Main sewer districts may be subdivided into special sewer districts in such manner that each special district shall include 1 or more lateral or branch sewers connecting with a main sewer, and such lands as in the opinion of the board, subject to the approval of the council, will be benefited by the construction thereof. When deemed necessary, special sewer districts, to include 1 or more local or branch sewers, and such lands as in the opinion of the board, subject to the approval of the council, will be benefited by the construction thereof, may be formed of territory not included in any main sewer district.