The terms sewage system, sewerage system, and disposal plant or plants as used in sections 18-501 to 18-511 are defined to mean and include any system or works above or below ground which has for its purpose any or all of the following: The removal, discharge, conduction, carrying, treatment, purification, or disposal of the liquid and solid waste of a city or village. It is intended that sections 18-501 to 18-512 may be employed in connection with sewage projects which do not include the erection or enlargement of a sewage disposal plant.