All industrial wastewater treatment permit applicants, all permittees who construct authorized treatment facility expansions, and all permittees who apply for increases in pollutant loadings, who propose to dispose of treated industrial wastewater effluent by discharge into the waters of the state in the Lake Houston watershed shall achieve effluent treatment levels commensurate with the goals of this section. All permittees within the Lake Houston watershed that are not covered by the preceding sentence shall achieve the effluent requirements of this section on or before July 1, 1988. Time extensions may be specified by the Texas Water Commission in wastewater discharge permits on a case-by-case basis where circumstances so dictate, but in no case will extend beyond January 1, 1990. The Texas Water Commission may require more stringent effluent limitations where advisable to protect water quality. The Texas Water Commission may authorize a variance to allow less stringent effluent limitations based on considerations consistent with the provisions of the Texas Water Code.