Contracts for construction, or for labor or materials entering into the construction of any improvement authorized by the district shall be awarded at public bidding except as herein otherwise provided. A notice calling for sealed proposals shall be published in such newspaper or newspapers of general circulation as the board shall designate for a period of not less than two weeks (three weekly issues) prior to the day of the opening of the bids. Such proposals shall be accompanied by a certified check for such amount as the board shall decide upon, to guarantee a compliance with the bid and shall be opened in public at the time and place designated in the notice. The contract shall be awarded to the lowest and best responsible bidder: PROVIDED, That the board shall have authority to reject any or all bids, in which event they shall readvertise for bids and, when no satisfactory bid is then received and with the written approval of the director, may proceed to construct the works by force account.