The Clearfield Municipal Authority (CMA) set out to accomplish a number of key goals when upgrading its regional wastewater treatment plant in Clearfield County, PA, yet few took on more importance than a new aeration blower system capable of efficiently and cost-effectively delivering proper aeration across a wide spectrum of daily and seasonal operating conditions, in addition to meeting the plant’s long-term aeration needs.
To achieve its goals for aeration, CMA partnered with engineering firm Gwin, Dobson & Foreman, Inc. and blower manufacturer Aerzen USA to design and install a system in that leverages three aeration blower technologies, including turbo blowers, hybrid blowers and positive displacement (PD) blowers. Each fulfills aeration and oxygen demands for separate and distinct applications, while helping CMA continue its strong record of compliance at the lowest possible cost.
Turbo blowers are one of three aeration technologies used at the Clearfield Wastewater Treatment Plant. Shown with the units from left: Randy Rongeux, CMA Treatment Plant Operator; Leo J. Drass, Senior Project Engineer, Gwin Dobson & Foreman, Inc.; and Eric Bennett, Product Manager, Controls, Aerzen USA.
Mandates and Ongoing Improvement Drive Upgrades
Originally built in 1958, the CMA wastewater treatment plant serves 14,000 residents, as well as businesses, in Clearfield Borough and surrounding portions of Lawrence Township, Clearfield County, Pa. From the start, the plant consistently met water quality and effluent parameters as specified in its National Pollutant Discharge Elimination System (NPDES) permit throughout. However, new mandates in 2010 and the need for ongoing improvements drove the need for plant upgrades.
Mandates required the plant to comply with annual nutrient loading limits of total nitrogen (TN) and total phosphorous (TP) in addition to secondary treatment limits in accordance with the Chesapeake Bay Tributary Strategy. CMA also needed to more adequately handle and treat wet weather flow, which historically burdened the facility and its overall treatment capabilities. The new mandates and the need to update the aging infrastructure led to a $35 million plant upgrade.
The plant initiated the upgrade project in 2014. The first phase of the project involved construction of a new treatment facility adjacent to the existing facility. Major components of the new facility included an influent wastewater pumping station, new pretreatment/headworks building, three BNR reactors, clarifiers, channel UV disinfection units, Return Activated Sludge (RAS) pump and chemical feed building, and upgrades to the instrumentation and controls systems.
In the second phase of the project, the plant’s digesters and control building were upgraded to a new sludge processing facility. In addition, anaerobic digesters were converted to aerobic digesters with the aid of the blowers. The first phase of the project was completed in fall 2016. The second phase went online in summer 2017.
Operating Conditions Call for Unique Blower Strategy
The updated plant is designed to handle an average daily flow of 4.5 million gallons per day (MGD), yet allow for a peak flow rate of 25 MGD. Additionally, the plant must handle a maximum monthly flow of 16 MGD. A particularly rainy day might result in the need for the plant to handle 12 MGD. In addition to managing flows, the plant must achieve nutrient discharge concentrations of 6.0 mg/l (TN) and 0.80 mg/l (TP).
The need to address wide swings in operating conditions – and hold down costs – dictated a unique approach for maximum aeration efficiency given that aeration energy consumption at wastewater treatment plants typically consume as much as 60% of all electrical usage.
Working with CMA and Aerzen, Gwin Dobson & Foreman (GD&F) developed an aeration blower strategy that includes separate technologies precisely matched to three key areas, each of which with its own unique operating conditions and aeration requirements. These include:
- Three BNR reactors.
- Two digester tanks.
- A sludge holding tank.
Turbo Blowers Ideal for Wide Flow Spectrum
At the plant, a pump station feeds sewage to a headworks building with fine screens and grit separators. Influent is then routed to a reactor feed distribution box with a gravity step feed system for wet weather flows. The distribution box feeds the reactors, each of which is engineered with six zones to achieve various levels of treatment as the wastewater passes through it.
One of three NRT Reactors at the CMA Wastewater Treatment Plant.
Aeration blowers for the reactors are essential for satisfying the plant’s Biological Oxygen Demand (BOD), which allows for aerobic bio-degradation of the pollutant components. When designing the aeration blower system for the reactors, GD&F needed to provide the correct amount of aeration needed, yet no more than necessary to avoid wasted energy.
