Water monitoring
or laboratory instrumentation. The latter is usually employed to measure grab samples (Table 2). Monitoring and proportional addition of sulfite- based 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 utilise. However, the major and obvious deficiency of such 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 analysers, built on a continuous or batch analysis method. Table 2 provides an overview of the criteria and expectations for such verification. To summarise, all currently available methods to monitor and control chlorination/dechlorination in water treatment have their positive and negative traits, and the utilities should carefully analyse 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 30ppb 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.
Figure 4. MBS feed response test.
Figure 5. Comparative accuracy test: ULR analyzer vs. Total Chlorine grab sample (Hach Method 8167). Flow through the analyser was also measured with internal flow meter and recorded in the data log. Three compared grab sample analyses did not show expected match even with regards to the grab sample accuracy depicted by vertical error bars. See details in Table 3.
Figure 3. Analyser installation point (first pass RO feedwater).
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TEST SETUP, RESULTS, AND DISCUSSION An online analyser using the DPD technology, to accurately detect and quantify chlorine concentrations in RO feed at below 30ppb, 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 analyser 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 an Analog Devices facility manufacturing microelectronics (semiconductors). The plant has several RO racks with over 200 individual cartridges with Granulated Activated Carbon (GAC) pretreatment and addition of metabisulfite to destroy extra chlorine residual in RO feedwater. The RO membranes are organised 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. Typical life expectancy of the membranes is three to five years. However, they are usually replaced 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, labour, and lost revenue. Every two to three years on average, the RO membrane users must run an autopsy of failed membranes 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 membrane lifespan and reducing operating cost can be economically justified. These considerations laid grounds for the facility
to try a new online analyser 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 analyser was installed in June 2020 at the first pass
April 2022 Instrumentation Monthly
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