Phosphorus recovery from urine with different magnesium resources in an air-agitated reactor

Phosphorus (P) recovery through struvite formation from urine has gained increasing attention as an approach to alleviate the potential shortage of P. The concentration of magnesium (Mg) is lower than those of ammonium and ortho-phosphate in urine, so the cost of adding Mg is a major economic constraint for P recovery. This study aims at evaluating the potential of seawater and bittern as Mg resources to recover P from urine in an air-agitated reactor. Results indicate that effects of seawater and bittern on P recovery from urine are comparable to that of magnesium chloride, with an average crystal size of around 34 μm under an Mg/P molar ratio of 1.3. The average crystal size of the struvite crystals obtained from high P concentration urine with different Mg resources showed no significant difference, whereas larger crystal size was obtained from low P concentration urine with bittern as Mg resource. Analysis of mainly rod-like crystals produced in the reactor indicates the presence of pure struvite (>98.0% by weight) with small amounts of calcium (<1.39% by weight). Bittern is recommended as a suitable Mg resource for P recovery because of its high Mg content and the convenience it offers in terms of transportation and storage.