Effect of Iron-Containing Phosphorus Removal Agents on Organics Dissolution and Anaerobic Digestion of Waste Activated Sludge.

Widespread chemical-enhanced phosphorus removal treatments usually result in waste activated sludge (WAS) containing excess phosphorus removal agents and byproducts. This study investigated four wastewater treatment plants (WWTPs) in China and took FeCl₃ as a representative agent to explore its effect on WAS organics dissolution and anaerobic digestion (AD). The results revealed a biphasic effect of Fe³⁺ on WAS: Disintegration dominated at low concentrations (e.g., 50-100 mg/L), and coagulation dominated at high concentrations (e.g., 1000 mg/L). High-concentration Fe³⁺ inhibited the AD performance by disrupting the system (pH and ORP), resulting in system acidification. Conversely, low-concentration Fe³⁺ promoted AD through synthetic effects (enhanced microbial activity and key gene expression, activated the DIR process, and mitigated acid accumulation), achieving maximum methane production at 50-mg Fe³⁺/L. This study enhances understanding of the effect of iron-containing phosphorus removal agents on the AD of WAS and recommends a staged dosing strategy for WWTPs to optimize performance.