Clarifier effluent troughs - How to design launders I overflow weirs

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Settling is the tendency for particles to fall through the liquid in which they are entrained.
The same principle is used by wastewater treatment plants to separate particles from wastewater. In clarifiers particles settle down as sludge to the basin bottom and the clearwater is released over effluent launders on top of the basin. Effluent launders are usually rectangular or circular troughs with overflow weirs and are made of stainless steel or FRP.
To ensure that all water can be discharged evenly from the clarifiers the sizing of effluent launders is very important. Obviously, the sizing depends especially on the maximum flow to the clarifiers but some other factors also play an important role. Friction forces between effluent water and launder surface as well as bottlenecks at the effluent discharge openings cause buildup of water in the effluent launders. The longer the launders are as more backwater buildup can be observed.
Furthermore, the spacing of effluent launders plays an important role in reducing water short circuiting within the clarifier basin. If the effluent launders are positioned unevenly in the basin or if the distance between effluent launders is too big water will flow along a nearly direct pathway from the inlet to the outlet, often resulting in shorter settling times in comparison with the calculated or presumed detention times. If tube settlers are used, their installation height must be considered for the effluent launder distances too. As a rule of thumb, the distance between effluent launder to effluent launder should be X, the distance between basin wall and effluent launder should be 1/2X and the distance between top of tube settler and top of effluent launder should approximately equal the distance between two launders divided by the square root of 3, divided by 2.
The water level in clarifiers is never completely even but at the same time the overflow of water over the effluent launder edges must be uniformly. To balance the differences in the water level out, effluent launders are usually equipped with a V or U notched overflow weir. The V or U notched weir regulates the overflow water amount and therefore accounts for uneven water levels in the clarifier basin.
In case the water flow to the clarifier basin is very turbulent and the indifference in the water level is even greater, the overflow weirs can also be made adjustable to ensure an even overflow.

Видео Clarifier effluent troughs - How to design launders I overflow weirs канала Karl-Uwe Schmitz