Pennsylvania is home to one of the first Argos® SBR plants to utilize a surface mounted process aerator/mixer, which is a significant improvement over the traditional diffused air network designs. The new layout offers substantial capital investment savings as well as significant reductions in noise, operations/maintenance, and energy consumption.
The design of this SBR is based on an average daily flow of 100,000 gpd of municipal wastewater. The plant layout consists of three rectangular concrete tanks — two batch reactor tanks measuring 13′ wide x 35′ long x 20′ deep and one sludge holding tank measuring 9′ wide x 25′ long x 20′ deep. Instead of diffused air, one 10 Hp Aire-O2 Triton® Process Aerator/Mixer is installed in each of the reactors. In addition, one 5 Hp Triton unit is installed in the 30,000 gallon sludge holding tank to provide aeration and mixing.
Each reactor contains a decanter with Tritons that are float-mounted and ride vertically up and down on stainless steel slide pole assemblies as the water level in the tanks change. Polyurethane rollers are integrated into the float assembly and enable the units to move smoothly along the slide poles without binding. This enables the equipment to be easily inspected and maintained from the surface of the tank. In fact, all of the equipment in the SBR is easily accessible from the top without ever needing to drain the basins for inspection or service.
The aerator used in the design is a unique piece of process equipment that has the capability to mix and aerate in one single unit and to do so completely independently of one another. The aerator is driven by a 900 rpm (60Hz) motor which significantly extends the life of the equipment. The motor drives a large power mix propeller which produces a high degree of mixing, ensuring that the tank contents are completely mixed from top to bottom. The aerators are not self-aspirating. All of the air produced by the aerator is fed by a very quiet on-board regenerative blower which pushes the air down the dynamically balanced hollow shaft of the mixer and injects the air into the water column producing a large volume of fine bubble diffused air. This also minimizes noise at the plant. In fact, this installation is located immediately adjacent to a Rails-to-Trails bike path that runs between Pittsburgh and Washington D.C. Terry Soster, President of KLH Engineers, noted that “there are many bikers that pass by the plant and the noise levels are imperceptible.” Noise levels within 10m of the plant are easily < 70 dbA without the need for any special covers or enclosures for any of the equipment. The air is driven deep into the tank by the power mix propeller providing for efficient contact of the air with the wastewater and long bubble hang times thereby maximizing oxygen transfer efficiency.
Each SBR cycle includes fill and anoxic mix where the mixer only portion of the aerator is in operation, fill and aeration where the full aerator/mixer is in operation, settling with the aerator/mixer in standby, and decant with the aerator/mixer in standby and the decanter in operation for a total of six cycles per day.
The current permit limits for the plant are CBOD = 25 mg/l and TSS = 30 mg/l. Average influent quality entering the treatment plant from December 2007 through July 2008 was CBOD = 198 mg/l and TSS = 152 mg/l Effluent quality during the same period has been far lower than permit limits and has averaged CBOD = 6.4 mg/1, TSS = 6.4 mg/l, and ammonia = 1.3 mg/l.
Since the aerator provides both the mixing and aeration in one unit and since that unit is installed directly in the tank, there is are substantial savings in capital investment both from an equipment supply perspective and an installation perspective versus conventional SBR designs utilizing diffused air. The cost of diffuser networks, piping networks, and supports, centrifugal or positive displacement blowers, and blower buildings or enclosures is eliminated and replaced by the process aerator/mixer at a much lower capital cost. The result is a much more compact plant that is quicker to install and easier to operate and maintain. Terry stated, “The use of the aerator devices has allowed the authority to free up valuable building space that would have been used for blowers and to use that space for much-needed storage that they did not have with the original plant design.”
Furthermore, as discharge permit requirements continue to get more and more strict throughout the United States, biological nutrient removal (BNR) is a certainty for most, if not all, plants in the future. A big advantage from a process perspective is that this plant can be easily upgraded in the future to enable the plant to nitrify and denitrify as well as facilitate biological phosphorus removal by simply changing the controls programming to allow for anaerobic and anoxic mixing steps during the SBR cycles. Terry observed that the SBR “provides flexibility in the future for a mixed non-aerated cycle that will allow the facility to comply with biological nutrient removal requirements without any additional capital expenditures.”
This new SBR design has provided a cost-effective plant that is easy to maintain and operate with low impact on the surrounding environment. And the process flexibility afforded by the innovative system ensures that the plant will be able to meet its permit requirements in a proactive manner well into the future.