Urine treatment and characterization

Urine was collected from the urine treatment system in the NEST building on the Eawag/Empa campus in Dübendorf. The system is equipped with waterless urinals and urine-diverting toilets (Save!, Laufen AG), which allow the separation of urine with approximately 200 mL flushing water entering the urine pipe per flush. The dilution with flushing water is around 1:1, calculated based on the salt concentration presented in Table 2 and in undiluted stored urine presented in Udert et al. (2006). This dilution is higher than normal because we had only few users during urine collection while the cleaning frequency remained the same. The urine was stored anaerobically in a tank in the basement with a variable hydraulic retention time of 2 to 8 weeks depending on the production of urine and the performance of the treatment. The urine taken from the storage tank is referred to as anaerobically stored urine. Biological degradation of organics was performed in a 60 L membrane aerated biofilm reactor (MABR) (Oxypilot, Oxymem) with a hydraulic retention time (HRT) of about 1.2 days. The pH of the anaerobically stored urine was 9 (Table 2), which inhibits nitrification (Fumasoli, 2016). The MABR was operated with continuous inflow of approximately 2 L h⁻¹ and continuous aeration with pressurized air at a flow rate of 200 L h⁻¹. The pressure was kept at 300 mbar, scouring was performed weekly and sludge removal of 2 L was performed monthly. The treatment in the MABR removed between 70% and 90% of the organics present in anaerobically stored urine, and after treatment it was referred to as organics-depleted urine. The concentration of the organics in the anaerobically stored urine varied over time due to changing user behavior, e.g. higher dilution during less usages while keeping the cleaning frequency. The characterization of the urine solutions used for the experiments is given in Table 2.