Common use of Waste Stabilization Clause in Contracts

Waste Stabilization. The anaerobic lagoon has emerged as the overwhelmingly predominant method used for the stabilization and storage of liquid swine manure. Methods of aerobic stabilization (e.g., oxidation ditches or aerated lagoons) were abandoned many years ago due to high electricity costs and operational problems such as foaming. Solids separation is typically not practiced at swine facilities. Several factors have contributed to the use of anaerobic lagoons for swine waste management. One is the ability to handle the manure as a liquid and use irrigation for land application. A second is the potential to reduce noxious odors by maximizing the complete reduction of complex organic compounds to CH4 and CO2, which are odorless gases. Finally, the use of anaerobic lagoons in the swine industry was driven, in part, by the potential to maximize nitrogen losses through NH3 volatilization thereby reducing land requirements for ultimate disposal. With the shift to phosphorus as the basis for determining acceptable land application rates for animal manures, maximizing nitrogen loss is ceasing to be an advantage. The design and operation of anaerobic lagoons for swine and other animal manure has the objective of maintaining stable populations of the microorganisms responsible for the reduction of complex organic compounds to CH4 and CO2. The microbial reduction of complex organic compounds to CH4 and CO2 is a two-step process, in which a variety of VOCs are formed as intermediates. Many of these VOCs, such butyric acid, are sources of noxious odors when not reduced further to CH4. Methanogenic microorganisms have slower growth rates than the microbes responsible for the formation of VOCs. Therefore, anaerobic lagoons must be designed and operated to maintain a balance between the populations of these microorganisms and methanogens to avoid accumulations of VOCs and releases of associated noxious odors. Both single cell and two cell systems are used for the stabilization and storage of swine manure. In single cell systems, stabilization and storage are combined. In a two-cell system, the first cell has a constant volume and provides stabilization while the second cell provides storage. With two cell systems, water for pit recharge or flushing is withdrawn from the second cell. In climates with low precipitation and high evaporation rates, there may be one or more additional cells for the ultimate disposal of excess liquid by evaporation. Anaerobic lagoons use bacterial digestion to decompose organic carbon into CH4, CO2, water, and residual solids. Periodic removal of settled solids will be necessary. Typically, lagoons are dredged every 10 to 15 years, and the sludge is applied to land. The design of lagoon treatment cells is similar to storage ponds with one exception. Lagoons are never completely emptied, except when accumulated solids are removed. Lagoons require permanent retention of what is known as the minimum treatment volume that should be reflected in design. Thus, lagoons must be larger in total volume than ponds that provide storage for the same volume of manure. Determination of minimum treatment volume for lagoons is based on Natural Resources Conservation Services recommended total volatile solids (TVS) loading rates and the daily TVS loading to the lagoon. For anaerobic lagoons, recommended rates range from 3 lb TVS per 1,000 ft3 per day in northern parts of Montana and North Dakota to 12 lb TVS per 1,000 ft3 per day in Puerto Rico and Hawaii. This is a reflection of the effect of temperature on the rate of microbial activity. The calculation of minimum treatment volume is simply the daily TVS loading to the lagoon divided by the recommended TVS loading rate for the geographical location of the lagoon (USDA, 1992). With open manure storage tanks, ponds, and lagoons, provision also is necessary to store the accumulation of normal precipitation directly falling into the structure less evaporation during the storage period. The storage requirement for normal precipitation less evaporation varies geographically. In addition, there are provisions for storage of precipitation from a 25- year, 24-hour storm event, which also varies geographically, with a minimum of one foot of free board remaining. Design values used for the accumulation of normal precipitation less evaporation are based on mean monthly precipitation values for the location of the storage facility obtained from the National Oceanic and Atmospheric Administration. In some situations, manure storage ponds or lagoons also may be used for the storage of runoff captured from open confinement areas. In these situations, provision for storage of runoff collected from normal precipitation during the storage period as well as from a 25-year, 24-hour storm event must be included in the design storage capacity of the pond. Expected annual and monthly runoff values for the continental U.S., expressed as percentages of normal precipitation, for paved and unpaved open lots can be found in the Agricultural Waste Management Field Handbook (USDA, 1992).