Foul Water Drainage & Surface Water Drainage: What’s the Difference?
What’s the Difference Between Foul Water Drainage and Surface Water Drainage?
- Foul water refers to sewage or water from a toilet, sink, or used for food preparation and washing.
- Surface water is rainwater collected from rooftops or drains.
Most properties generate two types of water that enter the surrounding drainage system: foul water drainage and surface water drainage. Foul water drainage includes waste from sinks, toilets, washing machines, and other similar appliances. This is sometimes referred to as grey water. On the other hand, surface water, also known as stormwater, is rainwater that is collected via gutters and drains around the house, such as from channel drains during rainfall.
You might assume both types of water follow the same path and are treated similarly, but that’s not the case. There are distinct differences in how foul water and surface water drainage are managed. It’s beneficial to understand these distinctions.
In this article, we'll delve deeper into foul water drainage and surface drainage, including their definitions, handling requirements, relevant regulations, and potential issues that can arise when these systems get mixed up.
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What Is Foul Water?
Foul water is potentially heavily contaminated. According to the UK government’s 'Drainage and waste disposal' statutory guidance (Building Regulations, Part H), it is defined as:
- Waste from a toilet, bidet, or appliance used for washing receptacles for foul waste; or
- Water that has been used for food preparation, cooking, or washing.
Foul waste can be generated by various appliances and systems within a property:
- Toilets
- Sinks
- Baths
- Showers
- Washing machines
- Dishwashers
Given its potentially contaminated nature with effluents, chemicals, or other harmful pollutants, foul water is typically directed through the main sewer network to a sewage treatment plant. For properties not connected to the mains drainage system, it is discharged into a septic tank, private wastewater treatment plant, or a cesspit.
What Is Surface Water?
In this context, surface water primarily refers to rainwater collected on your property.
In rural areas, much of the rainwater drains naturally into the ground. However, in urban settings, it is collected by drains and directed into natural watercourses like rivers, streams, or even the sea.
Since surface water hasn’t been used within the property, it is not contaminated. Therefore, it doesn’t require treatment at a wastewater facility. Instead, it is considered safe to direct back into soakaway drains and dispersed into the natural drainage of the area.
Technical Terms and Names
The terminology for drainage pipes and pipework varies depending on their location.
When above ground, these pipes are generally referred to as 'sanitary pipework.' Everything below ground level is called 'foul drains' or 'foul sewers.'
Don't worry—no one will give you a strange look if you refer to a sanitary pipe or vent pipe as a foul pipe. It’s just good to know these terms if you come across them in building regulations documents or legal paperwork.
As a general rule, a sewer pipe will be terracotta in color, while a grey water or surface water pipe may be black (or even grey or white if above ground).
Rules and Regulations
As mentioned earlier, foul and surface water are both covered under Part H of the UK Building Regulations. Section H1 specifically addresses foul water drainage, while Section H3 covers rainwater drainage. Due to its contaminating nature, a greater emphasis is placed on how foul discharge is handled compared to cleaner surface drainage.
Building regulations establish a 'priority order' for waste disposal. Property owners are encouraged to connect their drainage to the first available option from the following list:
- The public sewer
- A private sewer that connects to a public sewer
- A septic tank or wastewater treatment plant on the premises
- A cesspit
Why Is It Important to Keep Foul Water and Surface Water Drainage Systems Separate?
Since surface water typically drains into rivers, streams, or other watercourses, it’s evident why fluid contaminated with effluent and/or chemical waste shouldn’t follow the same path.
The consequences of contaminating a watercourse are severe. Not only does it impact the environment, but individuals or companies found guilty of pollution offenses can face unlimited fines, and in extreme cases, custodial sentences.
It’s not just pollutants entering a surface water sewer that pose a problem. When surface water enters foul water systems, the drainage and sewers can quickly become overwhelmed, potentially causing hazardous flooding. It can also create issues at sewage treatment facilities, leading to overflows, which often result in untreated foul water being discharged into public watercourses or the sea. Underground sewer pipes come in various sizes, with the main ones being 110mm and 160mm drainage pipe.
Why Do Surface and Foul Water Drainage Become Mixed?
Sometimes, a foul water drainage and surface drainage system become mixed unintentionally. Many older homes have combined foul and surface water underground drainage systems, sending wastewater through the same pipe into the main sewer.
There’s also a possibility that construction work can lead to mix-ups in foul and surface water sewers, often due to misconnections when the wrong type of wastewater is routed through the property’s drains.
If this applies to your property, don’t panic—you won’t have environmental health officers knocking on your door demanding access to your drain covers and inspection chambers! That said, it might be worth looking into and fixing if possible.
Foul Water Drainage Products
Now that we’ve covered some essential information about foul and surface drainage, let’s take a closer look at the products we offer that could be useful if you’re planning to install or overhaul your drainage system.
Sanitary Pipework
Items like bottle traps, waste traps, waste pipes, and overflow pipes make up the pipework that removes foul water from your home. Outside the property, it becomes a soil pipe stack that connects various outlets and directs the flow toward the sewer. A soil pipe stack is typically made up of sections of soil pipe, junctions, and the necessary fittings to secure them.
Building regulations also require that a soil stack design must be 'vented,' meaning gases or odors created must be allowed to escape in a way that’s clear of surrounding windows or doors. This is usually done with a dedicated vent pipe sitting above the roofline, covered with a protective cowl (usually at least 5 meters away from any roof or window).
Ultimately, the soil stack will connect to the local main sewer (underground) drains, carrying sewage away from the site.
On-Site Sewage Treatment
About 4-5% of UK properties have off-mains drainage systems, meaning it’s either impossible or impractical to connect them to the main system. While 4-5% might not seem like a large number, it equates to roughly one million homes—and that’s not including rural businesses.
For people living in homes like these, some form of on-site storage or treatment is essential. Septic tanks are a popular solution, but an increasing number of larger properties are opting for their own sewage treatment plants—a smaller version of the main system.
Drainage System Question? We’re Here to Help!
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Author Bio
Nathan Wilde
Nathan has been in the drainage and plastics industry for over 12 years. Having worked for both builders’ merchants and major manufacturers, Nathan has gained a significant amount of industry and product knowledge. At EasyMerchant, Nathan is dedicated to making tradespeople’s lives easier.
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