66 BACTERIA IN WATER 



(c) rate of filtration ; (d) the renewal of the filter-heds. After a 

 certain time the filter-bed becomes worn out and inefficient, and at 

 such times renewal is necessary. Not only may the age of the 

 filter act prejudicially, but the extra pressure required will tend to 

 force through it bacteria which ought to have remained in the filter. 



In 1890 a special study of filtration was made by the Massa- 

 chusetts State Board of Health, and in annual reports published 

 from 1890, a number of experiments are recorded which have proved 

 of classic importance, and which should be consulted by the student 

 or practical worker desiring to acquire a thorough grasp of the 

 principles of biological filtration. There it is shown that water can 

 be filtered through sand filters at the rate of 3,000,000 gallons per 

 acre daily and 99'95 per cent, of the bacteria removed. In actual 

 practice it was found that the finer sands were more effective than 

 the coarser, and imder moderate pressure 1 foot of sand was as effec- 

 tive as 5 feet. Over 80 per cent, of the bacteria removed were 

 found in the upper inch of sand and 55 per cent, in the upper 

 quarter inch. If the surface of the filter was scraped, it was shown 

 that an increased number of bacteria passed through the filter, which 

 was therefore much less effectual. Subsequently, Koch emphasised 

 the importance of this vital layer. But it was the Massachusetts 

 Board that first proved by experiment that the oxidation which occurs 

 in a filter-bed was due to the nitrifying organisms in the surface 

 or scum layer. "When nitrification is established in a filter, the rate 

 of filtration within certain limits was found to exert comparatively 

 little effect upon the removal of the organic matter. 



In 1893 Koch brought out his monograph upon Water Filtration 

 and Cholera, and his work had a deservedly great influence upon the 

 whole question. He showed how the careful filtration of water 

 supplied to Altona from the Elbe saved the town from the epidemic 

 of cholera which came upon Hamburg as a result of drinking 

 unfiltered water, although Altona is situated several miles below 

 Hamburg, and its drinking water is taken from the river after it has 

 received the sewage of the latter. 



Now, from his experience of water filtration, Koch arrived at 

 several important conclusions. In the first place, he maintained 

 that the jportion of the filter-hed which really removed micro-organisms 

 effectively was the slimy membranous organic layer upon the surface of 

 the sand. This layer is produced by a deposit from the still unpurified 

 water lying immediately above it. The most vital part of the filter- 

 bed is this organic layer, which, after formation, should not be dis- 

 turbed until it requires removal owing to its impermeability. A 

 filter-bed, as is well-known, consists of, say, 3 feet of sand and 1 

 foot of coarse gravel. The water to be filtered is collected into large 

 reservoirs, where subsidence by gravitation occurs. From thence it 



