Abstract

This article discusses the different technologies to control chlorination and dechlorination and how using the right method can help save cost.

Introduction

The use of membranes for water treatment, especially reverse osmosis (RO) filtration processes, has almost doubled in the past five years.¹ The use is widespread in many industries 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 38 ppb (based on 1000 ppm/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.2 To maintain this delicate balance, the membrane operators must accurately monitor oxidant concentration and the addition of dechlorinating agents, especially in the RO feedwater. It is also important to monitor the cumulative exposure of the membranes to the oxidizing 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.

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)1, illustrate that ORP provides relatively fast response to chlorine breakthroughs. However, its response to an excess of reducing agents, such as sodium bisulfite (SBS), can be long. Moreover, relying on the absolute values of ORP can be misleading due to the limitations of this technology and its relative nature. Seeing ORP levels as a direct correlation to chlorine concentration to quantify the response can lead to significant issues, regardless of the sensors used for monitoring. This is because ORP is a surrogate measurement and may not accurately reflect the actual chlorine concentration.

Another electrochemical method used by some utilities to control chlorination/dechlorination is amperometry, and sensors are 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 in controlling 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, for example, pH, flow, pressure, etc. The benefits provided by electrochemical sensors are reagentless 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 the change in the chlorine concentration earlier on. Nevertheless, the full accuracy of the measurement is achieved at approximately the same time for both methods. Any continuous measurement is characterized by the sensor’s response time, for example, T₉₀ or T₉₅, which represents the time to achieve 90% or 95% of the maximum signal level, or accuracy. This characteristic, usually specified between 60 seconds 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 ~100% accuracy in 100 seconds 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 utilized through different techniques represented by either process or laboratory instrumentation. The latter is usually employed to measure grab samples (Table 2). Monitoring and proportional addition of sulfitebased agents is mostly done with either grab sample analysis based on DPD, or in combination with continuous ORP measurement. Intermittent grab sample analysis leaves significant gaps in the monitoring and can suffer from the user technique, while the relative nature of ORP does not make it the method of choice.

From the technique standpoint, grab sample analysis provides more versatility because there are different chemical or electrochemical methods to utilize. However, the major and obvious deficiency of such a technique is its intermittent nature that cannot provide a continuous measurement, and therefore efficient control of the process, be it static or dynamic. Thus, the main objective of the grab sample analysis is to verify the performance of process analyzers, built on a continuous or batch analysis method. Table 2 provides an overview of the criteria and expectations for such verification. To summarize, all currently available methods to monitor and control chlorination/dechlorination in water treatment have their positive and negative traits, and the utilities should carefully analyze these to fit the application, as well as expectations.

Some facilities use process chlorine monitoring instrumentation, which cannot deliver the desired result based on the existing state of technology. There is a demand for a simple and reliable instrument to measure chlorine residual at the lower end of the range in a substantially continuous manner and with adequate accuracy. The method should be accurate below 30 ppb to ensure sufficient concentration of disinfectant to control biofouling and avoid underfeeding/overfeeding the dechlorinating agent. Such instrumentation can maintain the health and longevity of the membranes at lower costs associated with additional cleaning and dechlorination.

Test Setup, Results, and Discussion

An online analyzer using the DPD technology, to accurately detect and quantify chlorine concentrations in RO feed at below 30 ppb, was developed and tested at several facilities using membrane filtration. The new instrument can be connected to a SCADA system, automatically reports the results every 150 seconds, and calculates cumulative chlorine exposure. The analyzer was tested in RO applications ranging from drinking water, to reuse, to power and oil refining, to desalination and beverage production.

This study was conducted at Analog Devices’ facility manufacturing microelectronics (semiconductors). The plant has several RO racks with over 200 individual cartridges with granulated activated carbon (GAC) pretreatment and the addition of metabisulfite to destroy extra chlorine residual in RO feedwater. The RO membranes are organized in first and second pass RO filtration systems. Their health is usually monitored using flow rate, total dissolved solids (TDS), and silica concentration in permeate and reject. The typical life expectancy of the membranes is three to five years. However, they usually require replacing about six months earlier than expected. Around 30 membrane cartridges are replaced during a typical year, which is approximately $10,000, including the costs of membranes, labor, and lost revenue. Every two to three years on average, the RO membrane users must run an autopsy of failed membranes. This is usually done by contractors and it can cost a few extra thousand dollars. Therefore, any premature failure of RO membranes due to chlorine breakthrough is a costly problem. Thus, extending the membrane life span and reducing operating cost can be economically justified.

