Probing for Better Data

A new sensor is helping operators control aeration and provide more accurate readings

• By Bob Dabkowski
• Sep 01, 2006

Switching from a membrane-type dissolved oxygen (DO) probe to new, breakthrough luminescent technology has brought tighter control to the conventional aeration system serving the Holland Area Wastewater Treatment Plant. Plus, a four-month trial demonstrated that the new luminescent DO (LDO) probe performs well monitoring high-DO effluent from the facility's pure oxygen system. Until now, it had been difficult for the plant to find a reliable online measurement instrument for this environment.

Challenges
Maintaining optimum DO levels in its 606,000-gallon aeration basin had always been a challenge for the Holland Area Wastewater Treatment Plant. To control aeration and ensure proper DO concentrations, operators have relied on online DO measurement, but keeping these membrane-type DO probes operating consistently for long periods of time has always been troublesome. A recent switch to new DO measurement technology, however, has finally brought long-term accuracy and dependability to the process.

The 12 million gallons per day (mgd) Holland Area Wastewater Treatment Plant which serves about 70,000 people, receives wastewater from five townships and the City of Holland, Mich.

The plant is divided into the "East Plant" and "West Plant," which are essentially two separate secondary treatment systems that receive flows from the same primary clarifiers. The East Plant is a conventional activated-sludge system served by aeration prior to final clarification. The West Plant operates a high-purity oxygen-activated sludge process that uses pure oxygen gas rather than air as the aerating gas for secondary treatment.

Due to an abundance of meat processors and food industries in the area, raw water influent biochemical oxygen demand (BOD) can exceed 325 milligrams per liter (mg/L). The east and west sides of the Holland plant treat 35 and 65 percent of total flows, respectively. Final effluent is discharged to the nearby municipal power plant for use as cooling water.

DO Probe Troubles
The Holland plant had long been using stationary, membrane-type DO probes to monitor dissolved oxygen demand in its aeration system, which varies proportionally to the strength of the incoming waste stream. Operators routinely adjusted the plant's blowers during the day and night based on the probe's readings.

But keeping these probes operating was always a headache for plant staff, which had to continually clean and recalibrate them, as well as frequently replace their membranes. The most recent series of DO probes installed in the plant's aeration system had been cartridge-type DO sensors.

"We replaced the entire unit twice," said Wally Wittaniemi, an industrial electrician and instrument technician for Holland Board of Public Works. "The manufacturer said they'd last for at least 18 months, but they didn't hold up anywhere near that long."

Over the years, whenever one of the plant's stationary DO probes started to fail, operators would then begin using spot measurements from handheld DO metering units to supplement the necessary data to adjust the blowers. And as the performance of a stationary probe continued to deteriorate, scheduled spot DO readings would then become the plant's main basis for aeration control until the next new stationary probe was installed.

Although this "Plan B" method allowed the plant to consistently meet its discharge permit of no less than 5 mg/L DO in its final effluent, it was a vicious cycle, and it required adjusting the blowers slightly higher to play it safe. The plant wanted to consistently maintain tight DO control in its aeration process, not only to comply with strict treatment requirements, but also for optimizing process control and for achieving and maintaining cost-efficient power usage. In 2004, the plant decided to upgrade its stationary DO metering probe once again, but this time it chose a completely new technology.

The life of the luminescent technology sensor, the new instrument chosen, is considerably longer than that of traditional sensor technologies, partly because there are no membranes or electrolyte solutions that need replacement. The system self-calibrates, and cleanings are infrequent because the system produces accurate DO readings even with organic build-up on the sensor.

The probe is mounted inside the 104- x 52- x 15-foot aeration basin, on the end of a 2 inch-diameter PVC pipe situated between two of the basin's four mixing paddles and approximately halfway between the top of the water level and the basin floor.

"It's a pretty harsh environment in there, with all the air bubbles and the mixers running," Wittaniemi said, "But it hasn't caused any problems for this probe."

The probe is continuously read by a universal controller that communicates via a 4-20mA signal to the plant's SCADA system. The controller has a built-in datalogger that collects measurements at user-selectable intervals (one to 15 minutes), along with calibration and verification points, alarm history, and instrument setup changes for up to six months.

