As part of our continuing series answering frequently asked questions from customers, here are some important basics about that water industry workhorse, the dissolved oxygen and ozone analyzer.
Q. How do dissolved oxygen/ozone sensors work?
A. Dissolved oxygen and dissolved ozone sensors are amperometric sensors with a gas-permeable membrane stretched tightly over a cathode. A silver anode and an electrolyte solution complete the internal circuit. During operation, oxygen or ozone diffuses from the sample through the membrane to the cathode. A polarizing voltage applied to the cathode converts all the oxygen entering the sensor to hydroxide ions. The reaction produces a current, which the analyzer measures. The current is directly proportional to the rate at which oxygen/ozone reaches the cathode, which is ultimately proportional to the concentration of oxygen/ozone in the sample.
Q. Why are dissolved oxygen measurements necessary?
A. Dissolved oxygen is very important in the treatment of domestic wastewater, as well as industrial waste from such sources as the food, pulp and paper, chemical, and metals industries.
The primary function of dissolved oxygen in a waste stream is to enhance the oxidation process by providing oxygen to aerobic bacteria so they will be able to successfully perform their function of turning organic wastes into their inorganic byproducts, specifically, carbon dioxide, water, and sludge. This oxidation process, also known as the “activated sludge process,” is probably the most popular and widely used method of secondary waste treatment today and is normally employed downstream of a primary settling tank. The process takes place in an aeration basin and is accomplished by aeration (the bubbling of air or pure oxygen through the wastewater at this point in the treatment process). In this manner the oxygen, which is depleted by the bacteria, is replenished to allow the process to continue.
In order to keep this waste treatment process functioning properly, a certain amount of care must be taken to hold the dissolved oxygen level within an acceptable range and to avoid conditions detrimental to the process. It is also important to make the measurement at a representative location on a continuous basis to have a truly instantaneous measurement of the biological activity taking place in the aeration basin.
Q. What affects the accuracy of dissolved oxygen/ozone sensors?
A. Since the O2 and O3 sensors measure diffusion across a membrane, anything that affects the diffusion rate can make the reading inaccurate. Two major concerns are: 1) a coated membrane which slows down diffusion; and 2) inadequate flow which reduces the availability of fresh sample near the sensor tip.
The rate of diffusion of ozone through the membrane also depends on temperature, as this affects the membrane permeability. Rosemount Analytical dissolved oxygen and dissolved ozone sensors include a PT100 RTD sensor, which can measure the temperature and send it back to the analyzer for automatic compensation. If the temperature reading is not correct, the analyzer will signal an error.
Q. How to calibrate a dissolved ozone sensor?
A. Calibration is necessary to ensure measurement accuracy. It is recommended that you calibrate your equipment regularly – the more critical the data, the more frequent the calibrations.
Because ozone standards do not exist, the sensor must be calibrated against the results of a laboratory test run on a grab sample of the process liquid. Ozone solutions are unstable, so the sample must be tested immediately. Portable test kits are available from other manufacturers.
Q. How do I clean my dissolved oxygen/ozone sensors?
A. Periodic maintenance and cleaning is required for best performance of the sensor. Generally, the membrane and fill solution should be replaced every four to six months. Sensors installed in harsh or dirty environments require more frequent maintenance. When cleaning a dissolved oxygen or ozone sensor, do not rub or brush the membrane surface. Carefully rinse the sensor tip with water to remove surface coating. If that does not restore function, change the membrane cap and calibrate.
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Integrated analytical measurement systems are often complex, costly, and have multiple stakeholders involved throughout all phases of the system’s integration project – from conception to commissioning. With so many factors to consider, including stringent specifications, thorough documentation, and tight deadlines to meet, any minor delay can cause a major setback and place your bottom line at risk.
Most industrial companies aren’t equipped to go into the systems integration business in order to meet their analytical systems’ needs. Emerson is! With a unique combination of analytical expertise, process knowledge, and global resources, Emerson is a single-source provider of complete analytical solutions for liquid and process gas applications including integration of third party analyzers. From sample handling systems and standalone instrumentation panels and cabinets, to three-sided shelters and environmentally controlled walk-in enclosures, Emerson offers wide flexibility in system packaging to meet project and application requirements.
Far from just nuts and bolts hardware, Emerson manages systems projects through the Project Management Office (PMO) where highly trained systems engineers take care of everything from Front End Engineering Design (FEED) and consulting services to manufacturing, integration and testing, to commissioning and on-going lifecycle support.
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Download a copy of the guide HERE and get a better idea of how to take the risk and worry out of executing complex analytical systems projects. We also invite you to learn more about the Emerson advantage in systems integration by visiting Emerson.com/RosemountAnalyticalSystems.
