From Loo to River

Raindrops on the Roof: Where Rainwater Goes

Imagine the soft patter of rain on your roof. Each raindrop gathers with others in the gutter, forming rivulets that flow down drainpipes. This surface water, the rain running off roofs, pavements and roads, enters a separate set of drains designed just for rainfall. In modern neighbourhoods, these rainwater drains carry the water away, often leading straight to the nearest river, stream or the sea. In older towns and cities, the surface water is mixed with the water from your toilet, bath, shower, dishwasher and washing machine. This can cause problems, more about that later.

Look at the curb on a street during a downpour and you’ll see water gushing into metal-grated openings, these are surface water drains known as gullies, guiding the rain into underground pipes. In some places, the rain is funnelled into soakaways in the soil, gently seeping back to groundwaters. But in many towns and cities, it travels through pipes, merging with brooks and streams. Each raindrop, once fallen, becomes part of a hidden urban river, coursing beneath our streets on its way to join open waters.

Down the Drain: The Path of Wastewater

Now consider the water that leaves our homes, the flush of a toilet, the soapy surge from a washing machine, the swirl down a sink. This used water is foul water, and it carries the whisper of our daily lives: organic waste, soap suds, food scraps and detergent. Unlike rainwater, this wastewater cannot be allowed to flow straight into rivers, because it’s teeming with pollution and germs. When you flush the toilet or pull the plug in the bath, the water disappears from view but not to oblivion. It rushes through narrow pipes from your house into a larger sewer pipe under the road.. These foul water sewers are like buried streams of sewage, flowing under pavements and past gardens. They join a vast network of sewer tunnels that converge from across towns and cities, all leading to a sewage treatment works, known as water recycling centres now. Every time water vanishes down our drains, it is beginning a voyage through this network, bound for a place where it will be cleaned and made safe before rejoining the natural water cycle.

Beneath our feet, two kinds of water flow in different veins: one mostly clean, one dirty, each with its own destined path. But problems arise when they meet and mix, more on that below.

Two Paths for Water: Surface Drains vs. Foul Sewers

Modern engineering has given us a tale of two parallel systems. Most newer homes (built after around 1970) have separate drains – one set for rainwater, and another for wastewater. The rain that falls on your rooftop or trickles across your driveway enters the surface water drains, which carry it to a nearby river or brook, or into the ground. The used water from your household, the foul sewage, flows into a separate sewer pipe that leads to the treatment works. By keeping these flows apart, we protect our rivers from being routinely sullied by everyday sewage. In a perfect scenario, clean rainwater never mixes with dirty sewage. The rain runs off to nourish streams, while the sewage travels to be purified.

However, not every place benefits from this neat separation. When the first sewer networks were built in Britain during the Victorian era, they were designed as “combined” systems, all of the rainwater and wastewater went into the same drains and sewers, which in those days were simply aimed at the nearest river. The goal back then was urgent: to remove foul waste from the streets and homes quickly to combat disease, with little thought for the rivers receiving that foul mix. Many older towns and city centres still rely on these combined sewers, where a single pipe carries both stormwater and sewage. Properties built before the 1970s often have this kind of connection by default. Today, those combined systems usually feed into treatment works so that normal dirty dry-weather flow gets cleaned. Yet they carry a legacy of an older design, one that struggles to cope when the weather turns wild and the heavens open. In fact, around half of properties in Britain are still connected to combined or older-style drainage systems.

When the Rains Come: Storms and Overflows

On a calm day, the underground rivers of sewage and stormwater run their ordinary courses. But picture a sudden heavy rainfall, when the skies open and torrents of water cascade from roofs and roads. In areas with combined sewers, all that rainwater rushes into the same pipes that carry sewage. Very quickly, the volume of water can overwhelm the system. The combined sewer pipes fill to the brim; if nothing were done, this mix of rain and waste could back up through drains and flood homes, streets and gardens. To prevent such a nightmare, engineers long ago devised relief points in the system. Combined Sewer Overflows – often shortened to CSOs – act like safety valves. During heavy rainfall, when the capacity of pipes is exceeded, CSOs allow the excess diluted sewage to spill out into rivers or the sea rather than surging up into our kitchens and streets. In effect, the river or ocean becomes a pressure release zone for the sewer network.

