The first documented use of sand filters to purify the water supply dates to 1804, when the owner of a bleachery in Paisley, Scotland, John Gibb, installed an experimental filter created by engineer Robert Thom, selling his unwanted surplus to the public..
A slow sand filter consists basically of three different layers within a filter box.
These layers are from bottom to top: the underdrainage system, the gravel layer and the sand.
It is only the sand which plays any part in the treatment process.
The gravel layer Is arranged in four graded levels..
Rapid sand filters were first developed in the 1890s, and improved designs were developed by the 1920s.
The first modern rapid sand filtration plant was designed and built by George W.
Fuller in Little Falls, New Jersey..
Monitoring and Operation Requirements A slow sand filter must be cleaned when the fine sand becomes clogged, which is measured by the head loss.
The length of time between cleanings can range from several weeks to a year, depending on the raw water quality..
It will remove practically all the turbidity from water, together with virtually all harmful eggs, protozoa, bacteria and viruses without the addition of chemicals and may frequently be constructed largely with local materials.
A slow sand filter consists basically of three different layers within a filter box.
It is estimated that there are over 500,000 people in developing countries currently using slow-sand filters [ 16 ].
Slow-sand water filters have many advantages:
Slow sand filter design Slow sand filtration is a simple and effective technique for purifying surface water.
It will remove practically all the turbidity from water, together with virtually all harmful eggs, protozoa, bacteria and viruses without the addition of chemicals and may frequently be constructed largely with local materials.
Assuming a typical filter depth of 1 m and porosity of 0.5, the estimated hydraulic retention time for a slow sand filter would be 1.25 to 5 h. However considering the suggested 1 to 1.5 m of standing water above the sand layer , the HRT ranges from 2.5 to 12.5 h.
