172 BACTERIAL TREATMENT OF SEWAGE 



passed through a settling tank prior to its being brought on to the 

 contact beds. In short, the method resolved itself into one of an 

 open septic tank and multiple contact following the settlement or 

 screening out of grosser solids. The results surpassed even the most 

 sanguine expectations, even though the beds were filled four times 

 daily. If the two methods namely, the closed septic tank and 

 single contact, and the open tank and multiple contact are com- 

 pared, it is the opinion of the three experts named that " where both 

 systems are dealing with the same volume of sewage on the same 

 area, the advantage as regards efficiency belongs indisputably to the 

 double contact system." Boyce subsequently confirmed this conclusion 

 in favour of a combination of the anaerobic and aerobic processes, 

 provided that the septic process was perfected, and the suspended 

 sludge did not pass over on to the beds.* 



Before summarising the main conclusions which may now be 

 legitimately drawn from the Manchester experiments, a word or two 

 may be said concerning the characters of an efficient bacteria bed 

 and the management of storm-waters in sewage treatment. 



The material, or filtrant, of which the bed is composed may vary 

 within wide limits. Burnt clay, coke, clinkers, cinders, or various 

 forms of gravel may all be efficient, provided there is ample aeration 

 and porosity. The required organisms exist, of course, mainly in the 

 sewage, but they require abundant oxygen in order to perform their 

 function. To assist in maintaining this aeration the surface should 

 be raked over from time to time. It has been suggested that in 

 times of frost a layer of ice would prevent the action of the bed. 

 But in point of fact such obstruction would rarely occur, the 

 temperature of the sewage being sufficient both to prevent such 

 stoppage of the beds and also to maintain the necessary activity of 

 the bacteria, which, as we have already seen, require for their vitality 

 nutriment, oxygen, moisture, and a favourable temperature. From 

 December to April the average daily temperature of the Manchester 

 sewage was 55*5 F., whilst the average temperature of the sur- 

 rounding air was 45 "3 F. Hence the ice difficulty is naturally 

 overcome. Another point of importance in connection with aeration 

 is the allowance of sufficiently frequent and prolonged periods of 

 rest. Without such intervals the beds would of course become 

 clogged, and eventually inactive, because lacking in aerobic bacteria. 

 Though not absolutely a character of the beds, there is one further 

 point always to be borne in mind in securing their efficiency. It 

 is, that the sewage being applied to the bed should be as far as 

 possible uniform in consistence and freed from suspended matters 

 by sedimentation. Any suspended matter not so removed should be 

 retained as far as possible on the surface of the bed. 



* Royal Commission on Sewage Disposal, 1902, p. 11. 



