If revenue bonds are issued under this Article, the income and revenue derived from the operation of the system shall be deposited in a separate fund, in a separate bank or savings and loan association account, designated as the (Water, Sewer, or other name) Fund of (name of township). The fund shall be used only to pay the cost of operation and maintenance of the system, to pay the principal of and interest upon the revenue bonds of the township issued under this Article, and to provide a reasonable depreciation fund whose amount and application shall be fixed and determined in the ordinance providing for the issuance of the bonds.