Summer 2001
Settling basins for open feedlots
by Jeff Lorimor, Department of Ag and Biosystems Engineering
Current Iowa Department of Natural Resources rules require all open feedlots to have settling basins below them to settle out the settleable solids before any liquid is allowed to leave. Proper settling basin design ensures good performance and optimum operator convenience. Good design can help achieve both these requirements.
Settling can occur in basins, terraces, diversions, or other natural areas. The law specifies the following minimum requirements:
- Basins must be designed to settle solids from runoff from a 1-hour, 10-year storm (approximately 2.5 inches per hour).
- Liquid velocity in the basin must be reduced to 0.5 foot per second or less for at least 5 minutes.
- Settling basins must have 1 square foot of surface area for every 8 cubic feet of runoff per hour from the feedlot.
- Basins must include adequate capacity to store the settled solids between cleanouts.
Although these requirements sound confusing, we can use some approximations to make settling basin design relatively easy and still be sure to meet the requirements and be effective. The two most important criteria are the storm size and the basin surface area requirement.
Storm
size. The
1-hour, 10-year storm varies from 2.1 inches in the northeastern corner
of
Basin surface area. The second requirement, 8 square feet per cubic feet per hour runoff, coupled with the storm size, determines the minimum basin size. The following example shows a minimum settling basin design for a 1-acre lot using 100 percent runoff and 2.5 inches per hour:
(2.5 inches/hour)
x (1 acre) x (3,600 conversion factor*) = 9,000 cubic feet/hour
*1 acre-inch/hour = 1 cubic foot/second, and 3,600 seconds = 1 hour
By using the Department of Natural Resoruce’s requirement of 8 square feet of surface area for each cubic foot/hour, the minimum basin size = (9,000 cubic feet/hour)/8 = 1,125 square foot surface area.
We must add solids
storage to this area. (Notice that the ratio of the lot surface area to
the settling basin surface area is 43,560/1,125 = 39). The rule of thumb
for settling basins in
Now that we have determined the size of the basin, we need to design it to maximize its effectiveness and to be convenient to manage. To do so consider the following four criteria.
Slope. Use a very flat slope so the flow slows immediately when it hits the basin. Generally use from 0.5 percent (1/2 of 1 percent) or less slope.
Depth. Settling basins typically should be fairly shallow to allow easy access for cleanout. One to 2 feet is often used, but if the situation warrants, deeper is fine. The shallower it is the more rapidly the solids will dry down. Some situations require settling ponds or tanks rather than flat basins. If you go deeper to more of a settling tank or pond, you must have appropriate cleanout methods available such as a back hoe, dragline, or track-type tractor.
Geometry. Settling basins below small concrete lots may only be 8 to 10 feet in width. Minimum width is the width of your loader bucket. Basins below larger lots comprising several acres will more likely be 20 feet or more in width. The length is determined by the length necessary to intercept all the runoff across the bottom of the lot, or by the minimum surface area as calculated above.
Use a concrete pad where the majority of the solids will settle. Concrete facilitates clean out. Some producers use concrete pads, others use total concrete, and some use no concrete. When using total concrete a continuous vertical concrete curb along each side helps guide the scraper bucket for cleaning and protects the sides from eroding.
Outlet. Design the outlet so the top elevation is 6 inches below the berm. If the outlet plugs, the liquid will still exit at the outlet location. Either vertical or horizontal 0.5- to 1-inch slots work. Horizontal slots are somewhat easier to construct and manage. Location is not critical for the outlet. It can be near an end or in the middle. It should be located, however, so it does not interfere with cleanout, and it should not be directly next to the inlet. A concrete end-wall to push against works well for concrete basins. Tile risers used as outlets need protection from the cleanout machinery.
General guideline summary for settling basins
- Basin surface area should be at least 1/40th of the drainage area. Using 1/20–1/10th of the drainage area allows for better settling plus solids storage, and is ISU’s normal recommendation.
- Basins should be nearly flat. Use no more than 0.5 percent slope.
- Basins should not be extremely narrow. Wider, shorter designs are advantageous. The minimum width that should be used is the width of your loader bucket. Using a 16-foot minimum width often works well.
- Except for very large lots settling basins should have concrete bottoms to allow easy solids removal.
- Concrete curbs along the side(s) of the settling basin make scraping solids easier.
- Slotted outlets are most common. The outlet should allow dewatering of the collected solids.
- Outlets should generally be in the side of the basin rather than on the end. A concrete wall to push against on the end facilitates solids loading.
- Settling basins require continuing management and maintenance. They should have solids removed frequently to function properly.
