(1) All work ordered, the estimated cost of which is in excess of fifty thousand dollars, shall be let by contract and competitive bidding. Before awarding any such contract the board of commissioners shall publish a notice in a newspaper of general circulation where the district is located at least once thirteen days before the last date upon which bids will be received, inviting sealed proposals for such work, plans and specifications which must at the time of publication of such notice be on file in the office of the board of commissioners subject to the public inspection. The notice shall state generally the work to be done and shall call for proposals for doing the same to be sealed and filed with the board of commissioners on or before the day and hour named therein.

Each bid shall be accompanied by a certified or cashier's check or postal money order payable to the order of the county treasurer for a sum not less than five percent of the amount of the bid, or accompanied by a bid bond in an amount not less than five percent of the bid with a corporate surety licensed to do business in the state, conditioned that the bidder will pay the district as liquidated damages the amount specified in the bond, unless the bidder enters into a contract in accordance with the bidder's bid, and no bid shall be considered unless accompanied by such check, cash or bid bond. At the time and place named such bids shall be publicly opened and read and the board of commissioners shall proceed to canvass the bids and may let such contract to the lowest responsible bidder upon plans and specifications on file or to the best bidder submitting the bidder's own plans and specifications. The board of commissioners may reject all bids for good cause and readvertise and in such case all checks, cash or bid bonds shall be returned to the bidders. If the contract is let, then all checks, cash, or bid bonds shall be returned to the bidders, except that of the successful bidder, which shall be retained until a contract shall be entered into for doing the work, and a bond to perform such work furnished with sureties satisfactory to the board of commissioners in the full amount of the contract price between the bidder and the commission in accordance with the bid. If the bidder fails to enter into the contract in accordance with the bid and furnish the bond within ten days from the date at which the bidder is notified that the bidder is the successful bidder, the check, cash, or bid bonds and the amount thereof shall be forfeited to the district. If the bidder fails to enter into a contract in accordance with the bidder's bid, and the board of commissioners deems it necessary to take legal action to collect on any bid bond required by this section, then the district shall be entitled to collect from the bidder any legal expenses, including reasonable attorneys' fees occasioned thereby. A low bidder who claims error and fails to enter into a contract is prohibited from bidding on the same project if a second or subsequent call for bids is made for the project.

(3) Any purchase of materials, supplies, or equipment, with an estimated cost in excess of forty thousand dollars, shall be by contract. Any purchase of materials, supplies, or equipment, with an estimated cost of less than fifty thousand dollars shall be made using the process provided in RCW 39.04.190. Any purchase of materials, supplies, or equipment with an estimated cost of fifty thousand dollars or more shall be made by competitive bidding following the procedure for letting contracts for projects under subsection (1) of this section.

(4) As an alternative to requirements under subsection (3) of this section, a water-sewer district may let contracts for purchase of materials, supplies, or equipment with the suppliers designated on current state agency, county, city, or town purchasing rosters for the materials, supplies, or equipment, when the roster has been established in accordance with the competitive bidding law for purchases applicable to the state agency, county, city, or town. The price and terms for purchases shall be as described on the applicable roster.

(d) Invitations for unit price bids must include, for purposes of the bid evaluation, estimated quantities of the anticipated types of work or trades, and specify how the district will issue or release work assignments, work orders, or task authorizations pursuant to a unit priced contract for projects, tasks, or other work based on the hourly rates or unit prices bid by the contractor. Contracts must be awarded to the lowest responsible bidder as per RCW 39.04.010. Whenever possible, the district must invite at least one proposal from a minority or woman contractor who otherwise qualifies under this section.

(e) Unit price contractors shall pay prevailing wages for all work that would otherwise be subject to the requirements of chapter 39.12 RCW. Prevailing wages for all work performed pursuant to each work order must be the prevailing wage rates in effect at the beginning date for each contract year. Unit priced contracts must have prevailing wage rates updated annually. Intents and affidavits for prevailing wages paid must be submitted annually for all work completed within the previous twelve-month period of the unit priced contract.