Membrane capacitive deionization (MCDI) for advanced water treatment

With an ever increasing global population, the adequate supply of drinking water and irrigation water to produce food is becoming critical. Membrane Capacitive Deionization (MCDI) is an emerging electrochemical desalination technology where ions are removed from a feed stream by the application of an electrical potential. This work assesses MCDI use at bench, pilot and field scale for potable, agricultural and industrial wastewater applications and provides a performance and costing model that can be used for system design. Using a single electrode module system (pilot scale), MCDI was found to be suitable for applications with high hardness influent (3,400 μS/cm with 1,180 mg/L as CaCO3) with no decline in conductivity removal (average 1,506 μS/cm) or increase in electrode energy consumption (average 0.55 kWh/m3) over a one month trial. Comparisons between NaCl and high hardness influent demonstrated that under the same operating conditions, the MCDI current efficiency (λ) of the high hardness influent was greater than that of the NaCl influent. In general, the use of NaCl as a model solution for assessment of MCDI performance was found to be acceptable. The development and use of a simplified MCDI performance model coupled to a costing model resulted in cost estimations of $1.18/m3 for a small town potable supply and $1.71/m3 to $2.24/m3 (scenario dependent) for a larger scale industrial application with a higher influent conductivity. Agro-economic modelling of the application of MCDI for irrigated agriculture showed that crop annualised profit could be increased by up to $44,200/ha/yr for a greenhouse strawberry crop by using MCDI to treat a groundwater source with an electrical conductivity of 3,000 μS/cm. Studies performed at bench, pilot and field scale demonstrated that MCDI was able to remove up to 98.9% of uranium from a groundwater source containing 50 μg/L uranium. Uranium was preferentially removed over other ions with removal performance increasing as electrode flow between flowrate decreased and applied current increased. A field trial of desalination of treated domestic wastewater (influent conductivity of 1,156 μS/cm) using an MCDI unit containing six electrode modules demonstrated no performance decline when adjudged by conductivity removal (average 720 μS/cm, 62% removal rate), electrode λ (average 74%) or water recovery (average 84%) over a three month trial. The use of photovoltaics to power the electrode modules reduced the total system energy consumption from 1.28 kWh/m3 to 1.05 kWh/m3 and the inclusion of energy recovery devices on the electrode modules facilitated a 40% reduction in electrode energy consumption to 0.61 kWh/m3. MCDI was found to be a suitable desalination technology for a range of applications with further research to reduce the cost and increase the lifetime of electrode modules considered to be of primary importance