Septic Tank

septic tank gathers wastewater – waste generated by your toilet and bathroom, garbage disposal and washing machine – from your house and holds it underneath the yard. Solids remain in the tank while liquids exit into a buried drain field.

What is a septic tank and how does it work?

A septic tank is an economical system that is used for the partial treatment of wastewater generated from domestic properties and commercial buildings that aren’t connected to the main sewage system. If you have questions about what septic tanks are, how they work and what they are used for, the answers you need are here. Let's start at square one:

• your septic tank is made up of a series of components that are used to treat wastewater on-site, most likely in a rural area that is off the main sewage network (off-mains).
• your septic tank works by collecting wastewater from your home, retaining solid materials, removing wastewater pollutants, and returning the treated effluent to your property’s soil or the surrounding environment via infiltration.
• across the UK, people use many different terms to describe septic tanks. The most common are sewage tanks, septic systems, residential septic tanks and sewage treatment plants (note: a sewage treatment plant is a different system to a septic tank.

Now that you have the basics under your belt, read on to discover how standalone septic tanks and more advanced treatment systems work.

How does a septic tank work?

A septic tank works by collecting wastewater from your toilets and drains. It retains solids and scum within the tank. While some of the more resilient pollutants are biologically treated, the effluent liquid from your septic tank is driven to a drainage field. In your drainage field, bacteria further break down the residual pollutants and the now fully-treated effluent returns to your property’s soil and groundwater.

That is the high-level answer. For a fuller understanding of how a standalone septic tank works, we can break the wastewater treatment process into 10 steps:

• Domestic sewage from your toilet (called blackwater) and wastewater from your shower, bathtub, sink, washing machine, and dishwasher (called greywater) runs into the main drainage pipe that leads to your septic tank.
• Heavier solid matter sinks to the bottom of your septic tank and forms sludge.
• Lighter matter such as fats, oils, and greases (FOGs) float to the top of your septic tank to form scum.
• Anaerobic organisms (microorganisms that do not need oxygen) in your septic tank feed on organic wastewater pollutants, converting them to inert matter.
• Cleaner liquids pass through an effluent filter (or other retention mechanisms), further purifying the effluent.
• The filtered liquid flows through a pipe toward the drainage field (or a secondary filter)
• The perforated pipes from your drainage field allow the partially treated wastewater to seep into a layer of gravel.
• The microorganisms present, as well as other subsurface mechanisms, further break down contaminants as wastewater percolates through the layer of gravel and into your native soil.
• In your native soil, any remaining impurities are removed. These impurities may include harmful bacteria, parasites, viruses, and nutrients.
• Treated wastewater reaches groundwater, which eventually drains into nearby streams, rivers, and lakes.

Please note, prior to 2020 you could install a septic tank with or without a drainage field (and discharge to water bodies). However, in 2020, the environment agency changed the septic tank regulations and introduced the General Binding Rules. This means that any septic tank that discharges directly to a watercourse is no longer permitted (without obtaining a permit). You may need to upgrade your septic tank to a sewage treatment plant, or add a secondary filter, such as an Ecoflo biofilter to continue discharging to a water body; alternatively, a drainage field is required.