Full-scale conversion of a wastewater treatment plant from biological to chemical phosphorus removal for vivianite formation.

This study evaluates the operational impacts and vivianite formation potential of converting a full-scale wastewater treatment plant from Enhanced Biological Phosphorus Removal (EBPR) to Chemical Phosphorus Removal (CPR) with the aim of producing vivianite-rich sludge as a prerequisite for downstream phosphorus recovery in the form of vivianite. By increasing the Fe/P molar ratio in digested sludge from 0.5 to 1.6, up to 90 % of sludge phosphorus was bound as vivianite. Although the additional iron was dosed into the primary sludge line to target vivianite formation in the digester, substantial vivianite formation may have occurred mainly in the secondary system before anaerobic digestion. The transition to CPR improved effluent quality (86 % lower P and 34 % lower COD), reduced biogas H₂S levels from 332 ppm to 1 ppm, and increased dewatered cake dry matter from 24 % to 30 %. Mass balances indicated that dosing iron into the primary thickener triggered substantial reduction of Fe³⁺ to Fe²⁺, causing iron recirculation to the influent and possibly impairing primary settling. Microbial analysis showed pronounced but reversible shifts in community structure, while Polyphosphate-Accumulating Organisms (PAOs) remained stable before and after the return to EBPR operation. For future implementation, maintaining waterline iron dosing for effluent P control while dosing only the additional iron required for vivianite formation directly into the digester may improve process stability, but this strategy requires site-specific testing.