Page 262 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Filtration

Filtration is an important process that removes turbidity and algae from the
raw, untreated water. There are many different types of filter, including screens,
gravel filters, slow sand filters, rapid sand filters and pressure filters. We will
concentrate on three of these:

1 slow sand filters
2 rapid sand filters
3 pressure filters.

The difference between these types is not just a matter of the speed of the
filtration process, but the underlying principle of the method. Slow sand filtration
is a biological process and rapid sand filtration is a physical treatment process.

Slow sand filters
These are often preceded by micro-straining or coarse filtration. These filters
are used primarily to remove micro-organisms, algae and turbidity. It is a slow
but very reliable method of water treatment, often suited to small supplies,
provided that there is sufficient area to properly construct the filtration tanks.
Slow sand filters consist of tanks containing sand with a size range of 0.15 mm to
0.30 mm, and to a depth of around 0.5 m to 1.5 m. For single dwellings, circular
modular units, usually used in tandem, are available. These have a diameter of
around 1.25 m. As the raw water flows downwards through the sand, micro-
organisms and turbidity are removed by a simple filtration process in the top
few centimetres of sand. Eventually, a biological layer of sludge develops, which
is extremely effective at removing micro-organisms in the water. This layer of
sludge is known as the ‘schmutzdecke’. The treated water is then collected in
underdrains and pipework at the bottom of the tank. The schmutzdecke will
require removing at periods of between 2 and 10 weeks as the filtration process
slows. The use of tandem filters means that one filter can remain in service while
the other is cleaned and time allowed for the schmutzdecke to re-establish.
Slow sand filters should be sized to deliver between 0.1 m and 0.3 m of water
3
3
for every 1 m of filter per hour.
2
Bellmouth
Float-operated valve

Inlet

Schmutzdecke

Fine sand

Outlet

Perforated support tiles or underdrains Gravel
p Figure 5.9 Slow sand filter
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