What is the difference between an attenuation tank and a soakaway tank?

Posted by Callum Vallance-Poole, on May 9, 2022.

Difference between attenuation and soakaway tank

Attenuation tanks and soakaway tanks both hold water and appear to work in the same way – but there are some important differences that make them completely unique. Here we explain the difference between attenuation tanks and soakaway tanks to help you choose the right one for your project.

What is an attenuation tank?

An attenuation tank is used as an alternative to ponds, swales and wetlands by developers when there isn’t enough land space to build them. The tanks are large containers that are installed underground to work as a reservoir where excess surface water run-off can be stored. This helps to reduce the risk of localised flooding in residential areas before the excess rainwater is released at a controlled rate into a nearby water course (such as a river or reservoir) using a flow control. 

While there is a wide variety of ways to create an attenuation tank, geocellular tanks tend to be the most popular. They provide a modular solution that enables them to be constructed in a myriad of different ways, which ensures engineers have more flexibility. Geocellular crates also have a very high void ratio that helps to maximise space and reduce the amount of spoil from the excavation, which makes the process more cost-effective.

Attenuation tanks work by controlling the amount of water received by the system via a flow-control chamber. This means it can easily manage the amount of water coming into the drainage system, before releasing it at a rate that is appropriate for the surrounding environment. The possibility of floods occurring is far less likely which is good news for residents and nearby wildlife.

What is a soakaway tank?

A soakaway tank is more commonly referred to as a septic tank. The area where the wastewater is discharged from the septic tank is called a soakaway, although due to legal changes in January 2020, septic tanks must now only discharge into a drainage field, rather than a soakaway.

Septic tanks are underground tanks connected to a house plumbing system that works as a wastewater treatment plant. Waste produced by bathrooms, kitchen drains and laundry activities are sent to the septic tank which uses a combination of biological decomposition and drainage to process the material. They are typically used by properties that are not connected to main sewage networks due to their remote location in rural or coastal areas.

The system features a few different chambers and inlet and outlet pipes. When waste is sent to the tank it forms into three different layers. The bottom is the heaviest, containing thicker substances, with wastewater in the middle and oil and grease at the top. Bacteria naturally form to break down the solid waste, enabling the liquids to separate. It can then leave the tank and head out into the drainage field, where it percolates naturally in the soil, finally finding its way into groundwater below the surface.

What are the differences between an attenuation tank and a soakaway tank?

While attenuation tanks and soakaway tanks may appear similar and even perform similar functions in part, there is quite a big difference in how they remove water.

In a soakaway scheme, a permeable membrane is used to wrap around the system. Once the water has been collected, it is discharged so it can soak into the ground using an infiltration process that can support the natural rate of the surrounding environment. This prevents the ground from becoming waterlogged and flooding from occurring, so the water infiltrates the ground as and when saturation allows. A half empty time of 24 hours is usually ensured due to the design of the crate structure.  

When it comes to attenuation crates, water is managed by flow controls. This ensures a controlled volume can flow through into the main system at a regular rate, which will then prevent flooding further downstream.

What are the different types of attenuation crates?

There are several types of attenuation crates that can be used for different applications:

Single piece crates

All the components are contained in a single crate, without any separate chambers. It is supplied fully assembled, which makes installation much easier to plan and complete.

Stackable crates

A modular design that makes the setup process much easier as they can be constructed in a variety of different ways. Specialist equipment isn’t needed for the installation process, and they can also be stacked on top of each other if needed. 

Light duty crates

Often used for pedestrian and domestic vehicle loads, light duty crates can usually support up to 20 tonnes and are typically installed in garden and landscaping areas.

Heavy duty crates

These are for areas that experience large amounts of traffic, accepting weight loads above 60-70 tonnes, if required. Ideal for car parks or for busy residential areas.

Inspectable crates

An inspectable crate is designed to make maintenance a lot easier. This helps with sustainability and ensuring the system can maintain peak performance levels for longer.

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!