Before planning for one waste management system or another, operators should consider (1) waste nutrient value and required land application area and (2) waste handling, land application interval, and storage volume.
In a 24-hour confinement operation, the 100-cow herd generates about 62 pounds of nitrogen per day and more than 22,000 pounds per year. About 30 percent of this is lost in a waste storage pond, leaving around 15,000 pounds. This same dairy waste treated in a two-stage lagoon system would be reduced by anaerobic bacteria to about 5,500 pounds of nitrogen. At a land application rate of 200 pounds of nitrogen fertilizer per acre, more than two and one-half times as much cropland would be needed for a waste storage pond system compared to a two-stage lagoon system for the same number of cows. With a typical first year nitrogen availability factor using slurry-wastewater irrigation of 50 percent, this would be around 0.38 acre per cow needed with the storage pond system compared to 0.14 acre per cow for the lagoon system, a 24-acre difference with a 100-cow herd.
Treatment degradation of phosphorus and potassium is not so great with lagoons. Soils showing increasing accumulations of phosphorus may dictate land application based on phosphorus. This could double or even quadruple required land application area.
Storage volume requirements are affected by the method of waste collection and transfer and the land application interval. Freestall lane flush with wastewater recycle is standard practice in the two-stage lagoon-flush system. Wastewater recycle flush systems are subject to salt buildup, predominately magnesium ammonium phosphate (struvite). For information on addressing this problem, see Extension Circular ANR-860, "Controlling Salt Buildup In Wastewater Recycling Systems."
Wastewater recycle flush is sometimes used in waste storage pond systems but is not recommended. It requires the addition of 50 gallons of freshwater per cow per flush during periods of low storage pond levels following waste pumpout until pond wastewater levels build up to allow recycle. A way of adding this freshwater to the flush system must be provided, and great care must be taken to prevent an accidental plumbing cross-connection that could contaminate the dairy water supply with recycle wastewater from the flush system. This additional freshwater need must be figured into total storage volume requirements to maintain desired storage period. This more complicated liquid management for flushing after pumpout is one of the factors that makes waste storage pond systems less desirable than lagoon-flush systems.
Full-time freestall lane flushing with freshwater is definitely not recommended due to increased freshwater needs and increased wastewater volumes generated. One alternative is to use a tractor to scrape dairy waste into the storage pond. This would require the construction of a reinforced concrete ramp with equipment safety barriers. The storage pond would also need to be adjacent to the freestall barn.
Land application interval will greatly affect storage volume needed. This interval is determined primarily by the need to apply dairy waste to a growing crop at agronomic rates to reduce the chance of environmental pollution through runoff or deep percolation to ground water. In most of Alabama, the prospect of an actively growing crop needing fertilizer during the months of December through March is unlikely. This, and the need to give additional application flexibility, suggest that a land application interval or storage period of 180 days may be desirable from a water quality standpoint (see Extension Circular ANR-918, "Animal Waste Management Planning," for guidelines).