What is the difference between a septic tank and a sewage treatment plant?

A septic tank provides basic treatment through settlement and bacterial action, requiring a drainage field to complete treatment in the soil. A sewage treatment plant (also called a package treatment plant) provides higher-level treatment using aeration and produces cleaner effluent that can sometimes discharge directly to a watercourse with registration.

Which off-mains drainage system is best?

The best system depends on your site: Septic tanks suit rural properties with good soil for drainage fields. Treatment plants are better where soil is poor or space is limited, as they can discharge to water. Cesspits are a last resort where no discharge is possible, as they require frequent (and costly) emptying.

Do off-mains drainage systems need planning permission?

Most domestic installations are permitted development but require Building Control approval and may need Environment Agency registration. In conservation areas, AONBs, or near watercourses, you may need planning permission. Always check with your local authority before installation.

How often does a septic tank need emptying?

Most septic tanks need emptying every 12 to 24 months, depending on tank size and household usage. Treatment plants typically need sludge removal annually. Cesspits require much more frequent emptying—often every 4 to 6 weeks—as they have no treatment or discharge.

Off-Mains Drainage Systems UK: Complete Guide

Q: What is off-mains drainage?

Off-mains drainage refers to any private sewage treatment system used by properties not connected to the public sewer network. The three main types are septic tanks, sewage treatment plants, and cesspits. Around 4% of UK homes (over 1 million properties) rely on off-mains drainage.

If your property isn't connected to the public sewer network, you'll need a private drainage system. This guide explains the three main options—septic tanks, sewage treatment plants, and cesspits—and helps you understand which is right for your property.

Quick Answer Off-mains drainage means private sewage treatment. Septic tanks are most common and need a drainage field. Treatment plants produce cleaner effluent and can sometimes discharge to water. Cesspits store waste for collection and are the most expensive to run.

What is Off-Mains Drainage?

Around 4% of UK homes—over 1 million properties—are not connected to the mains sewer system. These properties rely on private drainage systems to treat and dispose of wastewater.

Off-mains drainage is most common in:

Rural areas where mains sewers don't reach
Older properties built before mains infrastructure
New developments in remote locations
Properties too far from public sewers to connect economically

The Three Types of Off-Mains Systems

1. Septic Tanks

A septic tank is an underground chamber that collects wastewater and allows solids to settle. Bacteria break down waste, and the liquid effluent flows to a drainage field where soil completes the treatment.

Septic Tank Key Points

Most common off-mains system in the UK
MUST discharge to a drainage field—not to water
Needs emptying every 12-24 months
Lower installation cost than treatment plants

Read our full septic tank guide →

2. Sewage Treatment Plants

Also called package treatment plants, these systems use aeration and biological processes to treat wastewater to a higher standard than septic tanks. They produce cleaner effluent that may be allowed to discharge to a watercourse.

Treatment Plant Key Points

Higher treatment standard than septic tanks
May discharge to watercourse (with registration)
Needs electricity to run pumps and aerators
Requires regular servicing and maintenance

Read our full treatment plant guide →

3. Cesspits (Cesspools)

A cesspit is a sealed tank with no outlet. All waste is stored until a tanker empties it—typically every 4-6 weeks for a family home. This makes them the most expensive to run.

Cesspit Key Points

No treatment or discharge—just storage
Very frequent (and costly) emptying required
Only suitable where no other option works
Minimum 18,000 litre capacity required

Read our cesspit emptying guide →

System Comparison

Feature	Septic Tank	Treatment Plant	Cesspit
Treatment Level	Basic (primary)	Advanced (secondary)	None
Discharge To	Ground only (drainage field)	Ground or watercourse	No discharge (tanker removal)
Emptying Frequency	12-24 months	12 months (sludge)	4-6 weeks
Electricity Needed	No	Yes	No
Annual Running Cost	Low-Medium	Medium	Very High

Choosing the Right System

Choose a Septic Tank If:

You have suitable soil for a drainage field (passes percolation test)
You have enough space for tank and drainage field
You want lower installation and running costs
You prefer a system that doesn't need electricity

Choose a Treatment Plant If:

Soil conditions don't suit a drainage field
You have limited space
You need to discharge to a watercourse
Higher effluent quality is required

Choose a Cesspit Only If:

No other system is feasible
Soil is completely unsuitable for infiltration
Watercourse discharge is not permitted
Property use is very low (holiday home)

Regulations and Compliance

All off-mains drainage systems in England must comply with the General Binding Rules. Key requirements:

No septic tank discharge to water — Since 2020, septic tanks cannot discharge to streams, rivers, or ditches
Treatment plants need registration — If discharging to water, you must register with the Environment Agency
Building Control approval — Required for new installations under Building Regulations Part H
Maintenance records — Keep evidence of regular emptying and servicing