Increasing Biological Phosphorus Removal: Case Study at the Colony, Texas

The City of The Colony, located on Lewisville Lake in North Texas, faces a future phosphorus discharge limit of 1 mg/L. Recent upgrades to the wastewater treatment plant (WWTP) included features for biological nutrient removal. Historical data indicated that the facility had the potential to achieve the 1 mgP/L discharge limit. Field sampling and process evaluation were conducted to determine the limiting factors of enhanced biological phosphorus removal (EBPR) at the facility. Operation as an anoxic-anaerobicoxic (A-A-O) process resulted in high levels of nitrate and oxygen in the return activated sludge, producing elevated oxygen reduction potentials (ORPs) in the first anaerobic zone and limiting phosphorus release. Operation as a Modified Johannesburg process produced ideal ORP conditions in both anaerobic zones, but phosphorus removal below 1 mgP/L was not achieved. Influent characterization indicated that a relatively low readily biodegradable chemical oxygen demand (rbCOD) was present in the influent relative to the influent phosphorus, limiting the amount of volatile fatty acids available for phosphate accumulating organisms (PAOs) in the anaerobic zone. Future improvements to limit the nitrate and oxygen in the RAS as well as to supplement the rbCOD content in the anaerobic zones would increase the potential for EBPR at The Colony WWTP.