Floc floating at the surface of a basin
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Clarifies the water causing a precipitate to form in the water which can be removed
using simple physical methods
Flocculation is a process which clarifies the water.
Clarifying means removing
any turbidity or colour so that the water is clear and colourless.
Clarification is done by causing
a precipitate to form in the water which can be removed using simple physical
methods.
Initially the precipitate forms
as very small particles but as the water is gently stirred, these particles
stick together to form bigger particles - this process is called flocculation.
Many of the small particles that
were originally present in the raw water adsorb onto the surface of these small
precipitate particles and so get incorporated into the larger particles that
coagulation produces.
In this way the coagulated
precipitate takes most of the suspended matter out of the water and is then
filtered off, generally by passing the mixture through a coarse sand filter or
sometimes through a mixture of sand and granulated anthracite (high
carbon and low volatiles coal).
Coagulants / flocculating agents that may be used
include:
Iron (III)
hydroxide. This is
formed by adding a solution of an iron (III) compound such as iron(III)
chloride to pre-treated water with a pH of 7 or greater.
Iron (III) hydroxide is extremely
insoluble and forms even at a pH as low as 7.
Commercial formulations of iron
salts were traditionally marketed in the UK under the name Cuprus.
Aluminium
hydroxide is also
widely used as the flocculating precipitate although there have been concerns
about possible health impacts and mis-handling led to a severe poisoning
incident in 1988 at Camelford in south-west UK when the
coagulant was introduced directly into the holding reservoir of final treated
water.
PolyDADMAC is an artificially
produced polymer and is one of a class of synthetic polymers that are
now widely used.
These polymers have a high
molecular weight and form very stable and readily removed flocs, but tend
to be more expensive in use compared to inorganic materials. The
materials can also be biodegradable.
Mechanical system to push floc out of the water basin |
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