How clean is our water?

Discover how a sewerage system works and why sometimes sewage gets pumped into our seas and rivers

What is a sewerage system?

A sewerage or wastewater collection system is a network of pipes, pumping stations and accessories that transport wastewater from its points of origin to a point of treatment and disposal.

There are three types of wastewater:

domestic sewage
industrial sewage
storm sewage.

A sewer releasing wastewater into river.

An underground river flowing in a round sewer tunnel.

Why do we need a sewerage system?

Every time we flush the toilet or wash something down the sink, we create sewage.

We do not want to release this wastewater into the environment because:

human waste contains bacteria that can cause disease. Once water becomes infected with these bacteria, it becomes a health hazard.
it contains suspended solids and chemicals that affect the environment by leading to excessive algae growth and a reduction of oxygen in the water which does not help support aquatic life.
it smells.

This is why wastewater treatment plants have been built and laws enforced against the release of raw sewage into the environment.

London's Abbey Mills Pumping Station built in 1868

History of Britain's sewerage system

Up until the 1800’s, the River Thames was essentially an open sewer in which London’s sewerage system discharged into.
In the 1800’s as London’s population started to grow, there were several cholera epidemics and during the summer of 1858 – the 'Great Stink of London' – the hot temperatures created an intolerable stench which prompted planning to create a modern sewerage system.
An extensive underground sewerage system was designed by elected Chief Engineer of the Metropolitan Board of Works, Joseph Bazalgette. The flow of foul water from old sewers and underground rivers was intercepted and diverted along new low-level sewers which were built behind embankments on the riverfront to take the sewage to the new treatment works.
Although the sewer was officially opened by Edward, Prince of Wales in 1865 (and several of the largest sewer channels named after members of the Royal Family), the whole project was not completed until 1875.
This system revolutionised the way we deal with sewage and brought forward a great advance in hygiene that was mirrored and reproduced across the UK.

Sewage treatment plant infographic vector illustration. Clean dirty water from home to pump station, biosolids, filter, cleaners to sea or ocean.

Did you know?

“The local historian Richard North noted in his History of Plymouth (1871) of the streets, that ‘in 1634 they were so filthy that a royal writ was sent to require them to be put in decent order’. North, again, is worth quoting on the subject of sewerage, which at an estimated cost of £35,000, transformed Plymouth from ‘one of the unhealthiest towns in England into one of the healthiest’“. – Dr James Gregory, Associate Professor of Modern British History

Discover more on the BA (Hons) History module 'Filth and the Victorians'

How does a sewerage system work?

Wastewater is taken away – wastewater disposed of via toilets, sinks, showers or baths are channelled into underground foul water drains and sewers which lead to sewage treatment works.
Screening – removes objects that should never have been flushed such as nappies, wipes, sanitary items, cotton buds and other large objects. Grit is also removed as part of this process.
Primary treatment – all human waste is separated at this stage. Wastewater is put into large settlement tanks where the solids will sink to the bottom of the tank forming sludge, where it is pumped away for further treatment.
Secondary treatment – smaller particles and bacteria are removed. Wastewater has air pumped into it which encourages good bacteria to break down the bad bacteria.
Final treatment – the treated wastewater is now passed through a final settlement tank where the good bacteria sinks to the bottom creating more sludge. The clean water filters out of the top of the tank.
Sludge treatment – most of the sludge collected will be treated and recycled for farmers to use on agricultural land and to generate energy.
Back to the river – treated wastewater return to rivers and streams.

Why does sewage get pumped into our seas and rivers?

There is increasingly more significant media coverage about sewage being pumped into our river and seas. But is this recent activity an exception or the norm?

Most of the UK has a combined sewerage system which means rainwater and wastewater are carried in the same pipes.

Usually, all the waste is carried to a sewage treatment works, but capacity can sometimes be exceeded during heavy rainfall, especially when dry ground is unable to quickly absorb water.

The system is designed to overflow occasionally and discharge excess wastewater into the sea and rivers. This practice is known as (CSOs) and is permitted.

