FILTRATION OF WATER.

New Zealand Times, Volume XXXI, Issue 4893, 27 November 1876, Page 3

FILTRATION OF WATER.

(From Chambers Journal.) ‘ The impurities of water are of two kinds—. chemical and mechanical, the former consisting of those pernicious substances which it holds in solution, the latter of the solid particles rendering it muddy or milky. So far as we are. aware the Wdstmere Lake contains no chemical, but it has been said that it is sometimes impregnated with mechanical impurities. We’ do not vouch for the accuracy of the latter statement, but if it be true, there is a remedy —filtration, which is that known as the Lancashire Ellter, which is thus described by Professor Clark : - -

“Supposing you have a flat horizontal sur-’ face to form the bottom of the filter—that is, puddled ; above this you spread gravel, and, in general, very large stones. In’ the space between these stones you have' the water received, and passing out by means of little tunnels near the bottom. These are formed in a variety of ways. A very good method of forming them has been recently practised, and answers very well—the using simply of agricultural drain-tiles.*- Those little tunnels are for letting off the filtered water all around them. If you have large stones, the interstices between those stones constitute a receptacle for bolding the filtered water. Above the large stones you havelarge gravel, thensmallergravel, till you come to sand. The whole of the cleansing part of the filter consists of sand. This is of a 1 larger grain than the common sea-sand, except such as we see in rocky districts at the mouth of rivers on the shore—large grained sand of a uniform size. The filter, I think, may be worked so low as four inches of the hand ; I think it is constructed at about fourteen inches. From four inches to fourteen inches is a’workable depth of sand. The solid matter intercepted does not penetrate perhaps so much as a quarter of an inch, so that by removing a very small film from the surface you get a clear filter. This removal is performed by a workman from time to time. I think that this process of filtration is efficacious in removing mechanical impurities to an extent that could scarcely bo believed without seeing the process. What dirty water is thus filtered, and used in some of the first manufactories of calico printers, where one would think good water' was at least very desirable, would not have been believed by me to be possible if observation had not made me familiar with the fact. Cleaning the filter is a matter of very small expense in a large manufactory, neither is the structure of the filter expensive. What is scraped off the top is set aside, and at the end of such a period as a year is washed and put back again on tlm surface of the filter, so that no renewal of fresh sand is necessary. Such is an outline of the Lancashire filter. ■

“ In preparing the filter great cafe is requisite in washing the sand, so as thoroughly to remove all fine particles of clay or mud, and all matters liable to disintegrate and yield impalpable powder in the process of filtering. The washing is performed by exposing the sand to a stream of water sufficient to carry of the lighter, matters without sweeping away the silicious particles. The test of the sand being thoroughly washed is to put a quantity of it into a tumbler of clear water ; and if the sand, after being stirred up, falls down and leaves the water as transparent as before, it may be considered perfectly clean. Should the sand not have attained -this degree of purity, or should any milkiness appear in the tumbler, the fine matter remaining will have the effect of rendering the filter inert. “ Of all the filters that have been tried,

none as yet succeeded so well as the plaiti sand filter. It is easily cleaned, and requires only the addition of fresh sand at intervals of time, A sand filter has been in operation at the Chelsea waterworks for twenty years. “The expense of filtering water on the large scale for a town or manufacturing supply is found to be at the rate of one penny for 3000 gallons. A consumption of 100 gallons a day might be filtered for a shilling a year. Considering the great improvement that all surface and river waters undergo by filtration, and the muddiness that such waters are liable to especially after rains, they ought always to tie subjected to the filtering process.”