Evaluation of a low-cost magnesium product for phosphorus recovery by struvite crystallization

Background: The development of a cost-effective process of struvite crystallization requires the selection of appropriate sources of alkali and magnesium. In this study, the effectiveness of two industrial grade products, MgO and Mg(OH)₂, as magnesium and alkali sources to recover phosphorus as struvite were investigated and compared in a first set of experiments. Subsequently, the use of industrial Mg(OH)₂ was compared in two different struvite crystallization systems, an upflow fluidized bed reactor (FBR) and a continuous stirred tank reactor (CSTR) coupled to a settler tank. Results: At the same operational conditions, the consumption of MgO was higher than Mg(OH)₂ consumption. Moreover, industrial Mg(OH)₂ consumption for FBR and the CSTR operation was 1.6 and 1.1 1 mol Mg added mol^-1 P precipitated, respectively. This difference was caused by the high mixing intensity and the higher contact time between the Mg(OH)₂ slurry and the influent in the CSTR, favouring the conversion. Conclusions: Both industrial grade magnesium products are promising options for struvite crystallization. However, Mg(OH)₂ was more effective than the starting material, MgO, to recover phosphorus. Struvite crystallization by adding an industrial grade Mg(OH)₂ could be economically viable with regard to alternative physico-chemical P removal processes using metal salts, increasing the attractiveness of this P recovery process.