Higher and foundation tiers

Water treatment

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 and the steps listed 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 local rivers.

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. In these FOGG channels the fat, oil, grease and grit are removed. The waste water enters grit chambers where the heavier particles such as grit or silt settle to the bottom of the channel and are removed. After this, the waste water moves to a separate tank where the fats, oils, and grease (FOG) that float on top of the waste water and are skimmed off and also removed.


3. Next the waste water enters a sedimentation tank where heavier solids, also known as 'primary sludge,' settle to the bottom of the tank. The effluent (partially treated water) then moves on to the next stage, the biological treatment stage.


4. Next, the effluent from the sedimentation tanks now enters the aeration tanks or for smaller treatment works a trickling filters system is commonly used, here a large revolving arm will slowly spray the effluent onto a large pebbles or plastic beads, which are covered in aerobic bacteria. In either method the bacteria present use the oxygen present in a process called aerobic respiration to 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 any threat to the aquatic organisms living there.


5. The sludge from the sedimentation process contains mostly organic matter and many harmful bacteria. This semi-solid slug is fed into large anaerobic digesters. These digesters contains many bacteria which will feed on the sludge and break it down, this process can take up to 2 weeks. The digester operates at a temperature of 37⁰C 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.


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.

Specialised cleaning

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