Integrating BiologicalPhosphorus Removal with High-RateActivated Sludge for Enhanced Settleability and Nutrient Managementat Short Solids Retention Times

High-rate activated sludge (HRAS) processes operate at reduced hydraulic retention time and solids retention time (SRT) to minimize mineralization and enhance sludge digestibility. Enhanced biological phosphorus removal (EBPR) employs phosphorus accumulating organisms (PAOs) to uptake soluble phosphorus from wastewater, preventing nutrient pollution. However, the slow growth rates of PAOs relative to the aggressive SRTs (<2 days) commonly used in HRAS present a potential conflict. This study aims to determine the feasible minimum aerobic SRT that maintains biological phosphorus (bio-P) removal, quantify phosphorus removal through biomass assimilation and bio-P pathways, and assess the impact of bio-P selection on HRAS sludge settleability. Two parallel bioreactors were operated for 246 days with real wastewater supplemented with acetate and phosphate to ensure a consistent feed source; one system was operated as an HRAS without EBPR and the other as an integrated HRAS and EBPR. Significantly, integrating EBPR with HRAS improved sludge settleability, leading to an enhancement in carbon capture. In continuous operation, bio-P performance deteriorated at aerobic SRT below 1.9 days and was strongly influenced by the influent’s volatile fatty acid to phosphorus ratio. Interestingly, Bio-P activity tests demonstrate the feasibility of integrating EBPR with HRAS at aerobic SRT as low as 1.1 days. These results highlight the cobenefits of EBPR integration, including enhanced phosphorus removal, carbon redirection, and settleability, underscoring the high potential for resource recovery from wastewater streams.