More dairy farms are considering or have already begun to use lagoons designed to manage the waste produced by their animals. While this is an acceptable way to minimize pollution of adjacent lands and water bodies, over time lagoons will begin to fill with waste solids which must be removed to maintain the lagoon's efficiency.

The dairy at the Hill Farm Research Station utilizes a two stage lagoon system. Solids are retained in the initial stage where anaerobic digestion reduces the solids via fermentation. The first stage has been pumped twice, about 8 to 9 years apart. On both occasions, some solids near the bottom of the lagoon could not be agitated sufficiently to suspend them, and they remained when pumping was completed. These residual solids reduced the capacity of the first stage below its estimated 1.5 million gallons. In addition, it was observed that some bedding materials, especially rice hulls, are not decomposed as readily as other materials and will decrease the intervals between pumping.

In 1999, the first stage lagoon needed to be cleaned out again. Waste solids had become so concentrated that overflow of solids along with the liquid from the first stage into the second stage lagoon was possible. Solids were concentrated, thick, and had begun to crust over on top of the liquid making it evident the lagoon was not functioning properly. Consideration was given to removing as much of the liquid as possible by pumping without agitation, allowing remaining solids to dry for a few days, and then removing them from the bottom by dredging. This would have allowed for a wider selection of fields on which the solids could have been spread. However, the cost of dredging and hauling semi-dry material by truck was prohibitive.

The decision was made to pump as much of the solids as possible onto nearby fields without greatly increasing environmental risks associated with soil phosphorus levels. Water from the second stage was to be pumped back into the first stage to help suspend as many of the solids as possible and to remove some solids from the second stage. Suspended solids were to be pumped onto lightly disked ryegrass fields prior to planting in the fall. These fields were to be disked again a few days after slurry application to incorporate solids into the soil to reduce the potential for runoff onto adjacent areas.

Equipment to accomplish this task was rented from Tuckasee Irrigation, Russellville, KY (800-725-5986). They provided an agitator, self-contained pump with motor, irrigation pipe, hose reel, and traveling gun to apply slurry to the fields. The Hill Farm provided one tractor to power the agitator and one used as an anchor for the traveling gun. First day procedures were devoted to setting up the agitator and stirring the first stage lagoon. Also, over 4,400 feet of irrigation pipe were laid to the most distant field. The traveling gun was capable of covering a 150-foot wide radius on either side, even when the slurry was pumped that distance. The agitator did an excellent job of stirring solids into a slurry even though the solids were concentrated. After about eight hours of agitation, pumping began. Continual agitation, however, made the solids become so thick that it was difficult for the pump to operate efficiently. Therefore, water from the second stage had to be added to the first stage sooner than was anticipated. A crew was required to be with the agitator and pump on a continual basis to keep the solids suspended, and as such, two12 hour shifts were required for several days.

Pumping continued until no additional slurry could be pumped from the first lagoon. Almost all water and some solids from the second lagoon had been pumped into the first stage, and there was no way to continue to agitate the first lagoon. It was estimated that 2 million gallons of slurry were pumped onto the fields.

Before pumping began, a dairy waste management plan was developed with the intent to minimize the potential for drastically increasing nutrient levels in soils around the dairy, for runoff into streams, and for inconveniencing neighbors. Each neighbor within a half mile radius of the fields was personally notified three days before agitation began. At that time they were advised of the plans, dates, and potential for some odor during the pumping operation

A soil test of all fields around the dairy was obtained before pumping in order to establish existing soil nutrient levels. In addition, a sample of waste in the lagoon was obtained beforehand to determine nutrient levels in the waste. These values were used to develop the part of the dairy waste management plan related to which fields would receive waste material and at what level. Based on these nutrient levels, certain fields were excluded from receiving lagoon waste while the amount to be applied on other fields was determined based on the level of nutrients, primarily phosphorus. The amount of slurry delivered was monitored by placement of four collection pans of about 1 square foot surface area each across the area that the gun traveled.

In addition, the plan dictated several other general areas that were to be monitored. The slurry stream from the gun was kept at least 100 feet from any drainage ways. Bermudagrass between the fields to which the slurry was applied and any nearby streams provided a filter strip. Application of slurry ceased during rainfall. The irrigation pipe and flexible hose containing the slurry was monitored on a regular basis during all pumping operations for any leaks. Field terraces were monitored routinely for any breaches.

Below are the procedures that were used to calculate the amount of nutrients that would be spread on each field. The first six inches of soil was used because soil samples were taken at six inch depths and disking occurred at that approximate depth. A spreadsheet was developed that could quickly handle the calculations for a large number of fields using the lagoon and soil nutrient levels, inches of slurry applied, and total acreage in each field as input values. Use of the spreadsheet made it easy to input the amount of slurry added to any field at any time to determine the approximate change in nutrient levels. This spreadsheet is available from the Hill Farm Research Station. Below is an example of the calculations using phosphorus as the nutrient.

lagoon phosphorus level = 0.016% as is basis or 160 ppm

assumptions:

1 inch of rain / acre = 27,000 gallons / acre; slurry weight of 10 lbs / gallon; therefore 1 inch of slurry / acre = 270,000 lbs

slurry incorporated in first 6 inches of soil; top six inches of soil weighs approximately 2,000,000 pounds

The decision was made initially that no field would be used that exceeded 135 ppm phosphorus before the application. Therefore, field 34 SW did not receive any slurry as indicated by zero inches of slurry added. Addition of two inches of slurry to field 48 added an estimated 43 ppm of phosphorus to provide a total of 148 ppm in that field. Estimated phosphorus removal by a ryegrass haylage crop was estimated to be 20 ppm when harvesting five tons of dry matter at 0.4 % phosphorus. Other fields were handled similarly but are not shown. When one inch of slurry was applied as in field NDE-N, the phosphorus removed was similar to the phosphorus added with the slurry. The dairy waste management plan did indicate that fields closer to the barns were becoming higher in phosphorus so spreading of dry manure has been moved away from those fields to others with lower phosphorus levels.

The pumping operation was a considered a success because the solids level in the first stage lagoon was reduced, and the lagoon began to function again as intended. However, not all solids could be removed, and it is estimated that the lagoon will have to be pumped again in about five years because of increasing amounts of residual solids remaining on the bottom after each pumping operation. Dredging will most likely have to take place next time to reduce residual solids and restore full volume to the first stage lagoon.

Experiences such as these at the Hill Farm Research Station help the LSU AgCenter develop best management practices and experiences that can be beneficial to dairy producers as they begin to consider their options when lagoons must be cleaned out. Voluntary use of best management practices and a dairy waste management plan will reduce the opportunity for compliance to become mandatory and regulated at either the state or federal level.

 Related Links:

	Dairy Waste Management Plan Spreadsheet
	Pictures of lagoon spreading operation
	Slide Presentation:  Research to Help Poultry Producers Protect Water Quality
	Slide Presentation to the Louisiana Farm Bureau Federation Poultry  Advisory Committee: Litter and Water Quality

Developed: August 8, 2000. Revised: August 19, 2000