**1. Introduction**

Energy production accounts for 10% of freshwater withdrawal globally [1]. However, a much larger fraction of total freshwater withdrawal is used for energy production in industrialized countries, e.g., USA (50%), Western Europe (50%) and China (86%) [1,2]. The majority of withdrawal is used for cooling in thermal energy production [3]. The consumption of cooling water in thermal power plants depends highly of geographical location, cooling and fuel type [4] and is generally high for nuclear, coal and gas fired plants. The use of large amounts of fresh water for power production contributes to the increasing pressure on local water resources [2–5]. Reducing the freshwater withdrawal for cooling is expected to result in a substantial reduction in the water footprint of the energy sector. In general, four types of cooling systems can be employed for electricity generation including once-trough, recirculating, dry and hybrid cooling [5]. Dry cooling relies on air as the coolant medium eliminating water withdrawal and consumption totally. The capital cost of dry cooling is approximately ten times higher than that of once-through cooling [5]; as a result, its use is generally restricted to cases where insufficient make up water supply is available. Once-trough cooling using fresh water is less favored due to its large thermal emission to the surface water body; and [6] the number of power plants utilizing wet (evaporative) cooling systems with an open recirculating cooling tower has therefore rapidly increased [7]. Recirculating cooling towers operate in a feed and bleed mode. Circulating water is evaporated to reject heat while fresh feed water is continuously added, and a fraction of the

circulating water is discharged as blowdown. Water withdrawal required for cooling depends on the maximum salt concentration that can be maintained in the recirculation water, which is typically limited by operational aspects such as mineral precipitation and scaling [5]. Scaling involves the precipitation of partially water-soluble salts such as calcium carbonate (CaCO3), which is driven by increasing calcium concentration, pH or alkalinity due to evaporation. The CaCO3 hardness of surface water used for cooling depends highly on the geohydrology of the aquifer and can range from soft (<60 mg/<sup>L</sup> CaCO3) to very hard (>180 mg/<sup>L</sup> CaCO3). Previously explored strategies to improve water efficiency of wet cooling towers include feed pretreatment [8,9], use of alternative feed sources [10], circulation water conditioning and acidification [11] and blowdown recuperation [12–14].

Feed pretreatment can be achieved with several processes or process trains including conventional clarification, ion exchange and membrane filtration. Membrane based water treatment technologies have essential advantages over ion exchange in terms of environmental indicators but produce more effluent [8]. Membrane capacitive deionization (MCDI) is an emerging electromembrane process that makes use of electrostatic adsorption to remove ions from a feed stream. An MCDI cell consists of two carbon electrodes covered with ion exchange membranes and separated by a flow channel. During purification, a feed stream is applied to the flow channel, and an electric potential is applied to the electrodes. Ions of opposite charge are attracted and electrosorbed in electrical double layers in the anode and cathode, respectively, and thus removed from the feed stream. Accumulation of ionic charge on the electrode increasingly compensates the applied potential until electrode regeneration is required. During regeneration, the electric polarity is reversed to cause desorption of adsorbed ions. This cycle of purification and regeneration produces two streams, desalinated water and brine. Ion exchange membranes are placed in front of the electrodes in MCDI to prevent that during regeneration ions of opposite charge are attracted from the bulk fluid. This would result in incomplete regeneration leading to a reduced electrode adsorption capacity and longer regeneration times during the next purification step [15]. Application of MCDI for surface or brackish water desalination is characterized by low energy consumption, high water recovery and low fouling propensity [16] in comparison to pressure driven processes. A recent pilot study from Tan et al. [17], using a similar MCDI module as used in this work, showed the possibility of further reducing energy consumption by 30% to 40% using an innovative energy recovery system. MCDI is expected to be less prone to fouling and scaling than other membrane-based desalination technologies [18]. The membranes in MCDI protect the carbon electrodes [19] and due to frequent electrode reversal, build-up of fouling is prevented in a similar way to electrodialysis reversal [15]. MCDI is therefore a potentially highly interesting technology for cooling tower feed pretreatment. MCDI is currently not ye<sup>t</sup> widely applied on a large scale but is considered a viable alternative for partial demineralization of low salinity streams [15,20,21]. A limited number of MCDI pilot studies has been published [15]. Dorji and coworkers performed a pilot scale test with MCDI as alternative for 2nd stage RO in seawater desalination. The results showed that MCDI can effectively remove bromide and dissolved salt at lower energy consumption (0.15 kWh m<sup>−</sup>3) compared with second stage RO (0.35 kWh m<sup>−</sup>3) at high water recovery [22]. Van Limpt and Van der Wal [9] performed an MCDI pilot study in which MCDI is used to desalinate tap water as feed for an industrial cooling tower (500 kW) and a residential cooling unit (4500 kW). Chemical savings of up to 85% and water savings up to 28% at low energy consumption (0.11–0.23 kWh m<sup>−</sup><sup>3</sup> produced water) were achieved in this study. MCDI water recovery was limited to 80% to prevent calcium carbonate precipitation. However, a preferential uptake of chloride and calcium (20%) was found resulting in a lower risk of CaCO3 scaling. The authors concluded that the energy consumption was similar to what is expected from reverse osmosis (RO). A similar range of energy consumption for RO is mentioned by Qin et al. [23] who developed a mathematical model to compare the energetic performance of MCDI and brackish water RO (BWRO). They concluded BWRO to be significantly more energy efficient than MCDI, at high salt rejections and moderate to high water salinities.

The potential of MCDI for the reduction of cooling water intake in thermal power plants is studied in this paper. An experiments-based approach is used to evaluate the combined performance of MCDI and a cooling tower. Lab scale experimental data is used in a response surface methodology to determine the optimal working conditions of the coupled MCDI-CT system in view of water use efficiency and cost. The resulting optima are reevaluated for real CT feed water samples in MCDI lab tests and subsequently in an MCDI-CT pilot case study.
