William C. Chandler is a project engineer at Lockwood, Andrews & Newnam Inc. Chandler can be reached at [email protected].
undefinedFounded in 1870, the city of Copperas Cove is located in Central Texas, roughly 70 miles southwest of Waco. Copperas Cove is primarily a residential community for the 30,000 people that call it home, but it also accommodates a burgeoning commercial center that services residents of the greater Coryell County, including the military troops stationed at neighboring Fort Hood. The city owns and operates three wastewater treatment plants—the Northwest, South and Northeast wastewater treatment plants—and treats sewage generated from Coryell County within the city limits.
The Northwest Wastewater Treatment Plant is the largest of the city’s three facilities with a permitted average discharge of 4 million gal per day (gpd). The Northwest plant was originally constructed in 1976 as an oxidation ditch process with two secondary clarifiers and a chlorine disinfection system. In 1989, the plant capacity was expanded with the construction of two aeration basins, two new secondary clarifiers, and an aerobic digester for operation as a conventional activated sludge process. The aeration and digester basins were equipped with coarse-bubble diffusers positioned on vertical draft tubes, which were mounted off of header pipes spanning each basin. The aeration system was fed by three 200-hp multistage centrifugal blowers, which were configured to run on manual timers that operators would set to maintain dissolved oxygen levels. In 2003, a coarse bar screen was installed to filter plant influent sewage, and the coarse-bubble diffusers were replaced by fine-bubble diffusers.
Unfortunately, the 2003 improvements did not alleviate existing operational issues. Even with coarse screening up front, rags and debris would easily make it through to the process tanks and inevitably accumulated on the air-drops and diffusers. On top of the ragging, impacts from larger debris and fouling of the fine-bubble diffuser orifices incurred damage such that only an estimated 85% of the diffusers were operational after only a few years of service. Between the reduced number of available diffusers, the inefficient blower equipment and the imprecise aeration control scheme, controlling effluent quality was a challenge and operation costs were high.
In 2014, the Austin engineering team from Lockwood, Andrews & Newnam Inc. (LAN) were contracted to design improvements aimed at relieving these issues. Four targets for improvement were quickly identified: new energy-efficient blowers, a modern aeration control scheme, finer screening at the headworks and a maintenance-friendly air diffusion system.
Reduced Energy Costs
The first order of business was replacing the existing 200-hp multistage centrifugal blowers with more efficient technology. In the existing configuration, the only means for blower discharge control was manually adjusting a butterfly valve installed on the discharge piping of each blower. By 2014, all existing butterfly valves were non-functional, and two of the three blowers were operating constantly to accommodate normal aeration demands. Tied to the existing manual control system, this arrangement was adequate at average influent flow rates, but left little excess capacity or redundancy during peak conditions, putting the city at risk for permit violations. Considering recent flooding events and anticipated growth in the service population, the city needed to improve the efficiency of each blower and increase total capacity to mitigate this risk.
Due to their superior wire-to-air efficiency and the discharge range available when paired with variable frequency drives (VFDs), high-speed turbo blowers were identified by LAN and the city as the ideal technology to replace the existing multistage centrifugal blowers. The increased efficiency of the turbo blowers meant that the motor power required per blower could be reduced by 50% from 200 hp to 100 hp. Despite reducing the power of each blower, the total available aeration capacity was doubled from approximately 6,000 to 12,000 standard cubic ft per minute. The four blowers were tied into a shared discharge header with independent distribution lines to the aeration basins and digester. The discharge header was valved such that all blowers could discharge to any one process basin at a given time. Under normal conditions, two blowers were dedicated for duty service to the aeration basins, one blower for duty service to the digesters and one blower as standby for either process. Overall, the new design satisfied the city’s objectives by providing additional capacity with energy-efficient equipment as well as redundancy for reduced operational risks.
After opting for the VFD-equipped turbo blowers, the existing timer-based aeration control process received a much-needed modernization to online dissolved oxygen monitoring for feedback control of the blowers. This improvement only required the installation of dedicated online dissolved oxygen probes in each process basin and reconfiguration of the control logic to accommodate modulation of blower VFDs based on real-time dissolved oxygen levels.
