Settling Basin: Definition & Design

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

In this lesson, you will learn what a settling basin is, its different parts, and what it is used for. You'll also get an understanding of what goes into the design of a settling basin.

Defining a Settling Basin

When thinking of a settling basin it's good to start with the word 'settling.' Settling basins rely on the concept that if something is heavier than water and is allowed to sit in still water for long enough, then its going to end up sinking to the bottom.

Think of a bathtub that you dump a dozen rubber duckies in. Think of stirring that bathtub continuously. The rubber duckies are going to bob and twist and turn, and the moment you stop they're going to rise to the top. That's because rubber duckies are light and buoyant.

Now think about pouring a cup of sand into the tub and stirring continuously. The sand is going to remain suspended and turn the tub water all murky. The moment you stop, though, the sand will settle to the bottom. Settling occurs because sand is heavier than water, and you allowed the water to be still.

Now there's one more thing, what if you stir the sand, then stop, but before the sand hits bottom you start stirring again? No settling is going to occur. The third important piece is that you let the water sit still long enough for the sand to hit bottom!

A settling basin is any kind of container, either a hole in the ground or a concrete pool, that holds murky water long enough for the sediments to settle out. Sediments are particles that don't dissolve in water. Another word for them is suspended solids. They primarily consist of things like sand, grit, soil, rocks, heavy metals, leaves, and other plant material.

Settling Basin Applications

Settling basins are used for agriculture, mining, and construction. As mentioned above, they consist of a place where water can pool. Concrete settling basins are very common. If you're not pouring concrete, then a depression in the ground lined with a waterproof liner will do the job. Other than your basin, you also need a way to convey turbid water to your basin. Some basins may have a way to discharge the clear water without suspending whatever has settled to the bottom.

Image of an Earthen Settling Basin.

Water can be conveyed to your basin through a pipe, trench, or by accurately locating the basin so that it captures water that is running off the landscape. You're usually trying to catch water laden with sediments. For example, if you're using it next to a mine, you're working to capture runoff from the mining operations that may carry coal particulates, heavy metals like lead and zinc, and sulfur-based sediments. All of these are harmful if released into the environment. For construction sites, a settling basin is invaluable in capturing soil and sand when it's washed away by rain.

Once suspended solids have settled to the bottom, you may want to release the treated water back into the environment. You can do this by installing a pipe that releases the water from the top layers of the basins. You could also have a spillway that only allows water on the surface to spill out of your basin.

Designing a Settling Basin

When designing your basin, what is important is the location, soil type, and the size of the basin.

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