FullRetention Separators

A full retention separator treats the entire flow from the drainage area, up to a defined maximum flow rate set by the automatic closure device, with any excess simply backing up rather than bypassing untreated. A bypass separator only treats a portion of the flow and diverts higher volumes around the separator, which makes full retention units the correct choice for higher-risk sites like fuel depots and forecourts where any spillage must be captured.

Sizing is based on the total paved or hardstanding area draining to the unit and the resulting peak rainfall flow rate, typically calculated to a specified storm return period. You'll also need to account for the site's pollution risk classification and any local water authority or Environment Agency requirements. We'd always recommend getting your drainage area and impermeable surface figures confirmed by your engineer or drainage consultant before ordering.

These are specified for sites with a genuine risk of hydrocarbon or chemical spillage entering surface water drainage, such as fuel distribution depots, filling station forecourts, vehicle workshops, hauliers' yards and large car parks. Building control, the Environment Agency or your local authority may make one a planning condition for developments of this type. If you're unsure whether your site needs a full retention or a lower-spec separator, it's worth checking your drainage strategy or planning conditions first.

Most units are manufactured in GRP (fibreglass) or polyethylene, both of which are lightweight, corrosion-resistant and well suited to below-ground installation. GRP tends to offer greater structural rigidity for deeper installations or higher traffic loading, while polyethylene units are lighter and easier to handle on site. The right choice depends on your ground conditions, invert depth and expected vehicle loading over the chamber.

These are below-ground units requiring excavation, a suitable prepared base (often a concrete slab or backfill as specified by the manufacturer), and correct connection to the inlet and outlet drainage runs. Access covers must be brought up to finished ground level to allow for the ongoing inspection and desludging the system needs. Installation should follow the manufacturer's guidance closely, as incorrect backfilling or bedding is the most common cause of problems later on.

Routine inspection is generally recommended every six months, with full desludging and cleaning carried out at least annually or sooner if the oil and silt levels reach capacity. High-risk sites such as busy forecourts may need more frequent attention. Regular servicing isn't just good practice — it's what keeps the separator performing to its design standard and keeps you compliant with your discharge consent.