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Let’s talk chlorine. Obviously, balancing chlorine in water treatment is a critical issue, so having a friendly relationship with your chlorine analyzers is important. Customers ask us a number of basic questions about chlorine and its analysis. Here are a few of these questions with the essential answers from our Rosemount Analytical experts.
Q. How do you measure chlorine in aqueous solutions?
A. Reagent-less chlorine measurement requires an amperometric sensor and an analyzer to convert the current signal to a ppm reading. Unless the pH never changes more than 0.2 pH units, a separate pH sensor is strongly recommended when performing chlorine measurement. The preferred installation is in a bypass line with the sensors installed in a low flow cell. Other alternatives are installation in a 1-1/2 inch tee or submersed in a tank.
Q. How do chlorine sensors work?
A. Amperometric sensors use electric currents or changes in electric current to detect ions in a solution. The amperometric sensor tip consists of a membrane stretched over a noble metal cathode. The chlorine in solution diffuses through the membrane to the surface of the cathode. A voltage applied to the cathode reduces the chlorine to chloride. This process consumes electrons, which come from a second electrode (the anode) inside the sensor. To measure the amount of chlorine in the solution, the analyzer measures the number of electrons consumed at the cathode (the current) which is directly proportional to the concentration of chlorine in the sample. Since the sensor is constantly consuming chlorine from the process, it is necessary to have a continuous flow in front of the sensor, or else all the chlorine in front of the cathode will be destroyed, and the sensor will read zero chlorine in the sample.
Q. What types of chlorine sensors are available for real-time measurement and process control?
A. Monochloramine, free chlorine, and total chlorine sensors and systems are available for process control. It’s important to match the application and kind of chlorine to the measurement system.
Q. How do I calibrate a chlorine sensor?
A. Because stable, diluted chlorine and monochloramine standards are generally not available, the sensor must be calibrated against the results of a laboratory test run on a grab sample of the process liquid. (Learn how to zero the Free Chlorine Measuring System with a Rosemount Analytical 499ACL sensor here.)
Q. What affects the accuracy of chlorine measurements?
A. Chlorine measurement accuracy can be affected by fluctuations in temperature as it changes membrane permeability rate, electrolyte conductivity, and the sample pH levels. The need for additional sensors is reduced with the automatic temperature compensation and low sample conductivity requirement on the Rosemount 499ACL (Option -01) free chlorine sensors. Click HERE to learn more about the Rosemount Analytical 499ACL Chlorine Sensor.
There are many more basic questions on chlorine measurement that can be critical to successful water treatment. If you have pressing questions, please comment below.
We’ll address some more chlorine questions in the future.
By Marc Mason, business development manager, liquid analysis, and Gregory Taylor, sales manager, Emerson Automation Solutions
I wanted to share some insights from our liquid analysis experts into the top questions asked by users like you about pH measurement. Today, I’ve selected a few of the questions asked the very most. Chances are, you or someone in your plant have had one or more of these questions puzzling you –
Q) What affects the accuracy of a pH calibration?
A) The first thing to consider when trying to get an accurate pH measurement is the proper calibration of your equipment. Make sure that you take the appropriate time to calibrate your pH meter or analyzer with a quality standard buffer solution.
Room temperature, buffer temperature, and sample temperature all impact the calibration process. Try to simulate the actual environment the sensor will be operating in for the best calibration results.
As the pH sensor depends on its glass tip to make readings, the cleanliness and the quality of the glass can also impact your accuracy. Time, heat, and harsh chemicals gradually eat away at the glass surface, changing its properties and degrading the quality of the reading.
Q) What is the slope of a sensor?
A) The slope (sensitivity) indicates how well the sensor responds to changes in pH. A theoretically ideal electrode slope has an mV change of 59.16 mV/pH at 25°C (77°F). As the electrode ages, its slope decreases and a sensor should be replaced when the slope reaches 48 to 50 mV/pH.
Q) Why isn’t a pH 10 buffer solution recommended for calibrating?
A) A pH 10 buffer solution absorbs carbon dioxide (CO2) from the air, which depresses the pH. When CO2 is absorbed in water, it forms carbonic acid, which in turn lowers the pH of the buffer. Thus, the true pH is less than the expected value, and the calibration slope is low. If the pH 10 buffer gives a low slope, repeat the calibration using a lower pH buffer. For example, use pH 4 and 7 buffer instead of pH 7 and 10 buffer. It’s important to keep in mind, however, that a 10 pH buffer is more susceptible to cross contamination between buffers during calibration and more affected by temperature than a 4 or 7 or other lower pH buffers, so at different temperatures, a 10 pH buffer may not actually be a 10. Applications with higher pH values often times are better served using a 10 pH buffer.
Q) Why should I monitor glass impedance and reference impedance?
