Upon providing a sewer system in any area of the district, the district may declare the further maintenance or use of cesspools, septic tanks, or other local means of sewage disposal in such area to be a public nuisance and may require all buildings used by human beings to be connected with the sewer system within such period, not less than 120 days from the completion of the sewer system, as may be prescribed by the district; provided that such buildings to be connected are within 150 feet of the system.