Electrocoagulation for phosphorus recovery from anaerobic effluent

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University of New Brunswick
A batch monopolar electrocoagulation system was developed to compare the performance of iron and magnesium electrocoagulation systems for phosphorus recovery from anaerobic bioreactor effluent and to understand the effects of parameters such as initial pH, retention time, current density, inter-electrode distance, as well as their interactions. For iron electrocoagulation system, removal of 98.05 % total phosphorus (TP) was observed at optimal operating conditions (pH=6.75, retention time=11.06 min, current density=300 A/m2, inter-electrode distance=1.5 cm). A kinetic study for TP removal revealed that at optimal conditions, phosphorus removal followed first-order kinetics (rate constant(K)=0.185 m/min). Phosphorus was recovered from the post-precipitated iron electrocoagulation sludge through combustion followed by acid leaching. Acid leaching tests using sulphuric acid resulted in around 91% of phosphorus recovery at a liquid-to-solid ratio of 100 mL/g with sulphuric acid. However, for magnesium electrocoagulation system, TP recovery of 97.3% was observed at optimal conditions (pH=8.4, retention time=35 min, current density=300 A/m2, and interelectrode distance=0.5 cm). The energy consumption for iron and magnesium electrocoagulation systems under optimal conditions was found to be 1.28 kWh/m³ and 2.35 kWh/m³, respectively. A kinetic study for TP removal revealed that at optimum operating conditions, TP removal followed second-order kinetics (rate constant(K)=0.0117 mg/(m2·min)). X-ray Diffraction analysis of the iron electrocoagulation sludge was found to be amorphous whereas for magnesium precipitate revealed the presence of struvite as the only crystalline compound.