| Resum: |
Human presence in outer space is currently supported by regular resupply of life support consumables from Earth. However, deep-space exploration or space habitation will depend on regenerative life support systems (RLSS) for in-situ oxygen, water and food production and waste management as resupply becomes practically impossible due to the long distance and transit time. Urine recycling is of key interest in RLSS to recover water and nutrients, which can serve as a fertiliser for plants and microalgae. Urine source separation and recycling also gains attention on Earth to shorten terrestrial nutrient cycles, which play a pivotal role in our food supply, but are currently pushed to their planetary boundaries by extensive synthetic fertiliser production and use. Nutrient recovery from waste streams could reduce the need for energy intensive ammonia production and mining of non-renewable phosphorus and potassium, and obviate the need for advanced nutrient removal to protect the environment. Amongst other streams, urine is targeted, as it presents the major nutrient source in domestic wastewater and has good fertilising properties. Other benefits stemming from urine source separation include the reduced water consumption for flushing and the decreased nutrient load and better effluent quality of wastewater treatment plants. The goal of this PhD thesis was to develop resource-efficient urine recycling technologies that i) can be implemented in RLSS, such as the Micro-Ecological Life Support System Alternative from the European Space Agency or, ii) used for on-site/decentralised urine treatment on Earth. As resources are scarce in space, the goal was to achieve maximum nutrient recovery with minimum energy expenditure and use of consumables in order to reduce payloads and to minimise the need for resupply. Different urine treatment trains combining biological and physico-chemical processes were investigated. Urine contains many valuable compounds, but the compositional complexity and instability present challenges for urine collection and treatment. Urea, the main nitrogen compound in urine, quickly hydrolyses into ammonia and (bi)carbonate, causing nutrient losses, odour nuisance, scaling and clogging by uncontrolled precipitation, and ammonia volatilisation. Therefore, an alkalinisation step was included to prevent ureolysis and to remove calcium and magnesium by controlled precipitation, thereby minimising the risk for scaling in the following treatment steps. In Chapter 2 and 3, NaOH was used to increase the pH of fresh urine, whereas Chapter 4 investigated the use of an electrochemical cell to avoid base consumption, the logistics associated with base storage and dosing, and the associated increase in salinity. Nitrification was applied to convert instable urea and/or volatile and toxic TAN into non-volatile nitrate in Chapter 2, 3 and 5. Three different reactors were employed: a pilot scale MBBR, a bench scale MABR and a bench scale MBBR. The MABR was preceded by a microbial electrolysis cell to remove the COD prior to nitrification. Full urine nitrification is preferred over partial nitrification because of the higher process stability and safety, but requires additional alkalinity to compensate for the proton release by nitrification. In Chapter 5, the nitrification reactor was coupled to a dynamically controlled electrochemical cell for in-situ OH- production as an alternative to base addition, enabling full nitrification. Nitrified urine can be applied as a fertiliser, but the nutrient concentrations are low compared to synthetic fertilisers. Hence, for terrestrial applications, a concentration step is preferred. Chapter 2 explored the feasibility of ED to concentrate nutrients, whereas, in Chapter 5, the electrochemical cell for pH control also functioned as concentration technology. Alternatively, nitrified urine can be valorised as culture medium for microalgae, which was investigated in Chapter 6. |
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