To achieve the appropriate balance, GD&F specified a system for the reactors that includes two 75 HP turbo blowers, each of which is rated to provide 1500 CFM at 10 psig, and two 150 HP turbo blowers, each of which rated to provide 2,700 CFM at 10 psig. Each blower is equipped with Variable Frequency Drive (VFD) controls and has a 2:1 turndown. The turbo blowers are located in a newly constructed building located next to the reactors.
Each turbo blower is designed with air-foil bearings, which rely on compressed air in the two radial bearings of the drive shaft and in the axial bearing for absorbing axial forces. The air-foil technology is based on the principle that in operation an air cushion forms automatically and thus without further energy input. The turbo blowers, as with other blowers, at the plant are also sound attenuated for quiet operation.
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GD&F opted for turbo blowers rather than PD or Hybrid blowers for the reactors because units are typically more efficient and cost effective in applications with narrow swings in turndown, which is the case with the reactors at the CMA plant where the operating pressure requirements are consistent and highly predictable.
The use of four turbo blowers with different horsepower and CFM ratings also offers a much-needed level of flexibility, said Eric Bennett, Product Manager, Controls, Aerzen USA.
“Realizing the wide range to cover, we decided to use turbo blowers with multiple frame sizes for the application to achieve a wider flow spectrum and better overlap between machines for optimization,” Bennett said, adding that an alternative system that uses blowers with the same horsepower ratings for each blower would not be as effective for flexible.
“For example,” Bennett said, “the 75 HP unit might operate for the majority of the time but when there is higher demand, it might be best to run with the a 150 HP machine or run the 150 HP blower in conjunction with 75 HP unit depending on the load. It’s about truly optimizing the aeration to the process demand on any given day.”
MOV Control Method Saves Energy
The turbo blowers are controlled using a dedicated flow-based, Most Open Valve (MOV) method to ensure air distribution at the lowest possible operating pressure, which in turn, reduces energy consumption.
MOV control relies on an error-gain algorithm that automatically calculates the total airflow required for each of the zones within each reactor to maintain the proper Dissolved Oxygen (DO) set point. To do so, probes in each zone measures DO levels and relay the data to a control system located in the building that houses the turbo blowers. The control system automatically monitors the DO levels and opens or closes valve actuators located within each zone to achieve the proper amount of airflow based on the DO set point for a specific zone. Fine bubble membrane diffusers are also used to aid aeration process.
Actuator valves are used to regulate the airflow for each of the six zones within each NRT reactor at the wastewater plant.
“The strategy allows everything to work together,” Bennett said. “The system opens a valve all the way for a zone that has the highest need for air and tapers the other valves that need less air to achieve the optimum flow. It’s following the system curve and operating at the pressure needed to efficiently achieve distribution rather than operating at a pressure that is higher than necessary.”
Given the need to conserve energy, the MOV control strategy is crucial for the plant, said Bennett. The MOV control strategy also follows best practices touted by the Environmental Protection Agency*.
“A constant-pressure system typically operates at 0.5 to 1.5 psi above static head pressure, but any pressure above static head is wasted energy,” he said, noting that a 1.5 psi increase can result in a 20% increase in power consumption. “The MOV system effectively minimizes the waste associated with a constant-pressure system.”
Hybrid Blowers Manage Pressure Swings
Waste sludge from the reactor is routed to aerobic digesters tanks where the goal of aeration is to prevent waste sludge from turning septic. Importantly, the aeration scenario in the digesters differs from the reactors in that they experience significant fluctuations in head pressure based on changes in water levels. With a pressure swing ranging from 4 to 12 psig, the tanks called for a specific aeration blower technology. In addition, the application called for on/off blower cycling.
Given the operating conditions of the digester tanks and the need to cost-effectively deliver proper aeration, GD&F specified two 100 HP twisted screw hybrid blowers for the application. Each is equipped with VFD controls and rated to provide 1800 CFM at 12 psig. Each blower also has a 3:1 turndown.
