Daily Dairy Waste Accumulation. Storage volume considerations should include daily dairy waste. A mature dairy cow can weigh 1,400 pounds and generate around 14 gallons (about 120 pounds or 1.9 cubic feet) of feces and urine each day with an average as-excreted solids content of around 12 percent. The total excrement for cows housed in freestall total confinement, along with milking wash wastes would bring the total to more than 2.5 cubic feet per cow per day. This is nearly 9 tons and more than 9 cubic yards of manure per day per 100 cows. For cows on pasture part-time, manure handling and storage needs would be less and in direct proportion to actual confinement time. A 100-cow herd on half-time pasture would accumulate nearly 6 cubic yards (5.5 tons) of manure per day in confinement, including milk wash wastes.
Milking Center Wastes. Depending on milking preparation procedures (such as automatic cow wash) and the use of milk center manure flush cleaning, daily milk center wastewater generated per cow can vary from 30 to 150 gallons. Typically dairy wastes from milking centers (milk house, parlor, and holding area) are very dilute. However, this additional wastewater does increase waste storage volume needed.
Additional Inputs. Storage pond volume must also include expected rainfall minus evaporation from the pond surface. In Alabama this is typically 12 to 14 inches annually but should be adjusted for the planned storage interval worst case and be based on local figures if possible. Volume must also include space for the 25-year-24-hour rainstorm event and any resulting watershed runoff into the pond. Roof and other rainfall runoff may or may not be included, depending on whether dilution water is needed for irrigation or not needed for tankwagon application of dairy waste.
In addition to the above inputs, the volume of the storage pond should also include an allowance of at least 1 foot of freeboard above expected full level for overflow cushion protection and an allowance of at least 2 feet of depth on the bottom of the pond for materials not removed during the normal agitation-pumpout procedure. See the figure for cross section of a dairy waste storage pond. Note the upper pumpdown stake. Pumping should begin when water reaches this level.
Cross section of a dairy waste storage pond.
Solids Accumulation. Dairy waste solids, particularly those from freestall housing bedding, can accumulate quickly in waste storage ponds. Solids accumulation requires longer, more thorough agitation at pumpout time to resuspend settled solids and special manure solids handling "chopper" pumps for transfer to tanker wagons or waste slurry irrigation systems. Solids can cause pumping problems, and over time can greatly reduce usable storage pond volume. Serious consideration should be given to the installation of solids separation equipment between the freestall housing and waste storage pond.
Properly designed gravity settling basins can remove up to 50 percent of the waste solids but must have a minimum of 4 to 6 feet elevation between the bottom of the barn collection channel and the maximum height of the storage pond liquid surface to work properly. They require periodic cleaning out with a tractor front end loader and work best when at least two are constructed side by side to allow alternating use and some manure solids drying out before cleaning.
Mechanical separators are typically either rotating or stationary screens and generally remove 20 to 30 percent of the waste solids. These separators require little attention and produce manure solids that may be easily recycled as bedding or land applied off-farm with solid manure spreaders.