Scroll below for answers to all your sewage treatment questions

Do I need a sewage treatment plant?

A sewage treatment plant is required if your property discharges to a river, stream, ditch or canal (known as surface waters).

Septic tanks cannot discharge to surface waters as the effluent (discharge water) is too polluting.  Septic tanks discharge septic effluent.

Both sewage treatment plants and septic tank can discharge to a drainage field but most people will chose to install a sewage treatment plant because the drainage field is smaller and will have a much longer lifespan.

Most people will choose a sewage treatment plant for a modern building as it is far less polluting than a septic tank and is sustainable whereas a septic system will inevitably fail.

Where can a septic tank discharge to?

A septic tank can only discharge to ground via a drainage field and not to a soakaway.

The majority of sites in the UK cannot have a drainage field and therefore cannot have a septic tank.

Where can a sewage treatment plant discharge to?

A sewage treatment plant can discharge to either surface waters (a river, stream, ditch or canal) or to ground (via a drainage field and not a soakaway).

All discharges must comply with the Environment Agency’s General Binding Rules.

What is a soakaway?

A soakaway is a large hole/void in the ground used for the attenuation (storage) and disposal of rainwater.

A soakaway may be a hole filled with rubble, gravel, drainage crates or other media or it may consist of pre cast concrete rings with holes in.

Soakaways are designed to store water and enable it to be absorbed into the ground.

Sewage treatment plants and septic tanks cannot discharge to soakaways without a bespoke permit.

What is a drainage field?

A drainage field is a network of pipes and gravel laid in the top metre of soil, that facilitates the treatment and disposal of sewage effluent (water discharged from a sewage treatment plant or septic tank).

The primary purpose of a drainage field is to clean sewage effluent so that the contamination of groundwater is reduced.  A drainage field is not just a means of getting rid of water like a soakaway is.  This is why sewage treatment plants and septic tanks cannot discharge to soakaways.

To treat sewage effluent, a drainage field must be aerobic (have oxygen) and this is why it must be within one meter of ground level.  Oxygen in the air only penetrates approximately the top one metre of soil.

What is the difference between a soakaway and a drainage field?

A soakaway is a large hole/void in the ground:

  • A hole filled with gravel/rubble.
  • A hole filled with crates.
  • Concrete rings with holes in.

A drainage field is a network of pipes and gravel laid within one metre of ground level.

Can a septic tank discharge to a soakaway?

No.  In the UK a septic tank can only discharge to ground via a drainage field.  If a septic tank discharges to a soakaway then this is spot pollution of groundwater.

Can a sewage treatment plant discharge to a soakaway?

No.  In the UK a sewage treatment plant can only discharge to ground via a drainage field.  If a sewage treatment plant discharges to a soakaway then this is spot pollution of groundwater.

How big does a drainage field need to be?

This question cannot be answered without more information.  Every drainage field is unique and sized for the specific ground conditions.  There is no such thing as an average size drainage field.

What size sewage treatment plant do I need?

The size of a domestic sewage treatment plant in the UK is determined by the number of bedrooms.  For singles houses use the following:

  • 1 bedroom = 2 person
  • 2 bedroom = 4 person
  • 3 bedroom = 5 person
  • 4 bedroom = 6 person
  • 5 bedroom = 7 person
  • 6 bedroom = 8 person

For multiple properties or commercial sites, please contact us on 01759 369 915.

What size septic tank do I need?

In the UK, the minimum size septic tank is 2,700L and this is suitable for up to 4 people.

For every additional person you increase the volume by 180L.

The minimum number of people the septic tank has to serve if determined by the number of bedrooms in the same way as a sewage treatment plant.

It is very important to note that the septic tank volume refers to the “working volume” or volume of water the tank holds and not the gross volume which includes the air space above the water.

Do I need Planning Permission for a sewage treatment plant?

Most local authorities in the UK require you to obtain Planning Permission and it is very rarely refused.

Do sewage treatment plants smell?

Some do and some don’t.  Any sewage treatment plant that has a Primary Tank cannot be classed as odourless because the Primary Tank will give off PTO (Primary Tank Odour) just like a septic tank.  The design of the sewage treatment plant is critical in determining how frequently PTO is likely to be emitted.  Some system such as Cyclone are deliberately designed so that PTO is a very rare event.

