No free service shall be furnished by any such system, project or combined system to the municipality owning it or to any agency, instrumentality or person. The reasonable cost and value of any service rendered to any such borrower by any such system, project or combined system shall be charged against the borrower and shall be paid for monthly as the service accrues from the current funds or from the proceeds of taxes which the borrower, within constitutional limitations, is hereby authorized and required to levy in an amount sufficient for that purpose, and such funds, when so paid, shall be accounted for in the same manner as other revenues of such system, project or combined system.