uptake, which were consistent with bench tests results. Nitrite and nitrate levels in the anoxic zones in the A2O process were found to be higher than those in the UCT process. Quantification of poly-phosphate accumulating organisms (PAOs) in the two processes using fluorescence in situ hybridization (FISH) indicated the Rhodocyclus-like PAOs abundance in the UCT process was higher than that in the A2O process by about 17%.