The commission shall make a grant to any municipality which, prior to May 1, 1967, constructed, rebuilt, acquired or expanded a pollution abatement facility, which grant shall be thirty per cent of the principal amount of bond or note obligations of such municipality, issued to finance such construction, rebuilding, acquisition or expansion and outstanding on said date, exclusive of all interest costs and for which grant application is made prior to October 1, 1969, on an application prescribed by the commission. Such grant shall be paid in equal annual installments at least thirty days prior to the date the municipality is obligated to make payment on such bonds or notes, provided any grant under this section shall be reduced by any amount payable to such municipality under the provisions of § 22a-439 for the same construction, rebuilding, acquisition or expansion project, such reduction to be prorated over the period remaining for the payment of such bonds or notes.