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

Christiaens, Marlies E.R., De Vrieze, Jo, Clinckemaillie, Lorenzo, Ganigué, Ramon, Rabaey, Korneel
Water research 2019 v.148 pp. 97-105
ammonia, anaerobic digestion, bacteria, calcium, cations, fermentation, hydrolysis, inoculum, magnesium, microbial communities, nitrogen, odor control, odors, sludge, temperature, urease, urine
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.