A Quick Guide to Off-Mains Sewage Systems

In our region, most properties are directly connected to the main sewer system. Homeowners are responsible for the drains on their land, while the drains and shared sewers beyond are maintained by the local water company. The wastewater charge covers our waste being taken away, and treated, via these channels.

In rural areas, however, some properties are not connected to this main sewer network. This might be because the existing pipes are too far away or cannot be extended to the property's location. Even if making the connection is possible, the owners may be put off by the cost and disruption involved.

Instead, these properties rely on private 'off-mains' sewage systems: cesspits, septic tanks, or sewage treatment plants. These systems require emptying and maintenance by specialist companies, and the owners take on these costs instead of paying a wastewater charge to their local water company.

In all three types of system, bacteria naturally break down the waste as they digest it for energy. Beyond this, the systems diverge in how they treat and discharge waste materials. Let's take a look at how they work!

Cesspit

Also known as a 'cesspool', a cesspit is essentially just a waste storage tank: its contents are held, untreated, until it is emptied. This means that cesspits need emptying more regularly than other systems - four to eight times a year, depending on their size and how many people are using them.

This need for more frequent emptying can make cesspits more costly to run than other systems. Moreover, because the waste they hold is untreated, they pose a greater risk to the environment if they fail. As such, they are generally only used when the location is unsuitable for anything else.

Septic tank

The most common type of system, a septic tank releases wastewater but doesn't actively treat it. During storage, the waste naturally separates into solid 'sludge' and surface 'scum'. The middle wastewater layer flows into a second compartment, where the process repeats, and then out of the system. The sludge and scum remain until the tank is emptied.

Upon exiting, the wastewater enters a drainage field like a gravel pit. Drainage fields differ, but are all designed to filter the wastewater before it enters the wider landscape, thereby giving bacteria a chance to further break down the pollutants it carries. Drainage fields have a limited lifespan, and also need to be maintained.

In the past, some septic tanks discharged their wastewater directly into watercourses like streams and rivers, but these arrangements became illegal in 2020. If your septic tank doesn't utilise a drainage field, you will most likely need to install one or replace the system with a sewage treatment plant.

Sewage treatment plant

Sometimes called 'small', 'domestic', or 'package', a sewage treatment plant works much like its commercial equivalent. As in a septic tank, gravity separates the wastewater from the sludge and scum. In this system, though, it enters an extra compartment where oxygen is introduced by an electric motor.

This aeration process helps bacteria to break down organic matter in the wastewater before it leaves the system. Since the wastewater is treated more effectively than in a septic tank, it can be discharged directly into a watercourse without passing through a drainage field (although one can be used to further filter the water). The solids and scum remain until the tank is emptied.

Sewage treatment plants are the newest sort of off-mains system, but are becoming increasingly popular. They need emptying less often than a cesspit and take up less space than a septic tank with a drainage field. They do, however, require servicing to ensure their mechanical parts are working correctly.

Know your responsibilities

If you have an off-mains sewage system on your property, there are laws that you must follow. Failure to do so could result in enforcement action from your Local Authority or the Environment Agency. Beyond your legal responsibilities, managing your system properly can improve its efficiency and reduce the risk of faults. This will help you to not only protect the environment, but also avoid more expensive repairs.

We're helping off-mains users to keep their systems safe, legal, and effective through the Septic Smart project. If you'd like to learn more about this pressing issue, why not check it out?

How to manage your off-mains sewage system

How off-mains sewage systems impact the environment

More about the Septic Smart project