Decanter Design in SBR Plants: Weir Travel, Floats and the Power-Free Option

What the equipment has to accomplish

A sequencing batch reactor (SBR) and its variants — CASS, CAST, ICEAS and DAT-IAT — treat municipal sewage and industrial wastewater in a single tank on a timed cycle. Filling, biological reaction and settling happen in sequence, and then the clarified supernatant has to leave. The decanter does that, and the constraint is severe: it must draw water from near the surface without disturbing the settled sludge blanket below or the floating scum layer above.

The industries that depend on it

Papermaking, alcohol, dye, pesticide, leather, viscose fiber, monosodium glutamate and sugar all run batch or semi-batch biological plants, and all of them face the same withdrawal problem. Get the decanting depth wrong and the effluent carries suspended solids that the biology already removed; get the travel speed wrong and the tank contents are stirred back into suspension at the worst possible moment.

Anatomy of the assembly

A typical decanter carries a decanting weir, branch pipes, a main pipe, a 360° rotating slewing support, sliding supports, a drive unit and an automatic control system. The weir is the working surface, the branch pipes collect what spills over it, the main pipe carries the flow out of the tank, and the slewing support provides the axis around which the whole assembly rotates.

One cycle, in order

During operation the drive moves the decanting weir downward in a variable-speed circular motion around the axis of the slewing support at the base of the unit. Clarified supernatant in the reactor flows over the weir into the branch pipes and out through the main pipe. Once decanting finishes, the weir rises at uniform speed around the same axis back to its standby position and waits there until the filling, biochemical reaction and settling stages have completed, at which point the next cycle begins.

Details that decide whether it stays reliable

• Adjustable decanting depth. The stroke is designed against the process requirement, so the unit takes the clear layer and stops above the blanket.
• PLC-based intelligent drive. On a discharge command the weir moves quickly from its parked position to below the water surface, drains the settled supernatant and repeats. A level switch at the lowest water level issues the return command and the unit rises rapidly to its initial position.
• Undisturbed surface. Within the designed loading range of the weir the liquid surface beneath it is not disturbed, and a float plus a scum baffle protect effluent quality.
• Balanced buoyancy. A special design keeps the weight of the decanter and the buoyancy acting on it essentially in balance, so drive power consumption stays very low.
• Rigid outlet joint. The connection to the main drainage pipe is a stainless-steel rigid joint, which avoids the high failure rate, short service life and heavy maintenance burden that flexible connections bring.
• Structural frame. The whole assembly sits on a frame sized for the stresses it meets in service, including the torque of a stalled drive.

The power-free WFT variant

One branch of the family operates without external power at all. Apart from an electric valve at the discharge outlet, no powered components are used anywhere in the machine. The WFT power-free decanter is a highly automated water treatment device, and its use with the SBR process for municipal sewage has been widely promoted in China, where the decanter is treated as the core equipment of the process.

Its main body adopts a U-type rotating support with all components in stainless steel, which gives a compact assembly and smooth running. The float uses a floating design that holds the decanting layer depth constant even as tank level keeps changing, so the unit keeps the best decanting effect throughout the drawdown. Decanting speed is uniform, the water surface stays calm and free of waves, the main drive produces no noise, and after decanting the unit floats up continuously as the level rises again.

Two more choices extend service life. The underwater rotating parts use a mechanical rotation device rather than a bearing arrangement exposed to grit, so the assembly can run for a long time without aging or breakage, and the main body contains no rubber components at all. During normal operation the decanting range can be adjusted freely between 0 and the maximum decanting depth through the electric valve, which is how the same unit covers a range of tank depths without mechanical change.

What to check when specifying

Four items cover most of the risk. Confirm the decanting depth against the actual sludge blanket height across a full cycle, not against the design drawing. Confirm weir loading at peak flow, because the surface disturbance limit is what keeps solids out of the effluent. Confirm the outlet joint type, since a flexible connection is the single most common maintenance complaint. And confirm material grade for every wetted part, because a stainless body with a mild-steel bracket fails at the bracket.

Why the weir carries the effluent quality

Almost every decanter failure shows up as suspended solids in the effluent, and almost every one of those traces back to the weir. If the weir travels too fast the surface is dragged down and the sludge blanket lifts; if it travels unevenly one side of a long weir takes more flow than the other, and the loaded end pulls solids. A uniform-speed rise, a float that follows the level and a baffle that holds back floating scum are cheap insurance against an expensive permit problem.