The county commission of any county in this state, in which is located an unincorporated village or district in which main and submain sewers have already been constructed or hereafter may be constructed or deemed necessary, shall have power to establish sewer districts as provided in section 249.450, and to provide for the construction of sewers therein, and to pay the costs thereof by levying special assessments against the lots, tracts or parcels of ground in said sewer districts, and to issue special tax bills evidencing such assessments.