Dissolved oxygen monitor
In today's world, escalating demands are placed on our scarce resources. Optimisation of resource usage through the appropriate application of technology can help meet this demand. Sewerage treatment is no exception, with designers and operators striving to optimise throughput, water quality, power usage, maintenance, and costs.
Power costs related to the operation of activated sludge aeration equipment generally runs from 30% to 60% of the total electrical power used by a typical sewerage facility. Dissolved oxygen control in the aeration process can save substantial amounts of power by applying only enough air for the biological process to function efficiently. An important additional benefit of accurate control is improved process efficiency and consistent clarifier operation.
The key to optimisation of the aeration process is accurate and reliable D.O. monitoring. The signals from on-line D.O. equipment provide the basic control parameter required for blower control. With dependable control inputs, aeration equipment can be modulated to maintain D.O. values at optimum levels.
Dissolved oxygen sensors were developed for commercial application in the late 1960s in response to the rapid increase in the use of activated sludge in wastewater treatment.
Early sensors were originally designed for portable and laboratory use and were later adapted to on-line instruments. A variety of sensor styles were developed, including the membraned sensors commonly used today, the thallium electrode sensors that are now obsolete, and naked electrode sensors using galvanic principles.
A galvanic sensor is a pair of dissimilar electrodes immersed in a sample or in electrolyte solution. The metals are chosen so that the electrode potentials are appropriate for reduction of oxygen at the cathode. Common cathode materials are gold, silver or platinum. The anode may be copper, iron or lead.
Galvanic electrodes generate their own polarizing voltage and can operate independently of the electronic circuit. This can be a major advantage in maintenance and troubleshooting
Voltammetric / Polarographic sensors
A voltammetric sensor also uses two dissimilar metals. The difference is that a bias voltage is applied across the electrodes. This voltage produces a current proportional to electroactive species present in the sample.
The silver electrode in these systems is more susceptible to H2S interference
Bare electrode sensors are susceptible to both poisoning and fouling. Because of this, the membraned type sensor has emerged as the clear choice for most dissolved oxygen measurement and control applications today.
The technique of using a membrane was patented in 1959 (1) by L.C. Clarke who designed a two-electrode voltammetric cell using a polyethylene membrane with a gold cathode and silver anode. This type of cell has become known as a Clarke cell. Membrane technology was applied to galvanic sensors using a silver cathode and lead anode by Mancy et al (2) and further developed by F.J. H. Mackereth in 1963 (3). This type of sensor is now known as a Mackereth cell.
Advantages of membraned sensors Accuracy
Two characteristics of membraned sensors make them very well suited to the task. First, membraned sensors are more accurate than other techniques. The polymer membrane that isolates the sensor from the measured sample eliminates any interferences. Naked electrode sensors can suffer from errors due to changing solution conductivity and can respond to other oxidizing chemical species.
Membraned sensors work as well in air as they do in water. This is important because it makes calibration extremely easy. Ambient air represents a relatively stable oxygen standard. sing this standard to calibrate a D.O. sensor reduces calibration errors to an absolute minimum.
Aeration Tank Application
The activated sludge process for wastewater treatment is essentially a biological process in which aerobic microorganisms consume the organic contaminants in the incoming wastewater. Organic carbon in the waste stream is the food on which these organisms live, and the dissolved oxygen in the wastewater is critical to the survival of these organisms.
Aeration of the activated sludge is achieved by mechanical surface aerators, diffused air systems or direct oxygen injection.
Excessively low D.O. levels will reduce the ability of organisms to metabolise the waste. High D.O. levels provide an environment where other types of organisms will begin to proliferate, and the result can be poor settling in the secondary clarifiers. For these reasons, D.O. levels need to be maintained within a narrow band, normally 0.5 to 2.0 PPM, for best performance in organic carbon removal.
Main problem with using sensors in aeration tanks
Previous attempts at automating the control of D.O. levels in the aeration process suffered from the one serious weakness of membraned D.O. sensors, which is biological slime formation on the membrane. For accurate measurement, the polymer membrane on the D.O. sensor must remain clean. Any type of build-up on the membrane is the equivalent of increasing the membrane thickness, resulting in low D.O. readings. The problem is more severe when the build-up is a biologically active slime, because the slime actually consumes oxygen as it diffuses through the slime layer. This results in even larger errors on the low side.
Another common problem is sensor "ragging. Strips of paper, hair and other fibrous matter can wrap itself around sensors.
Solutions to Fouling Problem
Various attempts have been made over the years to solve the D.O. sensor-fouling problem.
Mechanical wipers, brushes, were attached to sensors but often experienced fouling problems with the mechanical components.
A naked electrode system with a grindstone to clean the electrodes had some success, but sacrificed accuracy and was expensive to maintain.
The general problem with mechanical cleaning systems is that the cleaning system relies upon motors and other moving components; these components sometimes fail and will always need replacement eventually.
