The board may apportion all or a portion of the expenses of operation and maintenance of any sewage disposal facilities constructed by the district, and the interest on any bonds issued therefor, among the public agencies whose sewers are connected with district facilities. The board shall establish rules for determining the apportionment, which among other things shall be based on the quantity, quality, or characteristics of the sewage delivered by each public agency.
Ca. Pub. Util. Code § 12812