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Drainage Systems: Mains Drainage Versus On-site Drainage

sewage treatment plant drainage systems Biotec
Biotec Sewage Treatment Plant (Image credit: Kingspan Klargester / Biotec)

When it comes to drainage systems there are many regulations covering both foul water and rainwater or surface runoff and these can be gleaned from our Building Regulations. All good architects, designers, builders and groundworkers will be familiar with how this work is undertaken, but the ultimate responsibility of what waste goes where is always down to the client paying for the work. 

How Drainage Systems Work

Internally, there is a web of small waste pipes running from sinks, basins, showers and toilets. These all feed into a soil stack, which drops down into an underground pipe, which then, in theory, falls gently away from the house towards public sewers or an on-site tank, depending on how the system is set up. 

Above-ground waste pipes, routinely referred to as soil and vent systems, are usually installed by a plumber. The soil stack can be either internal or external and there may be more than one. Critically, they have to have some way of venting above roof level, in order to let any bad odours escape from the house. Each waste pipe feeding into the soil stack also has to have a trap built into it, usually a U-bend or an S-bend, which is designed to stop drain odours leaking out into the home. 

Below ground, the pipes are referred to as drainage and are normally installed by the groundworkers who build the foundations of a house. Generally speaking, if your site is close to an existing mains drainage, and access is straightforward, it makes really good sense to feed your waste into it. It is usually a cheaper solution than specifying on-site drainage, which is the alternative. Sometimes you need to carry out investigations to determine what your best option is.

(MORE: Drainage Surveys)

Who is in Charge of the Mains Drainage System?

Normally a district’s water company will be responsible for its drains, but in any case, this is the kind of information that routinely turns up on a legal search when a property or plot changes hands and it is never usually hard to find out. Whatever company has responsibility, they will be who you pay your water and sewage bills to. 

Their permission will have to be sought if you wish to make a drain connection and they will likely charge a considerable fee giving it, but they are unlikely to carry out this work themselves. 

How do I Find out Where the Nearest Drains Are?

Your local water company will hold maps of drainage runs, which will show the depth of the drains. These maps aren’t always completely accurate but they usually give a good indication of the state of play. The main drains are usually run under the street and connections to them often involve digging roadworks to gain access, the kind of event that requires traffic control and the like. Such works are expensive but, in the great scheme of things, they are still usually cheaper than the alternatives.

It is possible that you will have access to shared drains that collect foul waste from a group of homes before entering the main or public sewers in the road. This is typical on housing estates and for terraced homes. Finding out where these drains run can be a complicated undertaking and you may have to enlist the help of a drainage contractor with camera equipment to help build up your own map.

Alternatively, you might have to put in a long drain run across land you don’t own. In such a situation, you have to have permission from the owners and there may be a fee attached, and a hefty fee. Once the likely connection costs run over £15,000, it is time to start looking at on-site drainage options.

Connecting Drainage Systems

As long as you are working with gravity, then there is nothing, in theory, to stop you running a 100mm foul waste pipe (the conventional diameter) as far as you want. Foul drains should be run at small fall: between 1 in 40 and 1 in 110. If the pipe is much steeper than this, solids in the waste will become separated from the liquids and the pipe will require frequent rodding to stop it getting blocked. 

If the fall is steeper than 1 in 110, then you have to introduce an inspection cover for access at a point with a sudden drop of a metre or more.

Of course there is a cost to laying conventional drains as they have to be excavated, laid in pea shingle, and have access points. At distances of more
than 50m, as well as road opening fees, the overall cost starts to become prohibitive, and on-site drainage becomes a viable option.

(MORE: How to Build on a Sloping Site)

drainage systems for self build

The challenge for these self-builders was filling the tank with water at the same time as filling the surrounding hole with concrete to secure it in place. “This was the most stressful part of the entire build,” recalls homeowner Lucy. “We struggled to get the water in fast enough and the tank rose too high. We cut our losses as it would’ve been impossible to get out, and began again with a second tank — with more success this time!” (Image credit: Jeremy phillips)

What if the Main Drains are Located too High?

Having to work 'against gravity' introduces another complexity. The usual solution is to have a tank with macerator pump, often referred to as a lifting station. This collects the foul waste from your house and then pumps the waste through a small bore pipe up the slope and into mains drainage. This is also an option on very long drain runs where the cost of excavation rises becomes exorbitant. 

