342 MODERN METHODS OF WATER PURIFICATION. 



filtered water. One of the advantages of the Jewell system 

 with the Weston Controller is that the rate of filtration can be 

 changed so that you can treble the filter flow in a few minutes, 

 e.g., from 62,500,000 to 185,000,000 per diem per acre. 



At Newport Mr. Haynes, the Borough and Water Engineer, 

 enabled me to visit the installation at Pantyreos. Here, as 

 elsewhere, polarite had first been used and later oxidium, and 

 one gathered that the polarite had required renewal from time 

 to time, but that the oxidium had the advantage over polarite 

 that it was harder, and therefore less liable to break down and 

 wash away. I visited other places in South Wales and else- 

 where, where I was able to gather a good deal of information 

 about these and other " mechanical " filters (although the 

 Candy is not so much a mechanical as a " pressure " filter), 

 but I have said enough to show that all these modern English 

 filters, with water suited to them, are giving satisfaction. I 

 found other systems of filtration and combinations of systems, 

 such as the Howard filter, ripening" of slow sand filter beds 

 with alum, a method which had suggested itself to me and 

 which we had found useful in Bloemfontein to " ripen " filters 

 previously to my seeing it in England ; also combination of 

 coagulation tanks for sedimentation and sand beds, also pre- 

 filters, or roughing filters, and double filtration arrangements 

 in various places on the Continent. 



Now, to sum up and strike a balance between the various 

 filtration systems, it is necessary to consider carefully what is 

 required at present and what should be advised for the require- 

 ments of the future if the population of Bloemfontein expands 

 and the water supply needs to be increased. Prof. W. R. 

 Smith, in his paper on " Modern Filtration Methods," quotes 

 the data of yields of different filters tabulated as follows : — 



I — According to German standards, sometimes called Koch and Piefke's, 

 a Sand Filter should be worked to yield (nearly) 2 gallons per square 

 foot per hour. 



2 — Harder worked Sand Filters 3 to 6 gallons per square foot per hour. 



3 — Bell's Filters . . .. . . 70 ,, , 



4 — Jewell Filters . . 80 to 100 



5 — Candy's Filters . . 100 to 200 ,, , 



6 — Mather and Platts' Filters 300 



That is how quantity works out. What about quality? How 

 does the bacterial efficiency work out ? What is the efficiency 

 relationship of the methods? and, a long way behind the 

 efficiency, what is the cost. There are no two opinions upon the 

 subject of the efficiency of sand filters after subsidence of the 

 water in basins without coagulation, or, as we call it. chemical 

 precipitation, in waters which are free from turbidity or rela- 

 tively so. A sand filter in good working condition should give 

 a reduction of bacteria of 98 to 99 per cent., and in specially 

 favourable circumstances 99'5. and even a higher fraction mav 

 be attained for a time now and then. Theoretically, a water 

 could by special sand and specially slow filtering reach a com- 

 plete purification, but that is not in practice. These results 

 with sand filters on water with only natural subsidences are 

 possible in a large proportion of waters, particularly in many 



