Off Mains Drainage Solutions

Though it’s little celebrated, mains drainage was one of the major engineering feats of the 20th century. However by the 1970s, the roll-out programme was all but complete and it left many isolated rural communities unconnected. It is estimated that there are around one million homes in the UK which aren’t plumbed into mains drains. Joining their number each year will be a few thousand self-build plots which have to find an offmains solution to their foul waste. What are the options?

Feasibility of connecting to the mains

Most people would prefer to make a mains connection if possible. If the main drains are adjacent to your plot, it’s a no-brainer but there are many instances where the drains are some distance away or are maybe running higher than your outlet. It is possible to pump your waste uphill or over long distances, so before you decide to go for an off-mains solution, first check out the costs of making a mains connection. If it can’t be undertaken at a sensible price – say less than £10,000 – then you have to investigate the off-mains alternatives.

Septic tanks

This is the traditional choice because it is usually cheaper than the alternatives. A septic tank system typically costs between £3,000-£5,000 — not that much more than many average-sized main drain connections.

Septic tanks are relatively simple affairs, consisting of a large underground holding tank, usually made of concrete or GRP, which separates the solids from the liquids, or liquor. The solids need to be pumped out at least once a year whilst the liquids, which get semi-digested inside the tank, overflow into a series of soakaways, typically placed under the garden land. Septic tanks don’t have any moving parts and are low maintenance: the annual desludging costs around £100.

However, in order to install a septic tank you have to satisfy your building inspector that your ground is porous enough to handle the free draining liquids and that you won’t be contaminating any water courses. If your site fails the porosity test, then you have to look at alternative methods of disposal.

CLOCKWISE FROM LEFT: A reed bed filtering system requires a fair amount of ground but doesn’t use any motors or pumps; a septic tank such as the Alpha from Klargester or, bottom right, a treatment plant, are the more common options for those with smaller sites

Treatment plants

A treatment plant attempts to do rather more than a humble septic tank in that it cleans the foul waste to a much higher standard. This allows you to pipe the liquids into streams or water courses. A packaged treatment plant is usually much the same size as a septic tank, but they require a supply of electricity and demand an annual service as well. The Environment Agency become involved in the specification and require a ‘Consent to Discharge’ which they will only grant if they are satisfied that your system is suitable and will be properly maintained.

Package treatment plants are significantly more expensive than septic tanks — typically by around 50%. They are also more expensive to run because of the service contract, as well as the desludging costs.

Reed beds

A reed bed filtering system can sometimes be used as an alternative to a treatment plant. It does much the same job – i.e. purifies the waste – but without recourse to electric pumps or motors. However, they are not suitable for every site as you need a fair amount of ground — about the size of half a tennis court. They are also surprisingly expensive to install as the beds themselves have to be waterproof, which usually involves concrete tanking. They also require a settlement tank that needs desludging annually. Reed beds are also far from maintenance free: individual reeds can cost as much as £1 each and a reed bed system may use as many as 2,000. So, although reed bed systems can make an attractive feature in a large garden, they are not really for the faint hearted: they are best suited to enthusiasts who wish to design a garden around the beds, although they do hold great appeal for many self-builders.

Cesspools

If all else fails, the cesspool is the drainage system of last resort. A cesspool doesn’t have any outlets and instead it relies on frequent emptying by tanker. This is likely to happen several times a year and so it becomes an expensive way of disposing of your foul waste. Because there is nowhere for the liquids to run off into, the tank size is generally much larger than you would use with a treatment plant and this makes the installation costs at least as high as a septic tank.