Your Guide to Off-Mains Drainage

Panic can set in when you discover your potential plot has no access to foul drains. But the absence of a public or private sewer is by no means the end of the world and, in some cases, the on-site solutions that are available may actually work out cheaper.

This is especially so when the site is adjoining a busy road with the sewer within it, necessitating traffic lights and the employment of ‘approved contractors’ to carry out the work.

Consent to Discharge

Before any foul effluent is disposed of on site, permission is required from the Environment Agency. This involves the filling in of quite a sheaf of forms and the production of information that can only really be provided by the manufacturers of the various on-site solutions.

Porosity/Percolation Tests

Part of the information required for a consent to discharge both foul and surface water into the ground will require that same ground to be tested to see if it is capable of accepting the effluent. At its most simple, this process is quite easy for the self-builder to carry out themselves.

A hole is dug 600mm x 600mm to a depth of 300mm; then a hole is dug in the bottom of that, 300mm x 300mm and again 300mm deep. The lower hole is filled with water and left for 24 hours for the water to drain away; it is then refilled with water and the time taken for the water to drain away is timed.

The resulting findings need to be supplied to your architect or designer; but if the water fails to drain away within a reasonable period, the ground has poor porosity. That doesn’t, however, mean that all is lost and, almost certainly, a system could be devised to counter those effects.

The science of sewage disposal gets more sophisticated every year with new solutions readily available. Here are the main options:

Septic Tanks
A septic tank takes the raw effluent and allows it to settle within its chamber. This produces a relatively clear liquid at the top, which in some systems then passes across a ‘baffle’ into a second chamber where it continues to settle out. A crust will form at the surface, allowing for anaerobic bacteria to break down the contents into a non-sterile liquid. This is discharged, normally by gravity, into weeper or land drains in order to percolate within the subsoil. A consent to discharge is required from the Environment Agency. (This also applies to existing tanks.)

Generally, tanks use no power, although there are models that introduce air streams within the effluent to speed up the bacterio - logical process. The cost of a simple glassfibre septic tank and the associated works is likely to be between £1,500 and £2,500, plus the costs of the drainage around the house.

Septic tanks rely on the subsoil being able to accept the discharged effluent and cannot, therefore, be expected to work in heavy clay or high water table situations. The ‘sludge’ will need emptying every six months at a cost of around £145 for every 1,000 gallons.

Sewage Treatment Plants
Effluent enters a single chamber within which slowly rotating turbines or blades constantly expose it to both anaerobic and aerobic bacteria, in order to break it down to a more or less sterile effluent. This can be passed into a system of underground weeper drains, a filtration bed or, in some cases, a ditch or watercourse. Costs for a single house will range between £3,600 and £5,000 plus the cost of the drainage around the house. Again, a consent to discharge is required. Plants require a power source and sludge will need emptying every three to six months.

Cesspools
The on-site sewage system of last resort. It is simply a holding tank where the effluent is stored until such time as it is pumped out and disposed of off site. Installation for a single home will be £5,000 upwards. They will have to be emptied every two months at a cost of £145 for every 1,000 gallons.

Reed Beds
These are not a solution in their own right but can be employed in conjunction with a septic tank or mini sewage treatment system. They are particularly useful in poor porosity situations, where the final effluent is discharged into a reed bed or floating reeds on a pond. This may require some sort of pump to raise the effluent to the right level, unless the fall of the ground facilitates a gravity system. Reeds, however, are capable of neutralising considerable quantities of harmful bacteria and even heavy metals. The reed beds themselves can become a haven for wildlife and are unlikely to smell.

Surface Water
Usually, unless there is a combined sewer, it is not permitted to connect to the surfacewater drains in the carriageway. Surface water should be disposed of on site by means of soakaways, which can either be 1m-deep holes in the ground filled with rubble, or they can be sophisticated concrete rings and filtration beds. Some form of attenuation of surface water does need to be demonstrated: 100% in flood-prone areas, 55% in others.

This means that at the very least water butts or aquifers must be employed or else the run-off should go to ponds and/or reed beds. Until recently, the discharge of surface water was largely left to the common sense of those on site. But now it is not unusual for a planning condition to be imposed requiring that the surface-water drainage employs.

Sustainable Drainage Systems (SuDS). This requires that the drainage system, rather than simply moving rainwater as quickly as possible into drains or soakaways, endeavours to attenuate it into filter drains and ponds, whilst at the same time ensuring that even low-level contaminants are filtered out.

Driveways
Planning permission is needed to pave or resurface more than 5m2 of driveway, if that means that the rainwater falling onto it will drain into the road or into the road gullies or drains. However, if you use a permeable material or create the driveway or patios so that any water falling on them drains into soakaways within the site or to an area of ground within the site that is capable of accepting the run-off, planning permission is not required.

Further Reading:

• Drainage: Cost Guide 
• Groundworks & Foundations: Drainage
• Is Your Plot Connected?