"This may be a sign of the times, but as these metering systems get more and more advanced, they are getting much easier to set up," Wittaniemi said. "The LDO probe is basically a plug-and-play unit -- you plug it into the sc100 and turn it on and it uploads all its information to the controller. It's a pretty neat unit."

Although the controller is designed to receive data from up to two sensors simultaneously, the plant currently uses it for the LDO probe only. The additional port provides the plant flexibility for adding a second LDO probe later or for gaining broader parameter measurements, such as suspended solids, by simply plugging in the appropriate sensor.

Operators manually regulate the amount of air going into the basin based on DO readings from the stationary LDO probe. In addition to the current DO reading, air volume from the blowers is monitored by the SCADA system from an orifice plate-differential transmitter, and a gauge installed near the blowers' valve controls allows operators to see how much air they're putting into the basin. Alarms have been established using the controller's 4-20mA signal that alert operators if DO in the aeration basin falls below 5.5 mg/L or rises above 6.5 mg/L. An option for the plant in the future is to install an automatic positioner on the pneumatically controlled valve, allowing the blowers to be controlled automatically based on the current DO reading.

After more than two years of continuous operation inside the aeration basin, the LDO probe is performing as well as the day it was installed, according to Operations Supervisor Larry Horn.

"We have much tighter aeration control with the LDO probe than we did before with our other units, and with the alarms we have set up, we no longer have to adjust the blowers on the high side to be safe, which saves power," he said.

The only maintenance required for the probe has been to periodically wipe the end of the sensor. Replacement of the probe's only replacement part, an inexpensive sensor cap, has not yet been necessary.

Monitoring Oxygen Activated Process
In addition to providing reliable online DO measurement in the plant's conventional aeration process, a four-month trial using the LDO probe to continuously monitor final effluent from the oxygen-activated sludge portion of the plant (the "West Plant") further validated the durability and dependability of the LDO in extreme conditions.

Because of the very high DO produced by the high-purity oxygen-activated process, the plant ran into difficulty finding instruments to consistently measure it. It's not an uncommon problem. Operation of oxygen activation plants is often complicated by the fact that there have been few, if any, reliable online measurement instruments available for high-DO environments.

Membrane-type DO probes rely on the consumption of oxygen at one electrode and the resulting current flowing through electrolyte to the second electrode. This oxygen consumption creates a fouling buildup in membrane sensors and an oxygen gradient that slows down response.

"A membrane DO probe basically acts like a battery," Horn said. "The higher the DO, the quicker it discharges. Average conventional treatment plants have DO levels of around 2 to 4 mg/L in effluent coming out of their secondaries.

"But because we're feeding pure oxygen instead of air (94 percent oxygen versus 21 percent), final effluent DO levels here run between 8-15 mg/L. Oxygen consumes membrane DO probes, and the higher the oxygen levels, the faster they wear out."

Horn said membrane probes have typically lasted only a couple of months monitoring the plant's oxygen-activated sludge process. Due to the inability to continuously monitor DO in high-purity oxygen processes, operators of systems like Holland, Michigan's must typically rely heavily on personal experience.

"We currently use vent purity measurements from oxygen air analyzers, along with final effluent DO measurements from our conventionally aerated side of the plant, to base the control of our oxygen-activated system," Horn said. "The original design of our system was to use oxygen air analyzers, but over the years we have also tried many different DO sensors in the tanks and effluent because we felt the response would be much faster. But we never found a probe that could handle it, until now."

Horn said that because the probe performed well in the higher DO environment of the final effluent stream, LDO probes would be good candidates for controlling high-purity oxygen systems. The non-electrochemical sensor does not consume oxygen and does not suffer from the extreme drift that membrane probes can experience as electrodes and electrolytes are quickly depleted in a high DO environment.

"I can see how we could install five LDO probes, one in each of our system cells, to automatically control the cell vents," Horn said. "That way, we could modulate the level of oxygen produced based on the current DO levels within each cell. This approach had never been feasible for us before because the probes were never able to withstand the environment."

This article originally appeared in the 09/01/2006 issue of Environmental Protection.