What Kind of Off-Mains Drainage System Do I Have?

If you occupy a property with an off-mains drainage system that you did not personally have installed, you may not be aware of exactly how that system works. There are multiple different types of off-mains drainage system that your property might have, and each of them needs to be maintained in a different way.

It can be difficult to know how your property’s drainage system is set up, because by their nature these appliances are hidden away underground, and householders will not be able to identify which is which simply by inspecting the above-ground infrastructure. However, it is important to find this out in order to make sure your drainage system can continue to function properly.

Here, we will explore the different types of off-mains drainage systems, and highlight the ways in which householders and homeowners can tell which system they have on their property.

What are the different types of off-mains drainage systems?

Off-mains drainage systems are utilised by properties that cannot be connected to the main sewer network. They are most commonly installed on remote or rural premises that are not located close to the main sewers, and ensure that wastewater and sewage generated by the property can be safely stored, processed and disposed of.

There are three common types of off-mains drainage system that are used by these properties:

  • Septic tanks – these are the most common type of off-mains drainage system, consisting of a large underground tank that separates the waste into layers, and discharges wastewater into a drainage field or soakaway system
  • Sewage treatment plants – these function similarly to septic tanks, but utilise mechanical components to treat and purify the waste, allowing the effluent to be discharged directly into a local watercourse
  • Cesspits – also known as cesspools, these tanks simply collect and store a property’s wastewater and sewage with no processing or treatment involved, meaning they need to be emptied more regularly

How can I identify my off-mains drainage system?

All three of these types of off-mains drainage systems have their own unique maintenance and upkeep requirements, so it is essential that you know what kind of system you have and how to look after it properly.

Here are the easiest ways of identifying which type of system is in place on your property:

Septic tanks

  • Your property will have between two and four manhole covers on site, located in close proximity to each other
  • You will be able to see above-ground vent pipes, used to remove the fumes and gases from the tank and distribute them into the air
  • The tank will be connected to a drainage field or soakaway to discharge effluent
  • The equipment will usually need to be emptied out around once a year

To find out more about how to maintain your septic tank, take a look at our complete guide.

Sewage treatment plants

  • You will be able to see an electrical box mounted above ground, somewhere in the vicinity of the equipment
  • You may be able to hear a humming sound coming from the electrical supply
  • The tank may be equipped with an alarm system to alert you to potential problems
  • The treatment plant has a cover that is slightly raised above ground
  • The tank will need to be emptied out around once a year

Sewage treatment plant maintenance is largely similar to septic tank upkeep, but with the additional requirement to keep its electrical and mechanical components serviced each year as well.


  • Cesspits have only one manhole cover above ground
  • These tanks tend to fill up very quickly and require regular emptying

Keeping on top of cesspit maintenance is absolutely essential to avoid overflows or damage to the system. To learn more, take a look at our cesspit maintenance page.

Find out more

By following this advice and obtaining the right information from the seller, you will be able to get a clearer idea of what kind of off-mains drainage system your property uses, helping you keep the system properly maintained and protecting you against the potential risk of overflows, blockages and breakdowns.

If you are having a problem with your off-mains drainage system and want to ask for help from qualified drainage experts, give Lanes a call today. You can contact us 24/7 on 0800 526 488, or fill in our online enquiry form to request a call back at your convenience.