WATER 559 



Unfortunately the mud and slime of tropical rivers rapidly stops 

 the action of the best filters so that constant cleansing is necessary. 

 They should be scrubbed and boiled once weekly. 



The candles are very frail and readily break. A large stock of 

 them must be kept. 



Other filters are not considered to be as efficacious. 



A Domestic Filtration. 



A cemented reservoir is prepared, 5 to 6 feet deep (i to 2 metres) 

 and made up in layers of broken stones and pebbles, coarse gravel, 

 coarse sand and fine sand. Water should not flow through the filter 

 at more than 4 inches (100 mm.) an hour, and not more than 200,000 

 gallons per acre per hour. The finer the sand and the thicker the 

 laver the more efficacious it becomes. 



A sand filter affords : — 



(i) Mechanical obstruction to coarse particles. 



(2) Oxidation of organic matter. 



(3) Nitrification by micro-organisms on the surface layer. 



(4) Removal of turbidity. 



(5) Reduction of hardness and removal of iron. 



There is a slimy layer on the surface of all large filters containing 

 a gelatinous mass of intercepted bacilli, streptococci, micrococci, alga^ 

 and other bodies, while immediately below this film is a layer of 

 nitrifving organisms. By this film the pathogenic micro-organisms 

 are intercepted and destroyed, the organic matter broken up into car- 

 bonic acid and ammonia, the ammonia being converted into nitrous 

 and nitric acids bv the nitrifying organisms. When the slimy layer 

 becomes so thick that it prevents the water from passing through, 

 half an inch should be scraped off the surface and the under layer 

 distributed by means of a rake. When the sand layer is only one 

 inch thick, the bed should be made up again. The first water which 

 passes through before the slimy layer is formed (during the first 

 twentv-four hours) may be impure and should be rejected. 



Allow three feet (one metre) of water to stand in the filter for 

 twenty-four hours. 



There should not be more than 100 micro-organisms per c.c. in 

 the filtered water. 98 to 99 per cent, of those in the original water 

 should have been removed by the filter. 



Too much exposure to the sun favours the growth of algc-e, hence 

 filter beds should be covered over. By this means dust and mosquito 

 breeding will be avoided. 



Water should pass through a settling or storage tank before and 

 after filtration. The processes of filtration and sedimentation should 

 alwavs be combined. 