The submersible aeration mixer design allows the units to be independent. Taking one unit offline for maintenance does not disrupt the overall system performance.
No More De-Ragging
Though not directly related to the efficiency of the aeration system process, the inadequacy of the existing coarse screen was to blame for much of the required diffuser maintenance. To better prohibit passage of rags and large debris while minimizing operations intervention due to excessive screen blinding, a 3/8-in. mechanical bar screen was chosen as the replacement.
Settling on the diffuser design was a more difficult choice to make. Two designs emerged as the best alternatives to provide energy-efficient and resilient service in the aeration and digester processes: floor-mounted fine-bubble disk diffusers and slow-speed submerged turbine aerators.
Floor-mounted arrays of fine-bubble disk diffusers are commonly used for delivering process aeration in wastewater treatment applications. They are sold by numerous manufacturers and are available in a variety of specialized designs to suit a wide range of installation requirements. Fine-bubble aeration continues to be a popular alternative as it is simple, provides high oxygen transfer efficiency and can easily be retrofitted to existing basins. The primary problems with floor-mounted arrays of fine-bubble disk diffusers are that they can foul easily and that any maintenance requires the basin to be taken offline.
Slow-speed submerged turbine aerators have been used in wastewater treatment aeration for a few decades, although they are not as widespread as fine-bubble disk diffusers. The relatively limited use of this technology was largely due to design flaws of early-generation equipment, which typically comprised surface-floating non-submersible motors connected to an axial draft tube that provided air to a submerged mixer. In this arrangement, the substantial torque and hydraulic forces that were generated under normal operating conditions were imparted on the draft tube and shortened the life span of the equipment. Mixers would operate at low rotational speeds to mitigate these forces, which resulted in relatively low oxygen transfer efficiency. Additionally, the fixed draft-tube length limited application to processes that maintained a consistent water level.
To overcome these issues, the modern submersible aeration mixer design features a submersible motor affixed to a floor-seated shear structure that houses the mixer impeller. Process air is delivered directly to the impeller, which efficiently diffuses air to achieve oxygen transfer rates comparable to fine-bubble diffusers. The U.S. EPA reports average standard oxygen transfer efficiency (SOTE) and corresponding alphas (a measure of efficiency in wastewater process applications compared to operation in a clean-water environment) for fine-bubble diffusers as 9.5% with an alpha of 0.4. Submersible aeration mixers have demonstrated a SOTE of approximately 7.5% with an alpha of 0.76, indicating comparable design efficiency as fine-bubble diffusers with a greater resilience to changing process conditions.
Another substantial benefit of the submersible aeration mixer design is that the units are independent; they are not fixed to the ground or other units installed within the same tank. As such, a single unit can be independently removed for maintenance without having to take the basin offline. Additionally, the location of equipment can be easily changed for optimizing aeration distribution, even with the tank online. Considering these benefits, the slow-speed submerged turbine aerators best met the city’s criteria.
How It Worked Out
Due to the ease of installation for these improvements, construction proceeded quickly. The multistage centrifugal blowers and existing air distribution system were phased offline as the new turbo blowers and associated controls and piping were brought online. Each process tank was required to be taken offline once so that the existing diffusers could be removed and the tanks could be cleaned prior to installation of the submerged turbine aerators. When the plant was ready for installation, the new aerators were put into place by crane. In total, the construction phase for all the improvements took 120 days.
The combination of improved blower energy-efficiency, precision of the control system, and improved mixing and diffusion obtained by the aerator mixers is substantial. Since the improvements, total average monthly energy costs at the Northwest plant have dropped by nearly 25%—from $22,000 per month to $16,000.
Moving forward, there are additional opportunities for improved efficiency of the aeration system and the secondary treatment process as a whole. The fact that the submersible aerator mixer design can operate independently of the blowers and function only as a mixer is an added benefit. Considering this, blower operation could be made more efficient by initiating an on/off aeration control scheme to alternate between oxic and anoxic conditions for nitrification and denitrification while using the aerators to keep the basins mixed.
Ultimately, these improvements keep more money in the city’s pocket every month, and operators are confident that their process is working exactly as intended.
