The effect of zero-valent iron/Fe3+ coupling and reuse on the properties of anoxic sludge

Abstract

Mud-water separation efficiency and removal efficiencies of pollutants are relatively low in conventional activated sludge processes. In this study, zero-valent iron (ZVI) and Fe3+ were added into an activated sludge system and combined to improve sludge flocculation and effluent quality in an anoxic zone. The effects of ZVI/Fe3+ on removal efficiencies of nitrogen and phosphorus were investigated and the contact angle, Zeta potential and particle size of the sludge were analyzed. Changes in the ZVI crystal structure, element valence and corrosion rate were also explored. The results suggest that the addition of ZVI improved the removal efficiencies of chemical oxygen demand (COD) and total phosphorus (TP) by enhancing the electron transfer rate. Removal efficiencies of COD and TP were highest (72.66% and 98.66%, respectively) following the addition of 10 g/L of ZVI and 10 mg/L of Fe3+. Nitrate nitrogen (NO3−-N) was removed by ZVI/Fe3+ under biological and chemical action and its removal efficiency was directly proportional to ZVI content. Sludge flocculation after the addition of Fe3+ alone was better than that after the addition of ZVI alone. The combined action of ZVI/Fe3+ significantly increased particle size, Zeta potential and hydrophilicity of the sludge. Electron transfer was accelerated by the addition of 30 g/L of ZVI and 10 mg/L of Fe3+. More than 90% of the ZVI could be recovered, although flake and needle-like corrosion products existed on the surface of the recovered ZVI. The specific surface area of recovered ZVI increased and it could still effectively remove pollutants even when reused three times.