SEPTIC TANK METHOD 165 



offensive, and would rapidly lead to deoxygenation of the river water 

 if it were allowed to pass into the river. The above percentage 

 removal (51-3) was effected by coke-beds varying from 4 to 6 feet in 

 depth. A similar bed, 13 feet in depth, has proved more efficient, 

 and has for some time produced a percentage purification of 64 per 

 cent., while an old bed, 6 feet in depth, has given a percentage 

 purification of 86 per cent. A repetition of the treatment of the 

 effluent in a second similar coke-bed has produced an additional 

 purification of 19'3 per cent., giving a total purification of 70"6 per 

 cent. Effluents from chemical treatment would show a total 

 purification of under 20 per cent. It should be noted that the above 

 purification is reckoned on the dissolved impurity of the sewage; 

 the suspended solid matter is not taken into account. The bacterio- 

 logical condition of the effluent corresponds in the main with that 

 of the raw sewage. The total number of bacteria undergoes some 

 reduction in the coke-beds, but the different kinds of bacteria which 

 were present in the sewage are still represented in the effluent. 



From these and many other similar experiments, it has come to 

 be understood that the bacterial purification depends, as we have 

 seen, upon bwo main groups of organisms, namely, those that are 

 able to break down and liquefy solid organic matter, and those that 

 deal with it when in solution. Of the former group, some act best 

 under anaerobic conditions. N"o strict line of demarcation can be 

 drawn as to where one group begins and the other absolutely ends. 

 It is a complex co-operation, shared in by a large variety of 

 organisms classified roughly into these two groups. Systems may 

 be introduced in which the anaerobes are encouraged (as at Exeter), 

 or systems may be established on an aerobic basis (as at Sutton and 

 Manchester). Hence it may be accepted as finally settled that the 

 . bacterial treatment may be mainly an anaerobic one (Cameron, Scott- 

 Moncrieff, and others), or mainly an aerobic one (Dibdin, Fowler, and 

 others), or a mixture of the two. Whatever system is used, the two 

 great agencies of breaking down and oxidation must be allowed 

 ample opportunity. Probably we shall most clearly recount the 

 application of these principles by considering in some detail two 

 examples of the two typical methods of bacterial treatment. These 

 two examples are furnished in Cameron's Septic Tank Installation 

 (anaerobic), and at the Davyhulme Works, Manchester, in the 

 Multiple Contact Bacteria-ieds (aerobic). 



1. Septic Tank and Cultivation-bed Method (Cameron).— This 

 method has been adopted at Exeter and other places. The plant is 

 twofold, namely, a septic tank and several cultivation or bacteria beds. 



The septic tank is a large underground vault constructed of 

 concrete, cemented on exposed surfaces, and having a capacity of 

 thousands of gallons, according to the population. That at Exeter 



