Tex. Water Code § 59.022

The district may charge rates to persons and entities located outside the district's boundaries on terms, rates, and charges the board of directors may determine to be advisable. In setting rates for out-of-district customers, the board shall set rates sufficient to enable it to meet operation and maintenance expenses and to pay the principal of and interest on debt issued in connection with providing service and to provide a reasonable reserve for replacements to the district. In setting rates, the district may take into consideration past operation and debt service expenses.