Hydroxyapatite-facilitated microalgae-bacteria aggregates enable robust aeration-free nutrient removal from saline...

Microalgae-bacteria symbiotic systems offer a promising nature-based solution for wastewater treatment, yet their application is often constrained by poor biomass retention and limited phosphorus removal. In this study, an aeration-free upflow photobioreactor (UPBR) was developed to demonstrate a hydroxyapatite (HAP)-facilitated microalgae-bacteria system to simultaneously address these two constraints. The system was evaluated under realistic conditions, treating real saline domestic wastewater and considering seasonal temperature variations. Over >250 days of operation, the system achieved stable ammonium, total nitrogen, and phosphorus removal efficiencies of 99.3 ± 1.9%, 96.4 ± 6.0%, and 61.6 ± 11.9%, respectively, together with a COD removal efficiency of 65.3 ± 10.0%, consistently meeting local discharge standards. Stable nutrient removal performance was maintained at temperatures as low as 14 °C, demonstrating strong robustness against temperature stress. Pathway-decoupling batch tests and microbial community analysis consistently indicated an assimilation-dominant nitrogen transformation pathway, with minimal contribution from bacterial nitrification. Meanwhile, dispersed HAP precipitates under near-neutral pH facilitated the formation of mechanically robust, multi-nucleated aggregates with excellent settleability and structural stability. Overall, this study demonstrates that integrating chemical phosphorus removal with assimilation-dominant nitrogen transformation enables an energy-efficient and structurally stable microalgae-bacteria system for wastewater treatment under realistic conditions.