The figures given in this table show the effect of chlorination only; no other form of purification was used during the periods given, except at Toronto where a portion of the supply has been subjected to filtration.
It will be seen that since chlorination was adopted the typhoid death rates have been reduced by approximately 50 per cent and that the averages for the period after treatment are almost invariably less than 20 per 100,000, a figure that a few years ago was regarded as satisfactory. The average death rate for the last available year is 11 per 100,000, a result that is even more satisfactory and exceeds the anticipations of the most optimistic of sanitarians.
A portion of the reduction in the typhoid rates is no doubt due to improvements in general sanitary conditions but the reduction is much greater than can be accounted for in that manner alone and in many cases there was a sharp decline immediately following the commencement of chlorination.
In a few instances there is evidence that chlorination has reduced the typhoid rates of cities previously supplied with filtered water. Diagram X, drawn from data supplied by Dr. West, of the Torresdale Filtration Plant, shows the effect of disinfecting the filter effluents at Philadelphia.
During the years 1909-10-11, when practically the whole of the city supply was filtered, the average typhoid death rate was 18, but when the water was also chlorinated, in 1914-15-16, the rate was only 7, a reduction of 61 per cent.
The figures in Table XXXII show that the Torresdale filters, during 1915-16 were unable to adequately purify the water and that chlorination was necessary.
TABLE XXXII.--CHLORINATION OF FILTER EFFLUENTS
----+---------+---------+----------+---------------------------------- | | | | BACTERIA PER CUBIC CENTIMETER. | Oxygen | | +-----------------+---------------- |Consumed.| Colour. |Turbidity.| Untreated. | Treated. | | | +---------+-------+---------+------ | | | |Gelatine.| Agar. |Gelatine.| Agar. ----+---------+---------+----------+---------+-------+---------+------ 1915| 1.70 | 12 | 0.6 | 141 | 30 | 28 | 14 1916| 1.90 | 12 | Nil. | 88 | 23 | 38 | 11 ====+=========+=========+==========+=========+=======+=========+====== | _B. coli communis_ | | Per Cent Positive Tests. | +----------------------+-------------------------+ | Untreated. | Treated. | Added Chlorine +----------+-----------+------------+------------+ Parts Per |10 c.cms. | 1 c.cm. | 10 c.cms. | 1 c.cm. | Million. ----+----------+-----------+------------+------------+---------------- 1915| 66 | 24 | 5 | 0.3 | 0.18 1916| 49 | 16 | 7.4 | 1.9 | 0.15 ----+----------+-----------+------------+------------+----------------