Va. Code § 21-118.6
Whenever a county and one or more sanitary districts have entered into an agreement for a unified water supply or sewerage system, or unified water supply and sewerage system, and such county has agreed therein to pay from the revenues of such unified system the principal of and interest on all bonds issued for water or sewer purposes, or both, as the case may be, by such sanitary district or districts, and to impose such schedule of rates, rentals, fees and charges for the use and services of such unified system so as to produce revenues sufficient for such payment, such county may also apply the revenues derived from such rates, rentals, fees and charges to the payment of the cost of operation and maintenance of the unified system, to the cost of renewals and replacements and to any other lawful purposes connected with or pertaining to such unified system, including the making of additions to and expansions of such unified system, to the payment of the principal of and interest on any bonds issued by such county for the purpose of such unified system and to the creation and maintenance of such reserves as may be deemed necessary by such county to effect any financing for such unified system. The order of any priority of the application of such revenues to any of the foregoing purposes, including to the payment of the principal of and interest on such bonds of the sanitary districts, may be determined by the board of supervisors as the governing body of the county. If at any time the revenues derived from rates, rentals, fees and charges for the use and services of such unified system, are insufficient to provide for the operation and maintenance of the unified system, and for payment of principal of and interest on such bonded indebtedness of the sanitary district as the same shall become due, such sanitary district shall levy an annual tax upon all property in such sanitary district subject to local taxation to pay such principal and interest as the same shall become due. Nothing contained in the immediately preceding sentence shall, however, be construed to relieve the county of its obligations under any such agreement to impose rates, rentals, fees and charges for the use and services of such system sufficient to pay such costs of operation and maintenance and to provide for the payment of such principal and interest.
Va. Code § 21-118.6