Water monitoring Table 1 and Figure 1 (built upon a


comparative test conducted at a Wastewater Treatment Plant (WWTP) employing chlorination/dechlorination of final effluent before discharge), illustrate that ORP provides relatively fast response to chlorine breakthroughs. However, its response to excess of reducing agents, like Sodium Bisulfite (SBS), can be long. Moreover, relying on absolute values of ORP can be misleading due to limitations of this technology and its relative nature. Correlating ORP levels to chlorine concentration to quantify the response leads to severe problems, regardless of the sensors used to monitor, because ORP is a surrogate measurement.


Another electrochemical method used by some utilities to control chlorination/ dechlorination is amperometry and sensors built on this principle (Table 1). Unlike ORP, the amperometric technology provides a better correlation to chlorine concentration, being more selective. However, there are other potential issues in applying it successfully, especially to control the absence or very low concentrations of chlorine. This becomes a problem in intermittent applications because amperometric sensors must see oxidants in the sample to provide sustainable operation. Therefore, at intermittent sample flow or in consistent absence of chlorine, amperometric probes can lose their sensitivity to chlorine and require more frequent interactions. This happens due to various factors, from simple fouling of the probe surface, to developing layers of organic or inorganic coatings on the electrodes preventing necessary electrochemical reactions. When ORP or amperometric sensors are fully functional, their performance and accuracy depend on other parameters of the sample, e.g., pH, flow, pressure, etc. The benefits provided by electrochemical sensors are reagentless


Figure 2. Response of colorimetric (batch) and amperometric (continuous) analysers to rising chlorine concentration.


operation and fast response to rising chlorine levels based on the continuous nature of measurements. Visual comparison of such responses to rising chlorine levels (Figure 2) demonstrates the difference between continuous and batch analysis. The latter is represented by colorimetric technology and is based on the cyclic nature of the method that takes a sample, adds chemical reagents, and measures light absorbance, which usually takes one to two minutes to complete. Figure 2 shows the initial response of an amperometric sensor reported immediately that can help to reflect change in the chlorine concentration earlier on. Nevertheless, the full accuracy of the measurement is achieved in approximately the same time for both methods.


Any continuous measurement is characterised by the sensor’s response time, e.g., T90 or T95, which represents the time to achieve 90 per cent or 95 per cent of the maximum signal level, or accuracy. This characteristic, usually specified between 60 and 120 seconds, varies from sensor to sensor and depends on the sensor and sample conditions. For comparison, batch analysis of chlorine based on the standard Diethyl-p- Phenylene Diamine (DPD) method achieves approximately 100 per cent accuracy in 100 to 150 seconds and is independent of sample pH. Sample flow should be within the specified range and there are known interferences to the DPD colorimetric method to consider. Methods listed in Table 1 can be utilised through different techniques represented by either process,


TABLE 2. MAIN TECHNIQUES USED TO MONITOR CHLORINE RESIDUAL AND EXPECTATIONS APPLICABLE TO VERIFICATION.


PROCESS ANALYSIS GRAB SAMPLE ANALYSIS


MATCH CRITERIA (ONLINE & GRAB SAMPLE)


ORP sensor COMMON EXPECTATIONS Lab or portable ORP probe NA


Should not expect any match between process and lab ORP probes and


performance can be verified by using ORP standard solutions.


Amperometric sensor


Suitable colorimetric or titration method


Colorimetric analyser


Suitable colorimetric method/instrument


Readings within ±15% (EPA Method 334.0)


Readings within ±10% or X mg/L (the greater of the sum of specified accuracies or LODs [X] for comparable instruments)*


Amperometric sensor calibration (slope/offset) should be adjusted when the readings do not match.


Should not adjust analyser’s calibration based on comparison*. Should verify calibration with a set of appropriate chlorine standards, when needed.


* A less accurate reference method/instrument should not be used to verify the process analyser’s performance and adjust its calibration. Continued on page 40... Instrumentation Monthly April 2022 39


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