Enhancement of struvite pellets crystallization in a full-scale plant using an industrial grade magnesium product

A full-scale struvite crystallization system was operated for the treatment of the centrate obtained from the sludge anaerobic digester in a municipal wastewater treatment plant. Additionally, the feasibility of an industrial grade Mg(OH)₂ as a cheap magnesium and alkali source was also investigated. The struvite crystallization plant was operated for two different periods: period I, in which an influent with low phosphate concentration (34.0 mg P·L^-1) was fed to the crystallization plant; and period II, in which an influent with higher phosphate concentration (68.0 mg P·L^-1) was used. A high efficiency of phosphorus recovery by struvite crystallization was obtained, even when the effluent treated had a high level of alkalinity. Phosphorus recovery percentage was around 77%, with a phosphate concentration in the effluent between 10.0 and 30.0 mg P·L^-1. The experiments gained struvite pellets of 0.5-5.0 mm size. Moreover, the consumption of Mg(OH)₂ was estimated at 1.5 mol Mg added·mol P recovered^-1. Thus, industrial grade Mg(OH)₂ can be an economical alternative as magnesium and alkali sources for struvite crystallization at industrial scale.