(a) The effluent sets in §309.4 of this title (relating to Table 1, Effluent Limitations for Domestic Wastewater Treatment Plants) are intended to represent standard levels of treatment normally required for domestic wastewater treatment plants.     
      (b) Modifications to the effluent sets listed in §309.4 of this title may be considered by the commission when effluent limits more stringent than secondary treatment are required in order to maintain desired water quality levels.     
      (c) On a case-by-case basis, modifications to the effluent sets listed in §309.4 of this title may be considered by the commission for certain existing, natural systems which cannot consistently meet pH or total suspended solids (TSS) limitations due to the inherent variability of a particular system. Modifications to the effluent sets may be allowed for a natural system designed for treatment or polishing with a discharge directly into surface water in the state. Natural systems include, but are not necessarily limited to, aerated lagoons followed by stabilization ponds, facultative ponds, stabilization ponds, and constructed wetlands. For the purpose of this chapter, playa lakes are not considered natural systems. The commission will consider the following factors in approving a modification to the effluent sets:      
        (1) Any modification shall not allow a discharge which would cause a violation of the commission's surface water quality standards or any applicable total maximum daily loads or wasteload evaluation.       
        (2) A proposal for a modification must be supported by an engineering report, prepared and sealed by a licensed Texas professional engineer representing the permit applicant, which justifies the request for modification with specific information relating to the proposed design and that design's inherent limitations. For considering a request for modification of an existing system that cannot achieve permitted pH or TSS limitations, the engineering report must also document past efforts of design modification, operation, and maintenance, and include data showing for the past three years, influent and effluent hydraulic and organic loadings and the resultant effluent quality achieved.       
        (3) The commission may set narrative effluent limitations and effluent monitoring requirements as an alternative to a specific numerical effluent limitation when a specific numeric effluent limitation cannot be met because of, but not limited to, seasonal or operational factors. These narrative requirements shall ensure that necessary operational and maintenance actions are consistently carried out by the permittee to meet applicable water quality standards. The commission may request resumption of the original numerical limitations during the next permit renewal or amendment based on a review of the discharge effluent data.       
        (4) The commission may suspend setting a specific numerical effluent limitation for a temporary period of time not to exceed the remainder of the permit term, pending a review of the actual performance of a natural system's design as long as the facility meets paragraph (1) of this subsection. During any temporary suspension, the permittee must document that the system is operated and maintained for optimal performance in accordance with an operation and maintenance manual prepared in accordance with Chapter 217 of this title (relating to Design Criteria for Domestic Wastewater Systems) or Chapter 317 of this title (relating to Design Criteria Prior to 2008) as applicable and is meeting water quality standards. After review of performance data and related information submitted by the permittee in a permit application, for permit renewal or amendment, or when submitted at the request of the executive director, the commission may set specific numerical effluent limitations consistent with this subchapter and the performance documented for the particular system.       
     Source Note:        The provisions of this §309.2 adopted to be effective July 27, 1988, 13 TexReg 3514; amended to be effective June 5, 1998, 23 TexReg 5723; amended to be effective January 9, 2020, 45 TexReg 370