Updated / Tuesday, 28 Oct 2025 08:55

Be careful what you do with the dregs of your Pumpkin Spice Latte. Photo: Getty Images

Analysis: The coffee dregs we pour down the drain can create a serious pollution problem down the road for rivers and waterways

By Kevin Collins, The Open University

A woman was recently fined €170 by Richmond council in London for pouring coffee down a drain before getting on a bus. The fine has now been rescinded, but the incident has prompted many discussions about whether coffee discarded like this could cause environmental damage.

Over 2 billion cups of coffee are consumed every day worldwide. All that liquid has to go somewhere, whether you are at home, at work or running for a bus. While the welcome hit of caffeine is a morning ritual for many, it can be an unwelcome hit for the environment when disposed of.

An individual cup is insignificant, but millions of daily dregs poured down the drain would create a much bigger problem for rivers and watercourses, because we are adding to the caffeine levels already present in sewage from households. The more caffeine that goes in to these pipes, the more that could evade the treatment and reach rivers.

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Cups of coffee contain hundreds of chemical compounds. As well as caffeine (assuming you are not drinking decaf), many will include milk and sugar while some also contain cocoa, spices and other ingredients.

Of these, caffeine has the most impact, environmentally speaking. It does not break down quickly or easily, and is considered an emerging contaminant (scientists have only recently started testing for caffeine levels and it is not always monitored). But even back in 2003, caffeine was found to be polluting Swiss lakes and rivers.

However, don’t think this means it’s fine to pour decaf coffee down the drain. All coffee lowers the pH of water, and coffee also contains organic compounds which rob aquatic systems of oxygen as they decompose. The nutrients in coffee also encourage algae growth and may lead to additional oxygen depletion in rivers and lakes, which can stress and potentially reduce the lifespan of marine plants and animals.

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Why is caffeine such a problem?

Wastewater treatment plants vary in their ability and capacity to treat and remove caffeine – ranging from 60-100% depending on treatment types, plant design, season, temperature and other elements. This means even treated water can contain caffeine when it is returned to rivers and seas.

Heavy rains add to the problem if the capacity of sewage pipes is exceeded. When this happens, untreated wastewater is designed to divert directly into rivers and water courses to prevent sewage flooding of homes, businesses and treatment plants.

Whether from a street drain or toilets, some of the caffeine that we have consumed will eventually make its way into our rivers and aquatic environments. This is a problem in every part of the world, including in Antarctica. One study of 258 rivers in 104 countries found caffeine in over 50% of sites sampled.

Recent studies show that caffeine has an impact on the metabolism, growth and mobility of some freshwater algae, plants and aquatic fly larvae, potentially leading to their death. Caffeine can affect marine and plant life even in small amounts.

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What should and shouldn’t you put in a drain?

Street drains are part of our water system. Don’t put anything into a drain that you don’t want to see ending up in a river, lake, on a beach or in the sea.

This means no coffee or coffee grounds, food-based liquids, oils, paint or hot fats, detergents, bleaches, liquids from building work and so on. All these should be disposed of via the appropriate household bins or waste collection centres. Leave the street drains to do their single, simple job: collecting rainwater not wastewater. What’s good for your street drain is also good for your kitchen sink and good for the environment. If nothing else, be pragmatic: coffee grounds can easily block your kitchen sink.

So, what should you do with your coffee?

If you are constantly throwing away coffee water, perhaps try making less coffee. At home, you can dilute coffee water for use as a plant tonic. Coffee liquid and grounds can also be disposed of on gardens or any plant beds in small amounts with care.

While coffee grounds could add to the organic content of the soil, regularly adding grounds to the same patch of earth can cause a build up of caffeine and solids, which will be harmful to plants and soil function.

Otherwise, the best place for waste coffee is a compost heap or food waste recycling. If you don’t have access to these options, then put liquids or grounds into a container and put them in your bin.

A recent UK government inquiry concluded that improving the poor status of rivers and coasts requires major reform, policy changes and investment. But we, as individuals, are also part of how the water system works. We can help it by keeping coffee out of drains, out of our rivers and out of our environment.

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Kevin Collins is a Senior Lecturer in Environment & Systems at The Open University. This article was originally published by The Conversation.

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The views expressed here are those of the author and do not represent or reflect the views of RTÉ

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