With increased cost of Jand, construction and labour, "rapid" filth ers have been introduced; these filters are of less area than the slow sand beds, but filter more quickly and, consequently, require more frequent cleaning. This is effected by washing the sand in situ, the usual practice being to pass clear water in a reverse direction through the sand. In certain types of rapid filters, compressed air is employed to agitate the sand; in other types, revolving rakes are used. Rapid filters may be either "gravity", ¢.c., enclosed type: the latter are usually open-topped types, or "pressure"
adopted when it is desirable to place the filter on the pressure main A filter of this type is now at work at Chinkiang.
The process of filtration not only removes sediment but also a considerable number of bacteria, but an additiona] safeguard to the consumer is now afforded by sterilization. By far the greater number of water-borne bacteria are harmless, but a polluted water may contain bacteria capable of producing disease, chief among which is the type hoid bacillus, and it is the aim of the water engineer to eliminate these. An easily identified group of bacilli known as Bacillus Colt, though not in themselves dangerous, are indicative of sewage pollution and the possible presence of pathos genic organisms. It has also been proved that pathogenic organisms are more easily destroyed than those of the B. Coli group and, hence, a substantial reduction of the latter means a practical elimination of the former.
METHODS OF STERILIZATION, Various methods are available for the destruction of bacteria—boiling, which is impracticable on a large scale; exposure to ultra-violet rays; contact with ozonized air, both of which have presented difficulties when large quantities of water have had to be dealt with; and chlorination, The latter method is that most usually adopted. In- Shanghai, sterilization is effected by chlorination, bleaching powder being originally employed, but this has now been replaced by liquid chlorine. The does employed is extremely small, not exceeding one part in three million. With reliable chlorine control apparatus and skilled supervision, sterilization is an additional safeguard which no modern purification plant should be without.
There is a popular prejudice against putting chemicals' in water, largely due to ignorance of the Tact that natural water usually contains matters as truly chemical as those used in treating water and also that the amounts used are 80 minute that a consumer would need to drink hundreds of gallons to obtain a medicinal dose. The purim fication processes now in use in Shanghai-give a reduction in_bactoria cultivated at 37 degrees Centigrade in 99.83 per cent. and B. Coli absent in 100c.c. in 75 per cent of the samples taken. 7 The paper was accompanied by diagrams and photographs of the plant employed and various samples were exhibited.
Text read from the printed page and cleaned; old type leaves the occasional error.