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Waste Storage and Treatment Lagoons
Summary:

A lagoon, or waste storage pond, is typically made by excavating earth fill and constructing berms to provide temporary storage of animal waste that is primarily liquid in consistency. This storage capability is important for allowing the waste to be held until later periods when climate, weather, and soil conditions for land application, as well as plant requirements, are more optimal. Lagoons also provide treatment in addition to storage, and can reduce the amount of organics, pathogens, nutrients and odors in the waste (USEPA, 2001a). As an example application, anaerobic lagoons are commonly used for liquid swine waste in warmer climates, with the supernatant being used as recycled water for flushing the waste collection systems, with ultimate land application of the supernatant as fertilizer.

Lagoons have three distinct zones containing liquids, sludge, and solids. The liquid supernatant from the lagoons is often spread on cropland and pastures as fertilizer, and the sludge and solids are removed and disposed of less frequently. The potential water quality risks from lagoons include leakage into groundwater or surface water, overflow as a result of exceeding capacity or because of excessive precipitation, and in the event of a major berm breakage the release of a large volume of liquid waste to nearby streams.

Proper planning, design, and maintenance of waste lagoons are essential, as lagoons can contaminate groundwater if they are not constructed and maintained properly. Containment berm integrity and proper liquid freeboard levels must be maintained to prevent large-scale spills of liquid waste being released to nearby surface waters. Lagoons should be located in accordance with regulatory requirements for separation from water sources such as drinking water wells and surface waters. They should not be located in flood plains, and should be downgradient from drinking water wells. The liner for containing the waste is also very important, and should be constructed with a low-permeability liner made of synthetic material or geotextiles or formed by compacted clay or other soil material, typically an 18-inch layer of clay with a hydraulic conductivity of 10-7 cm/s or less (USEPA, 2001a).

Erosion of earthen containment systems can lead to seepage and subsequent contamination of groundwater, and to weakening of berm walls. Appropriate vegetative cover helps to maintain earthen berm integrity. Maintenance of liner and berm integrity is also important during activities that could cause damage such as the solid waste removal process. Two practices to protect the liner are building a concrete access ramp for waste removal equipment, and operating equipment under dry conditions by first removing all the liquids and letting the solids dry (USEPA, 2001a).

Maintaining an effective freeboard level in the lagoons is important for ensuring they won't overflow during precipitation events. Lagoons should be built to include capacity for some permanent storage, a period (e.g., six months) of generated waste storage capacity, the volume of storage for a 25-year, 24-hour storm event (a 25-year storm is one that has a one-in-25 chance of occurrence in a given year), plus a freeboard safety factor (e.g., 20 inches).

Breaching of lagoon containment walls can lead to large volumes of liquid wastes being discharged directly into nearby surface waters. Earthen walls are most susceptible for breaching during periods of heavy rains, which also help to increase the liquid level in the lagoon. As an example, break of the retaining wall of an 8-acre lagoon at Oceanview Farms in Onslow County, North Carolina in June 1995 spilled approximately 22-million gallons of liquid manure waste into the New River (Agricultural Animal Waste Task Force, 1996).