The Richmond Wastewater Treatment Facility was regularly meeting its permit limit of 1.0 mg/L effluent phosphorus.

The only trouble was that doing so required the addition of alum, which had once cost about $250,000, upstream of the final clarifiers.

To address that issue, the Richmond Sanitary District installed large-bubble mixing systems to create anoxic zones in selected aeration basins as part of a $14 million plant upgrade. The anoxic zones enable biological phosphorus removal and have nearly eliminated the feeding of alum.

CUTTING DOWN ON CHEMICALS

The city of Richmond lies in east-central Indiana next to the Ohio border. The sanitary district serves about 20 square miles in the city and surrounding Wayne County. Reducing chemical inputs has been a priority for the treatment facility, according to Todd Hobson, plant superintendent.

The large-bubble mixing systems were supplied by Pulsed Hydraulics, working with FACO, its Indiana distributor, and engineering consultant Donohue & Associates. The Richmond plant (18 mgd design, 8.9 mgd average) has an activated sludge extended aeration process with nine treatment trains, each with three passes labeled A, B and C, through which the flow proceeds by gravity.

The extended aeration equipment was aging and in poor condition, according to Larry Bell, vice president of sales with Pulsed Hydraulics. “Through the insight of plant operations management and the foresight of the city administration, they decided to go all in and improve the treatment facility and thereby improve the water quality going into the White River, while making the plant more energy efficient,” Bell observes.

A THOROUGH MIX

To achieve biological phosphorus removal, the plant upgrade team created two successive anoxic selector cells in Pass A of each treatment train. Water exiting the primary clarifiers goes through a splitter box with automated valves that divide the flow.

The nine Pass A (first-stage) basins each have two 20- by 30-foot anoxic selector cells, followed by a third cell of the same size with fine-bubble aeration. From there the flow proceeds downhill through Pass B and Pass C, also fully aerated.

The key to effective phosphorus removal, notes Bell, is thorough mixing in the anoxic zones. That is accomplished by the large-bubble mixing systems. It may seem counterintuitive to mix an anoxic zone by feeding air. However, the large bubbles produced by the Pulsed Hydraulics mixing system (about 36 inches in diameter) have miniscule surface area and therefore minimal oxygen transfer when compared with billions of fine bubbles.

The large-bubble system uses oil-lubricated rotary-screw compressors that deliver 0.2-second pulses of 50-80 psi air by way of poppet valves. The air goes out into the basins through 1-inch piping to 8-inch-diameter bubble-forming plates — two such plates on the floor of each anoxic selector zone.

Pulsing valves deliver two to six pulses per minute. The naturally buoyant large bubbles rise at about 4 feet per second, lifting the water and solids in the tank. The bubbles exit to the atmosphere at the surface, at which point the water and solids radiate outward in a circle; gravity then takes over and the downward force continues the mixing action.

PROOF OF PERFORMANCE

The sanitary district performed field testing on the Pass A anoxic selector cells. Testing was conducted at the center of each selector cell as representative of conditions in the rest of the cell, since the two bubble-forming plates have zones of influence that cover the entire cell area. Each location was sampled at three depths to verify the solids distribution.

Testing ran continuously for six hours while contractors were working with the gates in the flow splitter structures; this activity noticeably affected flows and therefore the waste loadings between the aeration basins immediately downstream.

In spite of this variable loading, the test data showed that the total suspended solids were evenly distributed in the anoxic selector cells, indicating that the large-bubble mixing systems were performing effectively.

In addition to the favorable TSS results from the testing, the dissolved oxygen measurements in all 18 selector cells were significantly below the 0.2 mg/L limit mentioned in the specification as the upper acceptable value for anoxic operation.

SIGNIFICANT SAVINGS

“The technology the city employed to achieve biological phosphorus removal is really nothing new,” Bell observes. “A complete mix enables the plant for the first time ever to use all the tank volume available. The plant upgrade as a whole has reduced the energy bill by approximately one-fourth, and they no longer have to feed alum or other chemicals to achieve their NPDES permit phosphorus limitation.”

Pat Smoker, sanitary district director, notes that before 2017, the district was feeding about 600 gallons per day of alum solution at the downstream end of the Pass C basins, just ahead of the channel carrying the flow to the splitter box that distributes the treated water to the four final clarifiers.

It was around 2017 when the district staff realized that, due to aging aeration infrastructure, the facility had accidentally created anoxic zones in Pass A basins — this allowed a reduction in the alum feed to about 100 gpd.

As the treatment plant upgrade was being designed, the goal was to set up controlled anoxic zones that would further reduce alum usage. As part of that design process, the large-bubble aeration system was ultimately selected.

“Right now, we’re putting in about 12 gallons of alum per day, just because our permit says we have to do it,” Hobson says. “We really don’t even need it. We were paying $5,280 per load of alum, and before 2017 we used to get a load every Friday. Now we get by with two loads a year.” Annual savings on alum compared to pre-2017 costs total about $250,000.

Effluent phosphorus averages about 0.37 mg/L, well below the 1.0 mg/L permit limit. Before the new mixing system was installed, effluent phosphorus averaged 0.7 mg/L.

Maintenance on the large-bubble aeration system has been minimal, says Hobson. At the recommendation of Pulsed Hydraulics, the plant team periodically boosts the pressure and size of the air pulses in the basins to enhance the mixing and prevent buildup of grit in the basin corners. About every two weeks, operators clean the ORP meters installed to monitor conditions in the anoxic zones.

“It’s a good system,” Hobson says. “So far it has worked really well.”