52 



SEWAGE AND ITS PURIFICATION 



The last column illustrates one advantage of stating the 

 results in parts per 100,000. As a gallon of water is 10 pounds, 

 they are translated at once into pounds per million gallons by 

 multiplying by 100. 



Incubator Tests. — From the very beginning it has been known 

 that a water which was good and sweet would " keep," and 

 that another which was bad in origin would " foul." C. Heisch, 

 in his " sugar test " of 1870, added pure cane-sugar to the 

 sample, and exposed it to sunlight, noticing growths and odours 

 developed. The incubation test was first developed by Dupre 

 in 1884. He stated in reports to the Local Government Board 

 that if a pure thoroughly aerated water be kept out of contact 

 with air for, say, ten days it will be found to have remained 

 fully aerated. Sewage-polluted water also, when sterilized by 

 heating, remains fully aerated. But if the water contained any 

 impurity capable of combining with oxygen, and also contained 

 living organisms, the amount of aeration would diminish. It 

 was hoped that the degree of diminution would give some 

 measure of the number of organisms present. This was at a 

 time when the number of organisms was more considered than 

 their species, functions, or activity. The process involved 

 determinations of free dissolved oxygen similar to those we 

 have described, and is still of considerable value. 



The fermentation in closed vessels resembles that at the 

 bottom of rivers and of deep ponds. But the dissolved oxygen 

 in saturated water only amounts to about i part by weight in 

 100,000, so could not be sufficient to oxidize the organic matter 

 of mixed liquids which, exposed to the air, would rapidly 

 improve. Thus a town sewage gave the following figures under 

 open and closed conditions, before and after passing through 

 a septic tank : 



Dissolved Oxygen in 

 c.c. PER Litre. 



Kept six days in closed full bottles 

 Exposed to air in a layer i foot deep : 



Twenty-four hours 



Forty-eight hours 



Raw Sewage. 

 .. 0-59 



Tank Effluent. 



0-43 



i-8i 



3-02 



2-o6 

 474 



