Septic Tank or Sewage Treatment Plant? We Help You Choose

However, this isn’t always a viable option even with the help of a sewage pumping station and in these instances, the only alternatives are a septic tank or a sewage treatment plant.

The two terms are frequently used interchangeably, but they are quite different in the way they function and their suitability for differing locations. In this blog post, we will examine the pros and cons of each system so you can make an informed decision about which you should install.

What is a septic tank?

For generations, properties in the UK relied on a cesspit to manage their sewage. This was a hole in the ground, covered to prevent odours and falls, and either sitting directly underneath an outside toilet or connected to indoor facilities by a pipe. The cesspit relied on the porosity of the surrounding soil to extract liquids from the sewage and the remaining solids were regularly removed.

Septic tanks took this concept one stage further with the addition of an outlet pipe that allowed liquids to drain from the tank while the solids settled to the bottom. The liquids would then be diverted to a soakaway pit, where they would be slowly absorbed into the soil. While the septic tank still required emptying, this was less frequent than with a cesspit.

Modern septic tanks have a dual-chamber design. The first chamber is used to collect solids, while liquid waste overflows into the second chamber from where it is discharged into a drainage field.

However, this design still didn’t prevent the wastewater from polluting soil and watercourses so by 2010, the UK’s Environment Agency was thinking about how they could improve water quality and reduce pollution. We’ve got an interesting blog post that explains the latest changes to regulations, which you can read here. 

Under the new regulations, a septic tank can no longer be connected to a soakaway system but must outflow to a drainage field that is designed and constructed in compliance with BS6297: 2007.

Pros and cons of a septic tank

The main advantage of a septic tank compared with a sewage treatment plant is that it is a simple design with no moving parts, little to go wrong and no need for an electrical power supply.

However, several factors make this a less attractive option. A septic tank can only drain wastewater into a drainage field, which requires a considerable amount of space. There are also limitations on where a septic tank can be sited. It must be away from buildings, boundaries, watercourses, underground cables or pipework and roads or driveways.

Not all soil types are suitable for a septic tank system. Clay soil, for example, doesn’t absorb or filter wastewater very efficiently and can lead to water contamination. Septic tanks cannot be installed in a Groundwater Protection Zone 1 area.

While a domestic system serving a single residence may not present a problem, septic tanks are also ranked by the number of persons they can serve, so this can limit the size of the building they can be used for.

Sewage Treatment Plants

Sewage treatment plants may outwardly seem similar to modern septic tanks, but they incorporate a three-stage water treatment process that leaves the wastewater discharge substantially cleaner than that of a septic tank. 

As with a septic tank, liquid and solid sewage discharge together into a primary holding chamber, where they are separated by gravity. Liquid waste flows into a second chamber, leaving solid waste in the holding chamber for periodic removal by a registered waste carrier.

In the second chamber, an electrical air pump introduces oxygen into the liquid waste. This encourages the growth of aerobic bacteria which breaks down any remaining solids in the wastewater. The bacteria-infused liquid is then pumped into a third chamber, where the bacteria sinks to the bottom and is then recycled back to the holding chamber.

The resulting wastewater is now clean enough to be discharged directly into a watercourse such as a lake, river, stream or ditch (providing consent has been obtained by the local authorities).

Pros and cons of a sewage treatment plant

A sewage treatment plant is far and away the most environmentally friendly option for off-grid sewage management and is compatible with any location or soil type. Sewage treatment plants can also be scaled to serve a larger number of persons than a septic tank.

However, they do have a few limitations.

The higher initial cost of a sewage treatment plant compared with that of a septic tank may be an obstacle, particularly if there is no nearby watercourse for the clean wastewater to discharge into.

A sewage treatment plant requires an electrical connection to power the air pump, which increases installation and ongoing maintenance costs.

As they contain moving and electrical components, sewage treatment plants naturally require regular servicing to keep them working efficiently and are more prone to wear and tear.

So, there you have the pros and cons of septic tanks and sewage treatment plants. The most suitable option for your property depends on a combination of factors that only you can decide on, but we hope we have made that decision easier for you.

We’re always happy to help if you have any further questions and you’re welcome to call our friendly team of drainage experts on 01420 555600 or email [email protected].

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!