This practice is intended as an emergency measure, used only when rainfall is unusually intense. The overflow water is mostly rain, but it carries some untreated sewage with it – a murky compromise to spare our communities from flooding. The Environmental Agency notes that these overflows of diluted sewage during heavy rain are a built-in aspect of old sewerage systems, not a malfunction. The Victorian engineers saw it as the lesser of two evils. However, the consequence is that at times of storm, our rivers and coastal waters receive pulses of pollution: water laced with raw waste from the sewers. If you’ve ever walked by a river after a big storm and noticed the water looking browner or carrying toilet paper or debris, that’s the sign of a recent overflow. In the UK today, there are thousands of these overflow points, and they are used more often than anyone would like. With climate change bringing heavier downpours, and urban growth adding more hard surfaces that funnel rain into drains, the pressure on combined sewers is increasing.

The result is too many moments when our rivers receive an unwanted surge of sewage. It’s a sobering part of the journey: water that started pure in the clouds can end up mixed with sewage effluent in the river, all because the system couldn’t hold back the flood.

Cleansing the Flow: How Sewage is Treated

Let’s turn to the brighter side of this cycle: what happens to the foul water once it reaches the sewage treatment works. These treatment plants are like modern alchemical workshops where dirty water is turned clear again. At the sewage works, wastewater passes through several cleaning and filtering processes so that it can be returned safely into rivers. First, the incoming sewage is screened to remove large objects and debris – all wet wipes (there is no such thing as a flushable wet wipe), cotton buds, and other rubbish that should never have been flushed (surprisingly even false teeth find there way here). The water then enters big settlement tanks in a stage called primary treatment. Here, the flow slows down, and gravity lets the solid waste (the sludge) sink to the bottom, while oils and lighter scum might float to the top. The partially cleared water in the middle moves on to secondary treatment. In secondary treatment, nature is harnessed in an engineered way: useful bacteria are encouraged to consume the remaining waste. Bubbling air through the water (in aeration tanks or percolating filters) gives these microbes the oxygen to thrive. They feast on the organic pollutants, breaking them down just as fallen leaves decay on a forest floor. By the time this biological process is done, most of the harmful content is gone. The water is then often passed through another settling stage, allowing the now-grown bacteria and any leftover particles to sink as a final sludge.

Some treatment works add a tertiary stage for extra purification. This might involve filtering the water through sand, using ultraviolet light to kill any lurking pathogens, or using chemicals or special filters to remove nutrients like phosphorus. Finally, the treated wastewater, now clarified and largely cleaned of its former nastiness, is released back into a local river or stream. The journey has come full circle: from river to tap in our home, to sewer, to treatment, and now from the treatment works into the river once more, destined eventually for the sea. The solid sludge collected along the way doesn’t go to waste either: it is often processed and used as fertiliser on farmland or even burned to generate energy. The entire process is carefully monitored. Environmental regulators set strict standards for the quality of water that flows out of treatment plants into rivers, although not all chemicals are monitored and have a standard, ensuring that it’s clean enough for wildlife to survive.

Yet even this cleaned water carries subtle remnants of its past life. Invisible to the eye, dissolved nutrients remain in the effluent, and these can cause trouble in large doses. Treatment works significantly reduce harmful bacteria and solids, but unless they have special stages, they may not remove all the nitrogen and phosphorus, the very ingredients of fertiliser. Thus, our story now turns to a quieter form of pollution, one that rides along with the treated water into rivers.

Nutrient Overload: Too Much of a Good Thing in Our Rivers

In the right amount, nutrients like nitrogen and phosphorus are part of a healthy river ecosystem, nourishing plant and microbial life. But when too much of these nutrients enter a river, they upset the natural balance. It’s like adding heaps of fertiliser to a garden pond, suddenly, algae and certain weeds grow wild, turning the water cloudy green and depleting it of oxygen. This phenomenon is known as eutrophication, though you might know it simply as an algal bloom or “pea soup” water.

In a river overwhelmed by nutrients, you may see mats of green algae or thick growth of duckweed, and if that persists, the water beneath becomes starved of light and oxygen. When the rampant algae eventually die off, their decay sucks even more oxygen out of the water, leaving fish gasping and aquatic insects struggling to survive. A nutrient-rich river can turn into an almost lifeless stream, its diversity of life smothered by a few overabundant species. It’s a case of “too much of a good thing”, the same nourishing elements that help crops grow can poison a river when concentrations run high.

