Experimental investigation of system performance for combined desalination processes with membrane capacitive deionisation (MCDI)

Abstract

The water supply in many coastal regions worldwide is affected by progressive salinization. Here, the use of desalination technologies is a viable solution for obtaining freshwater. In this thesis, two modular concepts for brackish water (BW) desalination by the use of membrane capacitive deionization (MCDI) and low-pressure reverse osmosis (LPRO) were developed and tested at laboratory and pilot-scales with two pilot plants installed in Vietnam. The two concepts were developed by using computer-based calculations (software: WAVE) and evaluated in a socioeconomic and environmental multi-criteria analysis. The first plant consisting of subsurface arsenic removal (SAR) as pre-treatment and MCDI for desalination was installed in Tra Vinh, in the Mekong Delta for the treatment of arseniccontaminated groundwater with a concentration of total dissolved solids (TDS) of 1.65 g/L. Results showed the feasibility of the modular concept for producing drinking water (TDS<0.45 g/L) with a specific energy consumption (SEC) of <3 kWh/m³. The relationship between feed salinity and specific ion removal of the MCDI was evaluated in real environment and compared with laboratory experiments. The use of renewable energies such as solar and wind for autonomous supply was proven feasible for these technologies. The second pilot plant was installed in a riverine estuary in the region of Cần Giờ, where no access to freshwater is available due to the progressive salinization of river water and groundwater. Here, river water showed TDS concentrations of up to 25 g/L. The combined system consisted of UF pre-treatment, LPRO and MCDI to produce drinking water and product water with TDS of <0.45 g/L and <1.5 g/L, respectively with a total SEC of 5.8 kWh/m³. Additionally, the performance of the LPRO was compared to seawater-RO (SWRO) in pilot trials, which showed a SEC of 5.5 kWh/m³. Although the SEC of single-stage SWRO was lower, the separate production of drinking and product water by LPRO+MCDI showed different advantages including a reduced SEC of 5.2 kWh/m³ for product water and additional 0.6 kWh/m³ for drinking water. Finally, an optimization of the LPRO+MCDI can be possible by increasing the desalination efficiency of the MCDI, increasing the efficiency of LPRO-pump and the MCDI power supply, and by aiming at feed water qualities with lower salinity.