A) Glass impedance refers to the impedance of the pH-sensitive glass membrane. The impedance of the glass membrane is a strong function of temperature. As temperature increases, the impedance decreases. The impedance of a typical glass electrode at 25°C is about 100MΩ. Most Rosemount Analytical pH sensors are 50 to 200MΩ, with exception of the PERpH-X pH sensors, which measure 400 to 1,000MΩ. A sharp decrease in the temperature-corrected impedance implies that the glass is cracked. A pH sensor that has a cracked glass will often read pH around 7, so if the pH meter is consistently reading 7 pH, it would be a good idea to calibrate the sensor to see if it is truly working. High glass impedance implies that the sensor is nearing the end of its life and should be replaced as soon as possible.
The major contributor to reference impedance is the resistance across the liquid junction plug. The resistance of the liquid junction should be less than 40kΩ. High impedance readings around 140+kΩ typically indicate that the junction is plugged or the filling solution/gel is depleted.
Q) What causes pH sensor poisoning?
A) The common reference electrode used in pH measurements consists of a silver wire coated with silver chloride in a fill solution of potassium chloride. The purpose of the potassium chloride is to maintain a reproducible concentration of silver ions in the fill solution, which in turn, results in a reproducible potential (voltage) on the silver-silver chloride wire. The mechanism of reference poisoning is a conversion of the reference from a silver-silver chloride based electrode to an electrode based on a different silver compound.
The ions (bromide, iodide, sulfide) form less soluble salt with silver than does chloride. When these ions enter the fill solution, they form insoluble precipitates with the silver ions in the fill solution. But there is no initial effect on the potential of the reference, because the silver ions lost to precipitation are replenished by silver ions dissolving off the silver chloride coating of the silver wire. It is not until the silver chloride coating is completely lost that a large change in the potential of the reference occurs. At this point, the reference electrode must be replaced.
Hopefully, the answers to these FAQs have touched on areas that you need to know. More information can be found here.
I’ll be back with more user questions here at Analytic Expert soon.
It’s easy to think of wastewater as the forgotten kid of industrial and municipal applications. The lower budgets and constrained personnel power often means wastewater is the last to take advantage of new technologies and solutions. And sometimes this can be a costly mistake as an installation at an Indonesian Liquid Natural Gas plant shows.
This LNG plant, the largest in Kalimantan, Indonesia, has a Corporate Social Responsibility (CSR) to ensure its wastewater quality meets environmental regulatory requirements before they discharge effluent into the sea – a demand shared by most industrial enterprises. This plant has been granted a gold rating in the Indonesian government PROPER* program and, in order to maintain its gold status, the company must continually strive to utilize innovative methods that ensure environmental compliance and sustainability. However, its wastewater treatment pond is located remotely, about 1 kilometer (km) from the LNG plant. Due to physical constraints and economic considerations, it had not been possible to implement online effluent pH measurement. The monitoring and reporting of process wastewater was done manually via a third-party Health, Safety & Environment (HSE) engineer. This method requires the HSE engineer to commute to the wastewater pond at least two to three times daily, collecting effluent grab samples and reporting data back to the local environmental agency.
From both the efficiency and compliance points of view, this method was unsatisfactory. The manual pH recording method is extremely time-consuming and accuracy of sample data can be unreliable. The risk of poor quality effluent being discharged between manual recording intervals is also a serious concern. Sample recordings can be missed when the HSE engineer is unable to go onsite due to safety issues such as severe weather conditions. Any uncaptured data poses a risk of violating regulatory statutes, resulting in penalties or operation suspension. Many municipal and industrial wastewater installations find themselves with this kind of challenge.
To solve the problem, this LNG plant implemented wireless – a technology some wastewater installations assume is out of their reach. The plant used the Rosemount™ 56 Dual Channel Transmitter with the Emerson Wireless 775 THUM™ Adapter and gained access to real-time online effluent pH monitoring. Previously, manual sample recordings took one to one-and-a-half hours to complete, and this was carried out two to three times per day, year-round. Replacing the manual method with a wireless solution saved approximately 1,000 hours of labor and travel time to the site. Yes, 1,000 hours! The return on investment was huge and immediate.
The remote diagnostic features of the wireless Rosemount 56 Liquid Analyzer enable maintenance engineers to quickly and easily identify and determine the cause of an issue, such as poor wastewater quality or a device malfunction. The data logger function provides data redundancy, mitigating the risk of losing data in the event of a power failure, and offers data recording for environmental audit reporting. Maintenance engineers can also download the process data and event logger from the analyzer to a memory stick for further analysis. Due to the success of this wireless solution, the plant plans to expand the monitoring scope to include turbidity and dissolved oxygen (DO) monitoring.
This example shows that few industrial applications can ignore the potential benefits of wireless technology. Wireless makes possible levels of automation unthinkable only a short time ago. Where are you using wireless in your installations?