Ureolytic phosphate precipitation from anaerobic effluents

In this work, the elimination of phosphate from industrial anaerobic effluents was evaluated at lab-scale. For that purpose, the ureolytic method previously developed for the precipitation of Ca2+ from wastewater as calcite was adapted for the precipitation of phosphate as struvite. In the first part of the study, computer simulations using MAPLE and PHREEQC were performed to model phosphate precipitation from wastewater as struvite. The results obtained showed that relative high concentrations of ammonium and magnesium are needed to precipitate phosphate as struvite. The total molar concentrations ratio of Mg2+: PO43--P:NH4+ required to decrease PO43--P concentrations from 20 to 6mg PO43--P/l at pH 8.4-8.5 was estimated on 4.6:1:8. In the second part of the study, lab-scale experiments with either synthetic wastewater or the anaerobic effluent from a vegetable processing industry were carried out in batch and continuous mode. Overall, the continuous operation at a hydraulic retention time (HRT) of 2.4 h and an added molar concentration [Mg2+]:[PO43 -P]:[ NH4+] ratio of 1.6:1:2.3 resulted in a constant pH value in the reactor (around 8.5) and an efficient phosphate removal (>90%) to residual levels of 1-2 mg PO43--P/l. Different operational conditions, such as the initial phosphate concentration, HRT and the use of CaCl2 or MgO instead of MgCl2, were analysed and the performance of the reactor was satisfactory under a broad range of them. Yet, overall, optimal results (higher phosphate removal) were obtained with MgCl2.