Cesspit vs septic tank vs treatment plant: what do you have?

The three system types

If your house isn’t connected to mains drainage, you have one of three things in the ground:

1. Cesspit

A fully sealed underground tank with no outflow. Wastewater goes in; nothing comes out except by tanker. Cesspits are simple, reliable, and expensive to run because they’re emptied frequently — typically every 4 to 8 weeks for a 4-person household.

2. Septic tank

A two- or three-chamber tank that separates solids from liquids by gravity. Solids settle to the bottom (sludge), grease floats to the top (scum), and partially clarified effluent flows out of the tank to a soakaway — a network of perforated pipes in trenches that lets the effluent percolate into the ground. Septic tanks need emptying every 6 to 12 months, but the soakaway also has to keep working.

3. Sewage treatment plant

An active treatment system with a motor or air pump that aerates the wastewater and uses bacteria to break it down. The discharge is much cleaner than from a septic tank — clean enough to discharge directly to a watercourse in some cases. Plants need annual desludging plus occasional servicing of the mechanical parts. Common brands: Klargester, Biodisc, Conder, WPL, Marsh, Vortex, Graf.

How to tell which one you have

Start with the cover. All three systems have a cover at ground level, usually round, often heavy. The cover alone won’t tell you which type — but what’s near it might.

• One cover, no other features visible: probably a cesspit. Sealed tanks don’t need anything else above ground.
• One or two covers plus a soakaway field downhill (sometimes visible as a slightly damp patch or different vegetation in summer): septic tank.
• A cover plus an electrical box, vent pipe, or a faint hum: sewage treatment plant. Plants need power for the air pump or rotor.

If you can’t tell, the deeds usually mention it. Failing that, send us a photo of the cover area on WhatsApp and we’ll often identify it from the layout.

Side-by-side comparison

	Cesspit	Septic tank	Treatment plant
Outflow	None	To soakaway	To soakaway or watercourse
Power needed	No	No	Yes
Emptying interval	4–8 weeks	6–12 months	Annual (desludge)
Discharge cleanliness	N/A	Partial	High (90%+ treated)
Ground footprint	Tank only	Tank + soakaway field	Tank + sometimes soakaway
Typical install cost (new)	Lower	Mid	Higher
Running cost	High (frequent emptying)	Low	Low–medium (electricity)

Legal context (England)

Septic tanks discharging directly to a watercourse have been illegal in England since 1 January 2020 under the General Binding Rules — they had to be either replaced with a treatment plant or rerouted to a soakaway by that date. Septic tanks discharging to a properly functioning soakaway are still legal. Cesspits are still legal. Treatment plants are legal provided they meet the relevant British Standard discharge consent.

If you’re buying a property and the deeds say “septic tank discharging to ditch,” that’s a problem — the system isn’t compliant and the seller is technically obliged to upgrade it.

Switching between system types

If you’ve moved into a property and the running cost of cesspit emptying is hurting you, an upgrade to a sewage treatment plant pays back over 5–10 years on most properties. The capital cost is meaningful (£3,000–£6,000 plus install) but emptying drops to once a year. We can’t install — we’re a tankering business — but we can recommend installers we’ve worked with cleanly.

Whatever you’ve got, we empty it

TankAway empties all three system types. Routine and emergency. Cesspits and septic tanks across our full coverage area; treatment plants on confirmation per job. Get a quote.

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Last updated 2026-05-07. Written by the TankAway operator. We empty tanks for a living — this is the advice we’d give a friend.