Common use of Manure Storage Clause in Contracts

Manure Storage. Most large hog farms have from 90 to 365 days of manure storage capacity (NPPC, 1996). Storage is in either an anaerobic lagoon or a storage facility. Typical storage facilities include deep pits, tanks, and earthen ponds. Anaerobic lagoons provide both manure stabilization and storage. The use of storage tanks and ponds generally is limited to operations with deep pits and pull-plug pits where manure is handled as a slurry. Pit recharge and flush systems typically use anaerobic lagoons, because of the need for supernatant for use as recharge or flush water. Anaerobic lagoons emit less VOCs and noxious odors than storage facilities, but emit more CH4. Storage facilities and anaerobic lagoons are operated differently. Storage facilities hold manure until the vessel is full and then are fully emptied at the next available opportunity. To maintain proper microbial balance, lagoons are sized for a design manure acceptance rate and are emptied on a schedule (but are never fully emptied). This section describes the types of lagoons and storage facilities used and the factors affecting their design. Storage facilities include deep pits (beneath confinement buildings), in-ground tanks, above-ground tanks, and earthen ponds. Most storage facilities are open to the atmosphere. Manure storage tanks and earthen ponds not only must have adequate capacity to store the manure produced during the storage period but must also store any process wastewaters or runoff that require storage. In addition, provision for storage of the volume of settled solids that will accumulate for the period between solids removal is necessary. Due to the size of storage structures for liquid and slurry type manures, it is difficult to completely mix and empty these facilities during draw down at the end of each storage period. Thus, an accumulation of settled solids will occur requiring a complete clean out of the facility periodically. Estimates of rates of settled solids accumulation for various manures can be found in the Agricultural Waste Management Field Handbook (USDA, 1992). The microbial processes responsible for CH4 and VOC formation also occur in storage tanks and ponds. However, the necessary balance in microbial populations for the complete reduction of organic carbon to CH4 and CO2 is never established due to higher organic loading rates and accumulations of high concentrations of VOCs, which inhibit CH4 formation. Thus, emissions of CH4 from manure storage tanks and ponds will be lower than at anaerobic lagoons, and emissions of VOCs will be higher. Rates of formation of NH3 and H2S will not differ, but emission rates may differ depending on hydraulic retention time, pH and the area of the liquid- atmosphere interface. The pH of storage facilities normally will be acidic due to the accumulation of organic acid, which will reduce the rate of NH3 emission but increase the rate of H2S emission. The reverse is true for anaerobic lagoons, which have pH values that typically are slightly above neutral. However, time and surface area are probably the more significant variables controlling the masses of NH3 and H2S emitted.