Sec. 4. (a) Revenue bonds issued for the payment of works of improvement for the collection, treatment, and disposal of sewage and other liquid wastes may provide that the principal and interest shall be paid:
(1) solely from the net revenue of the sewage works, which is gross revenues after deduction only for the reasonable expenses of operation and maintenance; or
(2) from a combination of net revenue and other money available to a district by:
(A) levy;
(B) special benefits taxes; or
(C) assessment of exceptional benefits.
(b) The board may covenant with the holders of the bonds to pay:
(1) a certain percentage of principal and interest from the revenue;
(2) a certain percentage from the other money to maintain a reasonable reserve from the other money that may be used for payment of principal and interest if the revenue is not sufficient; or
(3) both.