Effect of contact to the atmosphere and dilution on phosphorus recovery from human urine through struvite formation.

Phosphorus (P) in hydrolysed urine can be recovered through struvite formation. In the present study, batch experiments were conducted to investigate the effects of contact to the atmosphere (i.e. open and closed) and dilution [Vurine/(Vurine + Vwater)] (i.e. 100%, 50% and 25%) on P recovery from fresh urine through struvite formation with the addition of magnesium chloride (molar ratios of Mg/P = 1.3 and 2.0) after 32 d of storage. The P loss mainly occurred during the initial stages of precipitation with calcium and magnesium (5 d). The precipitates formed at the bottom of the jars were identified by X-ray diffraction to be struvite, hydroxyapatite and calcite. The results showed that the P recovery efficiency from urine solutions in open jars was lower than that in closed jars. It caused significant ammonia volatilization in open jars, resulting in higher nitrogen loss, lower pH values and lower supersaturation. The P recovery efficiency decreased with dilution, which is related to lower pH and lower supersaturation resulting from water dilution. An increase in the Mg/P ratio from 1.3 to 2.0 enhanced P recovery to some extent in urine solutions with different dilutions. The largest P recovery efficiency was 93.7% and 97.3% at an Mg/P ratio of 1.3 and 2.0 for the 100% urine solutions in closedjars, respectively. Scanning electron microscopy revealed smaller struvite particle sizes at lower dilutions (100% and 50% urine) compared with higher dilutions (25% urine).