62 The Filtration Works for the Trnprovement of the 



from the destruction of such matter. Now, by the ammonia 

 process the organic matter in a given quantity of the water is 

 destroyed, and the resultant ammonia is measured. The 

 ammonia is thus an index or measure of the organic matter 

 present in the water. It may be asked, however, why we get 

 the ammonia under the two headings of free and albuminoid. 

 Well, the answer to this is very simple. Some of the organic 

 matter may have already undergone decomposition, producing 

 ammonia. This ready-made ammonia is called free ammonia. 

 Then, there is some organic matter in the water which has 

 not yet decomposed. Vegetable matter, for instance, takes 

 more time to decompose than matter of animal origin. This 

 the analyst destroys by distillation, and obtains the resultant 

 ammonia, and the latter is called albuminoid ammonia. I 

 must not, however, pursue this part of the case further. I 

 merely wish to show that the organic matter in the water is in 

 most cases the dangerous element, and the ammonia process 

 professes to indicate how the water stands in this respect. 



The Water Commissioners, after a full examination of nearly 

 all the waterworks filters in the United Kingdom, determined 

 to follow the same lines as London, Dublin, Bradford, Edin- 

 burgh, Liverpool, and a number of smaller towns. In these 

 places the water authorities have adopted vertical filtration 

 through a layer of sand, resting on layers of gravel and 

 broken stones. The total thickness of the filtering materials 

 varies from four to eight feet. As will be readily understood, 

 the rate of filtration has a good deal to do with the success of 

 the process. Thus, if we pass through a filter a given quantity 

 of water in a day, it will be seen that the resultant water will 

 not be so clear and good as if a period of two days was allowed 

 for the operation. The speed of filtration is generally expressed 

 as so many gallons per twenty-four hours per square yard of 

 filter surface. In Edinburgh the rate is about i ,000 gallons per 

 square yard of sand surface in the day. Throughout England 

 the rate varies from 1,000 down to 500 gallons, and in the new 

 works about to be described a speed of 500 gallons per square 



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