The authority of sanitary sewer districts to construct a system or systems of sanitary sewers as provided in sections 249.010 to 249.420 shall include the authority to construct sewerage disposal plant or plants for the disposal of sewage, and the authority of lateral sewer district to construct a system of lateral sewers as provided in sections 249.010 to 249.420 shall include the authority to construct sewerage disposal plants for the disposal of sewage.