TENS of metres under central London a tunnel has been dug that stretches 25 kilometres at a cost of 拢4.3 billion. Boring was completed in April 2022 and the passage should start operation in 2025. The 7.2-metre-wide won鈥檛 carry people or vehicles though, but sewage.
This 鈥渟uper sewer鈥� will collect the that often overwhelm London鈥檚 ageing sewage system. Instead of being dumped into the Thames estuary, as happens now, the effluent will gush through the new tunnel to Europe鈥檚 largest sewage treatment works at Beckton, on the outskirts of the city.
The Thames Tideway is the most conspicuous example of the UK鈥檚 attempts to stop dumping sewage into rivers, amid growing outrage towards the privatised water companies responsible for the waste system. But this new infrastructure is just the tip of the iceberg when it comes to solving this crisis, which is linked to wider issues of how water is managed and even climate change. The good news is that technical solutions already exist, many of which are win-wins: helping to solve sewage overflows as well as problems such as flooding and drought.
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While many forms of pollution foul Britain鈥檚 rivers, sewage has grabbed the limelight. News programmes have repeatedly shown footage of dirty slurry tumbling into pristine river waters. The main culprits are combined sewer overflows (CSOs), designed for when water from heavy rain is carried into the sewage system and must be discharged due to its high volume 鈥� taking waste with it.
Sewage spills
Campaign and conservation groups such as , and have compiled data on CSOs in Britain. Their headline numbers don鈥檛 make pretty reading. Top of the Poops reports 鈥渁t least 361,103 鈥榮ewage spills鈥� into English rivers in 2022, lasting an almost unbelievable 2,207,527 hours鈥�.
It is tempting to assume that rates of sewage dumping must have increased, but we have only had systematic data collection from CSOs for a few years. Water companies were encouraged to install monitors by a , the then UK minister for natural environment and fisheries. Benyon wanted 鈥渢he vast majority鈥� of CSOs to be monitored by 2020. As a result, between 2016 and 2020. In March this year, the UK government reported that more than . This expansion of monitoring has revealed the current scale of sewage dumping.
The issue has become strongly politicised. A common narrative is that privatised water companies have siphoned off profits while failing to invest enough in infrastructure. In England, the and have 拢54 billion of debt. Yet Scottish Water remains publicly owned and it allowed . It also lags behind English companies in .
Instead, the problem of sewage dumping can be traced to Britain鈥檚 antiquated water management infrastructure. Many of its sewers date to the 19th century and carry both sewage and excess rainwater. 鈥淥ur stormwater and wastewater are collected by a single drainage system,鈥� says at University College London. The aim was to reduce the risk of flooding. 鈥淲hen it rains hard, rather than your property being flooded, it鈥檒l go into a sewer and be taken away,鈥� says at Cranfield University in the UK. The sewers transport the mix of municipal sewage and rainwater to wastewater plants (see 鈥淗ow sewage treatment works鈥�). The downside of this combined system is that heavy rainfall can overwhelm the wastewater treatment plants. If too much water flows into the system, the sewers back up, risking raw waste welling up into people鈥檚 homes.
Two major things have been done to reduce this threat. First, engineers estimate how big a flow a treatment plant might experience and design it to cope. 鈥淵ou have to build the sewage works bigger than you might do for the average flow,鈥� says Stephenson. Second, plants are fitted with an overflow system. When too much water enters, some is diverted into overflow pipes and out into rivers and other waterways. Inevitably, some sewage goes too. 鈥淭hey [CSOs] are deliberately discharged so you haven鈥檛 got sewage bubbling up into your toilet,鈥� he says. 鈥淵ou鈥檙e protecting properties, people and businesses from being flooded.鈥�
In other words, CSOs are an inherent part of a combined sewer, but they are supposed to be a rarely-used backstop. Furthermore, if they are operating properly, they are relatively safe. That is because overflows should only happen at times of really heavy rain, when rivers are running high. This waters down the discharged sewage to the levels seen after treatment.
Yet the system clearly isn鈥檛 working properly. In the past few years, several water companies have been prosecuted for polluting rivers and other water bodies in the absence of rain.
