The Commissioners shall from Time to Time, unless it be otherwise provided by the special Act, make the general Sewer Rate of such Amount as will with the special Sewer Rates raise Money sufficient not only to defray the current Expences of maintaining the Sewers that shall have been purchased or made, but also to keep; down the Interest of any Monies borrowed on Security of the special and general Sewer Rate, and to pay off the Principal of such Monies within a Period not longer than Thirty Years.