The average single person in the UK uses around 149 litres of
water per day, while an average household uses about 349 litres of water.
This includes water used for such things as: washing, cooking, cleaning, bathing and flushing toilets.
Once used this clean potable water becomes waste water or sewage and can no longer be safely released directly into rivers or the sea without further treatment.
Waste water from homes contains a mixture of sewage, bacteria, soap, detergents, fats, food waste and other dissolved substances.
However this is not the only source of waste water that needs to be treated before it can be safely released back into rivers or the sea; other sources include:
Factories and industrial processes which can produce large volumes of
waste water or effluents that may contain
pollutants such as
heavy metals, oils, dyes and other harmful chemicals.
Rainwater runoff from streets can carry soil, litter and other
pollutants such as fuel, oil or brake dust, soil and twigs into the drains and sewers.
Another large source of waste water is the runoff from farms and agriculture; this can include
water which has been contaminated by fertilisers, herbicides, pesticides and animal waste.
All of this contaminated water must be collected and treated carefully before it can safely be released back into the
environment. The images below summarise
the common areas in which water can be contaminated with various
pollutants.
Waste water treatment
The main aims of treating waste water at the water works are to remove any solid materials present, reduce the levels of harmful microorganisms
and lower the amount of organic matter present, that is material from living organisms and includes food waste, human and animal waste (sewage) and other substances that contain the element carbon that can be broken down by microorganisms.
The bacteria present in untreated sewage would, if it was released into rivers and lakes, simply use up large amounts of the dissolved oxygen in the rivers and lakes as they decompose the organic material present.
This reduction in oxygen levels would have devastating effects on the aquatic life in the rivers and lakes.
Without proper treatment, untreated sewage would quickly pollute rivers, kill aquatic organisms and spread harmful diseases such as cholera and typhoid.
Modern water treatment works combine several stages, including screening (using metal screens or grids of various sizes), sedimentation, and biological breakdown using microorganisms, along with the safe handling of the waste sludge and methane gas produced as by-products of waste water treatment.
Some of the methane gas from this process can even be used as a renewable fuel to power equipment at the works.
By following these steps, the water that leaves a treatment plant is clean enough to return safely to the natural environment without harming people or wildlife.
The image below, along with the description under it, outlines the main steps that take place at a typical water treatment works.
Waste water treatment
The image below outlines the main steps that take place in treating waste water before it is discharged back into rivers, lakes or the sea.
The image summarises the main steps that take place at the waste water treatment works but it should be noted that there are variations on the process outlined and different sites may operate slightly differently depending on their size and location.
The waste water enters the treatment works through the main sewer.
Here it passes through a series of metal grids or screens of various sizes to remove large objects such as
plastic waste, litter, condoms, nappies and other materials which should not really be flushed into the drains.
Next the waste water enters grit and settling channels (shown in the first image opposite). In the grit chambers, heavier
particles such as grit and silt settle to the bottom of the channel and are removed. The waste water
then moves to a separate tank where fats, oils and grease (FOG) that float on the surface are skimmed off and removed.
Next the waste water enters a sedimentation tank (shown in the second image opposite) where heavier solids, also known as
“primary sludge”, settle to the bottom of the tank. The effluent (partially treated water)
overflows from this sedimentation tank and then moves on to the next stage, the biological treatment stage.
Next, the effluent from the sedimentation tanks enters the
aeration tanks. In smaller treatment works, a trickling filter system may be used instead.
In an aeration tank, air is bubbled through the effluent. In a trickling filter, a large revolving arm
slowly sprays the effluent onto a bed of pebbles or plastic beads which are covered in
aerobic bacteria. In either method the
bacteria use the oxygen present and in a process called
aerobic respiration break down most of the remaining organic compounds in the
effluent. After treatment the water
is now clean enough to be fed back into a river, where it should not pose a threat to the aquatic organisms living there.
The sludge from the sedimentation process contains mostly organic matter and many harmful
bacteria. This semi-solid sludge is
fed into large anaerobic digesters. These digesters contain many
bacteria which feed on the sludge and break it down. This process can take up to
2 weeks. The digester operates at about 37 °C in an oxygen-free atmosphere to ensure optimum conditions for the microorganisms
present to break down the sludge. Methane gas is produced as a by-product from this
anaerobic digestion, and it is used as a power
source to operate much of the machinery at the water treatment works.
The sludge leaves the anaerobic digester and enters a dryer or a centrifuge, where up to 95% of the water is removed. The dried sludge
can be used in agriculture as a soil conditioner or fertiliser.
Self-check
Complete the activity below by simply clicking and placing the "Step cards" in the correct order in the left-hand column.
Press the check answers button when you're done; clicking on a step card will place it back in the left-hand column.
Ordering activity – stages in treating waste water
Click a step card on the left, then click a numbered box on the right to place it in order.
You can click a filled box to remove a step and place it somewhere else. When you think you are done,
click Check order.
Step cards
Put the steps in the correct order
1.
2.
3.
4.
5.
6.
Specialised cleaning
Factories and industries may produce waste water or
effluents which contain pollutants such as heavy metals,
oils, other organic substances and microorganisms from specialised industries such as meat processing. This
water may need additional steps to remove these pollutants before being discharged into a river.
This could include precipitation reactions to remove heavy metals followed by additional filtration.
It could also include treatment with chlorine or UV radiation to kill any microorganisms present,
or the use of membranes to remove certain pollutants.
Self-check
Which statement below is incorrect?
Spot the problem
Spot the problem – waste water treatment
One of the statements below about waste water treatment is incorrect.
Read them carefully and choose the statement that shows something being
done wrongly.
Key points
Key words – waste water treatment
Waste water / sewage – water from homes, toilets, sinks and industry that needs to be cleaned.
Effluent – partially treated water that flows from one stage of treatment to the next.
Sludge – semi-solid material that settles out during sedimentation.
Aerobic bacteria – microorganisms that use oxygen to break down organic matter.
Anaerobic digestion – breakdown of sludge by bacteria in the absence of oxygen, producing methane.
Biogas – a mixture of gases, mainly methane, produced from anaerobic digestion.
Waste water from homes and industry must be treated before it can be safely released back into rivers, lakes or the sea.
Screening removes large solids such as plastics, nappies, litter, rubbish and other debris that should not be flushed into drains.
Settling channels allow grit, silt and grease to separate from the waste water before further treatment.
In sedimentation tanks, heavier solids sink to form sludge while the liquid effluent moves on for biological treatment.
In aeration tanks or trickling filters, aerobic bacteria use oxygen to break down remaining organic material in the effluent.
Sludge from sedimentation is treated in anaerobic digesters where bacteria produce methane gas and reduce harmful microbes.
The partially digested sludge is dried, removing most of the water, and the solid residue can be used as a soil conditioner or fertiliser.
Cleaned water is discharged back into rivers where it is safe for aquatic life.
Some industrial waste water requires extra treatment to remove heavy metals, oils or harmful microorganisms before discharge.
and lower the amount of organic matter present, that is material from living organisms and includes food waste, human and animal waste (sewage) and other substances that contain the element carbon that can be broken down by microorganisms.
The bacteria present in untreated sewage would, if it was released into rivers and lakes, simply use up large amounts of the dissolved oxygen in the rivers and lakes as they decompose the organic material present.
This reduction in oxygen levels would have devastating effects on the aquatic life in the rivers and lakes.
Without proper treatment, untreated sewage would quickly pollute rivers, kill aquatic organisms and spread harmful diseases such as cholera and typhoid.
