SEWAGE OUTFALLS AND DISCHARGE 311 



flow of a sewer to five or more times its volume, but there is no 

 exact relation between the rainfall as ordinarily recorded and 

 the increment of flow at the outlet, the size, length, and in- 

 clination of the sewer greatly influencing the result. At Exeter 

 five-eighths of the ordinary rainfall is estimated to find its way 

 into the sewers. Mr. Silcock^ estimated from gaugings at King's 

 Lynn that " the ordinary dry- weather flow of sewage per acre 

 of a purely urban district, with an average population of seventy- 

 five persons per acre, consuming 20 gallons of water per head 

 per day, is 20 cubic feet (125 gallons) per hour. A rainfall of 

 i inch in 25 hours, or approximately i^q inch per hour, will 

 amount to the same discharge as the dry-weather flow per acre, 

 assuming that the streets are paved, and that only 50 per cent. 

 of the actual rainfall finds its way into the sewers. In other 

 words, a rainfall of yj^ inch per hour will double the ordinary 

 dry- weather flow. Now a rainfall at the rate of J inch, per hour 

 is a common occurrence, which would mean multiplying the 

 ordinary sewage flow by 25, and short storms at the rate of 

 h inch per hour are not infrequent when the ordinary sewage 

 flow is augmented 50 times. . . . For a town with a population of 

 100,000, if the whole of the sewage and rainfall had to be taken to 

 the purification works, the ordinary maximum sewage flow at 20 

 gallons per head would be at the rate of 4,000,000 gallons per 24 

 hours, and if the sewage were treated on bacterial intermittent 

 filters, 4 acres of filters would be required, but to deal with a 

 rainfall of J inch per hour would require 100 acres of filters, and 

 if the sewage had to be pumped it would require 25 engines 

 and pumps each capable of dealing with a dry-weather flow to 

 cope with the combined rainfall and sewage. It is therefore 

 evident that the whole of the rainfall cannot be taken to the 

 purification works, and that after a certain degree of dilution has 

 been reached, the storm-water must be discharged into the 

 streams." 



Both the quality and quantity of the local sewage have to be 

 considered in choosing a sewerage scheme. The efl"ect of rain 

 must not be considered as simple dilution, since the rain-water 

 carries the washings of the surfaces over which it has travelled. 

 Where the rock, or a clay-bed, is near the surface, the showers 

 will run off almost unchanged. From manured or peaty land 

 there will be an addition of brown humous liquids which are 

 particularly difficult to decolorize. See also pp. 6 and 27, and 



^ Leeds Sanitary Congress, 1897. 



