Tenn. Code Ann. § 54-12-310

The successful bidder shall be required to execute a bond, with sufficient sureties, payable to the county, for the use and benefit of the improvement district, in an amount equal to twenty-five percent (25%) of the estimated cost of the work, so let or that amount may be deposited in cash with the treasurer of the board of directors as security for the performance of the contract, and upon the execution of the bond or the making of the deposit, the deposit originally made with the bid shall be returned.