Phosphorus metabolism regulates the trade-off between phosphorus removal and sludge reduction

Sludge reduction decreases the phosphorus export flux through waste sludge discharge, thereby increasing the risk of effluent phosphorus instability. However, the regulatory role of phosphorus in sludge reduction remains unclear. Here, side-stream phosphorus recovery was introduced into an anaerobic side-stream reactor (ASSR) based sludge reduction system to redirect phosphorus export from sludge discharge to physicochemical recovery, enabling investigation of how phosphorus flux redistribution regulates sludge reduction. Two parallel systems, a conventional ASSR system (SBR-ASSR) and an ASSR system coupled with phosphorus recovery (SBR-ASSR-PR), were comparatively evaluated using phosphorus mass balance, endogenous respiration analysis, cryptic growth modeling, and metagenomic profiling. Phosphorus recovery increased total phosphorus removal from 77.8% to 97.3% and total nitrogen removal from 72.5% to 82.1%, while reducing the observed sludge yield by 28%. Phosphorus mass balance showed that 34.7% of influent phosphorus was rerouted through the recovery pathway, reducing phosphorus discharge via waste sludge from 74.9% to 57.5%. The resulting lower system phosphorus levels restructured microbial metabolic allocation, suppressing biosynthesis while enhancing decay and substrate reutilization, with the cryptic growth contribution increasing from 35.9% to 46.9%. Metagenomic profiling corroborated this metabolic shift, revealing significant changes in key genes and pathways related to phosphorus cycling, energy maintenance, and denitrification. These findings show that phosphorus metabolism can regulate microbial growth-decay allocation, and that side-stream phosphorus recovery can coordinate nutrient removal, phosphorus recovery, and sludge minimization by restructuring internal phosphorus fluxes and microbial metabolic allocation.