An integrated hollow fiber membrane contactor and chemical precipitation to recover N, P and K from human urine wastewater

In this study, an integrated hollow fiber membrane contactor (HFMC) and chemical precipitation process was developed for the simultaneous recovery of nitrogen (N), phosphorus (P) and potassium (K) from diluted human urine. The effect of operating parameters including solution pH as well as phosphate and magnesium dosages on the N, P and K recovery efficiency, was investigated. Those operating parameters were then further optimized by the response surface methodology (RSM). The pH decreased by about 0.5 during N capturing in the HFMC. When adding extra Mg and P to induce the chemical precipitation to recover P and K, the co-precipitation of MgKPO4.6 H2O, MgNaPO4.12 H2O, MgCO3(OH)2.2 H2O and Mg3(PO4)2.8 H2O occurred, which consumed more P and Mg and inhabited the recovery of K. RSM results showed the optimal pH for P and K recovery was 10.7 and the optimal ratio of Mg:K:P was 2.1:1:1.85. In addition, the optimal pH for HFMC can be determined to be 11.2. Under the optimal operating parameters, the N, P and K recovery efficiency was 97.02 ± 0.07%, 92.64 ± 0.29% and 73.47 ± 0.65%, respectively. The obtained liquid product in the HFMC process consists of 89.75% of (NH4)2HPO4 and 10.25% of NH4H2PO4. The solid product was a mixture of different precipitates rich in Mg, P and K. Two kinds of slow-release fertilizer, MgKPO4 and MgNH4PO4, accounted for 20% and 2% of the solid product, respectively. The liquid and solid products have the potential to be used as fertilizers in agriculture, horticulture and landscaping.