What is the test for fair and reasonable classification of water users?

It is well established that " 'a utility may, without being guilty of unlawful discrimination, classify its customers or patrons upon any reasonable basis....' " (Durant, supra, 39 Cal.App.2d at p. 139, 102 P.2d 759; see Toward Utility Rate Normalization v. Public Utilities Commission (1978) 22 Cal.3d 529, 543-544, 149 Cal.Rptr. 692, 585 P.2d 491; Kennedy v. City of Ukiah (1977) 69 Cal.App.3d 545, 553-554, 138 Cal.Rptr. 207.) The trial court found Ventura's three-tier classification scheme to be fair and reasonable. Plaintiffs, however, objected to Ventura's three-tier classification scheme based on what it perceived to be a feigned distinction in treating users of nonpotable, untreated water as a separate class.