HIGH NITRIFICATION OF SEWAGE EFFLUENTS. 



465 



accumulation in the filter itself, and this soon gives rise to trouble, 

 not only by choking it up, but on account of the undigested substances 

 having a tendency to lower its efficiency. 



On a large scale this element of rest in the form of an intermittent 

 discharge can be arranged by altering the rate at which the distributing 

 apparatus passes over any particular spot, but for the purposes of 

 experiment and producing an oxidized effluent upon a scale suitable 

 for use in a garden or under glass, I have devised an apparatus which 

 consists of a tilting apparatus discharging the sewage to be dealt with on 

 an even surface over a perforated tray, and the period between each 

 discharge is regulated by means of weights which are so placed as to 

 control the tilting of the tipper to any periods required. In order to avoid 

 the difficulty of dealing with a very small orifice I have also devised a 

 gauging box, in which the liquid is discharged over a sloping surface, 

 and then into a trough, in such a way that any required quantity may run 

 to waste, and a measured balance be thrown into the apparatus for distri- 

 bution. 



Turning your attention to the drawing (fig. 102), it should be observed 

 that as each of the filtering trays gives a definite result in the form 

 of chemical changes, it follows that the food supply of the organisms 

 must be correspondingly altered, and that there is therefore every reason 

 to believe that the organisms are changed both in character and kind 

 in correspondence with the altered environment which occurs in each 

 successive tray. 



This action, taken as a whole, is known as symbiotic, in which the 

 work of one organism is, as it were, handed over to another to complete. 

 In theory there should be distinct survivals, each produced by natural 

 selection, so that first the organic nitrogen is reduced to nitrogen as 

 ammonia, secondly to nitrogen as nitrites, and last of all to nitrates 

 in combination with a base such as lime or soda, which is the final 

 change from the organic to the original inorganic form. 



The table on the next page shows what actually occurred in the Ashtead 

 experiments. Each line gives the work that has been carried out in each 

 tray ; and, although it is difficult to identify all the organisms that produce 

 the various changes, it is quite certain that the conditions throughout are 

 inimical to the life of the anaerobic and putrefactive organisms that come 

 over from the cultivation tank, and it has been shown that at any rate 

 there is a great diminution in the numbers of the Bacillus coli communis, 

 which is a characteristic organism in crude and septic sewage. 



When the vast numbers and the varieties of the organisms engaged 

 in the work of purification are realized, and the fact that they carry out 

 the most complicated chemical changes, it will be readily understood that 

 the scientific aspects of the problem are quite inexhaustible. From a 

 practical point of view, on the other hand, nothing could be of more 

 primitive simplicity, and all that the gardener is called upon to deal 

 with is a few ordinary wooden boxes placed one above the other with 

 perforated bottoms and filled with coke. In cases where a sufficient 

 fall is available the sewage may be simply strained and discharged into 

 a tank fixed at a level that will allow of its contents passing on to the 

 distributor, or it may be pumped up by hand into a tank at convenient 



VOL. XXXIV. H H 



