Recovery of ammonium nitrate solution from urine wastewater via novel free nitrous acid (FNA)-mediated two-stage processes

Nitrogen (N) recovery from urine wastewater has been an important route to overcome intensive energy consumption in the conventional N-removal process. Hereby, this study reports a new process for recovering N in the form of ammonium nitrate (AN, i.e. NH₄NO₃), a common, stabilized type of fertilizer in agriculture. The process is achieved through two-stage biological–chemical reactions by using free nitrous acid (FNA) as an intermediate. In the first stage, a biological reactor was operated for 170 days, with ammonium-rich human urine wastewater as influent and gradually increased influent ammonium loading rates. Then a closed chemical reactor was designed to generate AN from the effluent of the biological reactor. Experimental results demonstrate that the two-stage biological-chemical process is technically and economically viable. Comprehensive modelling analyses show that FNA is the key to maintain stable partial nitritation in the biological reactor, and also critically affects the reaction rate of the chemical nitrite oxidation. pH adjustment of the bioreactor effluent is needed to increase an FNA concentration up to 250 mg HNO₂-N/L in the biological reactor effluent. As such, chemical nitrite oxidation can reach a substantially high rate, i.e. at 15 kg N/(m³·d), compared with the nitrite production rate of ∼0.5 kg N/(m³·d) achieved in the biological reactor. This result also indicates that the biological stage is a rate-limiting step in the overall process. Overall, the proposed FNA-based approach provides a promising option for nitrogen recovery from urine wastewater.