What weir plates, launders and scum baffles do

A clarifier or settlement tank only works if the water leaves it slowly and evenly. The weir plate is the level edge the water spills over; the launder is the channel behind it that collects the decant and carries it to the outlet. Between them they set the local approach velocity across the whole tank.

Scum baffles sit ahead of the weir and stop floating solids, grease and rag reaching the overflow. On a circular clarifier the baffle usually runs as a continuous board just inboard of the weir line; on rectangular tanks it is fixed across the channel width.

Get any of the three wrong and the settled sludge blanket is disturbed long before the water reaches the outlet.

Why weir condition affects settlement performance

Overflow weirs work on head. A weir edge that is high on one side and low on the other passes disproportionately more flow at the low point, because discharge over a weir rises faster than the head over it. The result is a hydraulic short circuit: a fast lane forms across the tank, residence time drops locally, and solids that should have settled are dragged over the plate.

Corroded, buckled or bowed plates do the same thing on a smaller scale, notch by notch. Deposits and biofilm build unevenly on a rough edge and change the discharge coefficient along the run. Once weir loading is uneven, upstream dosing generally does not make up the difference, because the problem is hydraulic rather than chemical.

V-notch weir plates and why level adjustment matters

V-notch (triangular) weirs are widely used on municipal and industrial clarifiers in preference to a straight edge. The notch concentrates the flow into discrete streams and makes the discharge far less sensitive to small errors in level, which is exactly what is wanted on a large tank that may settle or flex slightly over its life.

Notch geometry is only half of it. The plate has to be mounted so it can be levelled and re-levelled: slotted fixings, a packing arrangement behind the plate, and access to the bolts without emptying the tank where possible. Fabricated thermoplastic plates take slotted holes readily and can be drilled to match an existing bolt pattern rather than forcing new holes in a concrete or steel launder wall.

Thermoplastic against stainless steel and GRP

Material choice for weirs, launders and baffles is mostly a corrosion and handling argument. Published general properties for the common options run roughly as follows, and are a guide only — the duty, temperature and chemistry on the plant decide it.

Polypropylene (PP): good resistance to most treated waters, dilute acids and alkalis, light enough for two people to handle a long plate, easy to drill and trim on site, and readily welded
PVC: rigid, dimensionally stable, resistant to chlorinated and dosing chemistry commonly found around treatment works
Stainless steel: strong and stiff, but chloride pitting and crevice corrosion around fixings is a familiar problem in wastewater, and replacement plates are heavy to manoeuvre over a running tank
GRP: widely used for launders and covers; the resin system and any exposed fibre at cut edges govern its life

Beckox fabricates in thermoplastic. If a scheme specifies something outside that, talk to us about it rather than assume. The materials guide sets out what each polymer is generally suited to.

Scum baffles and boards

Baffle boards fail quietly. They warp, the fixings corrode, a section drops, and floating material starts short-cutting to the launder. Because the failure is partial rather than total, it often goes unnoticed until effluent samples move.

Fabricated PP or PVC boards can be made in manageable lengths with overlapping joints and stainless or plastic-backed fixings, so a single damaged section is replaced without stripping the whole run. Profiles can be matched to whatever is already bolted to the tank wall.

Fabricated tanks where a moulded vessel will not do

Rotomoulded vessels are excellent when the duty fits the catalogue. They stop being an option when the space is awkward, the volume is unusual, the fittings are numerous, or the tank has to be built in sections and assembled in a plant room with a narrow door.

That is where fabrication earns its place: rectangular, stepped and part-open process tanks and vats, effluent and settlement tanks, dosing and day tanks with the branch positions the pipework actually needs. Bunded chemical storage tanks are fabricated with integral secondary containment; the containment volume needed for a given vessel and substance should be confirmed against current guidance for the site. Where an existing concrete or steel structure is sound but the surface is not, a thermoplastic tank lining is often worth considering as an alternative to replacing the vessel.

Odour control at pumping stations and sludge handling

Hydrogen sulphide is the reason so much of a treatment works ends up in plastic. It is corrosive to concrete and steel, it is detectable at very low concentrations, and it is generated wherever water goes septic — wet wells, rising main discharges, sludge tanks, dewatering halls.

Covering the source and drawing the foul air to a treatment stage is the standard approach. Packed-tower and cross-flow fume scrubbers work by passing that air counter to a recirculating liquor, so the contaminant transfers into the liquid phase; caustic and oxidant dosing is the usual chemistry for sulphide. Ducting, covers, hoods and the transfer pipework can all be fabricated in the same materials, which keeps the corrosion problem out of the extract system as well as the tank.

Working to drawings for utilities and their contractors

Much of this work arrives as a drawing from a consultant or a framework contractor with a fixed outage window. Beckox has been fabricating to third-party drawings since 1982 — 44 years in the one trade — and designs, fabricates and installs from a single Poole works, so there is no handover between a shop and an unrelated install crew. Wessex Water is on our client list, alongside names such as Rolls-Royce, Babcock Marine and Merck.

Where the drawing and the tank disagree, which happens on older assets, we would rather resolve it before anything is cut.

Retrofit and replacement, measured on site

Replacing weirs and launder components on an existing clarifier is a measuring job first and a fabrication job second. Bolt centres drift, tank walls are rarely true, and the original drawings may not reflect what was built. We survey the run, take the fixing pattern as found, and fabricate to that rather than to an assumption — which means the parts fit as found rather than being adjusted on site during the outage.

Site consultations are free. Call 01202 736 725 to talk through a weir, launder or baffle replacement, or use the contact page to send drawings over and we will come and measure it.

PAGE REVIEWED — 30 July 2026

Weir Plates and Launders for Clarifiers and Settlement Tanks