In addition to taxes pledged to pay the principal of and interest on general obligation bonds issued to pay for costs of purchasing, acquiring, or constructing any public utility mentioned in RCW 35.67.020, or to make any additions, betterments, or alterations thereto, or extensions thereof, the city or town legislative body, may set aside into a special fund and pledge to the payment of such principal and interest any sums or amounts which may accrue from the collection of service rates and charges for the private and public use of said sewerage system or systems for the collection and disposal of refuse, in excess of the cost of operation and maintenance thereof as constructed or added to, and the same shall be applied solely to the payment of such interest and bonds. Such pledge of revenue shall constitute a binding obligation, according to its terms, to continue the collection of such revenue so long as such bonds or any of them are outstanding. If the rates and charges are sufficient to meet the debt service requirements on such bonds no general tax need be levied.