Microbial electrolysis followed by chemical precipitation for effective nutrients recovery from digested sludge centrate in WWTPs

A two-step sidestream process was investigated for nitrogen (N) and phosphorus (P) recovery from digested sludge centrate. In the first step, a dual-chamber microbial electrolysis cell (MEC) was used for N recovery on the cathode. In the second step, P was recovered as solid precipitates by the addition of Ca²⁺ or Mg²⁺ salts in the anodic effluent. The operation of MEC with centrate indicate that N transport from the anode to the cathode chamber is primarily driven by anodic electron transport rather than diffusional transport. Low concentration of readily biodegradable organics in centrate significantly hindered current density (<0.15 A/m²) and led to trivial N recovery on the cathode chamber. The addition of primary sludge fermentation liquor (25 vol%) with centrate as an exogenous source of readily biodegradable organics substantially increased current density up to 6.4 A/m², along with high TAN removal efficiency of 53 ± 5%. The energy requirement was calculated at 5.8 ± 0.1 kWh/kg-TAN; however, the recovered H₂ gas from the cathode was adequate to offset this energy input completely. The addition of Ca²⁺ salt at a Ca:P molar ratio of 3:1 was optimum for P recovery from the anodic effluent; Mg:P molar ratio of 2:1 was found to be optimum for Mg²⁺ salt addition. However, optimum doses of both salts resulted in maximum P recovery efficiency of ∼85%, while Mg²⁺ addition provided an additional 38% TAN removal. These results demonstrate that microbial electrolysis followed by chemical precipitation can promote sustainable nutrients recovery from centrate at municipal wastewater treatment plants where sludge fermentation has already been adopted to provide readily biodegradable carbon source in the biological nutrient removal process.