The process of cleaning our wastewater ensures that we have a fresh supply of safe water in the future, and minimises our impact on the environment by decontaminating our waste and turning it into a valuable resource.
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First Stop: Sewer
After we use water, we rarely consider its onward journey.
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Well Connected
Almost all homes and business are connected to a grid of sewers to collect household wastewater. Storm drains on streets are also connected to this network. These run beneath our feet for hundreds of miles.
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Different Sources
Pipes that come from domestic houses contain all types of water waste, from toilets and sinks to washing machines and bathwater. Industrial sources might contain organic waste or chemicals from factories.
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Ancient Pipes
These Victorian sewers are 150 years old and are built from red brick, by hand. Before the widespread building of sewers in the 1800s, waste overflowed in streets. This led to diseases like cholera becoming rampant, causing many deaths.
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Inflow
This wastewater treatment plant is able to serve 270,000 homes and is fed by many pipes from neighbouring towns and villages. After the branches of the different sources connect, the waste is pumped along a pipe up to 2.4m in diameter – large enough for a horse to walk through.
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Raising the Water Level
Once the water reaches the plant, it must be raised to the same level as the processing equipment. This means that the water is raised a total distance of 22m to the treatment area from the tunnel below.
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Working With Water
This is Richie. He is one of 5 staff members at the plant. Staff take it in turns to be on call at night – this means they have to react if an alarm is triggered in the plant.
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Water Source
At full capacity, the plant can process 3 tonnes of wastewater. It needs to be able to adapt to large volumes of waste quickly, as sewers can be in danger of overflowing during a heavy storm.
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Screening Process
The raw waste first goes through a filtration system, where it enters a chamber of screens. Here, 6mm holes separate larger particles – usually stray bits of plastic or other material that should never have ended up in the waste.
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The water then flows to the fat, oil grit and grease channels (FOGG), where air is injected into the liquid mix. This air forces the grease to the top of the mixture, and grit to the bottom. Here, grit is being collected, and will later be disposed of.
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Onward Journey
After the grit and grease is removed, the wastewater can move on to the next stage of decontamination. The grease and oil is converted into a biogas, which is then used as a fuel source.
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Grit Separation
Motors pump small particles of grit through these red pipes. Grit and grease are common byproducts of this process. Grit can come from rocks and sand that gets washed down drains, and grease can come from fast food restaurants.
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Safety Checks
Staff members have to carry out regular checks on their equipment. If the machines have a fault, it could trigger an alarm that stops the plant from working and cause a backlog of waste.
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Sludge Conversion
Sludge is the solid product that is removed at each stage of the treatment process. Chemicals are added to force large sludge particles to bind together so they can be removed easily.
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These larger particles are fed into anaerobic digesters – tanks where microbes feed on the bacteria present in sludge – and produce biogas, which can be used as a fuel source.
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Safety Wear
Safety precautions must be taken when working in an environment where bacteria might be present. If a machine is broken and needs internal repairs, staff must ensure their skin is covered before reaching into sludge tanks.
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Filtering Sludge
These paddles keep the sludge moving around a belt in this tank. This helps the larger particles bind together to be digested. After the biogas has been produced, the remaining sludge can be reused as farm fertiliser.
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The Right Consistency
Ferric chloride is added to bind the particles together, which then form flocs. Flocs are clumps of small particles, which in this case help to allow the larger particles to be removed from the mixture.
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Updating Results
In this laboratory space, staff must regularly take samples of sludge to ensure it is the correct consistency for being fed to the digester unit. This involves an in-depth knowledge of the science behind wastewater treatment.
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Richie has checked a sample from the sludge tank and is awaiting the result.
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Sample
Each of these bottles has a different sample taken from the sludge. This one is too watery and the one on the left is too lumpy – sludge that is ready to be digested will be somewhere in the middle.
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Testing Equipment
This machine tests the amount of moisture present in the sludge samples. If they are too watery, chemicals can be added to make them bind together to the right consistency.
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Odour Control
This shield funnels the smell from these samples towards a fan and away from the tester. The whole plant traps the air in every stage of the process and chemically scrubs it to neutralise odour.
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Aerated Filters
Lamella filters provide the second-last stage of the process. Hexagonal grids trap any remaining grime before the water flows through to the final treatment area.
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Filtered through biological aeration filter funnels (BAFFs), wastewater is forced upwards by compressed air through several metres of floating polystyrene beads. Microorganisms that grow on the beads feed on nutrients and oxygen, provided by the wastewater and air.
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The water is then released from the plant to the outflow tunnel.
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Water Difference
Staff work hard to ensure that water leaving the plant is safe to re-enter the environment. It is easy to see the difference the process has made. The outlet water is now clear of debris and sludge has been removed.
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Polystyrene Beads
Inside this pipe, water is forced upwards through the balls, as in this model. It then exits through a concrete filter, which allows water through, but prevents the balls from escaping into the outflow.
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Out in the Open
The final, safe water product travels 1.4km to the sea, where it joins a 2.5km outflow pipe. This pipe allows the treated water to be mixed with the seawater to begin the water cycle again.
