Tex. Water Code § 13.190

(a) A water and sewer utility may not directly or indirectly by any device or in any manner charge, demand, collect, or receive from any person a greater or lesser compensation for any service rendered or to be rendered by the utility than that prescribed in the schedule of rates of the utility applicable to that service when filed in the manner provided in this chapter, and a person may not knowingly receive or accept any service from a utility for a compensation greater or less than that prescribed in the schedules, provided that all rates being charged and collected by a utility on the effective date of this chapter may be continued until schedules are filed.

(b) This chapter does not prevent a cooperative corporation from returning to its members the whole or any part of the net earnings resulting from its operations in proportion to their purchases from or through the corporation.