Design considerations

Tank material types—The primary materials used to construct manure tanks are reinforced concrete, metal, and wood. Such tanks must be designed by a professional engineer and constructed by experienced contractors. A variety of manufactured, modular, and cast-in-place tanks are available from commercial suppliers. NRCS concurs in the standard detail drawings for these structures based on a review and approval of the drawings and supporting design calculations. A determination must be made that the site conditions are compatible with the design assumptions on which the design is based. Structures can also be designed on an individual site-specific basis.

Cast-in-place, reinforced concrete, the principal material used in below-ground tanks, can be used in above-ground tanks as well. Tanks can also be constructed of precast concrete panels that are bolted together. Circular tank panels are held in place with metal hoops. The panels are positioned on a concrete foundation or have footings cast as an integral part of the panel. Tank floors are cast-in-place slabs.

Other above-ground tanks are constructed of metal. Glass-fused steel panels are widely used. Such tanks are manufactured commercially and must be constructed by trained crews. Other kinds of metal panels are also used.

At least one company offers a wooden above-ground tank for liquid storage. The preservative treated boards have tongue-and-groove edges and are held in place using metal hoops similar to those used for concrete panel tanks. All manure tanks should meet the standards identified in the section on solid manure storage.

Sizing—Liquid waste storage ponds and structures should be sized to hold all of the manure, bedding, wastewater from milkhouse, flushing, and contaminated runoff that can be expected during the storage period. Equation 10-3 can be used to compute the waste volume:

where:

WV = Waste volume for storage period, ft3

TVM = Total volume of manure for storage period, ft3

(see equation 10-1) TWW= Total wastewater volume for storage period, ft3

TBV = Total bedding volume for storage period, ft3 (see equation 10-2)

Data on wastewater production are available in chapter 4 or from the farmer or rancher. Appendix 10C provides a method of estimating contaminated runoff volume.

In addition to the waste volume, waste storage tanks must, if uncovered, provide a depth to accommodate precipitation less evaporation on the storage surface during the most critical storage period. The most critical storage period is generally the consecutive months that represent the storage period that gives the greatest depth of precipitation less evaporation. Appendix 10C gives a method for estimating precipitation less evaporation. Waste storage tanks must also provide a depth of 0.5 feet for material not removed during emptying. A depth for freeboard of 0.5 feet is also recommended.

Waste storage ponds must also provide a depth to accommodate precipitation less evaporation during the most critical storage period. If the pond does not have a watershed, the depth of the 25-year, 24-hour precipitation on the pond surface must be included. Appendix 10B includes a map giving the precipitation amount for the 25-year, 24-hour precipitation. Frequently, waste storage ponds are designed to include outside runoff from watersheds. For these, the runoff volume of the 25-year, 24-hour storm must be included in the storage volume.

Appendix 10C gives a procedure for estimating the runoff volume from feedlots. The NRCS Engineering Handbook for Conservation Practice, chapter 2, may be used to estimate runoff volumes for other watershed areas.

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