By Michael Francis, global product manager, Emerson Automation Solutions
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.
Do you have any tips on using dissolved oxygen optimally?
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References
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