Why does my water smell like sewage?

21/03/2017 Océane Bidault

A smell of sewage, when you turn on your taps, is often the result of a build-up of bacteria in your plumbing system. When organic matter such as food, waste, hair or soap accumulates in your drains, bacteria grow and produce a gas that smells like rotten eggs. Turning on your taps disturbs the stagnant water in your plumbing, and your pipes emit the sewage-like smell.

An alternative source of the odour could be bacterial growth in your hot water cylinder, resulting from it running at a too-low heat.

What causes my water to smell like sewage?

Many waste materials such as soap, food and hair wash down sinks. They sit stagnant in your drainage system and provide the perfect environment for bacteria to grow. Due to a lack of oxygen, an anaerobic fermentation occurs, which produces a dense gas that gets stuck in the pipework. When you turn on your taps, the flow of water forces the gas upwards into the sink, making it seem like your water supply smells of sewage.

The smell results from hydrogen sulphide gas, which can cause skin and eye irritation at low levels and becomes toxic at higher levels. You will notice a musty odour from concentrations as low as 0.01ppm (parts per million), while the smell becomes an unbearable stench at 3ppm. Hydrogen sulphide is lethal above 300ppm, but you will detect the presence of the gas long before it poses a significant health risk.

If the smell only appears when you turn on your hot tap, then the issue could be with your hot water heating system. When your hot water system fails to reach a certain temperature, or if it has been switched off for an extended period, bacteria can accumulate. The bacteria present no direct health risk, but you will have to clean your hot water cylinder to remove the smell from your hot water tap.

What should I do if my water smells like sewage?

Your first step is to determine the source of the smell: fill a glass with cold water, step away from your sink and give it a whiff.

If the water itself does not smell, the issue is likely with your plumbing system. To kill the bacteria, you must disinfect both the sink and the drains using an appropriate cleaning product. Use a small brush, so you can scrub the length of the pipe where waste and bacteria accumulate.

If you fill another cup—this time with hot water—and you notice a foul smell, you have an issue with your water heater.

The government recommends that hot water systems reach at least 60°C to kill bacteria and prevent health hazards. Turn up your system to this level for a minimum period of 24 hours. Then, run your hot water taps to flush and cleanse the piping, being careful not to scold yourself.

If the above actions fail to remove the odour, talk to your neighbours to find out if those in your area report a sewage-like smell. If yes, the problem is widespread, and the UK Drinking Water Inspectorate recommends you contact your local water supplier immediately.

You can find emergency contact numbers either on your water bill or in the ‘contact us’ section of your water supplier’s website. To help your supplier identify the source of the smell, make a note of:

• When you first noticed the smell
• The nature and intensity of the odour
• Who else is experiencing the issue