Neither the commission nor any of the counties, cities or towns in whole or in part embraced within the district shall have power to mortgage, pledge, encumber or otherwise dispose of any part of the sewage disposal system of a commission, except such part or parts thereof as may be no longer necessary for the purposes of the commission, whether the same shall originally have been acquired by such commission or by one of the counties, cities or towns. The provisions of this section shall be deemed to constitute a contract with the holders of bonds of the commission. The sewage disposal system of a commission shall be exempt from any and all liability which may be incurred by, or imposed upon, the commission, or any county, city or town, which, in whole or in part, constitutes any part of any district.