How your home connects to the river

Why this matters for the Dart: once you know which pipe goes where, most of the advice on this site becomes obvious.

The key fact: most outdoor drains, road gullies and yard drains do not go to a sewage works. They usually go straight to a stream or river, untreated.

Nearly every home is served by two entirely separate drainage systems, and the difference between them explains almost everything else here.

1. The foul sewer carries wastewater from your toilet, sinks, bath, shower, washing machine and dishwasher to a sewage treatment works, where it is treated before the cleaned water is returned to a river or the sea.

2. Surface water drains carry rainwater from your roof, drive and road gullies. In most places they discharge directly into the nearest watercourse rather than to a treatment works ¹³. That is why community schemes paint a yellow fish beside roadside drains, as a reminder that "only rain down the drain": whatever enters a surface water drain can end up in the river exactly as it went in ¹⁵. In Devon, the Litter Free Coast and Sea partnership puts it plainly: many roadside gullies "go directly to a watercourse or the sea" ¹⁶.

Two complications are worth knowing about. In many older towns the two systems were built as one combined sewer, carrying sewage and rainwater in the same pipe. In heavy rain these pipes can be overwhelmed, so they are fitted with relief valves called storm overflows (you may also hear "combined sewer overflows" or CSOs), which release diluted, untreated sewage into the river rather than letting it back up into homes. Across England in 2024, storm overflows spilled 450,398 times, for a total of more than 3.6 million hours ⁹. Everything you do to keep fat, wet wipes and rainwater out of the sewers makes these overflows operate less often.

The second complication is the misconnection. A misconnection occurs when wastewater pipes from appliances such as washing machines, dishwashers or toilets are incorrectly connected to surface water drains instead of the foul sewer. As a result, polluted water is discharged directly into rivers whenever those appliances are used ¹³. Defra figures suggest up to 500,000 UK homes may have misconnections ¹⁴. The page on your drains & pipes shows you how to check your own home.

So the rule of thumb for everything that follows is this: everything that goes down an inside drain should be as river-friendly as possible, because sewage works cannot remove everything; and nothing but rain should ever go down an outside one.

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