BACTERIAL PURIFICATION 



271 



1. The nitrate has developed with extraordinary rapidity. 



2. The formation of nitrite is much less marked ; it rapidly 

 reaches a maximum and then declines. 



3. The free ammonia has been almost completely oxidized ; 

 at the same time it was noticed that the original yellowish 

 colour, black suspended matter and sewage odour had dis- 

 appeared.^ 



The following figures give the oxygen relations which I found 

 for the first and last trays : — 



Parts per 100,000. 



The organic matter therefore had been very rapidly reduced, 

 and the effluent was in a state of active natural purification by 

 means of its available oxygen. When allowed to stand, the 

 oxygen of the nitrate is utilized for the burning up of organic 

 matter, provided the latter has been properly fermented as in this 

 case, and the efiluent can be finally purified by a denitrifying 

 bed. 



In the trays described above the quantity of available oxygen 

 is obviously far greater than would be supplied by any process 

 of mere aeration, hence such an effluent can be easily " finished " 

 by a fine filter without fouling the latter, or can be beneficially 

 applied to a small area of land, or mixed with a river of moderate 

 volume not only without pollution, but possibly with an actual 

 benefit to the stream. 



The principle of dividing the bacteria into separate zones, 

 where each class can naturally choose its own habitat and work 

 successively to others, is paralleled by the rotation of crops. Of 

 the antagonism and symbiosis of bacteria we have already 

 spoken in Chapter V. 



1 It is stated that " by transposing the trays so as to upset the natural survival 

 of organisms in the sequence, the whole process was arrested, a high-coloured 

 and inferior effluent being the result, and one or two days were required to 

 re-establish the conditions that had been disturbed." 



