Chemistry Reference
In-Depth Information
1
water passed through screen
covered
storage
tank
2
aluminium sulfate added
water
in
pump
sulfur
dioxide
added
screen
3
coarse
sand filter
to
homes
and
factories
pump
sodium
hydroxide
added
4
sedimentation
tank
5
fine sand
filter
6
chlorine added
Figure 11.39
The processes involved in water treatment.
Sewage treatment
After we have used water, it must be treated again
before it can be returned to rivers, lakes and seas.
This multi-stage process known as sewage treatment
is shown in Figure 11.41.
6
Finally, a little chlorine gas is added, which
sterilises the water and kills any remaining bacteria.
Excess chlorine can be removed by the addition
of sulfur dioxide gas. The addition of chlorine gas
makes the water more acidic and so appropriate
amounts of sodium hydroxide solution are added.
Fluoride is sometimes added to water if there
is insufficient occurring naturally, as it helps to
prevent tooth decay.
1
screens
sew
age
2
settlement tank
The 'iron problem'
If the acidity level of the treated water is not
controlled, problems occur due to the precipitation
of iron(
iii
) hydroxide. These include:
●
vegetables turning brown
●
tea having an inky appearance and a bitter taste
●
clothes showing rusty stains after washing
(Figure 11.40).
5
sludge
(for either dumping
or conversion to fertiliser)
and methane gas
3
trickling filter
gravel
4
treated water is chlorinated
and returned to the river
Figure 11.41
The processes involved in sewage treatment.
Used water, sewage, contains waste products such
as human waste and washing-up debris as well as
everything else that we put down a drain or sink.
The processes that are involved in its treatment are
as follows.
1
Large screens remove large pieces of rubbish.
2
Sand and grit are separated in large sedimentation
tanks. The process is speeded up by adding
aluminium sulfate, which helps the solids to
Figure 11.40
The rusty stains on this pillowcase are due to iron (
iii
)
compounds in the water.
