SEWAGE 



339 



The effect of rain upon sewers even under the 

 separate system requires a very much larger 

 provision to be made for the conveyance of rainfall 

 than for the sewage proper, as the sewers are 

 affected by the rate at which rain falls, and 

 not by the given amount which falls in a day. 

 Under the ordinary rule of thumb calculations 

 the sewers should only admit a quantity not 

 exceeding a quarter of an inch in twenty-four 

 hours, which has been shown to be totally inade- 

 quate, and serious flooding has arisen in conse- 

 quence. Rainfalls in London have been recorded 

 at a rate exceeding 300 cubic feet per minute 

 per acre. On an average of four years' observations 

 made at Croydon it has been found that whenever 

 it rains so as to affect the sewers the rain falls at 

 the rate of 4 cubic feet per acre per minute ; 

 and rains have been known to increase the average 

 flow of sewage by over thirteen times its ordinary 

 normal volume. It is therefore important in 

 all systems of sewerage to determine the exact 

 area that shall contribute rainfall to the sewers. 

 In districts of considerable area the rate at which 

 sewers are affected by rain is very much less than 

 in smaller districto of limited area, as in the case 

 of large districts the distant rainfall has to traverse 

 a considerable length of sewer before it arrives at 

 the outfall. The abrupt increase of the flow in the 

 sewer may cause sewer-air to be discharged. But 

 long experience has now firmly established the fact 

 that the water-carriage system of removing sewage 

 is superior on the whole to all other systems. 



If the sewers are liable to decay or to leak 

 there is danger of the ground upon which houses 

 are built being fouled ; hence comes pollution of 

 the ground-water and the outbreak of various 

 ili-c.'iseg. The bricks used in the construction of 

 all sewer- works should be as impervious as possible, 

 and as a rule no bricks should be allowed to ! used 

 in the sewer-work in which the absorptive capacity 

 for water exceeds 12 per cent, of their weight. 

 The materials used in the jointing of brick sewers 

 should also be of the most permanent character, 

 and no other material except Portland cement 

 mortar has yet been discovered which will stand 

 tin- "heliacal action of sewage upon it. The smallest 

 sewers are as a rule made of glazed stoneware 

 pipes having various forms of joints. In some, 

 especially wet districts, cast-iron pipes jointed 

 with lead are used to form the sewers. No material 

 should be used in the construction of sewers which 

 will not allow of contraction and expansion by 

 change of temperature. In the case of house- 

 drains the changes of temperature are much more 

 considerable than in the case of sewers, as often in 

 house-drains lioiliiig water may at one period be 

 passing through and at another melted snow. 

 These changes of temperature affect the stability 

 ,of all sewer-work, and tend to pull it to pieces. 

 'I'll' 1 jointe therefore should be of such a character 

 if |M>ssil>le as not to present too much resistance, 

 and should be parallel, so that if the pipes move by 

 contraction or expansion the joint will not open 

 more at one point than at another. The ordinary 

 sucket joint when properly made is found to be one 

 of the best joints for either sewers or drains. 



The size of sewers must depend upon the popu- 

 lation, the volume of sewage, and the fall which can 

 lie given to them. The average dry- weather volume 

 of si-wage in most towns can be taken roughly at 

 30 gallons per head per day. In some places, how- 

 ever, it is very much less, in others considerably 

 exceeded. The dry-weather sewage is made up 

 by the volume of the water-supply of the district, 

 to which may be added in districts with a wet 

 gnbtoil a varying amount of leakage into the 

 sewers. There is a daily fluctuation in the 

 flow through sewers. Within a mile of the 



point of production of the sewage the volume in one 

 hour of maximum flow is at the rate of three times 

 that of the average flow during the whole twenty- 

 four hours, ami as a rule one-half the sewage 

 flows away in from six to eight hours per day. 



In order to make sewers self-cleansing, either by 

 the natural flow of sewage througli them or by 

 artificial means of flushing, they should in the 

 case of small circular sewers or sewers of less than 

 10 inches diameter never be laid with a less inclina- 

 tion than would give a velocity of 3 feet per second 

 through them ; circular sewers above 10 inches 

 diameter and up to 24 inches internal diameter 

 should always be laid at a slope that will give a 

 velocity of flow of not less than 2J feet per second ; 

 and in large sewers the rate of inclination should 

 lie such as to give a velocity of not less than 2 feet 

 per second. In house-drains the rate of inclination 

 ought to be such that the flow should not be less 

 than 4 feet per second. This means that a pipe 1 

 foot in diameter should not have a less inclination 

 than 1 in 160. The proper inclination of any smaller 

 size of pipe or drain to give a velocity of 4 feet per 

 second will be found by multiplying 160, which 

 expresses the proper inclination for a 1-foot sewer, 

 by the diameter of the sewer in feet. For instance, 

 a drain which is 6 inches or M foot in diameter 

 would require to have an inclination of 1 in 80 to 

 give the desired velocity. To give a velocity of 3 

 feet per second multiply the diameter in feet by 

 275 ; thus a 9-inch sewer = "75 feet should, to 

 give it a velocity of 3 feet per second, have an 

 inclination of 275 X '75 = 206 or 1 in 206. 

 When the velocity required is 2J feet per 

 second, then multiply the diameter in feet of 

 the sewer by 386 ; thus a sewer 2 feet in diameter 

 will require to have an inclination of 386 X 2 

 = 772 or 1 in 77ii. When the velocity required 

 is 2 feet per second the number to multiply the 

 diameter of the sewer will be 584. A sewer 

 therefore, 3 feet in diameter, would require to 

 have an inclination of 584 X 3 = 1752 or 1 in 1752, 

 or practically 3 feet per mile fall, to give it the 

 required velocity of 2 feet per second. Where 

 sewers cannot have a projier inclination so as to 

 render them self-cleansing with the ordinary flow 

 of sewage through them, flushing operations are 

 required. These consist either of the sudden 

 admittance of a large volume of water into the 

 sewer, or what is termed sectional flushing, by 

 means of penning back the sewage in sections i.e. 

 by erecting a dam in the sewer and allowing sewage 

 to accumulate behind it, suddenly removing the 

 dam and allowing it to flush out the lower section 

 of the sewer. 



All sewers require to be ventilated. But it is 

 by no means necessary to admit currents of air 

 through sewers for the purpose of ventilation ; for 

 it may be taken for granted that the admittance of 

 so much pure air into the sewer at one point of its 

 course means the expulsion of so much foul air 

 at another point. All that is required for the 

 purpose of ventilation of sewers is a series of 

 vents so as to allow the air to escajie where it 

 is apt to be compressed by either an increase of 

 flow in the sewers or an increase of temperature; 

 and to allow air to be admitted just as freely when 

 the tendency if\for the flow in the sewers to sub- 

 side and so create air-space. 



The simplest and probably one of the best 

 means of ventilating the sewers is by means of 

 pipes carried up to a sufficient altitude above the 

 level of the houses. In no case should any pipe 

 have direct connection with the houses themselves, 

 nor should a rain-water pipe he used for the pur- 

 pose of ventilation, as these pipes niay be blocked 

 by rain when most required for ventilation. Venti- 

 lating pipes should be free from all obstruction and 