As well as this, the idea of CSOs as a last-resort system is difficult to reconcile with the hundreds of thousands of dumping events every year. In the Netherlands, they only happen 鈥渙nce or twice a year鈥�, says at Delft University of Technology in the Netherlands. 鈥淚f it鈥檚 more often, yes that鈥檚 a problem.鈥�
While sewage isn鈥檛 the biggest source of pollution in Britain鈥檚 rivers (the leading cause is the runoff of slurry and sludge from farms), it is a threat to our health and the environment. Combined sewage and stormwater can include 鈥渉igh levels of bacteria, pathogens, pharmaceuticals and personal care products, heavy metals, sediments, microplastics and tyre wear particles鈥�, says Campos. 鈥淧athogens present in the untreated sewage can cause illnesses such as gastroenteritis, hepatitis and other waterborne diseases.鈥�
Not safe for swimming
The upshot is that Britain鈥檚 rivers aren鈥檛 reliably safe to swim in. Of 1500 rivers, only stretches of three are designated bathing sites 鈥� and even these aren鈥檛 judged to be safe for swimmers. There has also been a change in attitudes. 鈥淚 think people, quite rightly, are less tolerant of having a filthy stream running through the middle of their town than they used to be,鈥� says Stephenson.
England鈥檚 water companies are at least alive to the problem. In May, they apologised for the pollution and to clean up their act. Industry body Water UK says this amounts to a tripling of existing investment plans and should cut spills in England by up to 140,000 per year by 2030 compared with the 400,000 in 2020. The money is likely to come from water bills, which might need to rise by more than 拢50 per year to cover the costs.
So far, the companies haven鈥檛 given any details of their plans. What will it take to stop sewage flowing into Britain鈥檚 rivers?
Cleaning up Britain鈥檚 rivers
Britain鈥檚 wastewater treatment plants are perfectly capable of handling the volumes of sewage they receive and the technology is constantly improving. 鈥淚鈥檝e been at this for roughly five decades and the pace of change is faster than it鈥檚 ever been,鈥� says at the University of Michigan.
Instead, the problem is that the sewers can鈥檛 cope with the amount of water they receive at certain times and this indirectly causes the sewage to escape. 鈥淵ou have to deal with this volume,鈥� says van Loosdrecht.
The obvious solution is to stop using combined sewers, but that is far from trivial. New housing estates are already built with separate sewers and rainwater drainage. However, that still leaves much of the country with the old, combined set-up. Replacing it would be a huge, expensive project. 鈥淵ou鈥檝e got hundreds of billions of pounds of infrastructure built like that,鈥� says Stephenson.
It also isn鈥檛 a perfect solution. 鈥淓ven the rainwater itself flushes all kinds of dirt from the streets鈥�, including animal faeces, says van Loosdrecht. Large volumes of this contaminated water can still cause harm if regularly discharged into a river, so the rainwater may still need to be sent to a treatment plant 鈥� recreating the problem.
A better approach is to take a step back and consider the full range of problems facing Britain鈥檚 water. 鈥淲hat are the things that we can do that address the system as a whole?鈥� says Daigger. This holistic method, advocated by the United Nations, is called and is based on the premise that water is a vital resource for both people and ecosystems.
In the UK, this points to a simple truth. 鈥淲e have two problems: too much water or too [little] water,鈥� says Campos. While Britain is famously rainy, the water isn鈥檛 evenly spread in time or space. South-east England is particularly prone to droughts. Other regions, like the south-west, can receive intense rainfall from the Atlantic, leading to floods.
Global warming will worsen both problems. 鈥淐limate change is increasing the frequency and severity of storms,鈥� says Daigger. It will also make droughts more frequent and intense. As the weather becomes more changeable and extreme, more sewage will be forced out of combined sewers by extreme rains. 鈥淭hese overflows, which are a designed component of the system, are occurring more frequently and with higher volume.鈥�
In this context, says Campos, we need to collect more rainwater where it falls. 鈥淎void the stormwater being discharged into the drains,鈥� she says. That way, we can delay it entering the sewers, reducing the risk of overflows. The water can either be used immediately or stored to help with subsequent droughts.
How to store water
There are many ways to do this. We can collect the stormwater in tanks or artificial lakes that become amenities for communities. On a smaller scale, many more homes could have water butts attached to their drainpipes, with the water used for gardening.
