Evaluation and thermodynamic calculation of ureolytic magnesium ammonium phosphate precipitation from UASB effluent at pilot scale

The removal of phosphate as magnesium ammonium phosphate (MAP, struvite) has gained a lot of attention. A novel approach using ureolytic MAP crystallization (pH increase by means of bacterial ureases) has been tested on the anaerobic effluent of a potato processing company in a pilot plant and compared with NuReSys (R) technology (pH increase by means of NaOH). The pilot plant showed a high phosphate removal efficiency of 83 +/- 7%, resulting in a final effluent concentration of 13 +/- 7 mg . L-1 PO4-P. Calculating the evolution of the saturation index (SI) as a function of the remaining concentrations of Mg2+, PO4-P and NH4+ during precipitation in a batch reactor, resulted in a good estimation of the effluent PO4-P concentration of the pilot plant, operating under continuous mode. X-ray diffraction (XRD) analyses confirmed the presence of struvite in the small single crystals observed during experiments. The operational cost for the ureolytic MAP crystallization treating high phosphate concentrations (e.g. 100 mg . L-1 PO4-P) was calculated as 3.9 (sic) kg(-1) P-removed. This work shows that the ureolytic MAP crystallization, in combination with an autotrophic nitrogen removal process, is competitive with the NuReSys (R) technology in terms of operational cost and removal efficiency but further research is necessary to obtain larger crystals.