Aeration Upgrade

When Pennsylvania’s Moscow Sewer Authority (MSA) was researching methods to improve the dissolved oxygen (DO) levels in its wastewater treatment plant’s (WWTP) oxidation ditches in 2001, staff decided to replace an aging brush rotors system. The town replaced the rotors with the Aire-O2 Triton process aeration system and saw immediate results that exceeded its expectations and would meet future nitrogen permit requirements.

The community of Moscow is located in the northeast corner of Pennsylvania, approximately 10 miles southeast of Scranton. The current WWTP consists of an activated sludge system with three oxidation ditches in parallel operation. The total volume of all three ditches is 0.372 million gal per day (mgd), while the average size of each ditch is 180 ft by 24 ft by 6 ft deep.

Previously, two of the ovals were constructed with brush rotor devices for aeration and mixing. The brush rotors system, including bearings and gear reducers, required difficult, expensive and frequent maintenance, coupled with consistent failures. Rotor aerators also cause significant cooling due to their splashing nature and, in cold-weather climates, cause freezing issues. The splashing effect of the rotors decreases water temperatures dramatically, which leads to a decrease in aerobic activity. Rotor covers used to prevent icing issues and to control aerosoling may decrease the oxygen-transfer capabilities of brush rotors by reducing contact of the water with the free-moving ambient air, thus resulting in a reduced standard aeration efficiency.

Ralph DeLeo is the plant superintendent of the MSA and its wastewater treatment facility. “In July 2001, we were researching ways to increase the DO levels in the aeration tanks,” he said. “Following the advice given by the Department of Environmental Protection’s Operator Outreach Program and after another rotor system failure, the authority decided to totally replace the existing system.”

Process Aeration/Mixing

The Aire-O2 Triton process aeration system manufactured by Aeration Industries Intl. was chosen as the replacement aeration system. The units went online in December 2001. Two 7.5-hp units were installed in each ditch.

“Immediately, improvements occurred,” DeLeo said. “The DO levels rose, and ammonia levels dropped.”

The process aerator/mixer is a surface-mounted device that utilizes a directional mixer combined with a blower to inject fine-bubble diffused air beneath the water surface. Its strong horizontal mixing and circulation capabilities are ideally suited for oxidation ovals. The mixer and blower operate independently of one another, which allows the air flow to be controlled to meet process requirements without any impact on mixing performance.

In the process aerator’s aeration and mixing mode, air is pressurized using the high-efficiency regenerative blower and forced down a hollow shaft exiting from the diffuser in front of the mixing propeller, which dramatically increases oxygen dispersion and bubble residence time. The average bubble size is 2 mm, which is classified as fine bubble by the U.S. Environmental Protection Agency.

The unit can be bridge or float mounted, allowing for easy access, installation and routine maintenance.

“The units are virtually maintenance free, and the one time we needed a replacement part, it was shipped and received by us in less than 24 hours,” DeLeo said.

In 2008, the WWTP was expanded and upgraded to meet new permit requirements. A new oxidation ditch process with two 10-hp Aire-O2 Triton aerators was added to provide for nitrification and denitrification. All three ditches were equipped with oxidation reduction potential (ORP) control for nitrogen removal.

The Moscow plant has a daily average operating flow of 0.180 mgd, with a hydraulic capacity of more than 0.3 mgd. The average influent loading is biochemical oxygen demand (BOD) of 240 mg/L, total suspended solids (TSS) of 200 mg/L and total Kjehldahl nitrogen of 35 mg/L. The average effluent levels after filtration are consistently BOD of less than 2.8 mg/L, TSS of less than 4 mg/L, ammonia of less than 0.5 mg/L in summer, ammonia of less than 2 mg/L in winter, total phosphorus of less than 0.4 mg/L and nitrate and nitrite of less than 2 mg/L.

Winter Performance

“The units actually warm the mixed liquor in the aeration tanks,” DeLeo said. “This helped prevent the biomass from dying off in the winter and kept the clarifiers from freezing for the first time since the plant was put into operation.”

The rotor system splashed the wastewater into the air, causing an evaporative cooling effect. Hoods were placed over the rotors to minimize it, but they could not eliminate the freezing problems in winter.

The new subsurface aerators inject air below the water’s surface. This ability to mix without splashing and cooling the water proved to be an added bonus for winter performance efficiencies. DeLeo no longer has to worry about the secondary clarifiers freezing up in the winter months, which allows for consistent quality discharge and an ability to meet permit standards.

Future Sustainability

In 2001, Moscow officials made a decision to purchase an aeration system based on its oxygenation capability. Years later, its permit regulations tightened to include meeting stricter total nitrogen permits.

The Aire-O2 Triton aeration system has dual-mode operation that allows for process control for biological nutrient removal treatment for nitrification and denitrification by independent control of aeration and mixing modes. Both aeration and mixing functions can be accomplished in one unit independently of one another, so no separate mixers are needed for denitrification.

The Moscow plant utilizes this function to meet total nitrogen permit limits. The unit’s oxygenation capability is turned off to allow for denitrification in the mixing-only mode. In the mixing mode, the blower is turned off, which keeps solids in suspension and facilitates denitrification. This also allows for power savings when loads decrease by allowing the blower, and in some cases complete mixers, to be turned down or off based on oxygen demand while maintaining uniform solids suspension throughout the depths of the basin. The blower function at Moscow is controlled based on ORP levels. Monitoring of this parameter helps to indicate when the nitrates are consumed by the denitrification process in the mixing-only mode and signals the switch back to aeration mode.

The plant was upgraded economically to meet the Moscow community’s wastewater treatment needs a decade ago and was followed up with another improvement in 2008. Now the MSA has the design capacity to meet the strictest permit regulations for the next decade or more.

“The authority and the operators are very pleased with the performance of the plant and the Triton process aerators,” DeLeo said.

Cheri D. Cohen is vice president, marketing, for Aeration Industries Int'l. Cohen can be reached at cheri.cohen@aireo2.com or 952.448.6789.

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