In addition to the method of constructing sewers by contract as provided for in section 249.510, if the county commission deems it practicable to procure from federal or any other governmental source any part of the cost of the construction of main or submain or lateral sewers, or if for any other reason said county commission deems it more efficient and economical to construct any of such sewers without letting a contract therefor, the county commission may proceed to the construction of the same under the supervision of the sewer engineer. In such case, an accurate account of the cost thereof shall be kept by the county commission and the commission shall be authorized to accept any financial aid either by the contribution of labor or material, or both, and any amount so received shall be first applied to the cost of such work and the balance thereof shall be paid by special assessment apportioned as provided in section 249.510, and special tax bills shall be issued therefor. The tax bills authorized in this section may be issued in installments, or payable at one time, as provided in section 249.520 where work is done by a contractor. The county may bring suit to collect the said tax bills, or they may be assigned by the county commission on such terms as it deems advisable.