250.130.  Revenue bonds — amount of net revenue pledged to payment of principal and interest. — 1.  Whenever any such city, town or village or sewer district shall authorize and issue revenue bonds pursuant to this chapter an amount sufficient for the purpose of the net revenues of the sewerage system or of the combined system for the benefit of which such bonds are issued shall by operation of this chapter be pledged to the payment of the principal of and the interest on such bonds as the same shall mature and accrue.

  2.  The term "net revenues" shall be construed to mean all revenues derived from the operation of such system less the actual and necessary expenses of operation and maintenance of the system.

  3.  It shall be the mandatory duty of the officials of any such city, town or village or sewer district charged with the custody and management of the revenues to be derived from the operation of such system to provide for the prompt payment from such revenues of the principal of and the interest on any such revenue bonds as the same shall mature and accrue.