Breath of Fresh Air

A new aeration system at a Mississippi treatment plant enhances dissolved oxygen levels and significantly reduces maintenance
Breath of Fresh Air
Paul Surbey (left) and Gummi Jaegerfrom Triplepoint Water Technologies remove protective bags and prepare the membrane diffusers.

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Every month, operators at the West Jackson County Regional Land Treatment Facility in Ocean Springs, Miss., launched a boat into the 20-acre aeration cell in the treatment lagoon and spent up to six hours servicing eight splasher-type 25 hp surface aerators. In addition, the underwater power supply experienced frequent shorts, and the mooring cables holding the units to their concrete pads sometimes broke.

“About every three months, we’d find an aerator spinning wildly or flipped over and tangling itself in the cables,” says section manager Kevin Elliott. “A local shop rewound the motor, but it still took two weeks to get it back. We had spells where BOD hit the permit limit of 10 mg/l. That made us nervous.”

When Hurricane Katrina arrived in August 2005, it ripped the aerators from their moorings and threw them about. Two were ruined. In the aftermath, the utility received a $13 million grant to expand the plant from 5 to 7 mgd to handle future development. The authority hired William Rackley, P.E., project manager at Alan Plummer Associates in Fort Worth, Texas, to design the expansion.

“The project conception design called for upgrading the surface aerators, but we explained our problems to Bill and our preference for a technology that gave us fine- and coarse-bubble diffusion,” says Elliott. The combination was needed to properly aerate the 14-foot-deep cell, which measured 415 by 1,545 feet.

Rackley located the MARS 3000 aeration system from Triplepoint Water Technologies. The units, with Double Bubble technology, produce 5 to 7 pounds of oxygen per horsepower hour (lb/hp-hr). Since their activation in May 2011, the plant has achieved reductions of 57 percent in BOD and 91 percent in TSS. The system produces a dissolved oxygen level of over 3.0 mg/l with only two of four blowers operating, versus 1.0 mg/l for the old system.


Readying the site

The facility, next to the U.S. Fish and Wildlife’s Mississippi Sandhill Crane Refuge, uses a three-cell lagoon treatment process on 75 acres with a 190-acre constructed wetland and 1 mgd spray irrigation system. Its 250-acre biosolids facility also handles material from three nearby treatment plants for land application. Effluent from the wetland system discharges into Bayou Costapia.

To prepare for the up-grade, operators bypassed wastewater into the second treatment cell, then drained the aeration cell, transported the sludge to the head side with earth-moving equipment, and contained it behind a soil berm. Once the cell’s clay base dried, they added fill where needed and shot elevations to ensure that the floor was level.

A crew from DRM Utilities in Tuscaloosa, Ala., built 3-foot-square, 2.5-foot-deep forms on the floor and poured concrete pads on 35-foot centers for the 288 aeration units. Simultaneously, the crew bored 30 feet into the clay and poured four 12-inch-square pilings. They left 16 feet above ground to support the 20- by 48-foot, 12-inch-thick concrete base and screened enclosure for four Continental Blower 125 hp multistage centrifugal blowers, operating at 3,400 cfm/6 psi.

When the aeration units arrived, DRM workers bolted them to the pads as insurance against natural disasters. The units are typically portable and can be installed from the surface. The crew built the 10-inch Type 304 stainless steel air header with custom-welded manifolds on the floor of the cell, then ran 4-inch flexible weighted tubing to each aerator. Motorized valves on the blower manifolds regulate airflow. The workers also excavated a lagoon levee to accept the new 24-inch force main.


Sea trials

“Everyone was excited about opening the valves and allowing decanted liquid from the second cell to cover the aerators,” says Elliott. “Wayne Dennis, operation and maintenance manager, ran three blowers alternately for two weeks to test the system and super-oxygenate the water. We had some compliance issues while bypassing and wanted to put them behind us as soon as possible.”

Each diffuser has a flow rate of 25 cfm (7,200 cfm total), transferring oxygen at 16 percent efficiency at 8 feet deep, and mixing 6,585 gpm (1.9 million gpd total). They have a central coarse-bubble tube surrounded by 10 1-meter fine membrane bubble tube diffusers positioned 6 inches above the floor. The 22-inch-high units have four weighted legs and clear the pad by 4 inches.

Air enters the diffuser’s central static tube, creating a Venturi that circulates the water and liquefies sludge. Anticlogging technology prevents backflow. With coarse-bubble diffusion, 3- to 25-mm bubbles rise through the water column at 2 to 3 feet per second, causing turbulence and robust mixing.

While suspended in the water column, the liquid mixes with fine 1- to 2-mm bubbles rising at less than 1 foot per second from the self-cleaning membrane diffusers. The high surface-area-to-volume ratio of degraded debris and fine bubbles maximizes contact time and reduces BOD5.

When the plant resumed normal operation, flows of 3.5 to 5 mgd and high dissolved oxygen numbers enabled Dennis to turn off the third blower. “Operators walk out on a pier now to maintain the blowers,” says Elliott. “It takes an hour, and that, along with no longer having to repair the motors, has reduced the lagoon’s maintenance budget by 50 to 60 percent.”


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