Optimization of dissolved oxygen control system.

In 2011, Howden began operating the largest-ever installation of its Roots IntelliView controls system at Bird Island’s aeration basins.

Bird Island Wastewater Treatment Plant (WWTP) in Buffalo New York, had an inefficient aeration control system which ironically, had been installed in 1998 as an efficiency upgrade.

Challenge

The operating principle was that air flow to all 32 of the plant’s aeration basins, or zones, would be properly controlled by an average of several Dissolved Oxygen (DO) level measurements taken by DO probes in a few of the basins. However, changes in tank loadings and physicadynamics, along with differences in oxygen transfer rates between diffuser grids, prevented a uniform air flow in the aeration zones. The plant also struggled to accurately adjust the air flow from the blowers because the 60 second open/close travel time of the valve actuators was too short.

By 2008, the Buffalo Sewer Authority (BSA) decided it had to cut the energy consumption of the blowers, which used more energy than any other part of the plant. To make that happen, the plant would have to get a new system that independently controlled each aeration basin. Bird Island, the second largest wastewater treatment facility in New York State, with a dry weather design flow of 180 million gallons per day (MGD), had obtained four 5,000 HP Roots single-stage compressors back in 1980, two of which had been rerated to 3,000HP in 1995, and after looking at product options for two years, BSA turned to Howden for another solution to Bird Island’s aeration control problems.

Response

Howden worked with BSA to change the aeration control plan for Bird Island and also recommended the replacement of the old valve actuators with RCS actuators that had a 180-second open/close travel time. In the fall of 2011, Howden began operating the largest-ever installation of its Roots IntelliView Controls system at Bird Island’s aeration basins. The system, comprised of RCS actuators and a Roots Aeration Master Control Panel with a remote access VPN portal, has been successfully deployed at more than 300 wastewater treatment plants in the U.S.

Key to its operation is a flow-based configuration where individual DO probes calculate the air flow required for each basin. The accumulated data determines the total flow demand for all basins, and the necessary adjustment in air distribution is communicated to the blower panel. Part of this "DO-to-Flow" concept is the use of "true" Most-Open-Valve logic, where at least one valve always remains in the fully open position. This sets a positive domino effect in motion that eases system header pressure, lessens the load on the blowers, and finally reduces the amount of energy needed to move the required volume of air.

BSA applied to the New York State Energy Research and Development Authority (NYSERDA) for partial funding to defray the estimated $800,000 project cost. BSA documented an expected annual savings of 3.8 million kWh equaling $345,000. This would be accomplished by lowering average DO levels from 3.1 milligrams per liter (mg/L) to 1.5 mg/L. NYSERDA was willing to pay for up to 50 percent of the project, and conducted a measurement and verifi cation review to test those assumptions.

Outcome

The project delivered a reliable and highly efficient ventilation on demand system, reducing energy costs while ensuring complete safety. However, the exercise of retrofitting the associated PLCs and ancillary equipment to existing fans, especially in such a remote and challenging location, involved many man hours of engineering time, incurred significant transport costs, and necessitated around a week of lost production.

Roots IntelliView controls significantly improved the stability of the plant's aeration process, and because the system also handles fluctuations in loading, the plant has been able to operate some of its aeration basins at a DO set point well below the norm – 0.75 mg/L in the first pass of each basin – and keep DO levels within a very narrow range. This has provided additional energy savings. "The system offers a logical approach that makes decisions like a plant operator would when controlling DO levels in the activated sludge basin," says James Keller, former BSA treatment plant superintendent at Bird Island.

NYSERDA’s completed review showed that annual energy savings would be even greater than expected because of the aeration control system's success in holding DO levels down and limiting their fluctuations. The findings led NYSERDA to furnish half the total project cost. That generous incentive payment gave BSA a very quick full return on its investment.

 

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