One of the best examples of slow sand filtration of a water supply is that of Philadelphia, which has four plants, namely, at Lower Roxboro, with a daily capacity of 12,000,-000 gallons; Upper Roxboro, with a daily capacity of 16,000.000 gallons; the Belmont plant, with a daily capacity of 67,000,000 gallons; and the latest and newest one of Torresdale, with a daily capacity of 240,000,000 gallons. The total cost of the Torresdale slow sand filtration plant in Philadelphia was $9,208,000.

In general it may be said that the cost per acre for building a slow sand filter is about $60,000. The gross daily capacity of a 1-acre filter is approximately 3,000,000 gallons, although its average capacity is only about 2,500,000 gallons. If the daily consumption be 125 gallons per capita, an acre of filter would supply 20,000 people. The first cost would therefore be about $3 per person, and the interest on the investment amounts to 18 cents per annum for each person. It is estimated by the experts of the Geological Survey that if the water be boiled as a substitute for sand filtration, the cost in fuel for 6 weeks will be about the same as the annual cost for supplying water from the slow sand filtration, from which the bacteria are so thoroughly removed that it is almost as safe as boiled water. On the basis of the consumption of 125 gallons per person per day, the actual cost of slow sand filtration would be about 36 cents per person per annum.