Sanitary sewage and industrial waste will obviously be derived largely from the water supply. Accordingly, an estimate of the amount of such wastes to be expected must be prefaced by a study of water consumption, either under present conditions or at some time in the future. The proportion of the water consumed which will reach the sewer must be decided upon after careful consideration of local conditions. Water used for steam boilers in industries, air conditioning, and that used to water lawns and gardens may or may not reach the sewers. On the other hand, many industrial plants may have their own supplies but discharge their wastes into the sewers. Although the sewage may vary in individual cities from 70 to 130 percent of the water consumed, designers frequently assume that the average rate of sewage flow, including a moderate allowance for infiltration equals the average rate of water consumption.
Relation to Water Consumption