330 MODERN METHODS OF WATER PCRIFICATIOX. 



and the quantity of wash-water suppHed. Some of the plants 

 I saw were being" superintended by young graduate engineers, 

 with sufiiicient knowledge of chemistry and bacteriology to 

 carry out the simple routine chemical and bacteriological tests. 

 Geo. C. \\' hippie suggests that all students in water engineer- 

 ing should have this instruction, so as to fit them to operate 

 filters as well as design them. 



The important question is what is the relative bacterial puri- 

 fication efficiency of the slow sand as against the rapid mechani- 

 cal methods. Can the latter stand on the same footing", 

 sanitarily considered, as the long-tried, old-fashioned sand 

 filters ? For water such as ours is for most of the year Sanna's 

 Post and Mazel's Poort have both taught that without chemical 

 coagulation, i.e.. using them as English filters, the simple 

 settling" without chemicals and slow sand filtering resulted in 

 an impure water owing to the suspended clay clogging" the 

 filters, and secondary to that by heavy head letting" the tur- 

 bidity, clay, and germs through into the water pipes. In 

 general, with a suitable water and slow filtration, sand filtering" 

 has a very high standard of removal of bacteria by the reten- 

 tion of the bacteria in the surface layers or " schmutzdecke " 

 of the filter. I notice in the writings of some of the American 

 modern authorities that in their opinion the importance of the 

 "biological "' working of the upper layer has been much over- 

 estimated, and is regarded as somewhat of a "shibboleth." 



Where one is making theoretical experiments you can get 

 the water almost sterile by prolonged standing in settling pools 

 and slow enough sand filtration with proper pressure and 

 grading" of suitable filtering" material, but that is not practical 

 nor practicable, and with us and with our water it is impossible, 

 because of the nature of the water. That has been demon- 

 strated. Added to that we have never had anything" like ideal 

 sand: it is within the easy recollection of us all that "boil 

 your water for safety " has been advertised for years, and how 

 even in a white enamelled bath one could frequently not see 

 through as little as two inches of water when turbidity was 

 high, due to the slow sand filters being pressed, clogged and 

 overcome by the turbid uncoagulated water that was quite 

 unfit to be put on to the filters, and the filters were incapable 

 of dealing with it. as the four-figure average of bacteria per 

 c.c. of previous years proves. Even in 1905 the average num- 

 ber was over i.ooo bacteria per c.c. with the new waterworks 

 installed and in ^vorking order. 



Coagulation has come to stay, and is proving itself valuable 

 and more and more reliable. Both the " life " of the filters 

 and the service done by them is greatly increased by the com- 

 paratively clarified water with which they now deal, and they 

 have given excellent results, and have an increased capacity 

 on account of the improved water. I pointed this out in reply 

 to an expert opinion expressed that danger was to be feared 

 on account of pressing inadecjuacy of filter accommodation. 

 That opinion was incorrect and rested on the belief that the 

 occurrence of B. Coli on certain occasions or during a period 

 was due to filter deficiency. That that was not so was demon- 



