Where does wastewater come from?
Wastewater comes from water used in the home, in businesses and factories and from rain falling on roofs or the roads and pavements.
All wastewater follows the same route, wherever it comes from. It is guided down drains and into sewers that run under roads. These carry the water, now called sewage, to the waste treatment or sewage works.
Sewers can get blocked by fat that's poured down sinks or unflushable items such as wet wipes and sanitary products, all of which should go in the bin.
Screening
Lots of things get into the sewers that shouldn't. Condoms, cans, plastic bags, nappies... even false teeth and wallets.
All must be removed during the first stage of treatment, along with the gravel and debris washed in from the roads.
Primary settlement
Our next job is to take out all the other 'big' dirt. We do this by pumping the water into large, deep tanks called sedimentation tanks. Here, heavy particles of solid waste sink to the bottom to form a layer of sludge.
We then take the sludge away for treatment, after which farmers can use it as fertiliser. This leaves us with water that has a lot of invisible dirt that we need to take out in the next stage.
Biological treatment
In stage three, we put billions of bacteria to work to break the waste down into harmless substances. We do this in two ways:
Holding the water in large tanks and pumping in air continuously, so helping the bacteria grow and thrive (activated sludge)
Passing the water over beds of special stones (or filter beds) which are home to waste-eating bacteria.
Final settlement
The almost-clean water goes through one final stage of treatment, which allows us to take out any last remaining waste particles.
The usual way we do this is by passing the water through a special tank (called a humus tank). Sometimes we use a reed bed instead.
Unless the effluent is to be discharged to a sensitive environment which requires further treatment we return the water to rivers or the sea where it is ready to go through the cycle all over again.
Sludge treatment
Treating sewage produces a lot of solid matter called 'sludge'. This has to be treated before we can recycle it to farmland.
We use large tanks (known as digesters) where bacteria break the sludge down and release methane gas. We collect this and use it to generate electricity.
Bacteria removal
In some cases there is an extra stage of treatment. Whenever we're returning the effluent to an environment where algae can easily grow, we remove the nutrients that encourage such growth.
Unwanted bacteria and viruses are sterilised using ultraviolet light.