## 1 Introduction  
The current USA Environmental Protection Agency (EPA) effluent limitations guidelines and standards for the Steam Electric Power Generating Point Source Category's rule published in November 2015 mandates the limitation of toxic metals and other chemical compounds from coal-fired power plants [1]. On an annual basis, the rule incrementally reduces the amount of toxic metals, nutrients, and other pollutants that steam electric power plants are allowed to discharge (no more than 1.4 billion pounds or 635 million kg per year) and reduces water withdrawal by 57 billion gallons or 216 billion liters per year [1]. Currently, a Flue Gas Desulfurization (FGD) system is used to remove sulfur dioxide (SO\({}_{2}\)) from flue gas produced from coal-fired power plants. In the FGD system, limestone reacts with SO\({}_{2}\) and produces gypsum. The process also removes mercury (Hg), selenium (Se), arsenic (As) and other elements and compounds. An FGD- wastewater treatment (WWT) system also uses various chemicals, such as organosulfides for mercury, to remove solids (e.g., CaSO\({}_{4}\), Se, Hg, As, etc.,) from FGD wastewater. Chemicals required for the FGD wastewater treatment are stoichiometrically based on coal and limestone chemistry, various operational variables (e.g., cycle of absorber concentration, competing reactions, etc.,) and the numerical discharge limit regulated by the EPA. The plant operators periodically collect samples at various stages of the FGD wastewater process and measure numerous chemical species with hand-held kits that are later sent to commercial laboratory for further analysis. This entire process can be considered time consuming, costly, and tedious. Despite these factors, the level of detection and precision needed to detect contaminants at the designated limits require analysis by a commercial lab. The turnaround time on this analysis method may be weeks, making it difficult to control the process in a timely and efficient manner. This monitoring methodology is not acceptable for ensuring that discharge from the wastewater treatment plant is within current guidelines. An alternate option which utilities may elect in the projected EPA rule for 2023 is even stricter, where the allowable discharges of mercury, selenium, nitrates, and nitrites are significantly lower [1]. The state-of-the-art FGD wastewater treatment technologies (e.g., CH2M Hill's FGD wastewater treatment) will need to adapt new technologies to meet the discharge limits. Overall, real-time sensing of FGD wastewater during processing is needed to enable operators to make process adjustments in a timely manner for an acceptable, consistent discharge quality. Therefore, a real-time robust sensor module that detects water quality at various stages in the FGD wastewater treatment plant units is necessary to keep in compliance with the regulatory discharge limits. Ion selective electrode-based sensor-arrays provide a convenient solution to these problems as they target the species of interest in the FGD wastewater treatment process and provide real-time monitoring. This study evaluated several commercially available ion selective electrodes for their use in FGD wastewater systems. The goal of this study was to determine if ion selective electrodes (ISEs) were compatible with the general composition of FGD wastewater and if an accurate analytical methodology for FGD wastewater characterization could be constructed from both model solutions and actual FGD wastewater samples. As of 2015, more than 66 companies are manufacturing water related sensors in the world, but only a handful of them market ISE, which utilize special membranes that allow for the measurement of specific ions. An ISE (with its own internal reference electrode) is immersed in an aqueous solution containing the ions to be measured, together with a separate, external reference electrode. The external reference can be separate or incorporated in the body of the ISE to form a Combination Electrode.