The Use of Polyethylene Geomembranes in Lining
and Gas Collection Covers for Manure Lagoons

Kris P. Kriofske, P. E.
Poly-Flex, Inc., Grand Prairie, Tx

The U.S. EPA has required solid waste landfills to be lined with an impervious material for most of the past 15 years. The regulations also prohibit hydraulic heads of leachates in these liners to exceed one foot. Manure lagoons, by contrast, function at beads of up to 25 feet and, in some states, are not required to be lined unless the ground water is less than 2 feet from the pond bottom. This apparent gap in requirements is quickly closing with new and more stringent liner regulations being imposed on the concentrated livestock operation industry's waste water handling and containment.

Manure lagoons have been lined with clay and concrete in the past with plastic liners making an impact only recently. The leading plastic liners serving this market are the polyethylenes (PEs) known as HDPE and LLDPE, or high density and linear low density polyethylene respectively. This is the same material used to line and cover most landfills today. PEs are used at an annual rate of up to one billion square feet per year in the U.S.because of the combination of properties and prices. PE liners protect the environment by preventing the migration of pollutants to soil and ground water.

Polyethylene is an inert plastic, which is UV stable and can be installed exposed without a soil protective cover. It remains flexible at temperatures down to -100' F and is rodent resistant. It is not an expensive laminated product and contains no plasticizers that can migrate out of the plastic leaving an embrittled barrier. PEs are resistant to the high nitrate, arnmonia, and phosphoric acid content in concentrated animal waste and fertilizer. Since these same substances are highly corrosive to concrete, polyethylene is sometimes used to line cracked concrete containments. PE liners provide natural flexibility for conforming to subgrades. Long term warranties are provided to back up the above claims.

Manure lagoons are lined in three different techniques. The first and most common is the complete lining of the lagoon with a single liner covering the floor, side slopes and extending from the top of the slope to an anchor trench into which the edge of the liner is buried. The second method is the same as the first but with a double liner. The space between the two liners contains a drainage medium to collect any leakage which may occur in the primary liner. This space is continuously evacuated by pumping. The double liner is used only in highly sensitive environments. The third system is an erosion control liner wherein only the slopes are lined from a top anchor trench to a trench on the slope below the low water line. The rest of the lagoon is clay lined. This liner also serves to control slope vegetation growth, siltation of the lagoon, and wave erosion of slopes.

Concrete access pads and ramps are installed on the liner or the liner is attached thereto (see attached drawings). These ramps facilitate the use of a trailer-mounted agitator, which can be lowered into the lagoon. PE lined ponds can be easily cleaned by creating a slurry and pumping the liner clean. The liner can be repaired by a heat-applied patch made in a dry and clean area.

Manure lagoons can be covered by floating PE liners on the liquid surface allowing for the cost-effective construction of anaerobic digesters which use microorganisms to ferment and process waste producing methane gas. The PE cover prevents vector migration out of the lagoon preventing odors, vapors and gasses from entering the atmosphere. They also prevent rainwater dilution of the waste and the accumulation of airborne silts and debris in the lagoon. But the most desirable effect of the PE cover is to create an inflatable methane gas reservoir which collects and contains the gas from the digestion process and allows the gas to be disposed of by flaring on by converting it into electric power by running the gas through an internal combustion engine. Given the increased use of co-generated power following utility deregulation, there are increasing opportunities to create economic returns from the production of methane gas. This is the thrust of the AgSTAR Agency of the EPA.

Various types of plastic floating lagoon covers are being built today. The most common is a floating cover, which covers the entire lagoon, and has its perimeter buried in an anchor trench much like that used for a liner. Perforated gas collection pipes are located under the cover near the top of the slopes and above the liquid line. Weighted and sunken liner cover troughs are located near the center of the lagoon to collect runoff and snow melt and to compensate for liner cover dimensional growth or shrinkage caused by fluctuating water levels in the lagoon. These troughs are defined by floats. Surface weights or cables welded into the liner seams control the amount of liner inflation and/or direct the flow of cover runoff to the sunken trenches where float actuated pumps are located to remove the accumulated liquids.

Another cover design floats a liner on the surface but leaves a 3' to 5' wide perimeter of liquid surface exposed. This cover is tethered to the lagoon banks by a system of cables. Floats and gas evacuation troughs or raceways are built into the cover and some cover inflation is accommodated. Cover collected runoff is sometimes shed by draining through the cover into the lagoon by open ended vertical pipes through the cover or through a series of perforated collection pipes laid on the cover surface. Surface pipes are attached to a pumping system, which is usually manually activated.

Two other attachments to this paper explore the comparison of plastic liners to clay and to concrete. And finally, three drawings are attached showing details for construction. and installation of liners. Floating covers are unique to individual facilities and must be designed on a custom basis by a qualified engineer. Interested parties should contact the author for more information on this subject.

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