Water Purification

Excess Lime Process.

In 1912 a new method of purification called the excess lime process was described. In softening waters lime is added in amount equal to, or just short of, what is necessary to combine with the dissolved carbonic acid in the water and with the bicarbonates. Carbonate of lime is formed, which, being practically insoluble, is thrown down as a precipitate. This mechanically purifies the water to a considerable extent, but does not produce a true bactericidal effect. In the excess lime method slightly more lime is added, so as to leave the water caustically alkaline, and this produces a marked bactericidal action. The amount of excess necessary depends on the duration of con tact and the amount of impurities in the water, but one part of lime (as Ca0) per i00,000 parts of water is usually sufficient.

The following quotation may serve to illustrate the usefulness of the excess lime method : . . . the experiments carried out by us during the last two years at the Langford experimental station of the Southend Water Co., using water from two comparatively small rivers, show that by acting on Sir A. Houston's suggestion to use excess lime better results can be obtained than by the use of chlorine, since chlorine does not remove any of the organic matter in solution, whereas the excess lime will remove at least so% of this. Waters, therefore, which a few years ago would have been considered quite unsuitable for a supply, and which no system of purification then known would have rendered safe, can now be utilized.

The method proved highly successful (1917) at Accra (Gold Coast Colony) in dealing with an impure swamp water used for water works purposes. Its successful use in 1913 enabled Aber deen to decide on the retention of the River Dee as a source of water supply, thus saving over .£100,000 on alternative schemes. In 1914 it was shown at Sunbury that the raw River Thames could be purified to a wonderful extent. For example, the ten worst samples of the river water and of the outlet from the first tank gave, on the average, colour estimations of 155 and 37 respectively, a reduction of 76%. B. coli was found to be absent from io,000 c.c. of the treated water, on ten separate days.

The method has disadvantages : for example, the cost, the diffi culties attendant upon the neutralization of the excess of caustic alkalinity and the problem of disposing of lime sludge in the case of hard waters. On the other hand, the advantages are consider able. Questions of taste are eliminated. Hard waters may be

softened and soft waters hardened by the process. Hard impure waters are not only softened and rendered safe bacteriologi cally but improved greatly, as judged by physical and chemical standards.

Water Tests.

The physical, chemical and bacteriological tests used in the examination of waters have not altered very materially during recent years. Physically, observations are made of the colour (using a colour meter), turbidity (gravimetrically, or with some form of turbidimeter) and opacity (as shown, for example, by passing a beam of light through the water), etc. Chemically, the chief tests are still for ammoniacal, albuminoid and oxidized nitrogen, chlorides, oxygen absorbed from permanganate, hard ness, etc., although new tests, like the determination of the hydro gen-ion concentration, are being used extensively. Bacteriologi cally, the number of bacteria (especially at 37° C) and the B. coli test still hold the field. In connection with the latter test, a vast amount of work has been done, but we are still uncertain what significance should be attached to the various races of B. coli encountered in water analyses. The tendency is to insist on the possession of, say, two positive attributes (lactose+indol+) which characterize human faecal microbes and then judge the matter on a quantitative basis. On the whole, the attempts to differentiate between B. coli of human intestinal origin and those derived from the lower animals, fish and birds have been most disappointing. The modern teaching is that as it is economically possible, by ade quate purification processes, to eliminate (or nearly so) all lactose+indol+B. coli," this standard should always be aimed at. Speaking generally, nearly all are agreed that there should be no B. coli of the kind referred to in ioo c.c. of water in more than half of the samples examined.

The interpretation of results has altered considerably during recent years, especially in those cases where chlorination is prac tised. The tendency is rather in the direction of condoning certain imperfections of quality (e.g., the amount of organic matter present) provided that B. coli is killed. It is considered that if B coli is destroyed the occurrence of epidemic water-borne dis eases is impossible and that therefore other inferential indices of safety lose much of their significance. But those who cling to past traditions regard chlorination as a short cut to apparent rather than real purity.