Comparison of different K-struvite crystallization processes for simultaneous potassium and phosphate recovery from source-separated urine

Controlled K-struvite crystallization is an attractive technology to simultaneously recover phosphate and potassium from urine. This study investigated the recovery of phosphate and potassium from source-separated urine by K-struvite crystallization using different use models of low-grade MgO (LG-MgO): LG-MgO alone (model 1, M1), LG-MgO plus phosphorus acid (model 2, M2), and a pre-formed stabilizing agent by adding LG-MgO plus phosphorus acid (model 3, M3). Results showed that 100% phosphate and 25% K could be recovered from urine by M1. M2 at an MgO:K:P molar ratio of 4:1:1.6 provided a maximum P and K recovery efficiency at 100% and 70%. M3 achieved a same K-removal efficiency as M2, but the phosphate recovery efficiency was lower than that of M2 due to the dissolution of phosphate in the stabilizing agent. K-struvite crystallization was closely accompanied by severe co-precipitation of Na-struvite. Increasing the Na concentration markedly improved the ability of Na co-precipitation, but the variation of pH did not affect the competition precipitation of K and Na. When the Na:K molar ratio was >10, the precipitation of Na was more than that of K. A process performance evaluation indicated that M3 is more suitable for simultaneous K and P recovery from source-separated urine. [Display omitted] •K and P were recovered from urine by K-struvite precipitation.•High nutrients recovery efficiencies were achieved by using low-grade MgO (LG-MgO).•Na:K molar ratio has a marked effect on the competitive precipitation of Na and K.•Using the stabilizing agent formed by LG-MgO and H3PO4 was optimal for K recovery.