GENERAL FUNCTIONS OF MICROBES 17 



In six weeks loo grams of egg-white leave only one gram of 

 residue, whereas in stagnant sewage there remain 76 grams, 

 and in running water 83 grams. In three weeks meat loses 

 almost 50 per cent, of its weight, and in six weeks 96 per cent. 

 The body of an animal, immersed in it is for long protected 

 by the layer of subcutaneous fat ; but cleaned cartilage and 

 tendon lose in five weeks 65 to 99 per cent, of their weight ; 

 even wool and feathers decompose. 



As regards hydrocarbons, the fats are slowly split into fatty 

 acids and glycerine. Cabbages and potatoes are almost 

 completely destroyed in six weeks. A hempen rope which, 

 after five weeks of immersion in stagnant sewage or running 

 water, could still bear the weight of 1 2 kilograms, broke under 

 15 grams after the same period of immersion in the septic 

 tank. After three weeks newspaper begins to dissohe libera- 

 ting bubbles of gas. It is quite wrong to consider the septic 

 tank as operating like a simple settlement tank : it is rather a 

 sort of crucible in which the powerful microbes melt and 

 disintegrate the most resistant organic matter. The septic 

 tank liberates various gases, methane, hydrogen, nitrogen and 

 carbonic acid, a cubic metre of sewage furnishing from 40 to 

 70 litres of gas. 



The aerobic phase of the purification is accomplished by 

 bacterial beds, into which the organic matter, already dissolved 

 and transformed into ammoniacal compounds is discharged. 

 What takes place in them is an intense nitrification carried 

 on by the same microbes which in arable soil transform the 

 ammonia into mineral salts, nitrites and nitrates. 



The bacterial bed consists of a thick layer of clinker or slag, 

 and is filled with sewage for periods of an hour and a half to 

 two hours separated by intervals of four to six hours. Contact 

 may be renewed if necessary two or three times by passing the 

 effluent through a second or third bacterial bed. During 

 contact the organic matter fixes itself on the clinker, while 

 during the aeration period it is oxidised by the ferments which 

 take up the necessary oxygen from the air. 



The nitrification is balanced in the bacterial beds by a 



c 



