After the final acceptance of the work by the board of supervisors, the management of the utility shall automatically go to the three trustees previously appointed by the board of supervisors. The trustees of a benefited water district located in a county with a population of two hundred fifty thousand or less shall have power to levy an annual tax not to exceed thirteen and one-half cents per thousand dollars of assessed value of all taxable property in the district, for the maintenance of the system. However, the trustees of a benefited water district located in a county with a population of more than two hundred fifty thousand may levy an annual tax on the taxable value of all taxable property in the district in an amount as may be necessary for the maintenance of the system, with the approval of the board of supervisors. This levy shall be optional with the trustees. The trustees may purchase material and employ labor to properly maintain and operate the utility. The trustees shall be allowed necessary expenses in the discharge of their duties, but shall not receive any salary.