The introduction of a public waterworks almost invariably leads to a diminished death-rate from zymotic disease, and could the purity of the supply be maintained by nitration the health of the community would be permanently benefited. But, as has been shown, the sources of pollution are manifold and increasing. With the increase of population, the growth of manufactories, and the crowding of houses in the vicinity of storage reservoirs and their feeders, filtration and aeration become indispensable.

Again, it is becoming more and more recognized that streams receiving sewage are not purified, no matter how ample their volume or how rapid their flow. Chemical tests alone cannot be taken as a proof of purification. The poison of typhoid has been conveyed twenty-five miles by a river, and communicated to forty hospital patients who drank its waters. To quote from a high authority (Mass. State Board of Health, 1876): "If sewage contains the germs of disease, whatever they may be, no agency at present known, except a sufficiently high temperature, will effectually destroy them".Hence it is desirable, as Parry, one of the best English authorities, says, that filtration should be performed wholesale by the public authorities, rather than to leave it to individuals. Thus rich and poor alike are benefited, and it will not be necessary to trust to cheap and worthless appliances left in charge of careless domestics. Many towns and water companies filter their water by passing it through beds of broken stone, gravel, sand, charcoal or other material. These are often very extensive and costly, notably those of the London water company and at Berlin. The filter beds at Poughkeepsie cost over $75,000 for the plant alone.