Cal. Wat. Code § 31100
A district may acquire, construct, and operate facilities for the collection, treatment and disposal of sewage, waste and storm water of the district and its inhabitants and may contract with any public agency including but not limited to sanitation districts for sewer outfall facilities. A district also may acquire, construct, and operate facilities for the collection, treatment and disposal of sewage, waste and storm water of inhabitants outside its boundaries; provided that it shall not furnish any such service to the inhabitants of any other public agency without the consent of such other public agency expressed by resolution or ordinance. The term "public agency" as used in this section, shall include a city, county, city and county, public district, municipal or public corporation, state agency or other political subdivision of the state, but shall not include a public utility subject to the jurisdiction, control and regulation of the Public Utilities Commission under the provisions of Divisions 5 (commencing with Section 10001) and 6 (commencing with Section 11501) of the Public Utilities Code.
Ca. Water Code § 31100