How does the sewer system work?

Our sewer system is an essential part of modern life, keeping our homes and streets clean and hygienic.

But how does the vast infrastructure that exists beneath our feet actually work?

In this article we will take a closer look at the importance of our sewer system, how it was created and how the system deals with the foul water and surface water that enter our drains.

Why our sewer systems matter

Most of us don’t think too much about what happens to the water that collects in storm drains, gets flushed down the toilet or washes down the plughole after our morning shower. This is because most of us don’t have to think about it, as our modern sewer system takes care of all the hard work.

The only time we really think about our sewers and drains is when our sewer system isn’t as good, or isn’t doing what it is supposed to be doing. For instance, in the UK, we’re fortunate to have one of the best sanitation systems in the world. In developing countries, where facilities are not as sophisticated, open trench drainage channels transport waste from the home to treatment facilities – and often, these trenches can get very smelly. Some countries don’t even have the luxury of functioning sewer systems.

The sewer system we use today is essential in our day-to-day lives.

It provides us with a safe and hygienic way to dispose of waste, which in turn protects us from the risk of infection.

The manner in which our waste water is carried to water treatment centres means we aren’t burdened with the smell or sight of wastewater. In addition, the treatment of these waters means we have clean water with which to drink, irrigate land and so on.

But things weren’t always so simple.

The history of the sewer system

Sewer systems have existed in some form or another since 3200 BC. The Orkney Islands in Scotland show signs of early drainage systems, with lavatory-like plumbing fitted within wall recesses. However, most of these early systems caused a wide range of problems, from causing cholera epidemics to simply making a big stink.

Historical UK sewer systems

When the Romans occupied Britain from 43-410 AD, they brought with them their high standards of cleanliness. This included public bathhouses, complex drainage systems, latrines and aqueducts with which to clean the streets. When the Roman Empire fell, they took their high standards of hygiene with them.

Approaches to cleanliness in Britain became quite primitive afterwards and disease was rife. A basic sewer system existed in London in the 1550’s, but it was nothing more than some open ditches that ran towards the River Thames.

Beyond this, little else was done to transform the sewer systems until the outbreak of several cholera epidemics in the 1800s.

In 1854 Dr John Snow discovered that a well in the Broad Street neighbourhood in London was becoming contaminated with waste, identifying this as the source of the latest cholera epidemic, in Soho.

Four years later came ‘The Big Stink’ – when high temperatures made the capital smell. As the Thames was effectively an open cesspool, the stench became so eyewatering that Parliament was closed several times during this period.

This alerted officials to just how dire the sanitary systems were. The need to create a new sewer system was evidently an urgent one, and soon Joseph Bazalgette was appointed the chief engineer of the new system.

This new sewer system, which was much more effective, caused excitement for Queen Victoria, who ordered that a small rail line be constructed along the lengths of the larger tunnels to carry visitors. Gas lights, walkways and booths selling souvenirs were also constructed.

The system revolutionised the way we disposed of waste in the UK and was quickly adopted throughout the country. This system is the same one that still exists in London today.

The Parisian sewers

Perhaps one of the most famous sewer networks in the world could be the one underneath Paris. Described in Victor Hugo’s Les Misérables, the Parisian sewers have a colourful history, stretching back throughout the centuries.

Originally, wastewater was poured onto fields, or unpaved streets, which finally filtered back into the River Seine. Around 1200, Philippe Auguste – the King of France at the time – had the Parisian streets paved, and incorporated a drain in their centre.

In 1370, Hugues Aubriot, provost of Paris, had a vaulted, stone-walled sewer built in the ‘rue Montmartre’, one of the oldest areas of Paris. The sewer collected wastewater and discharged it into Menilmontant brook, a smaller river in Paris. Despite an improvement in overall sanitation, wastewater was still being drained in open air watercourses – producing a terrible stench.

It wasn’t until Louis XIV came into power that significant changes were made to the Parisian sewer system. He oversaw the construction of a large ring sewer on the right bank. But, simultaneously, he converted the river Bièvre into an open-air sewer to service left bank. This caused significant health problems for those using the Bièvre as their source of drinking water. Diseases associated with drinking dirty water, such as typhoid, run rampant.

Napoleon I took a significant step to modernising the Parisian sewer network, by building the first vaulted sewer system in the early 1800s. Napoleon III continued the work of his predecessor by commissioning Georges-Eugene Haussmann to renovate urban areas in the 1850s.

Hausmann employed the services of the engineer Belgrand to create a dual sewage network, one for distributing drinking water, and the other for taking away wastewater. By 1878, the project had built over 600 km of underground tunnels. Sewage was no longer put into the Seine, but instead, treated in a sewage treatment plant miles away.

From 1880 to 1913, buildings across Paris were connected to the sewers, which by 1914, resulted in 68% of all buildings in Paris having direct connections to the sewer.

Between 1914 and 1977, more than 1000 km of new sewers were built, to supplement and to build on Belgrand’s great work. It can be said that Belgrand was the person that was responsible for revolutionising the Parisian sewer network.

How the sewer system works

The reason why an effective sewer system was so evasive was largely due to a lack of understanding about how dirt and disease were related to one another. Thankfully this is something we now know a lot about and the processes behind a sewer system have been carefully designed as a result. From flushing your toilet to the final uses of the treated water, there are a lot of steps that take place to transport and treat the wastewater you create.

Wastewater, or foul water, is water disposed of via your toilet, sink, shower or bath. All of this water heads into special foul water drains and sewers located under our feet.

There are also distinct drains and sewers used for collecting surface water – rainwater and melted snow and ice – which is then transported back into our rivers and seas.

Arriving at the treatment station

Foul water is taken to water treatment stations, where it goes through the first treatment, called screening. Debris, such as rubbish and grit, is removed from the water to begin with and the water is held until the solids have separated from the liquids. These solids are then collected and often used as fertiliser.

Waste-eating bacteria is then used to break down the remaining waste into harmless matter. To do this, the water is either held in large tanks or passed over stones, such as granite, that are inhabited by these types of bacteria.

Finally, the water is passed through a special tank to get rid of any remaining waste particles. Depending on the composition of the water, this step may be replaced by treating the water with chlorine to kill off any remaining bacteria.

What happens to the treated water?

After the water has been treated it is usually returned to rivers and streams, or is discharged into the sea. The treated water is of such high quality that it often improves the quality of the water that is already in these bodies of water, helping to keep them as healthy as possible.

So long as our drains and sewers remain free of blockages then our wastewater is treated effectively, with a smooth process that has amazingly taken millennia to perfect.

It is for this reason we need to make sure we protect our ageing sewer system and ensure we keep it free of blockages as much as possible. We need to make sure we avoid at all costs putting the following ‘Big Offenders’ down our drains and toilets to keep the system healthy at all times:

  • Fats, oils and grease (FOG)
  • Wet wipes
  • Nappies and pads
  • Sanitary products
  • Tampons, applicators and wrappers
  • Condoms
  • Razor blades
  • Bandages and plasters
  • Dental floss
  • Medicines and syringes
  • Cotton buds
  • Hair

By keeping this in mind we can make sure our amazing sewer system remains clean, hygienic and effective for many more years to come.

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