Pinpointing DO concentration optimizes aerobic digestion for Illinois WWTPReplacing high-maintenance membrane probes with accurate luminescent technology for online measurement of dissolved oxygen has helped bring greater efficiency to the aerobic digestion process at the Paris, Ill. Wastewater Treatment Plant (WWTP).
Maintaining optimal dissolved oxygen (DO) concentrations in its twin aerobic digesters has long been a key objective at the 1.4-mgd plant in east-central Illinois.
However, the limitations of galvanic membrane-type sensor technology made it difficult for the plant to reach this goal consistently. It wasn’t until the facility switched to new measurement technology that operators could finally be confident in maintaining effective DO levels in the plant’s two-stage aerobic digestion process.
Switch to aerobic digestion
Paris’s wastewater treatment plant serves a population approaching 10,000.
In addition to domestic wastewater, about 20% of plant loadings come from light industry, including food processors, plastics manufacturers and fabrication centers. In 1997, the original anaerobic system began to require significant maintenance and repair and the plant was retrofitted from anaerobic to aerobic digestion.
The switch to aerobic digesters was made in order to streamline operations and produce higher-quality sludge. Switching to an aerobic process eliminated the issues associated with anaerobic operations, such as methane capture, and saved costs from several operational aspects. There is no heat exchanger required, and because no methane gas is produced, there is no need for explosion-proof piping or a gas capture apparatus.
Optimum aerobic digestion requires tight DO monitoring and control, however. During the aerobic digestion process, aeration must be controlled accurately in order to complete the treatment within the contact time available. Inadequate DO levels will retard the process and cause the essential bacteria to die.
Conversely, too much aeration creates an excessive energy demand. In municipal wastewater treatment plants, the energy bill is typically the first- or second-most expensive operational expense, so finding a way to optimize the aeration process can help manage costs while maintaining proper effluent quality.
Galvanic DO probes inadequate
Until 2005, the Paris WWTP relied on galvanic membrane probes installed in the digesters to provide DO measurements, but the operators had reason to doubt the accuracy of the readings. Constant concern that there might be a shortage of DO in the digesters led to a practice of overfeeding air as insurance against noncompliance or an expensive remediation operation in the event of process failure.
Membrane-type DO probes rely on the consumption of oxygen at one electrode and the resulting current flowing through an electrolyte to the second electrode. This oxygen consumption creates a fouling buildup in membrane sensors and an oxygen gradient that slows and ultimately distorts response.
At the Paris WWTP, even the probes’ temperature readings came under suspicion.
“We were seeing summertime readings of 30°F, and that error had a significant impact on our not trusting the accuracy of the reported DO level,” said Don Jordan, chief operator at the Paris WWTP.
The problem with inaccuracy was most pronounced after the electrolytes dried out and the galvanic probes had to be replaced. They could not be calibrated satisfactorily after the change and the result was a continued loss of operational efficiency.
In addition, changing out the probes was labor-intensive. Maintenance procedures could consume hours of operator time.
“The DO levels just didn’t look right,” Jordan said, “and the maintenance created a headache. We needed a more reliable DO monitoring method.”
Adopting LDO technology
In an effort to solve the problem, each 300,000-gal aerobic digester was fitted with a Hach LDO luminescent technology probe in June 2005. The two LDO probes function in series and communicate with a Hach sc100 controller via simple “plug-and-play” connections.
In more than a year of operation, the new sensors have proven their worth in terms of accuracy at the plant. Operators check the probe readings on the controller and make adjustments as needed to the blower outputs. Although many wastewater treatment plants use the LDO readings to automatically adjust blowers, operators at the Paris WWTP prefer having hands-on control. The plant’s blowers aerate both the aeration tanks and the digesters, so maintaining manual control applies to more than one part of the system. Plant operators usually make one or two adjustments during a 24-hour period.
“Most of the time the air valve is wide open on our primary digester,” said Jordan. “The secondary digester is where most of the adjustment takes place.”
No membranes, no reagents
Unlike galvanic membrane sensors that actually consume oxygen during a complicated measurement process, the LDO sensor relies on light transmission to determine DO levels. Also, the LDO probe requires no membrane or reagent.
Instead, the sensor is coated with a luminescent material. Blue light from an LED is transmitted to the sensor surface, which excites the luminescent material. As the material relaxes, it emits red light. The interval of time between the blue light transmission and the red light response correlates to the amount of oxygen present. The greater the oxygen concentration, the less time it takes for the red light to be emitted.
Using LDO sensors has eliminated worries about maintaining an effective DO level in the plant’s two-stage aerobic digestion process. The two digesters are an extension of the plant’s aeration tank system. The cycle is based on a 31-day retention time. The target DO level in the first digester is 0.1 mg/L. The goal is to achieve optimum digestion with minimal DO residual.
While a certain amount of mixing and aeration is required in the second digester to maintain homogeneity, at this point in the process, the product has stabilized. Plant operators typically target a residual DO reading of 2 mg/L for this digester, where the finished sludge is produced.
The secondary digester is shut off and allowed to become anoxic for 8 to 12 hour periods so that supernate can be decanted. This increases the volume capacity of the second digester and helps minimize the amount of chemical additions required to maintain the desired pH level of 6.5 to 7. During warm weather, the pH level tends to decrease. The accurate readings provided by the LDO sensors are critical to fine-tuning these processes and preventing unnecessary aeration.
Digestion slows during the winter due to colder temperatures, but the Paris WWTP has consistently measured fecal coliform counts well below regulatory requirements during these periods. The blowers put a significant amount of kinetic energy into the digesters, which transfers heat into the operation. Even in very harsh winters, the temperature usually remains around 50°F. In cold water, the DO level is usually higher, and the blowers can be slowed down as long as the digester temperature remains acceptable. The accuracy of the LDO sensors helps operators monitor this balance effectively.
The aerobic digester tanks are 45 ft in diameter and have a 31-ft operating depth. To provide aeration to these digesters, the plant is equipped with two 1,600-cfm and two 800-cfm blowers. The pole-mounted LDO probes are immersed approximately 6 to 8 ft from the surface in each digester. Operators manually adjust the blower rates based on the DO readings transmitted to the sc100 controller.
Two probes, one controller
The Hach sc100 controller continuously reads the LDO probes. The controller also has a built-in datalogger that collects measurements at user-selectable intervals (1 to 15 minutes), along with calibration and verification points, alarm history and instrument setup changes for up to six months. The controller is designed to receive data from up to two sensors simultaneously, and plug-and-play capabilities and multiple-parameter functionality will allow operators to easily switch probes between different processes.
“The Hach sc100 controller and LDO probes were easy to install and we estimate a savings of approximately 5% on our aeration power costs since switching over to the LDO probes,” Jordan said. “Taken together with our operation and maintenance savings, we have a significant overall process improvement.”
Maintaining the former membrane-type sensors had required frequent intervention, complicated membrane replacements and subsequent calibration difficulties that led to a general mistrust of the DO readings. By contrast, maintenance has been minimal with the LDO sensors. Over the past year, each sensor has been pulled from the tank and wiped clean only once. The entire operation took about 15 minutes. When the plant undergoes its next upgrade, operators hope to see another online LDO meter installed in the last stage prior to final clarification, to further fine-tune the treatment process.