Interactions and coupling mechanisms between Anammox, partial denitrification (PD) and hydroxyapatite (HAP) biomineralization in a PD/A-HAP system

[Display omitted] •Single-stage PD/A-HAP process was successfully started up for the first time.•Effective nitrogen and phosphorus removal was achieved in the PD/A-HAP system.•Reconciling biomass and HAP growth is essential for effective mass transfer.•Biofilm-encapsulated HAP granular sludge exhibits superior settleability. In this study, hydroxyapatite (HAP) crystallization was introduced into a partial denitrification/Anammox (PD/A) process to develop PD/A-HAP granules and achieve simultaneous removal of nitrogen and phosphorus from wastewater. During the formation of PD/A-HAP granules, an unbalanced relationship between the biomass growth and phosphorus crystallization was observed, which resulted in a mass transfer inhibition inside the system. PD and Anammox activities were both impacted by the impeded nutrient contact, and PD process was revealed to be more susceptible to inhibition. Through the regulation of COD and NO3–-N, a dynamic balance between microbial proliferation and phosphorus crystallization was achieved. As a result, high nitrogen removal efficiency (NRE) was attained at 97.8±1.2 % in the final stage of operation, accompanied by a phosphorus removal efficiency (PRE) of 59.4±2.9 %. Furthermore, the recovered phosphorus was transformed into HAP crystals and encapsulated by the PD/A biomass to form granules, which greatly promoted the settling performance of the sludge. This study underlines the importance of reconciling biomass and HAP growth for a strengthened PD/A granular sludge and an enhanced nitrogen removal system. The constructed PD/A-HAP process will expand the application of PD/A with its ability to remove phosphorus while efficiently removing nitrogen.