Our use of cookies Understanding sewage: How sewage systems and treatment works operate
Most homes and businesses in the UK are connected to the local sewerage system, providing a simple and effective way to flush toilets and drain sinks.
Behind this convenience lies a vast, largely invisible network of pipes, pumps and treatment works that carries our waste away, cleans it and returns it to our rivers and seas.
With sewage spills rarely out of the headlines, this article explains precisely how the sewage system works in the UK.
What is sewage?
Sewage is the wastewater produced by homes, businesses and industry.
It includes foul water from toilets, greywater from sinks, baths, showers, washing machines and dishwashers, and trade effluent from commercial and industrial processes such as rinse water from a car wash.
Despite appearances, sewage is roughly 99% water. The remaining 1% includes organic matter, suspended solids, nutrients, chemicals and bacteria. All of these must be removed by the sewerage system before the water can safely drain into rivers and the sea
What is the difference between sewage and sewerage?
The two words are very similar and easily confused.
- Sewage is the wastewater itself, produced by local properties.
- Sewerage is the infrastructure that carries it away, including the network of pipes, manholes and pumping stations that moves wastewater from properties to a treatment works.
Put simply, sewage is what flows through the pipes, and sewerage is the system of pipes it flows through.
What is the sewerage system?
The sewerage system is the entire network of infrastructure that removes wastewater from local properties, transports it, treats it and returns it safely to the natural water cycle.
The diagram below shows and explains the key components of the sewerage system in the UK.
The sewerage system includes both:
- Sewers, the pipework that transports sewage away from local properties.
- Sewage treatment works, the facilities that treat the wastewater before returning it to the environment.
In the UK, the sewerage system is owned by local water companies that are typically also responsible for the potable water network. Find out more in our guide to British water infrastructure.
How sewers work
Sewers are the pipework that connects individual homes and businesses in a local area to the local sewage treatment works.
Sewers are typically managed by the local water company. This section explains the key aspects of how the sewers work.
Wastewater enters the sewers from local properties
Individual properties in a local area have a direct connection to the sewerage system, which allows the sewers to receive:
- Domestic sewage from local homes
- Domestic sewage and trade effluent from local businesses
- Surface drainage of rainwater (in the case of combined sewers)
The occupiers of local properties pay to use the sewers by:
- Households paying domestic water rates to the local water company.
- Non-domestic properties paying business water rates to licensed business water suppliers.
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Gravity transports wastewater along the sewers
The sewage pipes are laid underground, generally following the same path as the road network. They are laid at a slight downhill gradient so that sewage flows without the need for pumping.
The sewers from individual roads join to form gradually larger pipes.
Manholes and inspection chambers
Along the route, sewers can be accessed through manholes that connect to the surface.
Manholes allow employees of the local water company to access the sewers for inspection, cleaning and the clearing of blockages.
Pumping stations
In particularly flat areas, or where the sewage must flow uphill, pumping stations are required to push wastewater against gravity.
Through the combination of gravity and pumping stations, all wastewater that enters the local sewerage network is transported to the local sewage treatment works.
Types of sewers in the UK
This section explains the three types of sewers typically used in UK towns and cities.
Foul sewers
Foul sewers are specifically designed to carry only wastewater produced by domestic and commercial buildings.
The foul sewer system collects wastewater from individual properties and delivers it to a local sewage treatment works.
Surface water sewers
Surface water sewers, also known as storm drains, are designed to carry rainwater and other surface run-off that lands on roofs, roads and other paved surfaces.
The water collected is not transported to a sewage treatment works but instead typically discharges directly into a local watercourse.
These sewers are designed to prevent flooding in urban areas during rainfall.
Combined sewers
Combined sewers carry wastewater and surface water in a single pipe, with the two mixed together and transported to a local sewage treatment works.
Combined sewers were the common approach taken in towns and cities until the 1950s.
The problem with combined sewers is that, during heavy rainfall, the sewage treatment works they supply can be overwhelmed. In this case, combined sewer overflows are used where the excess untreated sewage is dumped directly into a local river.
How sewage treatment works operate
This section explains the steps used at a sewage treatment works to convert wastewater into a form that can safely be released back into the environment.
1. Preliminary treatment: Screening and grit removal
Sewage arriving at a treatment works first passes through a series of mesh screens that capture large debris such as wet wipes and sanitary items that cannot be processed at the works. This material is raked off and taken away for disposal.
Following screening, the sewage is slowed down, allowing heavier inorganic particles such as grit and pebbles to sink to the bottom. Removing these items protects downstream equipment.
2. Primary treatment: Settlement
The initial treatment stage uses a natural settlement process, harnessing the power of gravity.
The screened wastewater is held in a large settlement tank. Heavier organic solids sink to the bottom to form “primary sludge”, which is scraped away. Fats, oils and grease float to the surface and are skimmed off.
This stage typically removes a significant proportion of the suspended solids. What remains is a liquid that still contains dissolved and fine suspended organic matter.
3. Secondary treatment: Biological
The next stage of the treatment process relies on microorganisms to consume dissolved organic pollutants.
At sewage treatment works in the UK, one of the following processes is typically used:
- Activated sludge – Sewage is held in an aeration tank where air is bubbled through, feeding a population of bacteria that consume organic matter.
- Trickling filter – Wastewater is sprayed over a surface coated in a microbial film that breaks down the organic matter as the water trickles through.
A further settlement process is used to remove any microorganisms remaining in the treated sewage.
4.Tertiary treatment: Polishing
Where treated sewage is returned to a sensitive waterway or an area used for public bathing, a final polishing process is used to treat it further.
The polishing process can include:
- Sand filtration to remove fine solids
- Chemical dosing to remove nutrients
- A disinfection process to kill pathogens
5. Discharges of treated sewage
The final treated wastewater is released into a nearby river, estuary or the sea. At this point, it returns to the natural water cycle.
6. Sludge treatment
The organic solids removed from the wastewater are usually broken down in an anaerobic digestion process, which converts the waste into:
- Organic fertiliser used for agriculture.
- Biomethane that can be used to generate electricity.
Find out more in our guide to how sewage is used to power Britain.
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