The Environment Agency allows water utilities to release sewage into rivers and streams after extreme weather events such as prolonged heavy rain, to protect properties from flooding and sewage from backing up into streets and homes.

The agency says that overflows are “not a sign that the system is faulty”, and that they are “a necessary part of the existing sewerage system.”

Sewage entering the water supply via a pipe

How often is sewage released?

According to the Environment Agency, sewage was pumped into rivers and seas – highlighting the use of overflows is not occasional, as it is meant to be.

In 2022, Ofwat, the water regulator for England and Wales, launched cases against six water companies over discharging sewage at times when this should not have happened.

Since Brexit, the UK is no longer bound by EU environmental laws, but those laws were not directly regulating the frequency of number of sewage discharges. These EU laws were replaced the UK's Environment Act 2021, which according to has brought “some useful changes” which include “efforts to stop companies filling our waterways with sewage.”

Campaigners continue to argue for greater investment in the capacity of sewage systems to cope with heavy demand during extreme weather events, which may become more frequent because of climate change.

Sir James Bevan, chief executive of the Environment Agency, said his organisation was “working actively with the water companies to ensure overflows are properly controlled“ and that the harm they do to the environment needed to be stopped.

Did you know? It's believed wet wipes make up at least 90% of the material causing sewers to block. Keeping drains free of wet wipes and ensuring fats and grease are not poured down the sink can reduce the need for sewage releases.

Providing science to protect our waters

For more than 30 years, Professor Sean Comber has seen countless changes around what the planet’s populations put in our rivers and seas.

We are a little island with more than 65 million people living on it, and we have been impacting the environment for thousands of years. As a result, there is no place in the UK that is truly pristine any more. But our scientists, industries and governments are continuously seeking to reduce our environmental impact. In that regard, in my field at least, we are no longer the dirty man of Europe.

Professor Sean Comber

While working at Water Research Centre (WRc) in the Thames Valley, Sean focused on the implementation of the Dangerous Substances Directive, looking at the chemicals humans were putting into the environment (principally via sewage treatment works) and the impact they had once they got there.

Sean and his team devised a source apportionment process (a tool called SAGIS) which could (at a 1km grid scale) show precisely where a chemical in the environment was coming from. That made it possible to distinguish whether it originated from point sources from industry and sewage works effluents, or diffuse sources such as agriculture or abandoned mines.

In addition to metals and phosphorus, Sean’s research has been examining the impact on our waterways of pharmaceuticals.

“Sewage works are predominantly designed to deal with paper, urine and faeces. The removal of chemicals such as pharmaceuticals, and metals for that matter, is happening by chance unless specific treatment is used.

Even though many pharmaceuticals are biodegraded to a high degree during treatment, a small concentration – very small in some cases, parts per trillion – will pass into or waterways owing to the volumes we use in our everyday lives.”

Example source apportionment from SAGIS for phosphorus in East Anglia

Professor Sean Comber (landscape, centred)

As an applied scientist, Sean fully appreciates that pharmaceutical companies are under huge pressure to develop new drugs at almost breakneck speed. The work on COVID-19 vaccines is a prime example. However, he believes there are always opportunities to continue to develop and improve upon the environmental assessments used in the drug development process.

Sean is passionate about nurturing the next generation of environmental scientists, with several of his former students already working for agencies at the forefront of national and international research and legislation.

“I want all my students to realise that chemicals do have an impact, but that there are a range of factors affecting that. Put simply, they need to see all sides of the story.

That’s why I get all my students out into the field and talking to industry, and I encourage them to think about producing work of a quality that could be peer-reviewed and published. I am doing my bit now, but it is today’s young people who have the real power to change the world.”

Research by Professor Sean Comber

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Providing sound science to protect our waters

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Biogeochemistry Research Centre

Researching the environmental behaviour, fate and impact of nutrients, metals and pharmaceuticals in terrestrial, atmospheric and aquatic systems.

The Biogeochemistry Research Centre comprises expert researchers and instrumentation, with acknowledged international leaders in organic geochemistry and environmental analytical chemistry and a strong focus on marine science and current and past ecosystems and climates.

Find out more about the Biogeochemistry Research Centre

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