(a) Rates and service regulations may be established by contract between a municipality and utility for a specified term, not exceeding 30 years, but only by and with the approval of the commission to be expressed by its order. Utilities may contract with each other and with persons who are not utilities in respect of the use of their properties and facilities, the sale or exchange of water, gas, electricity or other products or commodities, otherwise than pursuant to established rates, the distribution to the public of such products and commodities jointly or singly, and the territory within which such joint or single service shall be rendered and other matters deemed to be of mutual advantage, subject, however, in all such cases, to the approval of the commission; but no person shall participate in such distribution who is not a utility.

(b) Whenever any such contract shall be made, it shall, before becoming effective, be submitted to the commission. If the commission shall find the provisions of any such contract consistent with the public interest, it shall approve the same. Otherwise, it shall disapprove the same, and unless and until so approved, such contract shall be of no effect; but if it is approved, it shall in all respects be lawful. Whenever a utility provides for itself by contract, as above provided, a source of supply of any product or commodity which it would otherwise be under the duty to generate or manufacture, it shall, to such extent as the commission may order, be excused from the construction or maintenance of plant, facilities and equipment necessary for such generation or manufacture.