In laboratory experiments, source-separated urine was stabilised with nitrification and denitrified via nitritation and anaerobic ammonium oxidation. The highest total ammonia concentration in the influent was 7,300 gN/m3, the maximum pH 9.2. In a moving bed biofilm reactor (MBBR) with Kaldnes® biofilm carriers, we stabilised urine as a 1:1 ammonium nitrate solution. The maximum nitrification rate was 380 gN/m3/d corresponding to 1.7 gN/m2biofilm/d. Nitrite ammonium solutions were produced in a continuous flow stirred tank reactor (CSTR) with 4.8 days sludge retention time (SRT) at 30°C and in a sequencing batch reactor (SBR) with more than 30 days SRT. Nitrate build-up was negligible in both reactors. Nitritation rates were 780 gN/m3/d in the CSTR and 280 gN/m3/d in the SBR, respectively. However, shortening the cycles would increase nitritation in the SBR. High concentrations of nitrous acid, salts, and presumably hydroxylamine suppressed nitrite oxidation in the nitritation reactors. In all three nitrification reactors, maximally 50% of the influent total ammonia was oxidised without pH control. None of the common inhibition or limitation approaches could explain why ammonia oxidation always stopped at pH values around 6. In a batch experiment, we showed that source-separated urine can be denitrified autotrophically by anammox bacteria.

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