“The hybrid blowers provide the ability to operate at very high pressure and operate efficiently at those high pressures, yet they also offer tremendous turndown,” said Bennett. “By comparison, the turbo wouldn’t effectively handle the wide pressure swing and a hybrid blower is much more efficient than a traditional PD blower at high pressure.”
The hybrid blowers are located in a newly constructed blower room that also houses the PD blowers. The hybrid blowers and PD blowers work off of separate discharge headers to deliver air to each dedicated process.
Hybrid and PD blowers are housed together in a newly constructed blower room.
A programmable logic controller (PLC) controls the hybrid blowers. One unit provides air to both digester tanks. To do so, the system leverages the error-gain algorithm to automatically deliver airflow to the appropriate tank based on DO or Oxygen Reduction Potential (ORP) set points. The second hybrid provides redundancy.
PD Blowers Best Suited for Low-Pressure Application
The primary goal of aeration for sludge holding (SH) tanks and recycling tank, which are downstream from the digesters, is to ensure the wastewater remains mixed to support the aerobic process.
The operating pressure for the SH and recycling tanks is typically at or around approximately 5.0 psi or less. The low operating pressures did not warrant an investment in turbo blowers or hybrid blowers.
Given the low operating pressure and lower capital costs, GD&F specified two 50 HP PD blowers for the SH and recycling tanks. Each is equipped with VFD controls and rated to provide 1200 CFM at 5 psig. Each blower also has a 4:1 turndown, which makes the PD blowers more energy efficient than turbo blowers or hybrid blowers when operating at low pressures and minimum flows.
“Capital cost, power consumption, and turndown all made it a clear decision that best choice for this application is PD blowers. Selecting a blower based on maximum flow efficiency is not always best. You need to look at where the machine will be operating the majority of the time. Since the PD can hit the minimum flows beyond that of a turbo blower or hybrid blower, it becomes the most efficient machine for the application,” said Bennett.
The PD blowers use the same control technology used for the hybrid blowers. One blower is dedicated to the SH tanks, while another serves the recycling tank. Given the high turndown ratio of the blowers and the discharge header configuration, the plant has the option of running one unit to easily meet the aeration needs of both SH tanks and the recycling tank. The same maintains control based on established DO and ORP set points.
The PD blowers at the plant compress inlet air to 5.25 psi and route it through the main header (bottom) to SH tanks and the recycling tank.
Blower Strategy Delivers Needed Results
According to CMA Wastewater Treatment Plant Operator Randy Rongeux, the use of three separate aeration blower technologies contributes to the operation’s goal of holding down costs for sewage treatment while allowing the plant to comply with stringent regulations.
“I’m a tinkerer and I always try to run the operation as efficiently as possible,” Rongeux said. “The aeration blower strategy goes a long way toward helping us achieve our goals.”
Up-to-the-minute data for blowers is visible on the treatment plant’s Supervisory Control and Data Acquisition (SCADA) system shown here. In addition, the turbo blower building and the building housing the hybrid blowers and PD blowers are each equipped with a master control panel for easy monitoring and independent control of each system. Click here to enlarge.
About Aerzen USA
Aerzen USA is a wholly-owned division of the German manufacturer, Aerzener Maschinenfabrik GmbH, and has been a recognized world leader in the production of rotary positive displacement machines since 1868. Established in 1983, Aerzen USA is based in Coatesville, PA. For more information, visit www.aerzen.com.
About Gwin, Dobson & Foreman
Gwin, Dobson & Foreman, Inc. is a full service engineering firm located in Altoona, Pennsylvania. The firm specializes in water and wastewater engineering and has been serving clients throughout Pennsylvania, Maryland, West Virginia and Ohio since 1954. GD&F takes a hands-on, practical approach to their projects to serve the long-term needs of their clients. As with all process technology, GD&F thoroughly evaluates and selects the best blower technology to provide the most cost effective solution to their specific treatment requirements. For more information about Gwin, Dobson & Foreman, Inc. call Senior Project Manager Jim Balliet at 814-943-5214, or visit www.gdfengineers.com.
To read more about Aeration Blower Technology, please visit www.blowervacuumbestpractices.com/technology/aeration-blowers.
*Evaluation of energy conservation measures for wastewater treatment facilities. U.S. Environmental Protection Agency, Office of Wastewater Management - 2010