鈥淪an Francisco is one of the best examples of how a significant metropolitan area is addressing this,鈥� says Daigger. New rules enable developers to install rainwater collectors and for the captured water to be put to use 鈥� often for non-drinking purposes like flushing toilets. 鈥淭his gets down into things like plumbing codes and the building trades and so forth,鈥� says Daigger. 鈥淗ere鈥檚 quite a significant international city that鈥檚 moving very systematically in a water-short area.鈥�
Alternatively, we can harness nature. Buildings with green roofs naturally absorb some rainwater. Many new housing estates in the UK now have a swale: a wide, shallow ditch that captures rainwater. A swale enables this water to slowly drain away, rather than flowing quickly over paved surfaces straight into the nearest sewer or river.
Swales are just one example of a . 鈥淲e can also implement 鈥榬ain gardens鈥� in urban areas,鈥� says Campos. These are planted areas of ground that collect runoff. For example, in many British towns and cities, roundabouts that were once made solely of concrete have been turned into miniature gardens. There are plenty of other hard, paved areas that can be planted and this would lead to leafier cities as well as helping to control the flow of water.
Where space is available, artificial wetlands can also control water flow. The best designs have a layer of gravel or sand, which filters muck out of the water as it drains through, says Campos. The water can then trickle out into a river. Projects like these are being attempted in the UK. In May, South West Water announced it would to store water on their land, variously using ponds, woodlands and wetlands. The company has been given 拢1 million of funding for the scheme by Ofwat, the UK regulator for water and sewerage firms.
Campos emphasises that there is no one-size-fits-all solution. Wetlands probably aren鈥檛 viable in a densely-populated city like London, which is why the Thames Tideway tunnel has been built. But in smaller towns and rural areas, such huge infrastructure projects would be unsuitable. The challenge will be running Britain鈥檚 water system in a joined-up way. 鈥淭he technical solutions are there,鈥� says van Loosdrecht. 鈥淭he main issue is in the governance and the political decisions.鈥�
It remains to be seen whether the UK government will enact the kind of joined-up thinking needed to tackle not only the sewage problem, but wider issues of water resources. Last month, a handful of Londoners went on a subterranean journey to see the kind of interventions needed, venturing into the Thames Tideway before it comes into use. For the occasion, a stretch of the tunnel had been turned into a garden-like art installation. However, wastewater will shortly take its place 鈥� and the sooner we face up to the reality of the disposal of our sewage, the better.
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How sewage treatment works
The aim of a sewage treatment plant is to clean water sufficiently for it to be safely discharged into a body of water like a river. This treated water doesn't have to be as crisp and pure as a mountain spring, but it must be clean enough that it doesn't harm the ecosystem or pose a significant risk to people's health.
A wastewater treatment plant works in stages. First, filters remove "the gross solids, in both senses of the word: the nasty stuff, but also the big stuff", says Tom Stephenson at Cranfield University in the UK. This includes items such as condoms and supposedly flushable wet wipes, which shouldn't be flushed down toilets, but often are. A second step filters out small, but dense, particles like grit and sand.
The core challenge is the stuff that comes directly from the body, which forms the bulk of the pollution. "The big problem is the poo and the wee," says Stephenson. "You've got to remove the organic matter, you've got to remove the ammonia."
As a first step towards this, the sewage is stored in tanks. Here, gravity drags the denser organic particles to the bottom, where they form a sludge. This is treated separately: sometimes it is used to produce methane, which can be burned to generate energy, or it may become a soil improver.
Meanwhile, the bulk of the water is still contaminated with organic chemicals like fats and proteins, and with ammonia. Here, the trick is to harness microbes that break down the waste and get them to work quickly and efficiently. "You blow air or you provide oxygen and bacteria grow," says Stephenson. "They degrade the organic matter and, if you get the conditions right, you can also convert ammonia to nitrate, which is far less toxic."
Most people have never set foot in a wastewater treatment plant and so have a lot of misconceptions, says Stephenson. People imagine "a horrible brown water with lots of turds floating in it", he says. "It isn't. It's a slightly muddy river." And while the microbial processes do smell, the whiff resembles "strong earth" rather than a broken toilet.
Michael Marshall is a freelance writer聽based in Devon, UK
