Synergizing carbon and phosphorus recovery from wastewater: Integrating biofilm-based phosphorus removal in high-rate activated sludge

High-rate activated sludge operated at <2 days biomass age enhances carbon recovery from wastewater, but simultaneous biological recovery of phosphorus remains unachieved. Addressing the reported loss of phosphorus accumulating organisms (PAO) at such short biomass ages, this study investigated the integration of moving bed biofilms into high-rate activated sludge to enhance PAO retention. The results demonstrated sustained biofilm-based PAO activity and complete orthoP removal under short anaerobic-aerobic cycles with a hydraulic retention time of 2.7 h matching high-rate conditions. When combined with high-rate activated sludge in a sequencing batch reactor fed with acetate, complete orthoP removal was sustained. However, using synthetic wastewater promoted the growth of competing heterotrophic bacteria, reducing orthoP removal to 50-65 %. Biofilms served as a continuous source of PAO for the suspended biomass, which contributed to 46-55 % of the overall orthoP removal, even below 2 days biomass age. While acetate-fed microbial communities included known PAOs, using complex feed shifted the community toward less understood putative PAOs. Competition for acetate was likely compensated by a high fermentability of high-rate activated sludge, as PAO activity was maintained while reducing the acetate load in the feed from 20:1 to 5:1 g acetate⋅g P^-1. P release and uptake rates were accurately described by the biomass-specific acetate loading rate and the depletion of intracellular polyphosphate, respectively, providing predictive relationships for process optimization. Imposing an anaerobic-aerobic regime enhanced the carbon recovery of high-rate activated sludge from about 37 to 60 %. Integrating biofilms enabled efficient phosphorus removal while maintaining carbon recovery rates of 41-53 %, highlighting the synergistic benefits of this approach.