30 Tex. Admin. Code § 311.32
All domestic sewage treatment permit applicants, all permittees who construct authorized treatment facility expansions, and all permittees who apply for increases in their permitted effluent flows, who propose to dispose of treated sewage effluent by discharge into the waters of the state in the Lake Houston watershed shall, at a minimum, achieve the effluent treatment level specified in Effluent Set 2-N and A in § 309.4 of this title (relating to Table I-Effluent Standards for Domestic Wastewater Treatment Plants), except as otherwise provided in this section. All permittees within the Lake Houston watershed that are not covered by the preceding sentence shall achieve the treatment levels specified in Effluent Sets 2-N and A in § 309.4 of this title (relating to Table I-Effluent Standards for Domestic Wastewater Treatment Plants) 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 variances to allow less stringent effluent limitations as are necessary based on considerations consistent with the provisions of the Texas Water Code.
30 Tex. Admin. Code § 311.32