California Code, Health and Safety Code - HSC § 4742

It may join with any other district, city or other governmental agency in the purchase, ownership, use, construction, maintenance, or operation of a sewerage system or sewage disposal or treatment plant, or a refuse transfer or disposal system, or both, either within or without the district, or so join for any combination of these purposes, but no sewage disposal or treatment plant, shall be constructed or maintained in any city not in the district, except by consent granted by the unanimous vote of the governing body of the city.