SEWAGE PURIFICATION 289 



proportion between the easily oxidised matter and the more 

 difficultly oxidisable substances, or, in other words, while the 

 oxygen absorbed in four hours by the so-called ' four hours' 

 test ' may not greatly decrease, the oxygen absorbed in three 

 minutes will probably increase. 



Whether there is an increase or decrease in colloidal matter 

 depends upon circumstances. With highly concentrated 

 sewages such, e.g., as are met with under conditions of very 

 limited water supply, actual solution of colloidal matter un- 

 doubtedly takes place, owing to the breaking down, e.g., of 

 albuminoid substances into amino acids ; on the other hand, 

 with ordinary town sewage, it may readily be the case that 

 the colloidal substances in the effluent from the tank increase, 

 owing to the washing out of colloids from the sludge present 

 in the tank. In considering the changes taking place in 

 anaerobic tanks, the exact conditions of the installation must 

 always be carefully borne in mind. 



It was at one time considered essential that anaerobic 

 tanks should be closed, in order to prevent access of hght and 

 air ; numerous experiments have shown that this condition 

 is not necessary. It is obvious that beneath the immediate 

 surface of the sewage in the tank the conditions must be anae- 

 robic, and the covering of tanks is only called for by reasons 

 of sighthness, or to render more permanent the scum, which 

 generally forms owing to the throwing up of soUd matter by 

 the gases evolved during the fermentation of the sludge. A 

 cover, of course, is necessary if these gases are to be collected 

 and utilised as in the case quoted, but is of httle use for the 

 prevention of nuisance unless escaping gases are collected and 

 passed through a washing tower, e.g., of coke, down which a 

 spray of water passes. 



(c) Aerobic Decomposition.— In the early experiments of 

 Mr. Dibdin at Sutton in Surrey, during the later nineties, the 

 attempt was made directly to treat crude sewage on a coarse 



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