Strategies to enhance the recovery of phosphorus as struvite during the storage of urine

Abstract

Phosphorus is a non-renewable resource, and it will in future become increasingly scarce. Urine can be used as a valuable resource of phosphate. One way to create favourable conditions for recovering phosphorus as struvite from urine is to store urine and promote urea hydrolysis, before the recovery of phosphate. To develop a faster and higher-quality recovery process, urine storage was studied by applying: (i) hydrolyzed urine to fresh urine, (ii) low-rate aeration, and (iii) continuous stirring. Seawater was used as magnesium source for struvite crystallization. Results show that urea hydrolysis rate was enhanced when hydrolyzed urine was added to fresh urine during its storage. And as a consequence, the precipitation time of phosphate was reduced. After 14 days, around 80 % of the phosphate was recovered as struvite when the volumetric ratio of hydrolyzed urine/fresh urine was 0.15, compared with the 20 % of the phosphorus precipitated when fresh urine was stored. Moreover, the application of low-rate aeration to urine storage allow to reduce the time of phosphate precipitation from 14 days to 3 days, recovering around 88 % of the phosphate. But the aeration generated that a significant loss of ammonium from urine through ammonia volatilization when the storage time was higher than 3 days. In addition, the use of aeration seems to be more useful to enhance phosphate precipitation compared to the application of continuous stirring during urine storage. This study demonstrates that phosphorus recovery as struvite can be developed during urine storage with high recovery efficiencies and struvite quality.