The Impact of the Initial N Load on the Performance of the Integrated Technology for N and P Removal and Recovery from Wastewater

Wastewater is a valuable nutrient source for fertilizer production. In recent years, a numerous technologies for nitrogen (N) and phosphorus (P) recovery from wastewater in the form of so-called waste-derived fertilizers have evolved. Only struvite can precipitate both, N and P ions, but according to their molar ratio of 1:1, it is not feasible to close the N cycle effectively. Therefore, to increase the N uptake at wastewater treatment plants, a novel two-stage technology for N and P removal and recovery was proposed. In the first stage, the P chemical precipitation (in the struvite form) from the anaerobic sludge digester liquors occurs. The precipitated struvite is separated from the liquors, which are subsequently directed to a membrane distillation (MD) unit. In the MD the remaining N is transformed into gaseous ammonia (NH3), which is absorbed to sulfuric acid (H2SO4) circulating in the gas-permeable membranes (GPMs) providing ammonium sulphate (NH4)2SO4 recovery. This work aims to present the results of the research on the initial N load ranging from 500 to 2000 mg dm−3 on the nutrients removal and recovery efficiencies in the integrated operation of struvite precipitation and MD. The obtained results showed that with the increasing influent N concentration, the N and P recovery efficiencies risen. Up to 97% of P, and over 8.8 g N m−2d−1 was recovered in the struvite and (NH4)2SO4 forms, respectively.