Water monitoring


Reverse osmosis membrane protection facilitated by direct, accurate, and automatic measurement of ultra-Low range chlorine residual


By Vadim B. Malkov (Hach) and Collin VanderZanden (Analog Devices)


nowadays, from municipal water and wastewater treatment to Ultrapure Water (UPW) production in various industrial applications. Multiple studies demonstrate that prolonged exposure of RO membranes to chlorine exceeding 38ppb (based on 1,000ppm-hr over three years) is detrimental to the membrane structure and integrity, while the absence of the disinfectant promotes biofouling and causes loss of recovery. To maintain this delicate balance, the membrane operators must accurately monitor oxidant concentration and addition of dechlorinating agents, especially in the RO feedwater. It is also important to monitor the cumulative exposure of the membranes to the oxidising disinfectant to understand its impact on membrane efficiency and life span. To control chlorine residual, utilities use available methods and instrumentation, which may not provide adequate results because of infrequent, indirect, or inaccurate measurements.


T


he use of membranes for water treatment, especially Reverse Osmosis (RO) filtration processes, has almost doubled in the last five years. The use is widespread in many industries


Figure 1. Comparative test of responses of colorimetric and ORP sensors to chlorine presence/absence in WW dechlorination application.


TABLE 1. BENEFITS AND DEFICIENCIES OF CURRENTLY AVAILABLE TECHNOLOGIES TO CONTROL CHLORINATION AND DECHLORINATION PROCESSES.


ANALYTICAL TECHNOLOGY MEASUREMENT PRINCIPLE MAIN BENEFITS MAJOR DEFICIENCIES


Oxidation-Reduction Potential (ORP)


Electrochemistry (Potentiometry) – change in mV output proportional to change in oxidation potential


Amperometric


Electrochemistry (Amperometry) – change in current/voltage across electrodes proportional to chlorine concentration


Colorimetric


Colorimetry – change in colour intensity proportional to chlorine concentration


Fast response to appearance of oxidants in the water, reagentless


Fast reaction to changes in chlorine concentration in the water, reagentless


Direct and accurate measurement, independent of sample conditions, stable calibration


* See Figure 1 illustrating the non-linear response of ORP to chlorine presence/absence. 38 April 2022 Instrumentation Monthly


Indirect, non-specific, matrix influence (sample pH, flow/pressure, etc.), non-linear response*


Calibration-dependent, matrix influence (sample pH, flow/pressure, etc.), may lose sensitivity to chlorine


Non-continuous response (batch analysis), reagents.


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