Separators,Silt Traps, Washdown Separators

A full retention separator treats the entire flow of water passing through it, so it's used where the whole surface water run-off could be contaminated, such as forecourts. A bypass separator only treats the "first flush" of run-off, diverting the more heavily contaminated initial flow through the separator whilst higher volumes from heavy rain bypass treatment, which suits car parks and lower-risk sites.

Forecourt separators are specifically designed and rated to handle the higher hydrocarbon loads and fuel spill risk found at petrol stations and fuel storage sites, and are typically specified as full retention units. A standard separator may be adequate for car parks or general vehicle areas with lower contamination risk. Local authority and Environment Agency guidance, plus your site's discharge consent, will confirm which class you need.

Sizing depends on the peak flow rate from your washdown activity (litres per second), the drainage area, and the level of contamination expected — heavy vehicle or industrial washdown needs a larger unit than light cleaning duties. You'll also need to factor in silt storage capacity if sediment loading is high. We can help you work through these figures against the relevant British Standard (BS EN 858) before you order.

Silt traps and separators need regular inspection and periodic emptying by a licensed waste carrier to remove accumulated oil, silt and debris — how often depends on usage, but quarterly to annual checks are typical. Skipping maintenance reduces treatment efficiency and can lead to discharge non-compliance. Most systems have an access chamber or manhole to make desludging straightforward.

A silt trap alone only removes sediment — it won't treat oils or hydrocarbons, so if your site has any risk of fuel, oil or chemical contamination you'll need a separator downstream as well. Silt traps are often fitted upstream of a separator to protect it from silt build-up, extending its service life and reducing how often it needs emptying. Sites with clean surface water run-off, such as some car parks, may only need a silt trap.

These units are typically installed below ground with suitable backfill (usually pea shingle or as specified by the manufacturer) to manage groundwater and loading conditions, and access covers left at grade for inspection and emptying. Ground conditions, water table and traffic loading (e.g. pedestrian versus vehicle areas) all affect the installation method and cover class required. We'd always recommend using a competent groundworks contractor and checking the manufacturer's installation guide before digging.