These considerations laid grounds for the facility to try a new online analyzer using the DPD technology, which can detect and quantify chlorine concentrations in RO feed accurately below 30 ppb. The new instrument was considered ideal for installing and testing for at least three weeks. The analyzer was installed in June 2020 at the first pass RO system influent, after GAC beds and sodium metabisulfite (MBS) injection, with source water (city tap water) containing 3 ppm to 4 ppm chlorine before GAC.

After conducting the MBS response test (Figure 3), the plant personnel made the first observations, calculations, and preliminary conclusions leading to the extension of the test to learn more about the analyzer and its capabilities.

The main results of the first three weeks of testing showed the analyzer demon strating stable and accurate readings and fast reaction to changes in the MBS feed (Figure 3).

This facility normally calculates membrane life span based on the manufacturer’s recommendations to maintain chlorine level < 100 ppb and tries to keep it below 80 ppb with the target set at 30 ppb. The existing grab sample analysis method2 detects and measures chlorine above 20 ppb and was used to verify the performance of the ULR analyzer in a comparative test conducted in the extended trial (Figure 4).

Insufficient sample flow can affect the performance of any process analyzer, and therefore, the intermittent operation of RO skids, being a normal case, can present a big challenge. The internal flowmeter of the new ULR analyzer helped to overcome this challenge and maintained the instrument’s operation by placing the analyzer on standby when the sample flow was insufficient, and automatically restarting its operation when the flow was restored. This ensured the accuracy of the analyzer readings recorded in the internal logs, which were thoroughly analyzed to arrive at the right conclusions.

From the analysis of chlorine and flow data, graphically represented in Figure 4, it was clear that once the MBS feed was adjusted to lower rates based on the grab sample results, the discrepancy between the grab sample and online analyzer readings fell out of the expected tolerance (Table 2). This can be explained by comparing grab sample analysis details and specifications for both methods (Table 3).

Table 3 shows there were several grab samples taken for each comparison falling out of expected tolerance and the spread between the results for the same sampling was quite significant, up to 40 ppb. This indicates either fluctuations in the sample, accuracy of the lab analysis, or both. Therefore, the comparison between ULR chlorine readings (LOD = 8 ppb) and lab results (LOD = 20 ppb) should be considered marginally matching. Mainly, such discrepancies can be attributed to a higher probability of deviations in conducting grab sample analysis, because any test involving human interaction increases the chance of a random error. Based on this logic, statistics, and specifications, the ULR process analyzer was found to be producing accurate results, comparable to the reference grab sample analysis.

Simple data evaluation showed that, based on the analyzer readings, the dosage of the dechlorinating agent (for example MBS in this case) could be safely reduced and later eliminated without compromising the quality of the operations and risk of increasing biofouling of the membranes. Solely chemical cost savings can potentially return all investments in the analyzer at this facility in three to five years. However, once other direct and indirect savings (for example, reduction in CIP frequency, associated labor and chemicals, extended membrane life, reduction in production losses, etc.) are factored in, the ROI period becomes shorter and more appealing.

The instrument was left running at this facility for a long-term evaluation and after over a year-long test, more observations were collected. For example, the analyzer responded to a recent event related to a GAC tank failure (Figure 5).

The first pass RO feed comprised the combined effluent from all carbon beds (GAC tanks). Two out of four carbon beds each account for ~20% of the total f low and the other two for ~30% each. Sodium metabisulfite (if online) is injected downstream of the carbon beds and upstream of the RO membranes. The event presented in the graph (Figure 5) happened after the MBS feed stopped on June 6, 2021. It was discovered that one GAC tank’s effluent was bringing 150 ppb of chlorine to the combined sample and another—80 ppb at ~50% of the total f low. This contribution was immediately detected and recorded by the analyzer and once the media in the bad GAC tank was replaced (July 9, 2021), the chlorine concentration came down to the desired level of < 30 ppb as the grab sample analysis confirmed at 14:58 on July 9, 2021 (Figure 5).

Therefore, the introduction of the new analyzer has been instrumental in identifying potential issues with exhausted GAC media or the development of channels within carbon granules in the tanks, which can allow chlorine to bypass the intended treatment process. This is another potential benefit of the new instrument, especially when its outputs are connected to the facility’s SCADA system, or DCS, and the readings are used for decision support, if not for dechlorination control.

Conclusion

This article demonstrates the value of highly accurate direct chlorine measurements at minimal maintenance efforts and supports all chemical and labor cost savings elucidated by the instrument projecting ROI in approximately two years.

References

¹ “Reverse Osmosis Membrane Market—Growth, Trends, COVID-19 Impact, and Forecasts.” Research and Markets, April 2023.

² "Chemical Pretreatment for RO and NF.” Hydranautics, 2017.