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%.
Enhanced biological phosphorus removal (EBPR) has been applied worldwide in full-scale activated sludge facilities to achieve low effluent phosphorus (P) levels. However, process reliability varies among wastewater treatment plants (WWTPs) (Neethling at al., 2005; Gu et al., 2004; Stephens et al., 2004).
The City of Las Vegas’ Water Pollution Control Facility (LVWPCF) has a capacity of 78 mgd average daily flow, and it applies a combination of enhanced biological phosphorus removal, chemical treatment, and tertiary filtration to produce effluent total phosphorus (TP) below 0.2 mg/L. A lower effluent TP limit will likely be required in the future due to increased influent loadings and potential permitted waste load allocation reductions to reduce impacts to Lake Mead and the Colorado River. An optimization study was conducted at the facility from 2004 to 2005 to evaluate both short-term and long-term strategies for complying with future effluent TP limits; optimization of existing EBPR process was recommended as part of the compliance strategy.
The WPCF has a total capacity of 91 MGD average daily flow and is composed of six treatment trains or “plants.” Figure 1 shows a process flow diagram of the existing treatment scheme. Raw sewage is received and treated in a common screening and grit removal facility, after which the flow stream is split to six treatment plants. Plants 1 through 4 have a similar treatment scheme, each consisting of primary clarifiers followed by trickling filters and secondary clarifiers. Plants 1 and 2 are identical and have a capacity of 18 MGD each. Plants 3 and 4 have a capacity of 12.5 MGD each. Effluent from Plants 1 through 4 is combined and is then treated in nitrification aeration basins followed by final clarifiers. Plants 5 and 6 are called EBPR plants, which consist of common primary clarification followed by four separate trains of activated sludge treatment that provide biological nutrient removal of phosphorus and ammonia. Secondary effluent from Plants 1 through 6 is combined, filtered, and disinfected using sodium hypochlorite. Solids treatment consists of gravity thickening of primary sludge, centrifuge thickening of waste activated sludge, anaerobic digestion of combined primary and waste activated sludges, centrifuge dewatering of digested sludge, and off-haul of dewatered sludge to landfill. Primary sludge fermenters are used to produce addition VFAs for EBPR plants.