Batch Coagulation/Flocculation Reverse Sludge Vortex Problem

[TWCivils]

My water treatment guys are having some problems with a simple batch coagulation/flocculation system we built. After the flocculation stage we drain off the treated water via a floating outlet. However even if the tank is 1/4 full I'm noticing that a reverse vortex quickly forms that snakes up from the floor of the tank bringing with it the sludge. No amount of playing with baffle plates can stop the vortex forming - whatever you put in it's path the sludge vortex seems to duck around it. For a bit of background information, we have four 5,000 gallon tanks where we dose the incoming raw (river) water with with 1% aluminium sulphate solution using a venturi doser. The tank inlet is anged at 90 degrees so that we get the body of water rotating to aid flocculation and the flocs form and settle out pretty quickly (within 30 mins). The problem occurs when we have to draw off the treated (and clarified water). We've tried floating and static outlets of various configurations and directions. In every case we notice that when there is 3 ft of water depth remaning a snake of sludge starts to creep up from the sludge in the tank bottom up to the outlet and joins the clarifed water. Any ideas would be appreciated.I have to say we're stumped! THX.

 

[bimr]

You have two options.

1. Fabricate a much larger outlet device to slow the current of water into the outlet.

2. Remove the sludge from the bottom before you remove the water.