Filter / clinker bed
Trickling or percolating biological filter
What it is & how it works
A filter bed — traditionally a "clinker bed" — is a secondary treatment stage, often added downstream of a septic tank on older rural properties.
Settled liquid is spread over a bed of coarse media — clinker, gravel or modern plastic shapes. The surfaces grow a film of bacteria that digest the pollution as the water trickles past, before it moves on to be dispersed.
In its classic form it works by gravity with no moving parts and no power, which makes it robust and cheap to run — but it needs the upstream tank kept in good order, or it clogs.
The rundown
At a glance
- Size & capacity
- Sized by the load it serves. Many are older, generously-sized rural installations; the bed area sets how much it can handle.
- Coping with busy periods
- Moderate. The media buffers flows to a degree, but a heavy hydraulic surge gives the bacteria too little contact time to treat the water well.
- Bleach & chemicals
- The bacterial film is harmed by bleach, cleaners and biocides, just like a treatment plant — use them sparingly.
- Wipes, sanitary items & fats
- Solids escaping the upstream tank foul and block the media, leading to ponding on the surface. Keep wipes and fats out.
- Land footprint
- A built bed structure of moderate size, usually alongside the tank it serves.
- Water quality & where it goes
- Good secondary treatment — noticeably cleaner than a septic tank alone, which is why they were popular before package plants.
- Care & servicing
- Keep the liquid spreading evenly across the bed, clear any ponding or leaf litter, and de-sludge the tank that feeds it. Watch for the media fouling over time.
- Signs it is failing
- Water ponding on the surface of the bed, smells, flies, and poor-quality water leaving it.
Compare
Other systems
Septic tank
An underground tank that settles out solids, then sends the liquid to a drainage field to soak away.
Learn more → Treats the wastePackage treatment plant
Like a septic tank, but with a powered stage that fully treats the waste — clean enough to discharge to a watercourse.
Learn more → Stores onlyCesspool
A sealed tank that just stores sewage with no treatment and no outlet — it has to be emptied by tanker.
Learn more → Disperses the waterDrainage field
Not a tank — a network of buried pipes that lets treated water soak into the soil, where bacteria finish the job.
Learn more → Treats the wasteReed bed
A planted gravel bed where reeds and the life around their roots polish the water — low-energy, with a wildlife benefit.
Learn more →