(a) No bid will be considered by a county unless it is accompanied by a proposal guaranty in the form of a certified check or other acceptable security payable to the county for an amount deemed by the county in the public interest necessary to ensure that the successful bidder will execute the contract on which he bid.

(b) A proposal guaranty will be returned to a bidder upon receipt by the county of the bidder's written withdrawal of his bid if such receipt is before the time scheduled for the opening of bids. Upon the determination by a county of the lowest reliable bidder, the county will return the proposal guaranties to all bidders except that of the lowest reliable bidder. If no contract award is made within 30 days after the date set for the opening of bids, all bids shall be rejected and all proposal guaranties shall be returned unless the county and the successful bidder agree in writing to a longer period of time.