Course: BFM 203 Water Treatment and Control

Lecture

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Current in Basins

Basin currents may be caused by a number of factors; currents can cause short-circuiting that reduces the effectiveness of the settling process. (Short circuiting refers to the tendency of water to bypass the normal flow path through a basin and thus reach the outlet in less-than-normal detention time.) Surface currents may be caused by wind. Density currents may be caused by differences in temperature and suspended solids concentrations. Eddy currents are produced by the flow of water coming into and leaving a basin. Currents can be reduced by proper basin design and the use of basin covers.

Detention Time

"Detention time" refers to the actual time required for a small amount of water to pass through a sedimentation basin at a given flow rate. Note that actual and theoretical times may vary greatly, though actual detention time can come close to theoretical detention time in basins that have been engineered and constructed correctly to reduce short circuiting. A die test will show the degree of short circuiting. A serpentine pattern of walls or baffles in a basin, as shown in the photo to the right, is one method of reducing short circuiting.

Sedimentation Basin Zones

Sedimentation basins are designed with four specific zones as described here. The "inlet zone" is designed for a smooth transition from the flocculation basin. The primary design goal is to distribute flow evenly, minimizing short circuiting. The "settling zone" is the largest portion of a basin. It is designed to provided calm, undisturbed storage long enough (3 - 4 hours) to permit effective settling. The "sludge zone" is at the bottom of the basin. It serves as a temporary storage place for settled particles. The "outlet zone" is design for a smooth transition from the sedimentation basin over weirs that control water flow and promote uniform draw off. Regardless of basin type, design characteristics of each of the four specific zones are critical for high particle removal efficiency.

Outlet weirs (as shown in the photo to the left) come in many configurations, but all must be properly leveled to ensure a smooth, even flow from the sedimentation basin.

Basin Types

A variety of basin types exist; each basin type has pros and cons to be considered when designing a new treatment plant.

Rectangular basins have a high tolerance for shock loading, perform predictably, are cost effective, low maintenance, and result in minimal short circuiting. Double deck basins conserve area (floor space). In this configuration, two basins are stacked. Sludge removal is required on both decks.