On Site Drainage Systems

Connecting to mains drainage is not the only option — you can run your foul waste to a septic tank or a sewage treatment plant on your land. This is a very common method of disposing of waste in rural locations where mains drainage is not present. However, you do need land and you do need permission. The Environment Agency (or SEPA in Scotland) require you to register with them should you want to dispose of sewage on-site (the relevant information and forms can be found at www.gov.uk/permits-you-need-for-septic-tanks). If yours is a new installation, it will be subject to planning permission and building control and you will be expected to install a sewage treatment plant or something of equivalent standard. 

If however, you are inheriting an on-site drainage solution, you may be required to upgrade your installation as certain designs, specifically septic tanks discharging into water courses, are no longer permitted. 

Cesspit

The simplest and cheapest option to install, but unfortunately also and the least satisfactory, is a cesspit — a tank with no outlets, which is usually emptied by a lorry every few weeks. Generally, cesspits are only used on a temporary basis whilst other arrangements are put in place.

Septic tank

A septic tank separates the solids from the liquids and the liquids flow out of the tank into a drainage field, which is a trench or an interlinked series of trenches in which the liquids gradually leach into the ground or sometimes into water courses (now prohibited). The solids are pumped out into a lorry but this only need happen once or twice a year. Septic tanks are commonplace in country districts but these days new installations are required to meet higher standards.

Sewage treatment plant

Sewage treatment plants are normally electrically powered and use various techniques to improve on-site treatment so that liquids leaving the tank are much cleaner than you would get from a septic tank. If you are building a new installation, you will almost certainly be required to fit a treatment plant. The liquid waste should be clean enough to be able to be discharged into a pond or stream, but can also be released into a constructed drainage field / infiltration system or a reed bed, if you have space.

Reed bed

As mentioned above, reed beds aren’t usually a drainage system on their own and often form part of a sewage treatment set up, particularly where on plots with poorly draining soil. Bacteria digest sewage and clean the water. A typical four-bed house would need a fairly small reed bed, around 8-10m2. Horizontal and vertical flows are available and often systems combine the two.

What are the Costs of On-site Drainage Systems?

Prices for sewage treatment works start at around £3,000, but the overall installation costs is likely to be in the order of £15,000. This is a good yardstick for comparing the costs of on-site drainage to any available mains drainage connections. There are also ongoing costs to running an on-site solution, such as tank emptying, and treatment plants typically require an electricity connection, though this is not the case for all models.

(MORE: Drainage Cost Guide)

drainage systems and permeable paving

(Image credit: Unit 1Architects / Charlie Birchmore Photography)

Surface Water Drainage Options

Wherever possible, rainwater must be disposed off separately to foul water. This is because foul water systems are easily overloaded by the addition of rainwater, especially during heavy storms, and this can lead to the release of untreated sewage into our rivers. 

As rainwater doesn’t require any treatment, it can be disposed of in an underground soakaway or, failing that, into a nearby water course. Some streets have storm drains, separate from the sewers, and these may be an acceptable destination for your unwanted rainwater.

Soakaways used to be very simple affairs, often no more than a metre-deep hole in the ground filled with brick rubble and covered over with a paving slab and some topsoil. These days, you will be required to undertake an assessment of how much rainfall will be running off your roof and your paved areas so that you can ensure there is an adequate volume of covered space to deal with the very worst storm without flooding. 

You may also be required to install permeable paving on your driveway to stop flash flooding. These requirements are likely to form part of a planning permission for a new home

Rainwater Harvesting

Instead of relying on a soakaway as your primary means of dealing with rainwater, you can put it to work by running it into an underground tank for use around your house and grounds. This is called rainwater harvesting

In some ways similar to on-site treatment plants, rainwater harvesting requires a filter, a pump, and an overflow to a soakaway. In parts of the world where there is no mains water supply, all domestic water is harvested from rainfall, but this requires additional on-site treatment to be potable. In the UK, harvested rainwater is usually used for toilets, dishwashers, washing machines, and watering the garden.

Mark Brinkley

Mark is the author of the ever-popular Housebuilder’s Bible and an experienced builder. He’s just finished his latest self build.