The plan devised in accordance with section 6119.19 of the Revised Code shall be formed with a view to the division of the regional water and sewer district into as many sewer districts as are necessary for securing efficient sewerage. Each of the districts shall be designated by name or number and shall consist of one or more main sewers with the necessary branch or connecting sewers, the main sewers having their outlets in a proper place. The districts shall be so arranged as to be independent of each other so far as practicable.