Optimization of The Scaled-Up Coaxial-Electrode Copper Ionization Cell for Water Disinfection

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Gore-Datar, Nissim
Xie, Xing
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Over 800 million people worldwide lack access to clean, uncontaminated drinking water, and over 1.1 million people in America lack a piped water connection. The public health consequences of this issue have been worsened by the ongoing pandemic, which has made the availability of clean water for hand washing more important. Centralized solutions to this issue, such as chlorination or membrane filtration, are too costly and energy intensive for widespread application in the developing world, and sometimes even pose their own risks, such as the formation of carcinogenic disinfection by-products (DBPs). Point-of-use solutions such as chlorine tablets or UV disinfection are more practical, but can also be energy intensive and pose DBP risks. The coaxial-electrode copper ionization cell (CECIC) is a water disinfection system developed to fill this gap using the biocidal properties of copper aided by other mechanisms such as electrophoresis, strong localized electric fields, and in-situ generation of copper ions. The CECIC has been proven to be highly effective (>6-log inactivation of E. coli with ~200 μg/l Cu) when tested with DI water at low flow rates in a reactor with an effective volume of 10 ml. In order to meet real-world conditions, it is necessary to scale up the system to a larger prototype and test its performance with more conductive waters at higher flow rates. This presents several challenges, such as maintaining a strong localized electric field with a low voltage in spite of the larger radius (inter-electrode distance) of the cell, keeping copper concentrations low in spite of a higher rate of copper release in more conductive water, and ensuring high bacterial inactivation in spite of a reduced hydraulic retention time (HRT). On the other hand, the larger cell of the scaled-up CECIC also allows for more flexibility with the anode configuration. More wires can be installed parallel to the flow and equidistant from the axis to reduce the gap between the electrodes, in turn creating more regions with enhanced electric field strength. The anode can also be positioned at an angle to the flow so as to increase mixing and contact with bacteria. Various experiments are designed and conducted in order to test these configurations and optimize the performance of the scaled-up CECIC under high-conductivity, high-flow conditions. Configurations consisting of 1, 3 and 6 wires positioned parallel to the flow as well as 3 wires positioned inclined to the flow are tested for their response to different flow rates (100 – 250 ml/min) at the same voltage (3 V) and to different voltages (0.5 – 7 V) at the same flow rate (150 ml/min). The results of these experiments show that inclining the wires reduces the disinfection performance rather than increasing it, but do not clearly indicate whether increasing the number of wires helps improve performance. Further testing is carried out with the original configuration (1 coaxial wire) to ascertain the synergetic role played by the electric field and copper concentration gradients by controlling the current supplied to the cell. These experiments demonstrate that the synergetic effect does play an important role even in the scaled-up reactor, with the disinfection performance improving significantly as the electric field strength increased. Lastly, the scaled-up system is tested with real water samples (river water and rain water) that are pre-treated to remove experimental interference from suspended solids and pre-existing microorganisms while preserving the real water matrix. Over 99% of bacteria are inactivated by the CECIC in both cases, with an effluent copper concentration of ~550 μg/l. This performance is lower than that achieved at the same conditions with synthetic water, likely due to interference from dissolved substances in the real water. However, this demonstrates that the scaled-up CECIC can disinfect real water samples too, which is an important stepping stone to pilot studies and field deployment.
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