Infiltration is the water that enters sewers through poor joints, cracked pipes, and the walls of manholes. Inflow enters through perforated manhole covers, roof drains, drains from flooded cellars during runoff events. Because infiltration may be nonexistent during dry weather, the dry-weather flow may be considered as the sanitary sewage plus the industrial wastes. In wet weather, infiltration will be greatly increased as groundwater level rise, and may be augmented by the inflow from roofs which reaches the sewers by rain leaders from the roof gutters. Most cities prohibit such connections, but they are sometimes made illegally, and some may remain from a period when they were not forbidden. Some sewers may be located below the groundwater table and therefore have some infiltration at all time. Sewers that constructed in or close to stream beds are especially likely to have high infiltration.
The amount of infiltration to be expected will depend upon the care with which the sewer system is constructed, the height of the groundwater table, and the character of the soil. Special type of joints tends to reduce the infiltration. A soil that heaves with varying water content will pull joints apart and so permit water to enter. A pervious soil permits easy travel of percolating water to the sewers where it will travel along them until it reaches a crack or open joint. Since conditions of construction and soil differ widely, the infiltration found in sewer systems varies considerably. Sewer size apparently has little effect. The large sewers present more joint length for leakage, but the joints are more likely to be of better workmanship. Infiltration rates are likely to vary from 35 to 115 m3/km of sewer per day (15,000 to 50,00 gal/mi per day) in old systems, but even higher rates have been noted where sewers are below the water table and are poorly constructed. Specifications for sewer projects now limit infiltration to 45 1/km per day per mm of diameter (500 gal/mi per in day).
Since sewers deteriorate, however, engineers are liberal in estimating the infiltration for design purposes. The figures given are based upon length of the public sewers and do not include the house sewers which extend to the buildings. It should be recognized that they will also permit infiltration, and their construction should be carefully controlled. In order to obtain federal funds for the construction of sewage treatment plants it is necessary to demonstrate that the sewer system does not permit excessive infiltration or inflow.