Triton Process Aerators Meets Future Nitrogen Permit Regulations

Moscow2-PAWhen the a Pennsylvania Sewer Authority was researching methods to improve the dissolved oxygen levels in its wastewater treatment plant’s oxidation ditches in 2001, they decided to totally replace its aging brush rotor system. They replaced the rotors with the Aire-O2 Triton® process aeration system and saw immediately results that exceeded their expectations and would uniquely meet its future nitrogen permit requirements.

The current wastewater treatment plant consists of an activated sludge system with three oxidation ditches in parallel operation. The total volume of all three ditches is 0.372 MGD while the average size of each ditch is 180 ft x 24 ft x 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. Also, the rotor aerators 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/or 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 (SAE).

The Plant Superintendent explains, “In July 2001, we were researching ways to increase the dissolved oxygen levels in the aeration tanks in July 2001. 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.”

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Process Aeration/Mixing

The Aire-O2 Triton Process Aerator was chosen for the replacement aeration system. The units went online in December 2001. Two 7.5 Hp units were installed in each ditch. “Immediately, improvements occurred.” The superintendent then added, “The D.O. (dissolved oxygen) levels rose and ammonia levels dropped.”

The Triton 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. The Triton aerator/mixers’ strong horizontal mixing & circulation capability are ideally suited for oxidation ovals. The mixer and blower operate independent of one another, which allow 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 2.0 mm which is classified as fine bubble by the US EPA.

The Triton unit can be bridge or float mounted that allows for easy access, installation and easy routine maintenance. The superintendent adds, “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 twenty-four (24) hours.”

During 2008, the wastewater plant was expanded and upgraded to meet new permit requirements. A new oxidation ditch process with two 10 Hp Aire-O2 Triton aerators were added to provide for nitrification and denitrification. All three ditches were also equipped with ORP control for nitrogen removal. This plant has a daily average operating flow of 0.180 MGD with a hydraulic capacity of over 0.3 MGD. The average influent loading is BOD 240 mg/l, TSS 200 mg/l, and TKN of 35 mg/l. The average effluent levels (after filtration) are consistently BOD <2.8 mg/l, TSS <4.0 mg/l, NH3 < 0.5 mg/l (summer), NH3 < 2 mg/l (winter), TP < 0.4 mg/l, NO3+NO2 < 2.0 mg/l.

Winter Performance

The superintendent remarked, “Another aspect of the Aire-O2 Triton came as a pleasant surprise. The units actually warm the mixed liquor in the aeration tanks. This helped prevent the bio-mass 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 could not eliminate the freezing problems in winter. The Triton 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. They no longer have 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, city officials made a decision to purchase an aeration system based on its excellent oxygenation capability. Years later, its permit regulations tightened to include meeting total nitrogen permits. The ability to meet these tightened limits was yet another added bonus with their choice of aerator/mixer system.

The Aire-O2 Triton aeration system has dual-mode operation that allows for process control for Biological Nutrient Removal (BNR) 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. Thus, no separate mixers are needed for denitrification. This makes the Triton aerators ideal for oxidation ditches.

The plant utilizes this function to meet total nitrogen permit limits. The Triton 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 Triton’s blower function here is controlled based upon 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 economically upgraded to meet their community’s wastewater treatment needs a decade ago and was followed up with another improvement in 2008. Now, the Sewer Authority has a design capacity to meet the strictest permit regulations for the next decade or more! The superintendent concludes, “The Authority and the operators are very pleased with the performance of the plant and the Triton process aerators.”

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