6 SEWER DESIGN 



in excessive amounts, the result is nothing more serious than 

 a temporary inconvenience, and no damage is done, as 

 might be the case were sewers to be gorged with an excess of 

 rain. 



5. Finally, it is said that if the rain-water is kept out of the 

 sewers, periodic flushing, which is of great value, will be lost, 

 and in the case of the street serving as a storm-sewer there 

 will be yards and alleys too low to be drained into it, whereas 

 they could be drained into a storm-water sewer. 



To this answer is made that for the irregular flushing by 

 rain the regular use of flush-tanks can be substituted; and in 

 case the sewage has to be pumped or treated, instead of being 

 discharged directly into a river, the presence of the rain-water 

 is not only undesirable but absolutely forbidden. 



To sum up the reasons for selecting, for a city, sewers to 

 carry storm-water and sewage, or sewage only, the arguments 

 just cited may be reduced as follows: It is improbable that 

 any house-refuse that would go into a combined system would 

 be kept out of a separate system, so that the only contribution 

 to the former not allowed in the latter is the rain-water from the 

 roofs, yards, and streets and any large amount of manufactur- 

 ing refuse which might be rejected from a separate system on 

 account of the large proportion not requiring purification. If 

 the streets are decently cleaned, there is no reason for expecting 

 the rain to act as a scavenger, and it is better to dispose of the 

 street-sweepings by means of sweeping-machines than to allow 

 the rain to wash these accumulations into the sewer, to be cleaned 

 out by hand or discharged into a river or harbor, there to be 

 dredged out. There is, therefore, no sanitary reason why 

 rain-water should not be separately disposed of. 



As to the dangers from slime deposited on the walls of large 

 sewers the case is suppositionary, and the evil effects entirely 

 unproved. Notwithstanding numerous examinations of sewer- 

 air, no pathogenic germs have ever been found a negative argu- 

 ment, to be sure, but of some weight. Judged by chemical 

 standards, the air in sewers is generally better than that in schools, 



