Performance evaluation of hybrid RO-CDI desalination system for the production of potable and ultrapure water

Abstract

Water is the basic necessity of life. The demand of water supply is increasing with world’s population and industrialization. Water of specific purity is required for human, agricultural and industrial consumption. In many parts of the world the available ground/surface water is not sufficient for the industrial, agricultural and domestic needs. Desalinating brackish water (surface water, ground water, or waste water) and/or seawater is required for the provision of desired water quality. There are several desalination techniques used to purify water from different sources. Reverse osmosis (RO) is a prominent technique used to desalinate both seawater and brackish water. Capacitive deionization (CDI) is another technique that can be used to desalinate saline water. CDI is an emerging desalination technology, in which more research is conducted to explore the possibility of using it in purifying brackish water and seawater.

The focus of most of the research work (directly or indirectly) in this area is to reduce the energy consumption during desalination and increase the water recovery. This research is motivated by these objectives. In order to accomplish the prime objective to establish an energy efficient desalination system with enhanced water recovery, a hybrid RO-CDI system was analyzed. A hybrid desalination system is generally designed by combining two different desalination systems to benefit from the merits of individual systems. A hybrid system can be used to reduce the energy and increase the water recovery.

In this dissertation a theoretical framework for the evaluation of desalination capability of hybrid RO-CDI system is established. The assessment of hybrid system performance based on energy consumption and overall water recovery is carried out. Furthermore, the capability evaluation of hybrid system to desalinate brackish water/seawater to produce the water of desired quality is performed. A comparative analysis of hybrid RO-CDI system with existing desalination technologies based on energy consumption/water recovery is also included.

In order to enhance the performance prediction of RO model, a parameter estimation methodology was established. Simulation of experimental results were performed to confirm the reliability of: (i) methodology used to determine permeability and (ii) determined permeability correlations. The RO model was combined with CDI model to simulate the hybrid RO-CDI system.

In order to produce the ultrapure water from seawater, the permeate stream of RO was fed to the CDI cell. This arrangement of RO and CDI produces ultrapure water along-with potable water. Two different approaches (constant voltage and constant current approach) to operate the CDI cell in this arrangement were used to understand the significance of these approaches. It has been found that ultrapure water with TDS (total dissolved salts) less than 2 ppm and potable water with TDS less than 400 ppm were produced by using RO-CVOCD (constant voltage operated CDI). The specific energy consumption of RO-CVOCD for the production of fresh water and ultrapure water is 3.083 kWh/m3 and 3.087 kWh/m3 respectively. The energy consumption by RO-CVOCD is lower than the energy required to produce distilled water (~10 ppm) by thermal process. The specific energy consumption for the production of desalted water is generally the same using either RO-CCOCD (constant current operated CDI) or RO-CVOCD for the same feed concentration and feed flow rate. The advantages of using RO-CCOCD over RO-CVOCD include a longer adsorption time and an increased quality of ultrapure water (>18 MΩcm). The RO-CCOCD hybrid system is superior to the RO-CVOCD system in the production of ultrapure water for the systems with the same feed concentration and feed flow rates.

The water recovery from RO brine stream was studied by desalinating the brine stream using CDI cell. The combination of RO-CDI in this arrangement was studied for the desalination of brackish water. The CDI cell was operated at constant current and constant voltage approach. RO-CDI was compared with the simulated results of two stage RO. In comparison with two stage RO, the specific energy consumption of RO-CVOCD and RO-CCOCD is 19 % and 50.6 % lower respectively. The hybrid RO-CDI system is energy efficient operation for the production of fresh water from low salinity brackish water. The RO brine treatment using CCOCD gives better overall water recovery with reduced specific energy consumption than the two-stage RO and RO-CVOCD.