40 CFR § 35.2107 - Intermunicipal service agreements.
If the project will serve two or more municipalities, the applicant shall submit the executed intermunicipal agreements, contracts or other legally binding instruments necessary for the financing, building and operation of the proposed treatment works. At a minimum they must include the basis upon which costs are allocated, the formula by which costs are allocated, and the manner in which the cost allocation system will be administered. The Regional Administrator may waive this requirement provided the applicant can demonstrate:
(a) That such an agreement is already in place; or
(b) Evidence of historic service relationships for water supply, wastewater or other services between the affected communities regardless of the existence of formal agreements, and