Water jets, sometimes containing detergents, were also tried. The water jets proved ineffective at removing all the slime build up. Another problem with the systems is that long water lines are prone to freezing, this stops the cleaning system working in cold weather and causes permanent long-term damage to the water pumps.
The next attempted solution was to use air to clean the sensor heads. The advantage of such systems was that the cleaning medium was free and did not freeze. Early systems utilized a central compressor and piped air around a plant using solenoid valves to control the cleaning of individual sensors.
The solenoid valves systems required maintenance and the, often, long pipe runs meant that the pressure of air at the sensor head was not high enough for effective cleaning.
Integrated high pressure air cleaning
The fouling problem was effectively solved with the development by ATi of a high-pressure "air blast" cleaning system integrated into the D.O. monitor. The system works by periodically scouring the sensor membrane with pressurized air, which is delivered in close proximity to the membrane through an air nozzle, which is part of the sensor assembly. The burst of cleaning air acts as an eductor, forcing a high velocity stream of air and water directly across the sensor membrane. This stream effectively blasts away biological and non-biological deposits from the surface of the membrane, preserving a clean surface through which oxygen can permeate without losses. This process is assisted by the fact that the membrane is made of a relatively thick Teflon film to which most materials do not easily adhere.
ATi sensors typically operate for a year or more wityh no maintenance in activated sludge. Sensor bodies can become coated in slime and or hair but the sensors continue to work within specification as the air clean system maintains the membrane in a perfectly clean state.
Important Features of High Pressure Air-Blast cleaning Nozzle design
Because many wastewater plants suffer from high levels of fibrous material in the aeration process, the design of the air blast sensor is important. The sensor is designed to eliminate any protrusions that might serve to catch fibres that would then build up around the sensor. The nozzle is also designed with this in mind because build-ups on the nozzle can effectively defeat the cleaning function.
The air supply for the sensor cleaner is integrated into the D.O. monitor electronic enclosure, which is IP65 rated for outdoor service. Microprocessor electronics control the cleaning process and are easily configured by the user. The cleaning frequency is operator adjustable to meet specific plant conditions. While daily cleaning is sufficient for some plants, cleaning cycles of every 6 hours have been necessary in particularly difficult applications.
Protection of air lines
Finally, the mounting assembly is designed so that the airline and signal cable are protected inside the mounting pipe, eliminating another source of potential build-up.
The ATI sensor is mounted to the handrail using a special bracket that allows easy access to the sensor for maintenance and calibration.
The overall result is a sensor assembly that can operate in virtually any mixed liquor environment without regular service, providing accurate D.O. measurement 24 hours a day for many months without service of any kind. Normally, a calibration every 6 months is all thats needed.
D.O. monitor electronics provide isolated 4-20 mA outputs for both D.O. and temperature. In addition, two programmable alarm relays are available for remote alarm functions or for simple control applications.
During the cleaning cycle, all analogy outputs and control relays are held at pre-cycle values so that control equipment is not affected by the cleaning process. The cleaning cycle lasts for 1 minute and the outputs are then held for an additional 3 minutes to allow the sensor output to re-stabilize.
Maintenance & Calibration
Sensor As previously noted, one of the advantages of membraned sensors is ease of calibration using ambient air as a reference. Ambient air provides a relatively stable oxygen partial pressure that is determined primarily by the barometric pressure at the plant site. D.O. monitor electronics allow entry of the barometric pressure reading, which makes simple push button air calibration practical. D.O. systems can be calibrated easily without the need for saturation tables, and air calibration generally provides the most repeatable standard available.
With galvanic membraned D.O. sensors, about the only variable that affects calibration stability is the condition of the membrane surface. Because the "air blast" cleaner is so effective at scouring the membrane, the frequency of process D.O. analysers calibration is greatly reduced. Checking calibration about every 3 months is good practice, but relatively little calibration change is seen over periods of 6 months or more.
Preventive sensor maintenance is recommended every 6 months, but many plants operate for 12 months between sensor service. The sensor is designed so that the electrode assembly can be easily and quickly removed for service without disconnecting field wiring. Sensor service can normally be done in 10 minutes, and interchangeable sensor modules can virtually eliminate down time.
The air cleaning system requires no maintenance at all. The compressor only runs for 15-20 mins. to day giving a field lifetime of many years.
Automatic D.O. sensor cleaning using the "air blast" system has greatly improved the reliability of process D.O. monitors and made effective oxygen control a reality.
Experience at a variety of plants in North America the nited Kingdom and Continental Europe have shown the system to be both accurate and reliable, with over 1000 units installed and working.
The system has proven itself in fine bubble aeration systems, coarse bubble aeration systems, mechanical aeration systems, and pure oxygen ( NOX) systems. Even aerated ponds with severe algae fouling have been successfully monitored.
2. Mancy, K. H., Okun, D. A., Reilly, C.N.: A galvanic cell oxygen analyser. J Electroanal Chem., 4 (1962) 65-92
3. Mackereth, F.J.H. An improved galvanic cell for determination of oxygen concentration in fluids. J. Sci. Instrum., 1964, Vol.41. 38-41
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