Off Mains Drainage – What Are The Options & Differences?

Living the rural life can certainly be the dream for many people, but it does come with a few extra ‘to-dos’ for your home maintenance list. Typically, a property that’s particularly remote in the countryside will not have access to the main sewer network and will have its own self-contained drainage system.

The waste from a property will always go to a tank of some description. There are three main options of tank which can vary depending on the circumstance of the property as well as your budget:

Cesspit (or ‘cesspool’)

A cesspit/cesspool is essentially a holding tank with no outlet or treatment process incorporated. Waste enters the unit to be stored and is then de-sludged (emptied) every 6–8 weeks. They are generally used in delicate scenarios where there may be facilities/amenities nearby, such as drinking water supply.

Installation costs are low; however, they require significantly more maintenance than other systems and this comes at a regular cost. It can cost around £1500-£2500 per year to empty the system at regular intervals of 6-8 weeks. At this point, the maintenance costs exceed any positives the system offers, so it’s only a worthwhile option when it is a necessity.

Cesspits now, are banned in Scotland and are becoming a last resort for waste management in the rest of the UK.

Septic tank

Septic tanks have been around for a very long time, come in all kinds of variations and are probably still the most common off mains drainage option.

More refined than the cesspit option, a septic tank has the functionality to treat the liquid part of the sewage. They contain multiple chambers to allow for the solids to separate and settle so that the effluent can be discharged via an outlet.

Waste enters the chamber, where gravity separate’s the solids and liquids. Due to mass – the solids sink and form a compact base layer, and the effluent stays on top until it reaches the outlet level. The effluent is discharged into the surrounding land through a drainage field, typically using soakaway crates, where the effluent percolates through the soils and is broken down by natural bacteria.

Just like cesspools, the solids will need to be removed professionally but at fewer intervals. The removal cost is roughly £300-£600 per year, depending on if it needs desludging once or twice – being the average. A percolation test will need to be carried out to qualify the suitability of the land for a discharge system

Changes in legislation mean that septic tanks that dispose of water directly to a watercourse or ditch are legally obliged to ensure the water is clean enough to not cause pollution. This is most likely to result in being upgraded to a sewage treatment plant, as below. Alternatively, there is the option of adding soakaway crates to infiltrate through to a drainage field instead. You can read more on the details of this change in legislation in our Septic Tank Regulations: 2020 Legislation article and decide on the most suitable option that suits you.

Sewage treatment plant

Technological developments have led to the arrival of sewage treatment plants – the most sophisticated and advanced option of an off mains drainage tank.

A sewage treatment plant is essentially a self-contained unit that cleans all wastewater which enters the system and allows for a typical rating of 97% clean water. Unlike that of a septic tank, this can safely be discharged into a ditch, stream, or soakaway system without the environmental concern of contamination.

The process is very basic, with three dividing chambers put in place:

  1. The primary settlement chamber – separates the solids and moves the water to the second chamber
  2. The second being the secondary reactor - known as the biological zone, pumps compressed air into the system via a ‘blower unit’. This aeration process encourages naturally occurring bacteria and breaks down the wastewater, removing contaminants.
  3. Final chamber for discharge - acts as a holding cell for the treated wastewater until it meets its capacity and then discharges out of the system. 

The main benefits are that the sewage is treated to a higher quality than other systems, replacing mains methods, and the effluent has little to no impact on the surrounding environment. The only disadvantage being the necessity for a mains electrical connection. De-sludging will need to occur around once a year, typically costing roughly £300 on average.

It is expected that in the lead to the forthcoming 2020 legislation change, a sewage treatment plant will be the most common choice for most rural properties.

Need help with your off mains drainage?

We supply everything you would need for environmentally friendly, off mains drainage options; including sewage treatment plants and soakaway crates. Browse our catalogue of products today and order for free next day delivery. Alternatively, you can call our friendly drainage experts for more guidance on the products that will meet your needs exactly.