Settling media ahead of a membrane

When a clarifier sits in front of ultrafiltration and reverse osmosis, it is no longer judged on effluent clarity alone. It is judged on whether it can protect a membrane - including on the day the sea delivers an algal bloom.

Also searched as: desalination pretreatment settling media · lamella clarifier for SWRO pretreatment · tube settler desalination plant · seawater clarification lamella

Send the pretreatment scheme Specification sheet (PDF)

Why pretreatment changes the media brief

A municipal clarifier can tolerate a turbidity excursion. A membrane train cannot: the excursion either passes through to the membranes or is caught by the filters ahead of them, and either way the plant pays. So the settling stage is specified for its worst day, not its average.

That shifts three parameters:

Design flow	Include the backwash and reject streams returning to the head of the works - they are part of the hydraulic load on the settling stage, not a side loop
Solids load	Design against the bloom or storm event the site has recorded, not the annual mean. This is what sets the cell size
Outlet quality metric	The membrane supplier's target is expressed as a silt density index, so the settling stage is judged on the SDI it delivers rather than on visual clarity

On the media itself, our published figures are the same ones any clarifier uses: cells of Φ25–90 mm at 62–236 m²/m³ specific surface and 93–97 % porosity, installed at 60°, stacked 4–5 m, loaded at 3–5 t/m²·h with a channel velocity of 2.5–3.0 mm/s.

The bloom case, and why cell size is the answer

Algal solids are light, they flocculate unevenly, and they arrive in a wave. A fine cell that performs beautifully at the design load blinds when the wave arrives; a coarser cell keeps the flow pattern and lets the pack recover afterwards. In practice, seawater pretreatment plants that see blooms are specified one cell size larger than a river-water plant at the same flow - and the loss of specific surface is recovered by stacking deeper, which the 4–5 m band allows.

Two operational points worth writing into the specification:

Sludge removal rate. A bloom produces sludge fast. The hopper, the scraper and the sludge withdrawal capacity are part of the settling design and are usually the first thing to become the bottleneck, well before the media does.

Cleaning access. If the media is expected to be hosed down between events, specify the module handling arrangement at the same time as the media. A pack that cannot be lifted out will not be cleaned.

Material for seawater

Polypropylene is the standard. Our published service life is 3–4 years for PP and 8–10 years for the ethylene-propylene copolymer grades - and for a plant that expects to run twenty years between refurbishments, the copolymer is the rational choice on cost per year even though the first price is higher. Stainless steel is specified where abrasion and temperature matter; plate grade and service life are confirmed per project.

Frequently asked

Can lamella settling replace dissolved air flotation in desalination pretreatment?

They do different jobs. DAF removes light algal solids quickly; lamella settling removes flocculated solids with a much smaller footprint than a conventional clarifier. Many plants use both, with DAF ahead of the settling stage during blooms. Specify the media for the solids that reach it and let the process designer decide which stage carries which fraction.

How deep should the media pack be?

Published stacking height is 4-5 m. Going to the deeper end recovers specific surface when you have chosen a coarser cell for solids handling, at the cost of structural load on the support beams. Check the beam load with the structural designer before specifying the deeper stack.

Does the media remove dissolved contaminants?

No. Settling media removes settleable solids only. Dissolved salts, boron and organics are a membrane or post-treatment question, and no settling product affects them.

What should I send with an enquiry?

Design flow including returns, the recorded worst-case solids or turbidity event, the membrane supplier's SDI target, basin dimensions and the existing support arrangement. With those four we quote the media and the module count against your basin.