August 24, 2025
The Surprising Truth Behind Our Drains
Have you ever stood in a downpour and wondered where all that rainwater goes? It seems like a simple question, but the answer leads to a hidden world beneath our feet—a world that holds the key to protecting our local environment and preventing pollution. The surprising truth is that it all depends on the type of drainage system your property has, which is a legacy of how our towns and cities have been built over time.
Understanding this isn’t just a matter of curiosity; it’s a vital piece of knowledge for every homeowner. The type of system you have can directly impact our rivers, seas, and even the risk of flooding in your community. Let’s dive deep into the two primary scenarios and the critical problem that links them.
The Old Way – The Combined Sewer System
If your home was built before the 1970s, it’s highly likely you’re connected to a combined sewer system. In this older setup, a single, large pipe collects everything from your property:
- Foul water: This is the wastewater from your toilets, sinks, washing machines, and showers.
- Surface water: This is the rainwater that runs off your roof, driveway, and garden.
This combined mixture is transported in one pipe to a local sewage treatment plant. For decades, this was the standard, and it worked well enough during normal weather.
The major drawback, however, is what happens during heavy rain. The sheer volume of water can completely overwhelm the system’s capacity. To prevent this unpleasant mix of sewage and rainwater from backing up into streets and homes, these systems are designed with Combined Sewer Overflows (CSOs). These are essentially safety valves that discharge the excess, diluted sewage directly into our rivers and coastal waters. . While necessary to prevent flooding, CSOs are a significant and well-publicized source of pollution.
The Modern Way – The Separate Sewer System
In contrast, if your home is a new build, or if it was constructed more recently, you’ll be connected to a far more environmentally sound, separate sewer system. This is the system required by law for all new developments and is designed to keep things where they belong. It uses two distinct pipe networks:
The Foul Sewer: This pipe carries only your wastewater to a treatment plant. It is a sealed system designed to transport waste to be cleaned and safely returned to the environment.
The Surface Water Drain: This separate pipe carries only rainwater away. Since this water is relatively clean, it doesn’t need to be treated. Instead, it is channelled directly to a local watercourse like a stream or river, or it’s allowed to naturally soak into the ground.
This separation prevents pollution and reduces the strain on sewage treatment plants. But where exactly does the rainwater go if it’s not going to the treatment plant?
Public Surface Water Sewers: A dedicated network of pipes that channels clean rainwater directly to a local water body.
Soakaways: These are essentially large, rubble-filled pits dug on your property. Your rainwater pipes channel water into the soakaway, where it is held and slowly absorbed by the surrounding soil. This process replenishes groundwater and helps prevent surface flooding.
The Big Problem: Misconnections
Despite the clear benefits of separate systems, a critical and widespread problem exists: misconnections. This happens when an appliance, a sink, or even a toilet is accidentally plumbed into the surface water drain instead of the foul sewer.
This might seem like a small mistake, but the consequences are huge. It means that raw, untreated sewage is being sent directly into our rivers, streams, and seas every single day. . It’s estimated that thousands of UK homes have misconnected drains, often without the homeowner’s knowledge. The responsibility for fixing this lies squarely with the property owner.
What You Can Do About It
So, does rainwater go into your sewer? It only should if you have an older, combined system. If you have a separate system, it’s crucial to ensure it stays that way. Before you undertake any new plumbing work, from an extension to a new outdoor tap, you must know where that wastewater will end up.
Here are a few actionable steps you can take:
Check Your Property Records: Your property’s original building plans or a local authority drainage map may indicate what kind of system you have.
The Dye Test: A simple but effective way to check for a misconnection is to perform a dye test. You can purchase a special, non-toxic dye and pour it down an external drain or appliance. If the dye appears in a nearby river or stream, you have a misconnection.
Embrace Sustainable Drainage: Consider implementing Sustainable Drainage Systems (SuDS) on your property. Options like rain gardens, green roofs, and permeable driveways can help manage rainwater on-site, reducing the load on public sewer systems and helping to prevent localized flooding.
By understanding our drainage systems, we can all play a part in protecting our environment. The next time it rains, remember that where the water goes is up to us.
