Any contract in excess of $2,000 between a sanitary district, whether formed under this chapter or by private and special Act of the Legislature, and a contractor for the construction of facilities located on private property for the exclusive use of a private individual and for which the private individual is required to pay the total cost directly to the sanitary district, shall be awarded by a system of competitive bidding. Unless there are valid reasons to the contrary, the contracts shall be awarded to the lowest responsible bidder.   [PL 1973, c. 476, §2 (NEW).]