All septic tanks and sewage treatment plants must discharge their effluent either into a drainage field or, if permitted, a nearby water course - for example, a river, stream, estuary, or coastal water.

You can only discharge into a water course if you have a BS EN12566 -3 certified sewage treatment plant and permission from the Environment Agency.  You must have either a Standard Rules Permit or a Bespoke Permit, depending on the nature of your sewage discharge.

https://www.gov.uk/guidance/discharges-to-surface-water-and-groundwater-environmental-permits#apply-for-a-standard-rules-permit

• A soakaway is designed for rainwater discharges.
• A drainage field is designed for off-mains effluent drainage.
• Do not use crate systems.

Do not assume a soakaway will do the job of a drainage field! It won’t. 

A soakaway filled with gravel will not be able to filter wastewater effluent; it will very quickly back-up and/or rather than filtering down through the soil, the effluent will end up as a nasty smelling, sludgy layer across the surface of the ground. It is not advised to put in gravel but if the area is restricted due to sizing, a bespoke permit can be granted by environmental agency.

Septic tank drainage fields are 20% larger than Sewage Treatment Plants. This is because septic tanks are designed to allow settlement of solid materials and don’t significantly treat sewage.  The settlement will remove significant amounts of solids but only around 30% of the active pollutants.  Most of the process of purifying the effluent and removing pollutants is done within their drainage fields.  They also require a reedbed as part of their drainage field to help with the cleansing process.

Sewage treatment plants, on the other hand, remove around 95% of pollutants in the effluent before they discharge this into their drainage fields.  Because most of the pollutants are removed within the tank, their drainage fields can be far smaller, and they will last a lot longer. 

Note: adding a reedbed to a sewage treatment plant drainage field creates the current ‘gold standard’ of off-mains drainage, making the system suitable for the most sensitive of sites.

Yes, and these are legal restrictions set out in Building Regulations Document H (https://www.gov.uk/government/publications/drainage-and-waste-disposal-approved-document-h)

For drainage fields, the water-table must be at least one metre below the surface during the winter (ie. when it’s likely to be at its height).  If it is higher than this, you will need to install a sewage treatment plant. 

The ground conditions must also allow adequate filtration, at a set rate, determined by a percolation test (see below).

A drainage field should be:

• 10 metres from any watercourse
• 15 metres from any building
• 50 metres from any water supply
• Separate from any other water supply pipes or other underground services
• Clear of any access roads, driveways, or paved areas

If your property is at risk of flooding, you must install a sewage treatment plant to avoid pollution risks, unless approved by local building authorities.

The size of the drainage field you need will be determined by the size of the property, the number of occupants, and the amount of waste you produce collectively. 

A mathematical equation will be used by your installer to calculate the optimum size of drainage field to meet your needs:

At = p X Vp X 0.25 (Septic tank) 0.2 (Sewage Treatment Plant)

Key:

At = Area of the drainage field in m2
p = Number of people served by the tank
Vp = Percolation value.  (See below)

A major factor when deciding where to site a drainage field is the porosity of the underlying soil. This is determined by a percolation test, or ‘perc test’, which measures how long it takes a measured amount of water to drain away from a saturated hole dug into the ground. 

The best types of ground for drainage are chalky or loamy soils.  The least suitable are clay (or a high clay content) or silty soils. 

Your installer will be looking for a Vp score of between 15 and 100 – the number of seconds for a measured amount of water to drain away.  But that score also needs to factor in the number of people who will be producing waste. 

Remember, the water table must also be at least one metre below ground.

A drainage field incorporates a network of perforated pipes, which are laid in the top 700-800mm (minimum legal depth, 500mm) of the soil. 

These pipes will lie in drainage trenches which must be at least two metres apart and separated by undisturbed ground.  The pipes will be laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm. 

Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt.  There must also be an inspection chamber between the septic tank and the drainage field.