Some sewage treatment plants, such as Vortex, are 100% aerobic and cannot emit odour if working correctly.

The most likely cause of odour from any sewage treatment plant is system failure.

Are sewage treatment plants noisy?

Yes and no. Most sewage treatment plants use a motor to oxygenate the water.  This is typically an air blower as used in fish ponds. Such motors give off a constant hum and the level of noise is dependent on the make and model of the motor.  Larger motors will be noisier than small ones. 

It is often the case that noise is caused by the vibration of the motor and is not the motor itself.  If a motor is allowed to touch another surface then vibration can occur and be amplified.  This is usually easily corrected.

FilterPod is a completely silent sewage treatment plant because it does not use electricity and thus does not have a motor.

How do I know if my septic tank needs emptying?

There is no way to tell if a septic tank needs emptying just by looking at it.

Septic tanks need emptying when the sludge storage volume is reached.  This can only be done with a core sampler.

For trouble free operation, empty your septic tank at regular, set intervals and never leave it more than twelve months.

How do I know if my sewage treatment plant needs emptying?

Most sewage treatment plants will not give you an indication that they need emptying.

For activated sludge sewage treatment plants such as Vortex, an indication of them requiring an empty is when the effluent being discharged is no longer clear.

In sewage treatment plants with a Primary Tank such as Cyclone and FilterPod, the depth of sludge in the tank must be measured to accurately determine if the tank needs emptying.  This is the job or the service technician.

To avoid pollution and tank failure, the best plan is to empty and refill your tank at set intervals.

What are the Environment Agency General Bind Rules (GBRs)?

The Environment Agency’s General Binding Rules (GBRs) is a set of conditions that outline what the Environment Agency will accept without a Permit being required.  It is essentially a checklist.  If you tick all the boxes then you can make the discharge without needing to inform the Environment Agency or obtain a permit.

Does a sewage treatment plant need emptying?

Yes.  All sewage treatment plants require periodic emptying because they are not 100% efficient and do not break down everything that goes in.

Over time, sludge accumulates in the sewage treatment plant and this must be removed when it reaches a certain level.

Once a tank has been emptied it must always be fully refilled with water as fast as possible.

How does a sewage treatment plant work?

A sewage treatment plant works by enabling sewage to mix with oxygen.  Oxygenating sewage allows aerobic bacteria to establish and these bacteria feed off the pollutants, removing them in the process.

There are various methods of aerating sewage in a sewage treatment plant but the most common is to use an air blower to blow air into the water, enabling oxygenation of the sewage.

How often does a sewage treatment plant need emptying?

This depends on the type and size of sewage treatment plant you have and the number of people using it.  As rule, all domestic sewage treatment plants should be emptied and refilled every 6-12 months. 

Some sewage treatment plants are marketed as having emptying periods of “up to” 3 years or even 5 years but no domestic system in daily use will achieve this.  if the sewage treatment plant is receiving very little or intermittent use (such as a holiday home) then longer emptying periods are possible.

How often do you empty a septic tank?

As a rule, you should always have a septic tank emptied once per year regardless of any other circumstances.  Larger septic tanks, often for multiple houses, should be emptied every six months.  This is often a requirement of your building’s insurance.

All septic tanks have a maximum sludge capacity and when this is reached, the tank needs to be emptied.  The rate of sludge accumulation depends on the number of users and their lifestyle.  The sludge capacity of the tank depends on its design.

The sludge level in a septic tank can be measured using a Core Sampler but this often unnecessary.

It is very important that the tank is fully refilled immediately after it is emptied.  It must never be allowed to fill naturally.

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

Septic tanks do not use electricity and create a polluting septic effluent.  Sewage treatment plants usually use electricity and create non polluting treated effluent.

A septic tank can be seen as a simple “solids separator” as all they do is separate solid and liquid waste in sewage.  The solids form a sludge in the bottom of the tank and this sludge rots.  As the sludge rots it turns the water septic.  The septic effluent discharged from a septic tank is toxic to life.

A sewage treatment plant oxygenates sewage to enable the formation of aerobic bacteria.  Aerobic bacteria digest the pollutants in sewage and therefore clean it.  The effluent discharged from a sewage treatment plant should be colourless and odourless.  The effluent does still contain chemical and biological pollutants but is generally acceptable for discharge into rivers and streams.