Higher and foundation tiers

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 for 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.

1. The waste water enters the treatment works through the main sewer. Here it passes through a series of metal grids or screens to remove large objects such as plastic waste, condoms, nappies and other materials which should not really be flushed down into the drains.
2. Next the waste water enters a series of settling 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 skimmed off and removed.
3. Next the water enters a sedimentation tank; here the organic matter including raw sewage sinks to the bottom of the tank and forms a sludge. The effluent (dirty water) leaves the sedimentation tank as shown in the image above and enters the aeration tanks.
4. Next the water from the sedimentation tanks enter another tank which contains large pebbles or even small plastic beads which are covered in bacteria. The bacteria are fed a constant supply of air and undergo aerobic respiration. These bacteria fed on and break up most the remaining organic compounds and also kill the harmful 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.
5. 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.
6. The sludge leaves the anaerobic digester and enters a drier where up to 95% of the water is removed. The dried sludge can be used in agriculture as a soil conditioner or fertiliser.

Next