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Nutrient and energy recovery from urine

Keywords: urine, urine treatment, nutrient recovery, microbial fuel cells, energy production from urine, membrane capacitive deionization. In conventional wastewater treatment plants large amounts of energy are required for the removal and recovery of nutrients (i.e. nitrogen and phosphorus). Nitrogen (N) compounds are removed as inert nitrogen gas and phosphorus (P) is for example removed as iron phosphate. About 80% of the N and 50% of the P in wastewater originate from urine1, but urine only contributes about 1% to the volume of this wastewater. High nutrient concentrations can be found in urine when it is collected separately from other wastewater streams. In this thesis, the nutrient and energy recovery from urine was investigated. At first, urine samples were analyzed for their composition. This characterization showed that the composition of the organic fraction in these samples was always similar. The differences between the concentrations of specific organic compounds were caused by dilution, due to individual consumption patterns of people. Two alternatives to the state-of-the-art nutrient recovery concepts are evaluated. These alternatives are on the one hand membrane capacitive deionization (MCDI) and on the other hand struvite precipitation combined with a microbial fuel cell (MFC). The evaluation of the MCDI system showed that nutrients can be concentrated from diluted urine. With its relatively low energy demand, MCDI could be an alternative to electrodialysis. The evaluation of the phosphate recovery by struvite precipitation combined with ammonium recovery and energy production by an MFC showed that this concept is most promising. The highest ammonium recovery rate achieved was 9.57 gN m-2 d-1 at a current density of 2.6 A m-2 (0.67 W m-2) using real undiluted urine. The ammonium recovery and energy production by an MFC (-10 kJ gN-1) can be considered a breakthrough, as usually energy is needed to recover (i.e. ammonia stripping 32.5 kJ gN-1)1 or convert (i.e. Sharon-Anammox 16 kJ gN-1)1 ammonium. Predictions show that approximately 5.1 kg struvite and 7.3 kg ammonia-nitrogen can be recovered from one cubic meter of urine, while producing approximately 20 kWh. A comparison to state-of-the-art technology showed that this process can be a good alternative for nutrient recovery from urine. Furthermore, ammonium recovery and energy production by an MFC can possibly be applied to other wastewater streams.