With North Carolina State University (NCSU) researching new options to replace the traditional lagoon and sprayfield system, agricultural engineers have begun analyzing methods and costs for closing lagoons, storage basins and holding ponds.

The costs of closing are variable because of several factors. They include how well the lagoon was sited, constructed and maintained, and how much sludge and solids have built up over years of use, according to Don Jones, agricultural engineer at Purdue University.

Jones and four other land grant university agricultural engineers have authored a white paper on lagoon closures for NCSU's National Center for Manure and Waste Management.

“There is no common lagoon,” says Jones. “Most are different, even from those lagoons right down the road. The commonality is that they have nothing in common.”

A cost analysis of eight closures in North Carolina in 1998 revealed the cost/1,000 gal. storage space ranged from $5 to $32. Therefore, the cost to close a 750,000 gal. lagoon would range from $3,750 to $24,000.

Clean Out

There are three zones of manure in a basin or lagoon. The top layer is liquid, which is pumped out regularly. Proper handling of the bottom two layers, sludge and solids, is key to closing or converting the structure in an environmentally sensitive manner, Jones says (See Figure 1).

“Almost all of the nutrients that go into a well-sited, well-maintained lagoon end up one or two feet above the bottom of the lagoon as sludge,” he says.

Sludge and solids build up in a lagoon at the following rates:

  • Nursery - 3 cu. ft./pig/yr;

  • Grow/finish — 16 cu. ft./pig/yr;

  • Farrow-to-finish — 53 cu. ft./pig/ yr;

  • Farrow-to-wean — 14 cu. ft./pig/yr.

Jones estimates the cost of pumping and land-applying sludge at 30¢ to 38¢/cu. ft.

Cleaning out the sludge by running heavy equipment on the lagoon liner is not a good option. The engineers stress that this would have negative environmental effects.

Instead, Jones suggests a method developed in North Carolina. A bulldozer and tractor with a PTO agitator confine the sludge to a small portion of the lagoon, where it can be agitated, removed and applied at agronomic rates (See Figure 2).

After cleaning out the liquid, sludge and solids, producers have a number of options for their earthen structure. They include:

  1. Eliminate the structure by diverting all runoff and pipes, cleaning it out and then backfilling with soil and planting a cover crop. This option has advantages in that heavy equipment is needed only once and there is no expense in maintaining the structure. The disadvantage is that the producer must find a way to use all the nutrients stored in the sludge at one time, which requires a large number of crop acres for land application.

  2. Conversion to a pond, following the same steps and setting the maximum water level by adding a spillway or standpipe. In order to be used as a pond, the lagoon must be rinsed with clean water at least once.

  3. Breaching the berm can be used in hilly areas where the lagoon structure will naturally drain runoff water. Loading pipes must be diverted and the contents removed before breaching begins.

Environmental Risk

The environmental risk of closing a lagoon or basin lies with the sludge, Jones stresses.

“We assume that the risk is going to remain as long as the sludge is in the lagoon,” he says. “It doesn't end there because the sludge has a high level of phosphorus, salts and heavy metals, so we have to be careful when and where it is land-applied.”

The sludge must be tested before land application. On average, it contains 13% total solids, has 22 lb. of nitrogen, 5.5 lb. of ammonia and 49 lb. of phosphorus per 1,000 gal., but this varies widely from lagoon to lagoon.

The engineers stress that a site-specific plan and nutrient testing are needed to evaluate the environmental impact of closing or converting a lagoon or basin.