3. The exposure of water in divided currents to the air by passing it through a sieve has been proposed as a means of purifying water, but it is inefficient when trusted to alone. Plants in reservoirs help to absorb organic matter; and fish, by destroying small crustaceans, have been found useful. Hard waters do not bear exposure to light, as a thick green growth of chara occurs, which may block pipes, and give a bitter taste to the water.
4. The Addition of Chemical Substances.—(1) Clarke’s process consists in adding milk of lime, i.e. an emulsion of quicklime with water, to the water in the reservoir on a large scale. By this means calcium carbonate is precipitated, but no effect is produced on calcium and magnesium sulphates and chlorides. The hardness of the Thames water can thus be reduced from 16° to 3° or 4° (Clarke’s scale). The calcium carbonate carries down with it suspended and possibly dissolved organic matter. In the Porter-Clarke process lime-water, i.e. milk of lime diluted, and the excess of lime separated by settlement or filtration, is mixed with the water to be purified, the water being freed from the precipitated calcium carbonate either by subsidence or by being forced through a filter of stretched canvas.
(2) Carbonate of Soda added to boiling water throws down calcium carbonate, and possibly lead if present. Much less is required when added to boiling than to cold water. Maignen’s process consists in adding anti-calcaire powder, containing chiefly carbonate of soda, lime, and alum.
(3) Aluminous salts are very effectual in removing suspended organic matter, if the water contains calcium carbonate. On the addition of alum, calcium sulphate and aluminium hydrate are formed, both of which fall to the bottom, carrying with them other impurities. The amount of alum required is about 6 grains per gallon of water. If the water is not hard, a little calcium chloride and carbonate of soda should be put in before the alum is added, in order that a precipitable substance may be formed.
(4) Potassium permanganate readily removes the offensive smell of stagnant water, but it gives a yellow tint to the water. The addition of a little alum will help to carry down the decomposed permanganate.
(5) Perchloride of Iron, in the proportion of 2½ grains to a gallon of water, has been found to completely purify water from finely suspended organic matters and clay.
(6) More recently, other substances, such as iodine and hyposulphite of soda, have been recommended. These are supposed to act by sterilizing the water, and iodine in suitable quantities undoubtedly effects this.
Chemical processes for the purification of water, with the exception of the softening process, are not to be recommended for general use. Efficient filtration, or boiling, is safer than chemical treatment; and it would only be justifiable to trust to the latter, when, as in a military campaign, an attempt at purification was necessary, and no means were available for filtering or boiling water.
7. Filtration.—The object of filtration is to remove the impurities of water. The most dangerous impurities are suspended in it, especially the microbes causing infectious diseases. Hence the most perfect filter is the one which most completely prevents the passage through it of microbes. If the water supply is pure, domestic filtration is not only useless, but likely to do more harm than good. This is true for such upland surface waters as those supplied to Liverpool, Glasgow, and Manchester; for such deep well-water supplies as those of Brighton (deep chalk), of Nottingham (new red sandstone), and others, when pumped from wells remote from inhabited houses. For upland surface waters known to attack lead pipes, filtration through charcoal or spongy iron may be advisable; for river water, filtration through a germ-proof filter is best.
Filtration on a large scale is generally carried on as follows:—A preliminary step consists in collecting the water into settling reservoirs, wherein the more bulky substances subside. The water is then filtered through beds of gravel and sand, containing perforated tubular drains below, into which the filtered water flows. The drains are covered by a bed of gravel about 3 feet deep, over which is spread a layer of sand about 1½ to 2 feet deep. Sharp angular particles of sand are the best; and the gravel should gradually increase in its coarseness as it descends.