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This study investigates the feasibility of electricity production in a solid oxide fuel cell using methane recovered from groundwater as the fuel. Methane must be removed from groundwater for the production of drinking water to, amongst others, avoid bacterial regrowth. Instead of releasing methane to the atmosphere or converting it to carbon dioxide by flaring, methane can also be recovered by vacuum stripping and served as a fuel. However, the electrical efficiency of currently used combustion-based technologies fuelled with methane-rich gas is limited to 35% due to the low heating value of the recovered gas (70 mol. % methane) and power derating due to the presence of carbon dioxide (25 mol.%). We propose to use a solid oxide fuel cell to use the methane-rich gas as fuel. Solid Oxide Fuel Cells are fuel-flexible and potentially attain higher electrical efficiencies up to 60%. To this end, specific gas processing, including cleaning and methane reforming, is required to allow for durable operation in a solid oxide fuel cell. We assessed whether electricity could be generated by a solid oxide fuel cell using methane recovered from a full-scale drinking water treatment plant as a fuel. The groundwater had a methane concentration of 45 mg∙L-1, and the recovered gas by vacuum towers contained 70 mol% methane. We used a gas cleaning reactor with impregnated activated carbon to remove hydrogen sulfide traces from the methane-rich gas. Thermodynamic calculations showed that additional steam is required to achieve a high methane reforming. The added steam and the carbon dioxide content in the recovered gas simultaneously contribute to the methane reforming to prevent carbon deposition. The measured open circuit potential corresponded with the theoretical Nernst voltage, implying high methane reforming in the solid oxide fuel cell. The achieved power density of the cell fuelled with the methane-rich gas (mixed with steam) was 27% less than the hydrogen-fuelled cell. Ultimately, 51.2% of the power demand of the plant can be covered by replacing the gas engine in a drinking water treatment with a 915 kW solid oxide fuel cell system fuelled by the methane recovered from the groundwater, while the greenhouse gas emission can be reduced by 17.6%.

The application of zero liquid discharge (ZLD) results in the generation of solid residual streams, which are often not fit for reuse. In this study, we assessed the separation of natural organic matter (NOM) and sodium chloride (NaCl) by nanofiltration (NF), electrodialysis (ED) and ion exchange (IEX) in reverse osmosis brine (RO-brine) and by the extraction of impurities from salt (SALEX) in the generated mixed solids of a full-scale ZLD water treatment plant. The NaCl recovery by NF, ED and IEX ranged 69-99% and the rejection of NOM ranged 18–19%, 43–65% and 53–76%, respectively. The recovery of NaCl by SALEX ranged 52–99%, while the rejection of NOM ranged 59–92%. The results show that NOM and NaCl can be sepa-rated both in RO-brine and mixed solids, opening opportunities for recovery of reusable salt from brines in ZLD.

Total ammoniacal nitrogen (TAN) is considered to be a pollutant, but is also a versatile resource. This review presents an overview of the TAN recovery potentials from nitrogen (N)-loaded residual streams by discussing the sources, recovery technologies and potential applications. The first section of the review addresses the fate of TAN after its production. The second section describes the identification and categorisation of N-loaded (≥0.5 g L−1 of reduced N) residual streams based on total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), TAN, and TAN/TKN ratio. Category 1 represents streams with a low TAN/TKN ratio ( 1 g L−1) that require a decrease of the TSS prior to TAN recovery, whereas category 3 represents streams with a high TAN/TKN ratio (≥0.5) and low TSS (≤1 g L−1) that are suitable for direct TAN recovery. The third section focuses on the key processes and limitations of AD, which is identified as a suitable technology to increase the TAN/TKN ratio by converting organic-N to TAN. In the fourth section, TAN recovery technologies are evaluated in terms of the feed composition tolerance, the required inputs (energy, chemicals, etc.) and obtained outputs of TAN (chemical form, concentration, etc.). Finally, in the fifth section, the use of recovered TAN for three major potential applications (fertilizer, fuel, and resource for chemical and biochemical processes) is discussed. This review presents an overview of possible TAN recovery strategies based on the available technologies, but the choice of the recovery strategy shall ultimately depend on the product characteristics required by the application. The major challenges identified in this review are the lack of information on enhancing the conversion of organic-N into TAN by AD, the difficulties in comparing the performance and required input of the recovery technologies, and the deficiency of information on the required concentration and quality of the final TAN products for reuse.

Ammonium (NH4+) can be recovered from water for fertiliser production or even energy production purposes. Because NH4+ recovery is more effective at increased concentrations, electrodialysis (ED) can be used to concentrate NH4+ from side streams, such as sludge reject water, and simultaneously achieve high NH4+ removal efficiencies. However, the effect of osmosis and back-diffusion increases when the NH4+ concentration gradient between the diluate and the concentrate stream increases, resulting in a limitation of the concentration factor and an increase in energy consumption for NH4+ removal. In this study, we showed that operation at dynamic current density (DCD) reduced the effect of osmosis and back-diffusion, due to a 75% decrease of the operational run time, compared to operation at a fixed current density (FCD). The concentration factor increased from 4.5 for an FCD to 6.7 for DCD, while the energy consumption of 90% NH4+ removal from synthetic sludge reject water at DCD remained stable at 5.4 MJ·kg-N−1.

Removal of ammoniacal nitrogen from residual waters traditionally relies on energy-consuming biochemical processes, while more novel alternative strategies focus on the recovery of total ammoniacal nitrogen for the production of fertilisers or the generation of energy. The recovery of total ammoniacal nitrogen as gaseous ammonia is more effective at high ammonia concentrations in the liquid feed. Typically, chemicals are added to increase the solution pH, whilebipolar membrane electrodialysis can be used to convert salt solutions to acid and base solutions by using only electricity. In this study, we used bipolar membrane electrodialysis to remove ammonium from water and to simultaneously produce concentrated dissolved ammonia, without using chemicals. The energy consumption and current efficiency to transport ammonium from the diluate (the feed water) were assessed throughout sequencing batch experiments.The total ammoniacal nitrogen removal efficiency for bipolar membrane electrodialysis ranged between 85 and 91% and the energy consumption was stable at 19 MJ·kg-N−1, taking both electrochemical and pumping energy into account. The base pH increased from 7.8 to 9.8 and thetotal ammoniacal nitrogen concentration increased from 1.5 to 7.3 g L−1, corresponding to a final ammonia concentration of 4.5 g L−1 in the base. Leakage of hydroxide, diffusion of dissolved ammonia and ionic species from the base to the diluate all contributed to a loss in current efficiency. Due to the increase in operational run time and concentration gradients throughout the sequencing batch experiments, the current efficiency decreased from 69 to 54%. We showed thatbipolar membrane electrodialysis can effectively be used to simultaneously remove ammonium from water and produce concentrated dissolved ammonia while avoiding the use of chemicals. Moreover, the energy consumption was competitive with that of the combination of electrodialysis and the addition of chemicals (22 MJ·kg-N−1).