The board of directors shall be responsible for the operation of all hospitals owned or leased by the district, according to the best interests of the public health and shall make and enforce all rules, regulations, and bylaws necessary for the administration, government, protection, and maintenance of hospitals under their management and all property belonging thereto and may prescribe the terms upon which patients may be admitted thereto. Such hospitals shall not contract to care for indigent county patients at below the cost for care. In fixing the rates the board shall, insofar as possible, establish such rates as will permit the hospital to be operated upon a self-supporting basis. The board may establish different rates for residents of the district than for persons who do not reside within the district. Minimum standards of operation as prescribed in sections 23-3528 to 23-3552 shall be established and enforced by the board of directors.