Drainage field cost

A septic tank drainage field is part of a septic system. It transports the wastewater back to the soil. After treatment and separation within the septic tank, water flows into a drainage field where it’ll organically filter through gravel, soil, and sand in a purification process before reaching the water table.

Often, the first indication of a leach field issue is the smell of sewage in your property or a swampy area in the garden.

Read on to find out the costs of replacing a septic tank drainage field.

What is a septic tank drainage field?

A septic tank drainage field – also known as a leach drain or leach field – sits beneath the ground. It’s made up of a variety of perforated pipes which border the septic tank. It eliminates pollutants and contaminations from liquid once it makes its way into the septic tank.

How much does a new septic drain field cost?

Septic tank drainage field costs vary depending on its size and type. On average, you’ll pay about £3,700 for the supply and installation of a 2,800 litre septic tank. For a larger septic tank (4,800 litres), you can expect to pay around £5,300 for supply and installation.

The cost to install a septic drain field ranges from £3,000 – £7,000.

How does a drainage field work?

The wastewater and waste from your property will flow into the septic tank, and thanks to the bacteria present, the water starts decomposing. The solid waste then falls to the foot of the septic tank and becomes sludge.

Fats, oils, grease, and other liquid waste floats on the top of the septic tank and the bacteria will break it down. The liquid waste then flows into the drainage field via underground pipework, whilst the solid waste stays at the foot of the septic tank where the bacteria continues breaking it down.

Finally, the liquid flows through the draining field’s pipework before running into the earth, where it’s naturally purified by the soil.

What’s the difference between a soakaway and a drainage field?

A rainwater soakaway is used to enable rainwater to infiltrate through the ground. Usually, it’s built by fitting soakaway crates within the earth to enable a substantial volume of water to come in and gradually empty away. You can only use a soakaway for surface water and rainwater.

A drainage field on the other hand is a loop of slotted pipes on and around a bed of gravel. It allows wastewater to run through the bed of gravel so that any bacteria will form there. This in turn removes contaminants from the water before it enters the ground.

Drainage field calculation

Apply your percolation test results to determine the floor area needed for your leach field. Use our drainage field calculator to work out the size of the field:

• At (floor area) = p (number of occupants in your property) x Vp (percolation value in mm/secs) x 0.25.

Drainage field design

• It should be a minimum of 10 metres from any permeable drain or watercourse
• It must be a minimum of 50 metres from a water abstraction point
• It must be adequately far away from other leach fields, soakaways, or mounds
• It should be a minimum of 15 metres from any construction
• To protect abstracted drinking water, it shouldn’t be in an Environment Agency Source Protection Zone
• If you do not own sufficient land to install a drainage field, consider a sewage treatment plant as an alternative to a septic tank and drainage field.
• The design and installation of a drainage field should follow relevant British standards and building regulations

Septic tank leach field sizes explained

Leach fields should be at least 50 metres from a water abstraction point, 10 metres from a watercourse, and 15 metres from any construction.

FAQs

What is the purpose of a drain field?

A drain field is a covered, shallow pit made up of unsaturated soil. Without one, a septic tank would just overspill, produce overflow, and create a bad smell in your garden.

How do you build a drain field?

Building a drain field is best left to a drainage and sewer installation specialist. You can find out all the costs associated with fitting a septic tank in our expert guide. Be aware that you’ll need to have a septic tank inspection done, which involves a thorough check of the pipework and tank. This often requires emptying the tank, too, with tradespeople using CCTV technology to inspect pipework.

If, however, you’re confident to do this yourself, here are a few pointers to bear in mind:

• Choose your location (make sure it’s away from your property)
• Get in touch with the authorities, as you may need a permit, and also, the site may need to be inspected before the work’s carried out
• Ensure the soil’s right
• Have the soil tested
• Begin digging!