SEWERAGE 385 



discharge a considerable volume of sewage, which should be 

 provided for. The provision necessary for subsoil water has 

 already been referred to. That the amount of actual sewage 

 will generally be less than the water supply will be evident 

 when it is considered that all the water used for sprinkling, and 

 some of that used for cleaning, either soaks into the ground 

 or evaporates. In manufacturing districts, also, considerable 

 quantities of water are used that do not reach the sewers. 



The common practice among American engineers is to 

 proportion the sewers of the separate system so that, when 

 running half full, they will discharge a quantity of sewage 

 equal to the maximum hourly water consumption, this maxi- 

 mum being taken equal to 1.5 times the average. The remain- 

 ing capacity is reserved for extreme variations in flow and for 

 ventilation. The conditions of flow are then as follows: 



Average daily flow, 100%; sewer one-third full. 



Average maximum daily flow, 150%; sewer one-half full. 



Total capacity of sewer, 300% ; sewer full. 



The average daily flow is assumed to be such as may reason- 

 ably be expected when the territory is fairly well developed and 

 the buildings all connected with the sewers. 



EXAMPLE. What capacity should the main sewer of a city of 

 25,000 population have, the water consumption being 85 gal. 

 per head each day, assuming the sewer to be flowing half full? 



SOLUTION. The total water consumption is 25,000X85 = 

 2,125,000 gal. per da. The discharge from the sewer is 2,125,000 

 X 1.50 = 3, 187,500 gal. per da. Reducing this quantity to 

 cubic feet per second, the capacity of the sewer is found to be 



3,187,500 



= 4.9 cu. ft. per sec. 



7.48X24X60X60 



SEWER COMPUTATIONS 



Sewer computations are made by Chezy's and Kutter's for- 

 mulas, given under the heading Hydraulics. For sewer work, 

 two values of n in Kutter's formula are used: .013 for vitrified 

 pipe and .015 for concrete and brick sewers. For both of 

 these values, the accompanying tables give velocities and dis- 

 charges for sewers of various sizes laid on different grades. 



