Application Of Membrane Capacitive Deionization Technology In Cooling Towers
By Piotr Dlugolecki, PhD, Aurora Connorton-Spragg, and Carlos Camero
Cooling towers provide cooled water for applications ranging from heating, ventilation, air-conditioning, and industrial processes. Stored heat in the recirculation water of the cooling tower is released through the process of evaporation and as the water evaporates, the solids that are dissolved in the water are left behind. As a consequence, concentration of dissolved solids in the cooling tower increase over time and it may result in corrosion and scaling because of high concentrations of chloride, calcium, and alkalinity. In order to reduce the scaling and corrosion potential in the cooling tower, it is common practice to add chemicals such as antiscalants and corrosion inhibitors to the recirculation water.
Once the conductivity of this water reaches a certain threshold value, it is discharged as a blowdown stream. This process leads to a discharge of large volumes of wastewater containing high levels of chemicals, which has a significant environmental impact when not properly disposed. In order to minimize this discharge of wastewater and use of chemicals, total dissolved solids (TDS) from the recirculation water need to be removed. TDS removal from the recirculation water can be achieved by deionizing the recirculation water, deionizing the feedwater that enters the cooling tower, or deionizing the blowdown water.
The use of reverse osmosis (RO) (1-3) has been suggested for deionization of the cooling tower recirculation water and for deionization of the blowdown water. The use of RO seems to be impractical because of low water recoveries as well as other technical drawbacks, such as silica fouling on the membranes. Furthermore, the cooling tower recirculation and blowdown stream contains high levels of foulants, requiring extensive pre-filtration steps such as ultrafiltration to prevent membrane fouling (2, 3).
There is an alternative and innovative method to reduce the chemicals and water use of a cooling tower by deionizing the feedwater by membrane capacitive deionization (MCDI). MCDI uses capacitive electrodes and ion-exchange (IX) membranes to remove ions such as chloride and calcium from various water sources, including, tap, well, and even seawater (4-9). The key difference between MCDI technology and standard capacitive deionization (CDI) technologies is the use of IX membranes, which translates into much higher ion removal efficiencies and water recoveries (5, 6, 8).
In addition, the IX membranes increase the ion storage capacity of the carbon electrodes by up to 40% because of the additional ion storage capacity that is available inside the macro-pores (pores greater than >50 micron [µm]) of the carbon electrodes (6). Furthermore, the membranes reduce the sensitivity of the electrodes for scaling and fouling by forming a physical barrier between the fouling-sensitive electrodes and the flow channel, resulting in a greater lifetime of the MCDI module.
MCDI, because of its tunable salt removal, ability to start without long start-up sequences, and high water recovery, can be implemented on the make-up water, which is in contrast to RO systems that are installed on the recirculation loop or blowdown water. There are four distinct advantages of MCDI compared to RO for the cooling tower application; the first being that it has a low fouling potential to silica. The second is the high water recovery, often above 80%. This equates to a more efficient use of the feedwater and higher overall water savings in the cooling tower system. The third advantage is that MCDI requires minimal pre-filtration compared to RO (10). The fourth benefit is the low energy use, allowing for lower operational costs (11).
The objective of this article is to demonstrate the feasibility of MCDI to produce desalinated water for cooling tower makeup. An additional focus is to demonstrate that MCDI is a robust technology that can treat raw river water (Black River, OH, USA) with minimum pre-filtration and cleaning.
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