1. The board of trustees for the district shall let contracts for all work to be done, excepting in case of repairs or emergencies requiring prompt attention, in the construction of trunk sewers and sewage treatment plants under the authority of sections 204.250 to 204.470, the expense of which will exceed five hundred dollars, to the lowest responsible bidder therefor, upon not less than twenty days’ notice of the letting, given by publication in a newspaper of general circulation in the district, and in the discretion of the board, in one or more newspapers of general circulation among contractors. The board shall have the power and authority to reject any and all bids and readvertise the work.

2. The board of trustees, subject to the concurrence of the advisory board established by sections 204.250 to 204.470, shall also have the power to enter into agreements with persons, firms or corporations of known standing and competence for the execution and preparation of the surveys, maps and plans needed and required by the board, and also for the laying out and superintendence of work to be constructed under the authority of sections 204.250 to 204.470, but no single agreement so made shall cover more than one piece or class of work.