Lagoon cover helps Memphis capture gas

To control odors and capture methane gas, Memphis, Tenn., under a design/build contract with Black & Veatch, Kansas City, Mo., built an innovative cover over a 15-acre wastewater lagoon at the T.E. Maxon plant, one of two major plants owned and operated by the city.

The plant uses land application as an economic means of sludge disposal with a portion of the sludge digested in a lagoon prior to disposal.

“We use the lagoon as an anaerobic digester to reduce sludge, odors and pathogens,” says Jerry Collins, administrator of environmental engineering for the division of public works.

Although it is reasonably well isolated from residential development, odor from the plant had become a concern, prompting the city to construct a synthetic membrane cover for the lagoon.

In addition to the cover, the project included collection and disposal of the gas produced in the lagoon, new sludge withdrawal lines, a new pumping station and a new force main to convey the sludge to another lagoon.

High density polyethylene (HDPE) was selected for the pond cover because of the wide band of temperatures at which the material can be welded. HDPE contains no plasticizers and therefore remains flexible even after long-term exposure to the weather.

Other advantages are that repairs can be made to the membrane without using solvents or adhesives; HDPE is lighter than water and floats on the surface of the lagoon; thermal expansion of HDPE is significantly less than other materials, and puncture resistance is high. The membrane is 45 mils thick and is reinforced with polyester scrim.

The cover was designed to drain rainwater from the site with the design quantity based on the amount of precipitation produced by a storm lasting 24 hours and occurring once every 25 years. Sump pumps mounted on the cover are used to move the water from the surface into the lagoon.

Wind forces on the cover cause both uplift and a horizontal drag force which the cover must withstand. Uplift is caused by the wind blowing across the cover but is resisted by the weight of the cover, adhesion between the cover and the water and the suction which develops as the cover starts to pull away from the water surface.

The horizontal drag force is resisted by anchoring the cover to a concrete ring wall installed around the perimeter of the lagoon.

The cover was designed to accommodate up to 2 feet of change in the liquid level in the lagoon. Connections to the ring wall at the edge of the lagoon and to onshore gas piping had to have the flexibility to handle the 2-foot variation.

The system was also designed to collect the 2,000 cu. ft.-per-minute of methane gas generated by the lagoon. The gas rises to the surface and is collected in a series of collectors supported by floats. It is then conveyed to a pipe at the perimeter of the lagoon, flows to a flare and is burned to control the odors associated with the gas. In the future, methane could be used to generate electricity that would be used at the wastewater plant.

The cover was shipped to Memphis in prefabricated panels of up to 10,000 square feet. It was assembled on the bank of the lagoon and installed in two pieces, each covering half of the lagoon. To avoid removing the lagoon from service, the cover was installed with the lagoon full of wastewater sludge.

Quality assurance and control programs were implemented. Quality control starts during manufacture of the membrane and covers material transport and storage, fabrication and construction.

Nondestructive testing was done on all the seams, and destructive seam testing was done at regular intervals. Both peel and shear tests were used to confirm construction quality.

The installed cost of the lagoon and associated work was $3.6 million, including the lagoon cover, the new sludge suction lines, sludge force main, sludge pumping station, blowers, blowing house and flare system.

“There is no question that the cover eliminates odors associated with large-scale anaerobic digestion,” Collins says. “We are very pleased with the results.”