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Floating decanters: drawing clear water from SBRs, lagoons and digesters
A decanter draws clear liquid from the upper part of a tank, leaving floating scum above and settled sludge below. In batch reactors, lagoons and digesters the level keeps changing, so the decanter has to follow the surface. Floating decanters do this with pontoons or floats that keep the inlet a set distance below the water. This guide covers floating decanters for tanks and lagoons, not decanter centrifuges.
At a glance
Purpose: withdraw clear water or liquor without drawing in scum or disturbing settled sludge.
How: a floating inlet rides on the surface, and a pivoting arm or telescopic pipe carries the flow to a fixed outlet.
Where: sequencing batch reactors (SBRs), settling ponds, lagoons, ballast and bilge water tanks, digesters and sludge thickeners.
How a floating decanter works
The inlet sits just below the surface, under any floating scum and well above the sludge. Floats keep it at that depth as the level falls during decanting. The decanted liquid flows through a pivoting discharge pipe, a swivel joint or a telescopic pipe to an outlet in the tank wall or floor. Because the inlet always takes from the clearest layer, the quality of the decanted water stays steady from the start of the decant to the end.
The rate of withdrawal matters as much as the position. If liquid is drawn too fast or from too narrow an opening, the local velocity can lift sludge from the blanket below. Spreading the inlet over a long weir and controlling the decant rate avoid this.
Three designs compared
Design | Inlet | Best for | Notes |
|---|---|---|---|
Floating weir box on twin pontoons, with a pivoting discharge pipe | Ballast and bilge water tanks, SBRs, settling ponds, tanks with variable levels | Simple and robust | |
Long slotted weir pipe across the tank width, supported by end floats | SBR plants, equalisation and settling tanks, lagoon outlets | Even withdrawal across the width; can be lifted clear of the water between cycles | |
Floating ring with a weir cup and a telescopic pipe | Digesters, sludge thickening and holding tanks | Keeps the draw-off in the clear layer above the sludge |
Decanting in a sequencing batch reactor
An SBR treats wastewater in one tank through a repeating cycle: fill, react, settle, decant and idle. During the settle stage the sludge sinks and clear water forms above it. During decant, the decanter withdraws that clear water while the level drops. During fill and react, the tank is mixed and aerated, so the decanter must not take in mixed liquor. That is why SBR decanters are often fitted with an actuator arm that lifts the weir clear of the water, or closes it, outside the decant stage.
Checklist: what to send us
Tank or lagoon dimensions
Top and bottom water levels during decanting
Decant flow rate and the time allowed per cycle
Liquid type and whether scum or oil is present
Outlet position and level
Passive operation or motorised, with any control signals required
Preferred material
Frequently asked questions
Why not use a fixed weir?
A fixed weir only takes water at one level. A floating decanter works across the whole level range and keeps drawing from the clear layer as the level falls.
Will the decanter draw in sludge?
Not if the lowest decant level and the decant rate are set correctly for the tank. We size the inlet so that the withdrawal velocity stays low.
Can decanting be automatic?
Yes. Motorised decanters can be controlled by the plant PLC to start, follow and stop the decant on level and time signals.
Is this the same as a decanter centrifuge?
No. A decanter centrifuge is a machine that dewaters sludge by spinning it. A floating decanter is a tank fitting that draws clear liquid off the top.
Each decanter is sized to the tank and cycle. See our decanters page for the range. Send us your details and we will come back with a proposal. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.
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