Anaerobic ureolysis of source-separated urine for NH3 recovery enables direct removal of divalent ions at the toilet

Abstract

Source-separated urine is of interest for nutrient recovery. Most nitrogen recovery technologies rely on ammonia (NH₃) as input, which requires ureolysis. As urease positive bacteria are widespread, source-separated urine is unstable, not only leading to NH₃ release but also loss, odor nuisance, and downstream scaling. Hence, ureolysis ideally occurs in a closed controlled environment close to the toilet. We characterized microbial-induced ureolysis, subsequent divalent cation precipitation, and fermentation in anaerobic sequencing batch reactors (SBRs) at 15 °C and 28 °C. Temperatures were a proxy for urine hydrolysis in a wet well at street level or in the toilet, respectively. The need for inoculation and the metabolic stability was assessed by inoculation with autofermented urine or a mixture of anaerobic digestion and fermentation sludge. The highest specific ureolysis rates in the SBRs were achieved at 28 °C: 2107 ± 395 and 1948 ± 1121 mg N g VSS^-1 d^-1, for the mixed and autofermented inoculum, respectively. For Ca²⁺ and Mg²⁺ precipitation, and organics fermentation, autofermented urine at 28 °C performed best with 47.9 ± 16.4 mg Ca²⁺ g VSS^-1 d^-1, 8.2 ± 4.6 mg Mg²⁺ g VSS^-1 d^-1, and 623 ± 129 mg VFA-COD g VSS^-1 d^-1, respectively. This indicates the hydrolysis reactor should be close to the toilet. The selected inoculum did not impact ureolysis, whereas both Ca²⁺ and Mg²⁺ precipitation and fermentation were better in the SBRs with autofermented urine. Ureolysis was identified as the only process significantly impacting the microbial community, indicating external inoculation would not be required. A urine hydrolysis reactor in the toilet without external inoculation could thus serve as a controlled environment to release NH₃ and remove divalent cations to prevent scaling in downstream transport and processing. For practical implementation in a household toilet, the reactor should be designed for user-friendly precipitate discharge and odor control.

Keywords: Precipitation; Resource recovery; Scaling; Source-separation; Ureolysis; Urine hydrolysis.