The study on the separate collection and nutrients recovery of urine in municipal wastewater

In consideration of the high N and P contents of urine in municipal wastewater, a technical idea of collecting urine separated from municipal wastewater for recovery of N and P is put forward to enhance the poor performance of sewage treatment plants in N and P removal due to high contents of N and P in municipal wastewater, the relatively low content of organic carbon source as well as the unbalanced ratio of C/N. By adding activated sludge in the collected urine, the decomposition of urea and polyphosphates in urine and their transformation to ammonia nitrogen and phosphate is highly promoted, which provides basic conditions for collecting N and P from urine through magnesium-ammonium-phosphate precipitation (also known as struvite formation). Under the conditions where pH = 5 and sedimentation time = 5 min, MgCl₂ and sodium dihydrogen phosphate are added as magnesium salt and phosphorous source with Mg:N:P proportion of 1.4:1:1 (molar ratio); the best recovery efficiency is obtained at which the recovery rate of N and P are nearly 95 and 98%, respectively. According to the analysis result of X-ray diffraction, the main precipitate is NH₄MgPO₄•6H₂O with a fraction of 70% among all recovery products with excellent values as fertilizers.