35 ILCS 200/11-15
To determine 33 1/3% of the fair cash value of any certified pollution control facilities in assessing those facilities, the Department shall take into consideration the actual or probable net earnings attributable to the facilities in question, capitalized on the basis of their productive earning value to their owner; the probable net value which could be realized by their owner if the facilities were removed and sold at a fair, voluntary sale, giving due account to the expense of removal and condition of the particular facilities in question; and other information as the Department may consider as bearing on the fair cash value of the facilities to their owner, consistent with the principles set forth in this Section. For the purposes of this Code, earnings shall be attributed to a pollution control facility only to the extent that its operation results in the production of a commercially saleable by-product or increases the production or reduces the production costs of the products or services otherwise sold by the owner of such facility.
35 ILCS 200/11-15