XLVIII. J. M. SMAIL. 



6. When the effluent from a filter does not correspond 

 with the hygienic requirements it must not be used, unless 

 the cause of the unsatisfactory working has already been 

 removed during the period covered by the bacterial examin- 

 ations. In case a filter, for more than a very short time, 

 yields a poor effluent, it is not to be used until the cause 

 of the trouble is found and corrected. It is, however, 

 recognised from past experience that sometimes unavoid- 

 able conditions (high water, etc.) render it impossible, from 

 an engineering standpoint, to secure an effluent of the 

 standard quality. In such cases it will be necessary to use 

 a poorer quality of water ; but, at the same time, it such 

 conditions arise as outbreaks of epidemics, suitable notice 

 should be given of the condition of the water. 



7. Every single filter must be so built that, when an 

 inferior effluent results which does not conform to the 

 standard, it can be disconnected from the pure water pipes, 

 and the filtrate allowed to run to waste. This wasting 

 should, as a rule, take place, so far as the arrangement of 

 the works will permit, (1) immediately after scraping filter; 

 and (2) after replacing the sand to the original depth. The 

 superintendent must himself judge, from previous experi- 

 ence acquired by the continual bacteriological examinations, 

 whether it is necessary to waste the water after these 

 operations, and if so, how long a time will probably elapse 

 before the water reaches the standard purity. 



8. The best sand filtration requires a liberal area of filter 

 surface, allowing plenty of reserve, to secure under all 

 local conditions, a moderate rate of filtration adapted to 

 the character of the raw water. 



9. Every single filter shall be independently regulated, 

 and the rate of filtration, loss of head, and character of 

 the effluent shall be known. Also each filter shall, by 

 itself, be capable of being completely emptied, and, after 





