WATER PURIFICATION BY STORAGE.
The water examination committee of the Metropolitan water board of London, England, has presented an exhaustive report respecting the work carried out at the board’s laboratory during the twelve months ended March 31 last. The report gave, in considerable detail, the results obtained in the testing of 11,993 samples of London waters, and also dealt at length with the important question of the value of storage. Dr. Houston stated in regard to this question that there was a consensus of opinion among bacteriologists that pathogenic microbes did not multiply in storage reservoirs, but gradually lost their vitality. The time required to effect the destruction of these bacteria was matter for controversy, and although the American authorities considered a few days might suffice, most British bacteriologists placed the limit at a much longer period. The essential point was that each day’s storage made for safety, and that a time arrived when, eventually, all the pathogenic bacteria perished. In such a case, subsequent filtration is required to improve only the chemical and physical qualities of a water already incapable of giving rise to epidemic disease. Until the safety limit had been determined (and it doubtless varied according to the quality of the water and the season of the year), it was obviously desirable to store water for as long a time as was economically possible. In this connection, it might be asked whether storage, if too prolonged, might not actually lead to deterioration in the quality of a water. The fact, however, remained that in relation to danger to health over-storage was believed to be impossible. Undoubtedly from the epidemiological point of view, apart from questions of cost of pumping, the storage reservoirs should be so worked as to give each drop of river water abstracted for waterworks purposes the maximum number of days’ detention in store. Dr. Houston also draws attention to the difference between “active and passive storage” and the great importance of active storage from the epidemiological point of view.