By John Becker
In today's world, escalating demands are placed on scarce resources. Optimization 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 optimize throughput, water quality, power usage, maintenance and costs.
Power costs related to the operation of activated sludge aeration equipment generally runs from 30 percent to 60 percent 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 optimization of the aeration process is accurate and reliable DO monitoring. The signals from on-line DO equipment provide the basic control parameter required for blower control. With dependable control inputs, aeration equipment can be modulated to maintain DO values at optimum levels.
The membraned type sensor has emerged as the clear choice for most dissolved oxygen measurement and control applications. Two characteristics of membraned sensors make them 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. Second, membraned sensors work as well in air as they do in water. This is important because it makes calibration easy. Ambient air represents a relatively stable oxygen standard. Using this standard to calibrate a DO sensor reduces calibration errors to a minimum.
Sensor Operation
A membraned dissolved oxygen sensor can best be thought of as an oxygen driven battery. Two electrodes are contained in a sensor body immersed in a common electrolyte. A polymer membrane isolates the electrode system from the measured sample, eliminating the possibility of electrode contamination. In operation, molecular oxygen diffuses through the membrane and is reduced to hydroxide ion on the surface of the working electrode.
Two types of membraned sensors are commonly used, polarographic sensors and galvanic sensors. While the basic operation of the sensor is the same in either case, there are differences in electrode materials and in the complexity of the electronics required for operating the sensor. Polarographic sensors, normally with gold and silver electrodes, require a bias voltage from the electronics in order to function. In addition, the silver electrode in these systems is more susceptible to H2S interference. Galvanic electrodes, typically platinum and lead, generate their own polarizing voltage and can operate independently of the electronic circuit. This can be a major advantage in maintenance and troubleshooting.
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. Excessively low DO levels will reduce the ability of organisms to metabolize the waste. High DO 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, DO levels need to be maintained within a narrow band, normally 0.5 to 2.0 ppm, for best performance in organic carbon removal.
Previous attempts at automating the control of DO levels in the aeration process suffered from the one serious weakness of membraned DO sensors, which is biological slime formation on the membrane. For accurate measurement, the polymer membrane on the DO sensor must remain clean. Any type of buildup on the membrane is the equivalent of increasing the membrane thickness, resulting in low DO readings. The problem is more severe when the buildup is a biologically active slime, because the slime consumes oxygen as it diffuses through the slime layer. This results in even larger errors on the low side.
One method of dealing with the problem is a high pressure "air blast" cleaning system integrated into the DO monitor. The systems 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 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 typically made of a relatively thick Teflon film to which most materials do not easily adhere.
One example of such a dissolved oxygen monitor is the D15/60 Auto-Clean DO Monitor from ATI, Analytical Technology Inc.
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 ATI sensor is designed to eliminate any protrusions that might catch fibers that would then build up around the sensor. Its nozzle also is designed with this in mind because buildups on the nozzle can effectively defeat the cleaning function. Finally, its mounting assembly is designed so that the air line and signal cable are protected inside the mounting pipe, eliminating another source of potential buildup. The overall result is a sensor assembly that can operate in virtually any mixed liquor environment, providing accurate DO measurement for months without service of any kind. Normally, preventive inspection every 6 months is all that's needed.
The air supply for the sensor cleaner is integrated into the DO monitor electronic enclosure, which is NEMA 4X 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.
The monitor's electronics provide isolated 4-20 mA outputs for both DO and temperature. In addition, two programmable alarm relays are available for remote alarm functions or for simple control applications.
During the cleaning cycle, all analog 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 one minute and the outputs are then held for an additional three minutes to allow the sensor output to restabilize.
Maintenance and Calibration
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. The ATI monitor's electronics allow entry of the barometric pressure reading which makes push button air calibration practical. The system can be calibrated without the need for saturation tables.
With galvanic membraned DO sensors, about the only variable that affects calibration stability is the condition of the membrane surface. Because the "air blast" cleaner is effective at scouring the membrane, the frequency of process DO analyzers calibration is greatly reduced. Checking calibration about every three months is good practice, but relatively little calibration change is seen over periods of six months or more.
The air cleaning system requires no maintenance at all. Preventive sensor maintenance is recommended every six months, but many plants operate for 12 months between sensor service. The sensor is designed so that the electrode assembly can be 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.
John Becker is President of Analytical Technology Inc. He has a BA in Chemistry from Carleton College, and has 25 years of experience with on-line analyzers and their application to the water and wastewater industry.
Booth 2559
