Many authorities consider that mechanical filters are not so effective in removing bacteria as the slow sand filters, but when the source of supply is unexceptionable epidemiologically this is a matter of small importance. The point is that by the use of a coagulant they can achieve results, as regards clarification, prac tically impossible in the case of slow sand filters. As regards action on lead, this is a serious matter, as lead is a cumulative poison. Fortunately, it is possible by the use of lime, in suitable doses, to render such waters absolutely safe for domestic
Turning now to more modern methods of purification, the chlorination of water at Lincoln during the typhoid epidemic (about i,000 cases and ioo deaths) there in 1905 marked a for ward step to which special attention must now be directed.
In the United States and Canada chlorination, for some years past, has been the rule, not the exception. In England, progress has been much slower, but the World War greatly altered things, thanks to the initiative of men like Hor rocks and Sims Woodhead. Further, the bold step taken by the Metropolitan Water Board in chlorinating so huge a volume of water as over io0,000,000 gal. a day proved a great incentive to further investigation of this water purification.
The "chlorinators" plead that inasmuch as they destroy practically all the non-sporing bacteria of intestinal origin they incidentally kill all the microbes, associated with epidemic water borne disease. It is a bold claim, but apparently sound on the basis of current knowledge. The "anti-chlorinators" urge that a "doped" water is prima facie open to condemnation, and assert that there have been conspicuous failures in the chlorination processes in the past due to break-downs in the plant, or to dan gerous compromises between doses sufficient to sterilize the water and yet small enough to avoid taste troubles. They further claim that natural processes (e.g., storage and slow sand filtration) remove at least 98% of the total number of bacteria in the original water of whatever sort they may be, and that prolonged experience has shown that this is an absolute protection from the diseases associated with the ingestion of impure water.
A third school claims that all purification processes are merely a retrograde movement—that safety lies alone in choosing vir gin, uncontaminated sources of water supply, requiring no sort of purification. It is possible to sympathize strongly with this exalted attitude of mind, but practically we must face, to an increasing extent, the necessity of rendering impure waters safe for domestic use. As regards the "chlorinators" and the "anti chlorinators," there is much to be said on both sides, and each case should be judged on its own merits. Chlorination is an exceedingly cheap process ; less than one shilling per 1,000,000 gal. of water treated may suffice. It is disputed how the chlorine acts. Some say it has merely an oxidation effect, others claim that it has an intrinsic bactericidal action. The dose is usually from 0.25 to 0.5 of available chlorine per 1,000,000 (2.5 to 5 lb. of chlorine
per 1,000,000 gallons). The time required for sterilization varies according to the dose and the quality of the water. A few minutes may suffice, but one to five hours or more should be aimed at.
The chlorine can be administered as a soluble hypochlorite (e.g., alkaline sodium hypochlorite), or as a solution of bleaching powder (chloride of lime), or as a solution made from the gas liquefied in and liberated from metal cylinders. The latter process is now most extensively used, and there are some highly ingenious forms of apparatus for measuring accu rately the gas as it flows from the cylinders through the chlorine apparatus on its way to the vessels or towers used for its final solution. Whatever method is adopted, it is highly important that the mixture of the chlorine and the water to be treated should be rapid and complete.
The success of the treatment is determined by the destruction of B. coli, a non-sporing excremental microbe, slightly more hardy than the typhoid bacillus and the cholera vibrio. It is also gauged by the circumstance that there are places where the incidence of water-borne diseases has been modified to a most gratifying extent since, and apparently as a result of, the intro duction of chlorination processes.
Taste troubles have been a most serious factor in the problem. Frequently, in consequence, the dose has been reduced below the limits of safe sterilization. Recently, however, knowledge has increased by leaps and bounds. It is now known that the presence of certain bodies (e.g., phenoloid sub stances) in excessively minute amount (less than 1 in 1,000 mil lions) may be the root of the trouble. These impurities may arise from atmospheric contaminations, or be conveyed by liquid pollu tions (e.g., washings from roads, etc.). Fortunately, valuable remedies (taste preventers) have been found, e.g., potassium per manganate and ammonia (dose about 0.2 per 1,000,000 the latter in terms of nitrogen). Even the organic matter, naturally present in waters, is a taste preventer, or "remover" of real value. There is no reliable indication that chlorinated water has any deleterious effect on man, the lower animals, fish-life or horticultural operations. On the whole, the same may be said as regards its alleged injurious effect on metals. Chlorination is a factor of great importance in water purification, although this admission may be coloured with certain cautious limitations. It can increase the margin of safety and can bring almost any water to any pitch of epidemiological perfection required. In the language of "the man in the street," chlorine, in doses of 0.25 to 0.5 per 1,000,000, can render dan gerous waters safe without giving them (at all events in con junction with taste removers) any unpleasant taste, or conferring on them any undesirable characteristics. Beyond all question, chlorination has come to stay, although it may be wise to regard it as a most valuable adjunct to other purification processes rather than as an absolute panacea.