Where are these excess nutrients coming from? A significant portion comes from us, carried by that very wastewater we send down the drains. Human waste is naturally rich in nitrogen and phosphorus, our urine and faeces contain these nutrients because they come from the food we’ve eaten. Additionally, the detergents and cleaning products we use can contain phosphorus (for example, phosphate additives in laundry detergent or dishwashing powder), which end up in the sewage as well. In fact, about a quarter of the phosphorus in household wastewater used to come from detergents alone before regulations began to limit phosphate use. Even with some restrictions now in place, phosphates from detergents and the natural waste in sewage still contribute nutrient pollution to our waters. Modern sewage treatment does remove some nutrients, but unless advanced nutrient-removal processes are employed, a lot of nitrogen and phosphorus flows out with the treated effluent. This means that every time treated sewage is discharged into a river, it can act like a mild fertiliser dosing the water. And when untreated or partially treated sewage overflows during storms, it dumps an even more concentrated pulse of nutrients (along with harmful bacteria and other pollutants).

Scientists studying British rivers have found that sewage pollution, even when treated, is a major driver of nutrient overload, promoting the growth of nuisance algae and disrupting river ecosystems. The nutrients from our waste create conditions that certain hardy organisms (like filamentous algae or “sewage fungus”) love, but more sensitive aquatic life cannot tolerate. Our detergents and toilets are thus unwittingly linked to those blooms of algae or slimy growths you might notice in some streams.

Nutrient pollution has become one of the biggest threats to healthy waterways in the country, and it’s not just from our sewers.

From Field to Stream: Agriculture’s Impact on Nutrients

While sewage effluent is one source of nutrient pollution, agriculture is another major contributor and in many areas the dominant one. Picture vast fields in the countryside: to grow healthy crops, farmers spread fertilisers rich in nitrogen and phosphorus, or they add manure from livestock which is also loaded with nutrients. In an ideal world, every bit of those added nutrients would be soaked up by crops and locked into the soil. Farmers use these nutrient-rich materials to improve the soil for better growing, that is their intended purpose. Ideally, the fertiliser nourishes the plants, and the excess stays put in the earth. But the reality often unfolds differently. After heavy rain or over the wet winter months, the rainwater percolating through the soil can pick up leftover nutrients and carry them off the fields. The nutrients become vagabonds, dissolved in runoff that trickles into ditches, streams, and eventually rivers. When it rains, the nutrients can be washed from the fields into our waterways undoing the farmer’s intentions and contributing to the same kind of nutrient overload that sewage does.

This agricultural runoff is a diffuse, widespread source of pollution, there isn’t one pipe to point to, as the water flows off hundreds of fields and into countless streams. But cumulatively, it has a huge impact. In England, it’s estimated that diffuse farming pollution (mainly nutrients) affects around 40% of water bodies, an even greater share than the water industry’s sewage sources. The vision of lush green fields belies a hidden export of fertiliser nutrients moving downstream. On paper, there are guidelines and rules, farmers are encouraged or required to use “good agricultural practice” to minimise runoff. They can do things like planting cover crops to hold soil in place, creating buffer strips of grass or woodland along rivers to catch runoff, and timing fertiliser application to avoid rain. Ideally, such measures would keep most of the nutrients on the land. In reality, however, economics, weather, and sometimes a lack of enforcement mean that a lot of farms still lose significant nutrients to the water. A sudden storm after fertiliser has been spread can wash a portion of it straight into the nearest brook. Slurry from livestock yards can seep into streams if not properly contained. The result is that our rivers receive a double helping of nutrient pollution: one serving from the sewage effluent of our towns, and another from the fertilised fields of our countryside.

Towards Cleaner Waters: Working with Nature

Standing by a river’s edge, watching its water sparkle in the sunlight, it’s hard to imagine the complex journey that water has taken and the challenges it faces. Yet understanding this journey is the first step toward cherishing and protecting our rivers and seas. We’ve followed the path from raindrop to river, seeing how in modern systems the rainwater and foul water are wisely kept apart, and how in older systems their fates are entwined. We’ve peeked into the subterranean world where sewers carry our waste away, heard the echo of water rushing through pipes beneath the road, and learned how those waters are cleansed at the treatment works before returning to the wild. We’ve also uncovered the delicate issue of nutrient pollution, an often invisible imbalance caused by the very substances that help life grow. This excess of nutrients, from our homes and our farms, is a reminder that even things that seem benign (a flush of the loo, the greening of a field) can have unintended consequences downstream.

As readers who have traveled along on this narrative, you might look at your sink, your street, or your local stream a bit differently now. There is a quiet poetry in the way water cycles from sky to land to sea, and we are part of that story every day. The next time rain drums on your roof, you might picture its journey under the pavement and out to the river. The next time you flush, you might recall the distant treatment works where that water will be purified. And when you stroll by a river and notice the plants growing in the current, you’ll understand how delicately we must tread to keep those waters clear and alive. Our rivers and seas are the lifeblood of the landscape, and by learning about their hidden connections to our homes and habits, we are better placed to keep them healthy.