Section 249.340 - Contractual procedure and work specifications1. Whenever it shall be ordered by the board of trustees of the sewer district that any sewer or system of sewers in the incorporated sewer district be constructed in accordance with the provisions of sections 249.010 to 249.420 and the engineer's estimated cost thereof exceeds the sum of five hundred dollars the said board of trustees shall order its said engineer to prepare and file with the secretary of said board of trustees all necessary maps, plans, specifications and profiles and the estimated cost of the work. Said board of trustees may approve or reject the maps, plans, specifications and profiles and have others prepared and filed.2. When the maps, plans, specifications and profiles have been approved, the said board of trustees shall order its engineer to advertise the letting of the contract, proposed to be let, by advertisement in some newspaper that has a general circulation in the district wherein the contract is to be executed which said advertisements shall be published once a week for three consecutive weeks, the last insertion to be within ten days of the day of the letting.3. All bids should be in writing accompanied by instructions to bidders which shall be furnished by the engineer of said board of trustees upon application. All bids on sewer work shall state the unit price upon which the same are based. All bids shall be sealed and filed with the secretary of said board of trustees, and, on the day and at the hour named in the advertisement, shall be publicly opened and read in the presence of the board of trustees and the engineer of said board and shall then be recorded in detail in some suitable book. All bids shall be accompanied by a certified check equal to ten percent of the engineer's estimate of cost, payable to the said board of trustees, or a bidder's bond executed by some surety company authorized to do business in this state or other good and sufficient surety in a like sum shall be given, as guarantee on the part of the bidder that if his bid be accepted he will, within ten days after receipt of notice of such acceptance, enter into contract and bond with good and sufficient sureties to be approved by the board to do the work advertised, and in case of default, forfeit and pay the sum of ten percent of the engineer's estimate of cost. The contract shall be awarded to the lowest and best bidder. The said board of trustees may in its discretion reject any and all bids. Any bid in excess of the engineer's estimate of the cost of the work to be done shall be rejected.4. When it shall be decided by order of record to accept any bid the said board of trustees shall order a contract to be entered into between the bidder and the said board of trustees. The contract shall have attached to and made a part thereof the proposal sheet, instructions to bidders, the bid, maps, plans, specification, and profiles. Whenever the contract is executed and approved by order of record and endorsement thereon it shall be filed and preserved as a permanent record in the office of the said board of trustees.5. It shall be incorporated in the contract that the said board of trustees shall reserve the right to make any additions to, omissions from, changes in, or substitution for the work or materials called for by drawings and specifications, without notice to the surety on the bond given to secure the faithful performance of the terms of the contract. The bidder must agree that before the sewer district shall be liable for any additional work or material the board of trustees of said sewer district must first order the same and the cost thereof must be agreed upon in writing and entered of record before such additional work shall apply in case of omissions, deductions or changes, and the unit price shall be the basis of the valuation of such changes. In case of disagreement upon the cost or price of any addition, omission or change ordered or so desired, then it is expressly agreed that the decision of the engineer of said board of trustees shall be received and accepted as fixing definitely and finally the cost of such change and when so fixed the said board of trustees shall enter of record such change. It shall also be provided in the contract that the contractor will furnish and promptly pay for all labor employed and materials used in the performance of such contract, and pay all bills incurred by said contractor in performance of said contract or contracts.