Like all forms of waste disposal, when it’s working well we don’t give a second though to where our water goes after it has been sent swirling down the plug hole, toilet or rainwater drain. But we’ve just sent it on a journey through a series of transportation and treatment processes that will eventually see it good enough to drink. We thought we’d take a little journey alongside it to see how complex – and yet somehow simple – the process is.
So hold your noses ... we’re going in!
A Mercifully Brief History of Sewers
Many of us think the Romans were pioneers in the use of underground sewage systems, along with their masterful feats of water supply engineering, under-floor central heating, concrete and highways. But to say they invented sewerage systems wouldn’t be true. There’s evidence of one being built by the Minoans on Crete almost 4000 years ago in their capital Knossos (near modern-day Heraklion); the Greeks developed more complex systems; and on the other side of the Atlantic the Ancient Mayans even had a system that predates Rome (although it’s a fair guess that they used different contractors).
By modern standards, these were all pretty basic, running off into rivers and seas with no real thought for treatment, but compared to the sanitary status quo of Britain’s Medieval period (think buckets and windows), they were in the space age. Indeed, we had to wait for the Victorians to arrive before anything approaching a modern system started to appear under the streets of British towns and cities. Outbreaks of cholera and other diseases were common.
Two Different Types of Waste Water
There are two separate types of sewer. First is the foul water sewage, which contains the waste from toilets, baths, wash basins, kitchen sinks, dishwashers, showers etc. Second, there are surface water sewers, which carry essentially harmless rainwater away to rivers and soakaways. These can also course rainwater directly from roofs (via guttering).
There are also combined sewers, where both types are mixed together underground and sent for treatment. The problem with this type is that it leads to unnecessary treatment and can cause overflow during rain storms, meaning raw sewage from homes can spill onto streets. New developments tend to keep the two types separate, but some towns and cities still have combined systems.
Water for washing often goes into the same exposed outdoor drains that carry away roof and surface water, but the waste from the toilet is entirely sealed, with no access to the air until it reaches the sewage plant.
Keeping the Smells Away from the Nose
With so much waste water in the drains, they would send pretty dreadful stinks right into our homes were it not for measures designed to cause a seal in the pipe. The simplest and cleverest solution is to use the water itself. By placing a section of pipe in a U-shape (a trap, or U-bend), there’s always an amount of water filling the bottom. This acts as a seal for unpleasant-smelling and harmful gases from the drain pipes. And because they are replenished every time you flush or drain a sink, they don’t have time to become particularly stagnant or smelly themselves.
Along a drain, water will pass through several traps. There’s one right underneath the toilet and sink, and there will be traps in the underground pipework outside.
Whose Responsibility is a Drainage or Sewage Pipe?
The owner of the property is responsible for all drainage pipework from plug holes and toilets and that beneath their property’s boundaries, right up to the point where it has a junction with another property’s pipework. At this point it becomes the responsibility of the local water company. In the UK there are twelve regional water companies, serving the following regions:
- Scotland – Scottish Water
- Northern Ireland – Northern Ireland Water
- North East England – Northumbrian Water
- North West England – United Utilities
- Yorkshire and the Humber – Yorkshire Water
- Wales – Dwr Cymru Welsh Water
- Midlands – Severn Trent Water
- East of England – Anglian Water
- Greater London and Thames Valley – Thames Water
- South West England – South West Water
- South West England – Wessex Water
- South East England – Southern Water
Note that unadopted roads often have different regulations; even if there are multiple properties on the road, sewers might be the joint responsibility of some or all of the properties.
Into the Main Sewer
Sewage flows into the main sewer, which is usually found underneath roads (much to the consternation of motorists, who will experience roadworks when it needs to be repaired). The general story from now on is that sewers get larger and larger as they join with others and take the wastewater of more properties. Sometimes sewage will need to pass through a pumping station to cope with capacity or raise the elevation.
Eventually the sewage will reach the treatment plant.
Screening of Raw Sewage
The first stage of the sewage treatment process is the removal of large objects. This is done by letting the sewage run through what is effectively a large sieve. All those things that you shouldn’t have put down the drain in the first place (nappies, paper, etc.) get trapped by the screening process. Because they would quickly clog up, mechanical draggers pull up all the trapped waste every few minutes for disposal in landfill.
After the large-object screening, grit and smaller objects are removed in a similar process.
If you’ve ever driven past a sewage works you’ll recognise the large circles that sit outside them. Check out a satellite image of most towns and you’ll see dozens of them on the outskirts. They are settlement tanks, where the mixture of water and excrement is allowed to sit relatively motionless so that the solids sink to the bottom. Scrapers move very slowly around the tanks, pushing the collected sludge into an outlet in the middle where it can be taken away and used as fertiliser, composted or sent to anaerobic digestion tanks, which creates enough methane to fire a power station – and many of them do.
The water flows over the top and moves on to the next stage of the process.
Secondary Treatment – Aeration
Although by now most of the solid matter will have been removed, the water that emerges from the settlement tanks is far from being safe to drink as it still contains potentially dangerous microorganisms. The next part of the treatment involves sending the water into aeration tanks, where oxygen is pumped into the water to encourage the “friendly” microorganisms to thrive and eat the dangerous ones. It’s a remarkably successful process, which kills most of the dangerous organisms.
The water is then sent to another settlement process, where the dead bacteria sink to the bottom to form a sludge, which can be sent into the first sludge process.
Many sewage works have one final stage, which involves letting the water trickle through sand in an artificial emulation of the way water filters itself in nature.
Back to Nature
The remaining water is now clean enough to be sent to the river or ocean, where it becomes part of the ecosystem. Eventually it’ll form clouds, fall as rain and some of it will find its way to a waterworks where it will be treated and end up in somebody’s cup of tea.
And now we all know what will happen to it next!