3i6 SEWAGE AND ITS PURIFICATION 



untreated suspended matter to the outlet. Mr. Martin shows 

 that at the Barrhead Works the velocity of flow in the tanks 

 with three times the volume of the dry-weather flow passing 

 through them only amounts to i^ in. per minute. With six 

 times the dry weather flow, therefore, the velocity does not 

 exceed 3I in. per minute, which is so slow a rate as to be power- 

 less to disturb solid matter in the tank. Martin's arrangement 

 of weir and regulated flow by modules is shown in Fig. 58. 



In bacterial treatment, whether by tanks, filters, or land, it is 

 known that the resolving organisms are most active, especially 

 in the final or oxidizing stage, when the liquid is of moderate 

 strength, since undue concentration retards the growths, while 

 too rapid a flow is liable to wash them away. Chemical pro- 

 cesses of precipitation or sterilization on the other hand find 

 concentration an advantage, so much so that some of them, as 

 for instance the Liernur process, require the aid of evaporation. 

 It is clear, therefore, that in both cases heavy dilution is to be 

 avoided, if possible, for other reasons than that it increases the 

 size of the works. Modern practice suggests the importance 

 of making the sewers watertight, and of disposing separately of 

 subsoil water, which is usually innocent of contamination. 



The conclusions, therefore, are : — 



1. Subsoil and deep drainage water can as a rule be separately 

 pumped and discharged, if the sewers are properly constructed. 

 The relief that may result to a system of sewage treatment has 

 been shown in a recent instance at Rathmines, Dublin, where 

 the normal sewage of 2J million gallons per day is swelled to 

 6 milHons by subsoil drainage. A similar observation has been 

 made in other towns. 



2. Road washings are dangerous, and especially under the 

 separate system should be sterilized in situ by sprinkling the 

 surface with effective disinfectants. 



3. Storm water should be impounded and stored or other- 

 wise treated, and must never be allowed to pass in large 

 volumes directly into a stream. The larger the volume of 

 liquid the more dilute it will generally be, and proportionately 

 so much less of a disinfectant will be required. 



Position of Outfalls. 



This is not always a matter of choice, and is affected by 

 questions of cost and engineering. Among the varieties of 

 final discharge are : — 



