Where sewage is applied to the surface of the ground upon which crops are raised the process is called "sewage farming." This varies but little from ordinary irrigation where clean water is used instead of sewage. The land employed for this purpose should have a rather light and porous soil, and the crops should be such as require a large amount of moisture. The application of from 5,000 to 10,000 gallons of sewage per day per acre is considered a liberal allowance. On the basis of 100 gallons of sewage per head of population this would mean that one acre would care for a population of from 50 to 100 people.

Sub-Surface Irrigation. This system is employed only upon a small scale and chiefly for private dwellings, public institutions and for small communities where for any reason surface disposal would be objectionable. The sewage is distributed through agricultural drain tiles laid with open joints and placed only a few inches below the surface. Provision should be made for changing the disposal area as often as the soil may require by turning the sewage into sub-divisions of the distributing pipes.