Filtration processes are used principally for the removal of particulate material in water including clays and silts, micro-organisms and precipitates of organics and metal ions. The process of filtration involves passing water, containing some physical impurity, through a granular bed of media at a relatively slow velocity. The media retains most of the contaminants whilst allowing the water to flow. The particles that are removed are typically much smaller (0.1-50 m) than the size of the filter media (500-2000 m) such that virtually no simple straining occurs and removal is based on particles colliding and sticking to filter grains as the water flows past.
Filtration through granular media, particularly sand, is one of the oldest and most widely used water treatment processes. Early reported applications of the concept include the Romans in 150 AD and the Italians in 1685. However, its first reported use as a municipal sand filter dates back to 1804 when a slow sand filter was installed in Paisley in the UK. In 1827, Robert Thom was granted a patent for a filter with a false floor and cleaning by backwash, and in 1829 the engineer James Simpson installed a similar design, which required scrapping rather than backwashing of the so called English filter, in Chelsea, UK. The immediate benefits in terms of control of waterborne diseases such as cholera and typhoid led to the wide-scale uptake of the technology across the UK. In the late half of the nineteenth century the need for large quantities of safe drinking water in the USA led to the development of coarser, more rapid filters. The first filters were designed by trial and error and although Darcy published his work in 1856 it was not until 1937 that the Kozeny-Carman equation, which describes flow through the bed, first appeared. Consequently, filters were designed using 'rules of thumb' developed from the early empirical designs and, although research into granular bed filters continues today, Introduction to Potable Water Treatment Processes Simon A. Parsons and Bruce Jefferson