Theory Of Filtration

Water for municipal supply may be classed, according to the source from which it is obtained, as surface waters or as ground waters. Waters obtained from streams, rivers, lakes, or impounding reservoirs are surface waters; generally such waters are soft and when filtered are the best kind of waters for both domestic and for industrial purposes. As surface water exists in nature, however, it is never bacterially pure and is seldom clear; it generally carries considerable matter both in suspension and in solution and sometimes is contaminated by specific germs of disease. The amount of suspended matter in surface water varies considerably, being greatest after heavy rains which wash the finely divided soil and earth down into streams, lakes and reservoirs. Water that contains large quantities of matter in suspension is unsuitable for domestic and for most industrial purposes and should be filtered before using.

Filtration is both a straining and a biological process in which most of the suspended matter and part of the hardness, color, and organic matter in raw water are removed. This is effected by passing the raw water through a thick bed of fine sand that is covered by a still finer jellylike layer which entangles and holds any suspended matter brought in contact with it. The efficiency of a filter depends largely on this jelly-like layer and a filter is not at its best until a suitable layer has formed. Under ordinary conditions to naturally form such a layer would take about twenty days, and to obviate such delay and bring a filter to its full bacterial efficiency in from twenty to thirty minutes, coagulants are used to artificially produce the jelly layer.

The coagulants generally used are sulphate of alumina (common alum) and sulphate of iron. When sulphate of alumina is added to water it decomposes into its component parts, sulphuric acid and alumina; the sulphuric acid combines with lime, magnesia, or any other base present in the water, while the alumina forms a flaky precipitate that gathers together and holds whatever suspended matter it encounters, thus forming in a few hours a layer that without the use of coagulant would require weeks to form. The thicker the layer of sediment, the greater the bacterial efficiency of a filter, but usually after from twelve to twenty-four hours' operation, the sediment layer becomes so thick that sufficient water cannot pass through, and the filter bed must then be cleaned.