328 MODERN METHODS OF WATER PURIFICATION. 



ments of the water an adequate coagulant, and allowing- it to 

 act in coagulating basins, the water is filtered rapidly thiough 

 specially-prepared filtering material and strainers ^^uth vari- 

 ous mechanical appliances for securing removal of the impuri- 

 ties caught in the filtering material by agitation of he sand 

 and washing oli the dirt which runs away m -^^^tteis placed 

 in the plant? I saw in America about 12 inches above the sur- 



^^The^fference 'in rate between the English (slow sand) and 

 the American (rapid mechanical) filtration methods is easiest 

 understood by saying that the rate of the former woiked at 

 S"e German standard%et by Koch is 2,500,000 gallons per acre 

 ner day and the American is at the rate of a normal ot 

 ?2 ,000,000 gallons per acre daily, and a " high of 185.000 000 

 o-aions per acre daily. Expressed as ''area'; differences it 

 ^vould read that the 'English slow sand titration requires 4 

 Ifour-tenths) to 'S (one half) of an acre foi hhiation ot 

 I 000,000 gallons of water daily, and the American does this 

 Work as -normal," as for instance at the works at Little halls 

 Uirough a fiher the area of which is 24 feet by 15 feet or pii 

 in measure of time to make it clearer, the American filter can 

 filter within one-third (-'normal" 2 gallons per square foot 

 per minute, normal English 3 gallons per square foot pei 

 hour) of as much water in a minute per square foot as the 

 English slow sand does in an hour and can do it as well, m he 

 case of a turbid water as the Modder River is, provided it has 

 been preceded bx adequate and properly-carried-out coagula- 

 tion, iccording to the opinion of some leading American au- 

 thorities most familiar with its use. But careful contiol is 



Filtering installations of this type are giving complete satis- 

 faction in Alexandria and other places in Egypt with the water 

 of the Nile, and at Delagoa with the water of the Lmbeloos, 

 to mention two places in Africa. At Alexandria m particular 

 some work which has become classical was done by two Ger- 

 man Professors, Drs. Bitter and Gottschhch. the former en- 

 o-ao-ed in Public Health work at Cairo and the latter at Alexan- 

 dria in investigating the efficiency of the rapid American filtra- 

 tion at the fii^st small installation, which has now been much 

 expanded. The Jewell Filter Company, I was informed, were 

 prepared to take the financial risk of the first installation till 

 it should have proved successful. The work was done under 

 what was almost hourly bacteriological observation at first 

 and the report is of deep interest to all who have to do with 

 water supply. The observers approached this system with its 

 easy working and regulators with the correct sceptic attitude 

 which should be adopted in face of so revolutionary a differ- 

 ence in filtering rate. There are considerable difference, 

 between the analyses of our water and the Nile water at the 

 season I visited the works, but it is a turbid river like ours, 

 though huge in volume, and liable to contamination similar 

 to ot^rs. though the pollution, great as it is, is diluted by the 

 laro-e volume of water. Their experiments were varied and 

 numerous before the new filter won their admiration and 

 approval. 



