MODERN METHODS OF WATER PoRIFKATIOX. 339. 



turbidity, and after the coagulation sand tilters were used. 

 Also the results are interesting. In one report by the Analyst 

 of Bolton the amount of purification effected by this mechanical 

 filtration reached lOO per cent, of suspended matters, but this 

 is, doubtless, exceptional. As in many places the authorities 

 have been highly satisfied with the results of the change from 

 sand filters to mechanical, and the price of this filtration is 

 half of that of sand filters. 



On account of the fact that I had been very favourably im- 

 pressed by what I had read and heard of the Candy filters, I 

 took special interest in this form of filtration, and in seeing as 

 much as I could of the working. The Candy filter makes no 

 use of added chemical coagulant, and in this way differs from 

 the others previously discussed. The Candy filters are steel 

 cylinders of various sizes, and made to resist any desired pres- 

 sure, as this filter is a " pressure " filter. It is also called an 

 " oxidising " filter. Sand, grit and pebbles, all specially 

 selected and graded, are used as its filtering medium, in addi- 

 tion to the special material called "oxidium." which last is 

 Candy's patent. The depth and layers of the filtering mediai 

 varied in different installations which I saw. In Newport they 

 were using 6 feet; in other places I visited I found 5 feet thick- 

 ness. In some of the installations polarite is still used; in 

 others this is supplanted by " oxidium." Polarite and oxidium 

 are similar in appearance, but are said to differ, oxidium being' 

 harder and giving better purification results. Dr. Thresh 

 shows a comparative purification of polluted water by the 

 polarite and oxidium as follows : — 



Parts per 100,100, 



Sample of Filtered through Filtered through- 

 Product Waste Polarite Oxidium 

 Free ammonia q-qioo 0-0050 o-ooo8 

 Organic ammonia o-oii6 00056 o-ooi6 



These results are quoted by the Candy people to show that 

 oxidium has much greater purifying power than polarite, and 

 rest on the authority of an eminent expert. The " oxidium " 

 I saw looked very like the polarite I saw, and I thought they 

 were the same, but we all — even analysts — come under the 

 limitation " humanum est errare." 



There is an impression current that the Candy filters have 

 no sand in their filtering layers; this is quite inaccurate, as any- 

 one could state who has seen the inside of a Candy filter, or 

 the materials with which it is filled. The filtering materials " 

 consist of specially selected and graded silicious sand, grit, 

 pebbles and oxidium, but the aeration and the oxidium are the 

 points specially insisted on in that type of filter as the powerful 

 purifying agents. It is claimed that the pressure compresses 

 atmospheric oxygen into the water and into the pores of the 

 polarite in the older installations, and in the new form of 

 Candy the oxidium, which is said to be an indestructible, 

 porous, silicious material resembling spongy platinum. The 

 arrangement of the layers is not always alike; in some there- 

 are coarse layers in the upper part of the filter, which act as- 



