Page 263 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 5 Cold water systems

Rapid (gravity) sand filters KEY TERM
Rapid sand filters are predominantly used to remove the floc from coagulated Floc: a collection of
water, but they can also be used to successfully remove algae, iron, manganese loosely bound particles or
and water turbidity from raw water. materials. These are bound
together by the coagulation
Rapid sand filters are usually constructed from rectangular tanks containing process for easy removal
coarse silica sand with a size range of 0.5 mm to 1 mm laid to a depth of from the water.
between 0.6 m and 1 m. As the water flows downwards through the filter,
the solids remain in the upper part of the sand bed where they become
concentrated. The treated water collects at the bottom of the filter and
flows through nozzles in the floor. The accumulated solids are removed either
manually every 24 hours or automatically when the head loss reaches a
predetermined level. This is achieved by backwashing.
A variety of proprietary units are available containing filtering media of
different types and sizes. In some filters, the water flows upwards, improving
the efficiency.

Inlet

Filter media

Backwash waste

Filtered water

Gravel
Wash water and air supply

p Figure 5.10 Rapid gravity filter

Pressure filters
These are sometimes used where it is important to maintain a head of pressure
to remove the need to pump the water into the supply. The filter bed is enclosed
in a cylindrical pressure vessel. Some small pressure filters are capable of
delivering as much as 15 m /h. The cylinder is typically made of specially coated
3
steel, and smaller units can be manufactured from glass-reinforced plastic. They
operate in a similar way to the rapid sand filter.

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