What is a domestic sewage treatment plant?

Question

Looking at a house off mains drainage. What's the difference between a treatment plant and a septic tank and which do I actually need?

Kieran · Accepted Answer

A sewage treatment plant and a septic tank both deal with wastewater from properties that are not connected to the mains sewer, but they work differently.

A septic tank mainly separates and partially treats the waste, with the liquid effluent normally discharging into a suitable drainage field. A sewage treatment plant goes further by using a biological treatment process to produce a much cleaner final effluent.

Which one you need will depend on the property, ground conditions, available space, where the treated water can discharge and the relevant environmental regulations. For an existing property, I would also establish what system is currently installed and whether it is compliant before purchasing the house.

If you are looking at buying an off-mains property, it is worth having the existing drainage system inspected before purchase, as replacing or upgrading an unsuitable system can be a significant expense

I hope this helps

Brian · Answer

A domestic sewage treatment plant is an on-site system that fully treats wastewater from a single home or small property — where mains sewers aren’t available. It cleans the water so it’s safe to discharge into a stream, ditch, or soakaway.

Dean · Answer

A septic tank uses gravity to separate solids from liquids, they also use no electricity. And a sewerage treatment plant uses mechanical components to introduce oxygen so it creates an environment for bacteria to break down and purify water, this requires constant electricity. A domestic sewerage treatment plant treats household water to a much higher standard.

Max · Answer

A treatment plant cleans the waste water as of a septic tank only uses gravity to separate the waste

Daniel · Answer

A domestic sewage treatment plant is basically a more advanced alternative to a septic tank for properties that aren't connected to the mains sewer. A septic tank mainly separates the solids and allows the liquid effluent to discharge into a suitable drainage field, whereas a sewage treatment plant actually treats the wastewater using a biological process before discharging much cleaner effluent.

Which one you need depends on the property, ground conditions, where the discharge is going and the existing drainage setup. If you're buying a house off mains drainage, I'd definitely recommend having the existing system inspected first to check its condition, capacity, discharge point and whether it complies with current regulations before deciding whether it needs upgrading or replacing.

Prentice · Answer

A treatment plant cleans the waste water as of a septic tank only uses gravity to separate the waste"

Mark · Answer

A sewage treatment plant uses air and bacteria to clean the water
A septic tank mostly just separates solids and still needs a soakaway
a domestic sewage treatment plant,needs power it cleans it much more thoroughly

Razvan · Answer

A septic tank collects wastewater and allows solids to settle, but the liquid discharged still needs to soak away through a suitable drainage field. A sewage treatment plant treats the wastewater to a higher standard before discharging it to a suitable location.

Which one you need depends on the property, the ground conditions, available space and the existing drainage arrangements. A treatment plant is often the better option where a suitable soakaway isn’t possible, but it still needs a compliant discharge point.

Before buying the house, I’d recommend checking the existing system, its condition and whether it complies with current Environment Agency regulations. If you’re unsure, get a drainage specialist to inspect it first.

Nico · Answer

A domestic sewage treatment plant is essentially a small-scale sewage works for a property that cannot connect to the mains sewer.

The main difference between a septic tank and a treatment plant is how much treatment takes place before the wastewater is discharged.

A septic tank mainly separates the waste. Solids settle at the bottom of the tank while the liquid effluent flows out into a correctly designed drainage field, where the surrounding soil provides further treatment. A septic tank should not discharge directly into a ditch, stream or watercourse.

A sewage treatment plant goes considerably further. It normally uses aeration and naturally occurring bacteria to biologically treat the sewage, producing much cleaner effluent. Depending on the site and applicable requirements, the treated water can discharge into a suitable drainage field or, where permitted, an appropriate watercourse. Treatment plants normally require electricity, periodic servicing and desludging.

Which system you actually need depends entirely on the property.

Before recommending either, I would establish:

• Whether connection to the mains sewer is reasonably possible
• What system is currently installed
• Where the existing system discharges
• Condition and capacity of the tank/plant
• Whether there is an existing drainage field and its condition
• Number of bedrooms/expected occupancy
• Available land and ground conditions
• Location of nearby buildings, boundaries and watercourses
• Whether the existing installation is compliant

If the property already has a sound septic tank connected to a correctly designed and functioning drainage field, there may be no reason to replace it.

However, if the existing system is failing, discharging incorrectly or a suitable drainage field isn't possible, a modern packaged sewage treatment plant may be the better option.

As you're considering purchasing the property, I would strongly recommend having the off-mains drainage investigated before committing. We can inspect the existing system and, where required, carry out CCTV surveying and drain tracing to establish the drainage layout, condition and discharge point.

From there, we can advise whether the existing system is suitable or whether upgrading/replacement is required.

What is a domestic sewage treatment plant?

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