Tex. Water Code § 30.030

(a) A public agency may pay for the waste collection, transportation, treatment, and disposal services with income from its waterworks system, sanitary sewer system, or both systems, or its combined water and sanitary sewer system, as prescribed by the contract. In the alternative, a joint board defined as a public agency in Section 30.003, Subdivision (3), may pay for these services from any revenue or other funds within its control specified in the contract if the city councils of the cities which created the joint board approve, by ordinance, the contract between the joint board and the district. These payments constitute an operating expense of each system whose revenue is so used.

(b) The obligation of contract payments on the income of the public agency's water system is subordinate to the obligation imposed by any bonds that are payable solely from the water system net revenue and that are outstanding at the time the contract is made, unless the ordinance or resolution authorizing the bonds expressly reserved the right to give the contract payments a priority over the bond requirements.

(c) If a public agency having taxing power holds an election substantially according to the applicable provisions of Chapter 1251, Government Code, relating to the issuance of bonds by cities and it is determined that the public agency is authorized to levy an ad valorem tax to make all or part of the payments under a contract with a district, then the contract is an obligation against the taxing power of the public agency to the extent authorized, and payments under the contract may be payable from and constitute solely an obligation against the taxing powers of the city or may be payable both from taxes and from revenue prescribed in the contract. Otherwise, neither the district nor the holders of the district's bonds are entitled to demand payment of the public agency's obligation out of any tax revenue.