Higher and foundation tier
The average single person in the UK uses 149 litres of water per day with an average household using 349 litres
per day. This includes the water used for such things as showers, baths, toilets and from appliances such as washing
machines and dishwashers. All this waste water must be treated before it is discharged back into rivers. The diagram
below gives an outlines of how the waste water from homes and industry is treated to make it clean enough to be feed back
into a river.
Factories and industries may produce waste water or effluents which contains certain pollutants. This could include
be heavy metals, oils or other organic substances and micro-organisms from industries e.g meat producers . This water may need
additional steps to remove these pollutants before the water is discharged into a river. This could include
precipitation reactions to remove heavy metals followed by additional filtration. It could include treatment with chlorine or UV radiation to kill micro-organisms or the use of membranes to remove certain pollutants.
- The waste water enters the treatment works through the main sewer. Here it passes through a series of metal grids or
screen to remove large objects such as plastic waste, condoms, nappies and other materials which should not really be
flushed down drains.
- Next the waste water enters a series of channels, sometimes called FOGG channels, fat, oil, grease and grit are
removed here. The grit or silt settles on the bottom of the channel and is removed, the fats, oil and grease float on
top of the waste water and they are also removed.
- Next the water enters a sedimentation tank, here the organic matter, includes raw sewage sinks to the bottom of the
tank and forms a sludge. The effluent (dirty water) leaves the sedimentation tank and enters the aeration tanks.
- Next the water from the sedimentation tanks enter another tank which contains large pebbles or even small plastic
beads, these are covered in bacteria. The bacteria are fed a constant supply of air and undergo aerobic respiration.
These bacteria fed on and remove most the remaining organic compounds and anaerobic bacteria in the effluent. The water
is now clean enough to be fed back into a river, where it should not pose any threat to the marine organisms living there.
- The sludge from the sedimentation contains mostly organic matter and many harmful bacteria. This semi-solid slug is
fed into an anaerobic digester. This contains many bacteria which will feed on the sludge. This process can take up to
2 weeks, the digester operates at 370C in an oxygen free atmosphere to ensure optimum conditions for the micro-organisms
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 machinery at the water treatment works.
- The sludge leaves the anaerobic digester and enters a drier where up to 95% of the water is removed. The dried sludge
is used in agriculture as a soil conditioner or fertiliser.