Comparison investigation on phosphate recovery from sludge anaerobic supernatant using the electrocoagulation process and chemical precipitation

Abstract

In this study, the electrocoagulation process was employed to recover phosphate from sludge anaerobic supernatant. Prior to the electrocoagulation experiments, a series of experiments were first performed to investigate the effects of solution pH, reaction time, alkalinity and reagent dosage on the recovery of phosphate by utilizing Fe2+, Fe3+ and Al3+ salts as the precipitators. The findings revealed that the solution pH, alkalinity and reagent dosage obviously affected the phosphate recovery by these salts, whereas the reaction time had a negligible effect on the phosphate recovery by Fe3+ and Al3+ salts. However, when the Fe2+ salt was used as the precipitator, the effect of the reaction time could not be ignored, and the phosphate recovery efficiency increased from 58% to 71.6% with increasing reaction time from 5 min to 30 min. The recovery efficiencies of phosphate by the Fe2+, Fe3+ and Al3+ salts followed the order of preference of Al3+ > Fe3+ > Fe2+ at the same reagent dosage. The results of the electrocoagulation experiments indicated that the current density and the initial solution pH could significantly influence the recovery of phosphate by the electrolysis of iron and aluminum electrodes. Under identical conditions, the recovery efficiency of phosphate by electrocoagulation using iron electrodes was much higher than that by aluminum electrodes. Additionally, the aeration experiments demonstrated that the aeration during iron electrode electrolysis can significantly improve the phosphate recovery. An economic analysis indicated that the use of an iron plate as the electrocoagulation electrode in phosphate recovery was more economic and feasible compared to the use of the aluminum electrode and Fe2+, Fe3+ and Al3+ salts.