In the UK each of us uses an average of about 150 litres of
water per day. Most of it is taken by rivers, but before it
can be piped to our taps, it needs to go through a multistage
treatment
process. Rainfall is seasonal and increasingly unpredictable. So we often use storage
reservoirs to make sure there is always a supply for treatment.
1st stage: Screening (13.09 minute)
By passing water through mesh screens, we remove sticks, water weeds and other large
foreign objects. The air takes some odours and gas from the water and allows some
dissolved metal salts to separate and to be filtered out. We add air by cascading the water
down a tower, spraying it into the air or bubbling air through it. At this stage there are still
some fine matter suspended in the water, along with colour and bacteria.
2nd stage: Coagulation and Flocculation (13.30 minute)
We add a precisely controlled amount of a coagulant, like Ferric or Aluminium Sulphate. This
reacts with the material in water making it all stick together to form what is known as floc.
To make this flocculation happen, water and the coagulant have to be mixed together very
quickly and thoroughly in a special device called a flash-mixer.
3rd stage: Clarification or Sedimentation (14.41 minute)
Next comes the clarification process, where the floc forms itself into sludge and is separated
in a specially designed tank called a clarifier. This sludge layer is called the sludge blanket; to
control the blanket, sludge is periodically drained off, concentrated and removed for safe
disposal. Once the sediment sinks to the bottom of the tank, it has to be removed or it will
build up and cause problems at the next step of the treatment process. Some sediment
basins have cone shaped bottoms, where the sediment is collected. Others have bottoms
that slope to one end or have squeegee baffles that very slightly scrape the bottom
collecting the sediment at one end. The sediment is then pumped out into a sludge basin.
Factors affecting good sedimentation are surface area of the basin and the time. Time can
be extended within the settlement basin with bubbling that makes the water flow very
slowly in a zigzag way from one side of the basin to the other.