Quantification and mitigation of the negative impact of calcium on struvite purity

Abstract

The dairy industry is a major economic sector in New Zealand with a large amount of wastewater being produced annually. These wastewaters are rich in phosphorus (P), and can be used as a potential source for struvite crystallisation for fertilizer production and environmental pollution abatement. However, this technique is hindered by the presence of foreign ions, especially calcium, which is present in high concentrations in dairy waste. This work aims to resolve the negative extent of the effect of calcium on struvite crystallisation to establish quantitative recommendations around where P recovery using this method is feasible. The impact of calcium concentration on the purity, morphology and particle size was assessed using laboratory experiments, mimicking the composition of typical dairy wastewaters, at a pH of 9 and room temperature. The results indicate that only at high Ca:Mg concentration ratios of more than one did the calcium adversely affect the product morphology and the fraction of struvite in the final product. There was negligible effect when the calcium concentration was low. Furthermore, a high ammoniac nitrogen concentration mitigated the negative impact, and therefore has to be taken into consideration when wastewaters with both high calcium and nitrogen content are considered as feedstock.