Aeration Industries International (AII)

Oxidation Ditch Expansion Comes With Host of Benefits case study


Courtesy of Courtesy of Aeration Industries International (AII)

This wastewater treatment plant located in Texas was originally designed in 2000 but within several years, the city needed to double the treatment capacities of the facility.

The wastewater treatment plant was designed with a plant capacity of 1.5 MGD utilizing a single ditch with the extended aeration activated sludge process for secondary treatment to facilitate removal of Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS) and Ammonia (NH4–N). An expansion followed to increase the plant capacity to 3.0 MGD.

The initial construction included a single ditch with brush rotors. In 2007 the rotors were proving insufficient to provide adequate aeration to sustain acceptable residual oxygen levels in the oxidation ditch. Numerous bearing replacements and multiple shaft failures, coupled with time consuming and costly operation and maintenance (O&M) requirements, led the city to seek alternative solutions to increase aeration in the basin.

In 2008, they found that the Aire-O2 Triton® process aerator/mixers offered a number of benefits over the existing rotor system. The Assistant City Manager reported, “The simple installation of the Tritons (without any additional structural and mechanical installation or modification) made the equipment an adequate fit for the failed surface brush rotor aerators.”

The installation of the equipment proved to be simple and required only readily available equipment. Float-mounted units further simplified the installation process. The units took only 1–2 hours to fully assemble and after installation helped improve mixing in the basin and successfully raised the residual oxygen to acceptable levels.

Following the success of the interim project, the city looked to address the expanding residential growth they were experiencing. They had to double the capacity of plant from to 3.0 MGD. All too familiar with the costly repairs and extensive downtime experienced with the brush rotors, they wanted to avoid installing rotors in the new ditch. Pleased with the ease of installation and reduced O&M of the Tritons, the city again looked to Aeration Industries® to propose a solution for the expansion. Aeration Industries® proposed the Tri-Oval® Oxidation Ditch System for the new ditch, again using Triton process technology.

The ditch system offered a number of benefits to the city. As before, structural work was limited and the O&M on the equipment would be minimal compared to brush rotors. The system offered redundancy and also provided for consistency with the equipment. They could be fully disassembled and reassembled in approximately one hour and spare parts for the units could be stored on-shelf, all of which would virtually eliminate any downtime in the ditch in case of an unexpected mechanical problem.

On the process side, the Tritons offered the ability to operate in aeration and mixing mode or mixing-only mode by turning the regenerative blower on or off, as dictated by residual oxygen set points to reduce energy consumption. Dissolved Oxygen (DO) probes located in each ditch, coupled with a Programmable Logic Controller (PLC) would work together to control the blower operating status. The system can also be adjusted to create anoxic conditions in the future. This offers two distinct benefits, the first of which is control of filamentous bacteria. Anoxic conditions cause strain on the filamentous bacteria and help minimize sludge bulking issues in the secondary clarifiers.

Anoxic conditions also help remove nitrate from the wastewater. This was not an immediate concern for the city, but the Tri-Oval system would afford them the flexibility to address the issue if nitrate removal became a requirement in the future.

Like most projects, this one faced challenges after the initial installation of the new Tri-Oval system. Once the construction was complete and the new process was put on line, operator was quick to realize they were not achieving the desired residual oxygen levels with the new technology in the new ditch. After testing, it was determined that the system was receiving loading concentrations in excess of what the system had originally been designed to treat. Fortunately the selection criteria and process used for the initial improvement would again prove beneficial. Aeration Industries® was able to respond quickly to offer a solution to address the higher than expected loading. Minimal installation requirements again made it easy to install two additional units in each ditch to supplement the original design and provide the air needed to compensate for the increased loading rate.

The Treatment Plant installed a total of 14 x 37.5 Hp total process aerator/mixers. The plant’s effluent BOD is 3 mg/l; TSS 3 mg/l and NH3 is 0.1, all well below permit limits. Today, the city officials and the plant operator are satisfied with the performance of their new system. Capital costs associated with the project were minimized because of the limited structural work required. Time, costs and downtime associated with operation and maintenance have been reduced significantly. All of which leave the city with the confidence they can quickly and easily respond to emergency situations and consistently meet effluent requirements.

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