Section 217.211 - Constructed Wetlands-Subsurface Flow System General Design(a) A wetted subsurface media must allow adequate root penetration. The type of subsurface media must be identified in the engineering report.(b) The operational water depth of a subsurface flow system (SFS) wetland must not exceed the lesser of: (1) 18 inches at design flow; or(2) the maximum normal root depth of the emergent plant species used in the SFS wetland.(c) Seasonal draw down of the water level must be performed to encourage deeper root penetration into the wetted media.(d) Plant spacing must be sufficient to allow maturity of a wetlands flora ecosystem, but must not exceed 36 inches on center.(e) Configuration. An SFS wetland must include the following minimum configuration standards: (1) Multiple cells. An SFS wetland must include multiple cells that can be operated independently, allowing individual cells to be removed from service while maintaining system operations.(2) Cell Size. SFS wetland cells must meet permit effluent limitations, even when any single cell is removed from service.(3) Hydraulic profile.(A) An SFS wetland must maintain between 6.0 inches and 9.0 inches of dry media cover at design flow. The wetland must have at least 2.0 inches of upstream media cover during peak flow conditions, and not more than 12.0 inches of upstream media cover during diurnal low flow conditions.(B) An SFS wetland hydraulic profile must be based on Figure 1: 30 TAC §217.211(e)(3)(B), unless an alternate design method is justified in the engineering report. The engineering report must include the method, the source of the method, and all supporting calculations and documentation. Attached Graphic
Attached Graphic
(4) Maximum depth. The maximum wetted media depth of an SFS wetland is the lesser of:
(A) 24 inches at design flow; or(B) the maximum normal root depth for a planned primary population of emergent plant species.(5) Minimum slope. An SFS wetland cell must have an adequate bottom slope to facilitate drainage for maintenance and to maintain media water depth over the entire cell length under all operational flow conditions.(6) Parallel trains. An SFS wetland must have parallel treatment trains to increase operational flexibility.(f) Flow Distribution. An SFS wetland must be designed with effective flow distribution and collection by meeting the following standards for inlets and outlets, submergence, maintenance, and staged influent feed. (1) Inlets and Outlets. (A) The inlet and outlet system of an SFS wetland cell must ensure uniform distribution of influent flow and uniform collection of effluent flow across an entire cell.(B) The inlet and outlet devices of an SFS wetland cell must not cause locally high velocities that could result in movement of wetland media.(C) Each inlet and outlet system must be adjustable to allow variation in operational water level and flooding of a cell for weed control.(2) Submergence. Each inlet and outlet of an SFS wetland must be below the media surface.(3) Maintenance. Inlet and outlet devices must allow inspection, cleaning, and maintenance.(4) Staged influent feed. If an average influent five-day biochemical oxygen demand concentration greater than 200 milligrams per liter is anticipated, an SFS wetland must allow for staged influent feed to improve process control.(g) Organic Loading and Treatment Efficiency. (1) A constructed wetland process must be based on the design organic loading of the influent to the constructed wetland.(2) The design of an SFS wetland must be based on the organic removal treatment efficiency for an SFS wetland on Equation H.5. in Figure: 30 TAC §217.211(g)(2), unless an alternate method to determine the organic removal treatment efficiency is justified in the engineering report. The engineering report must include the method, the source of the method, and all supporting calculations. Attached Graphic
(h) Temperature. An SFS wetland must be able to treat the wastewater treatment facility's wastewater at all water temperatures.(i) Vector Control. Vegetation maintenance, including removal of excessive plant litter and detritus, is required to limit mosquito production.(j) Media Design. SFS wetland media must meet the following minimum requirements. (1) The media must be hard rock, slag, or other clean, comparable media material.(2) The media must contain less than 0.1% by weight of clay, sand, and other fine materials.(3) The media materials must have a Mohs hardness of at least 5.0.(4) The media must be resistant to acidic conditions.(5) Synthetic media is a non-conforming or innovative technology and is subject to the requirements of § 217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals).(6) Media gradation and uniformity must be used to determine the SFS wetland's hydraulic conductivity.(7) The media must be placed in an SFS wetland by light-weight equipment to prevent introduction of clay or other undesirable materials, to avoid compaction, clogging of the media, and damage to the liner.(8) If an SFS wetland has gravel media larger than 1.5 inches in diameter, it must include a top layer of smaller gravel to encourage healthy plant rooting. The layer of smaller gravel must be above the normally saturated media zone. Also, an SFS wetland must include a transitional (medium grade) layer between small gravel and coarse gravel to minimize small gravel migration into lower void spaces.30 Tex. Admin. Code § 217.211
The provisions of this §217.211 adopted to be effective August 28, 2008, 33 TexReg 6843; Amended by Texas Register, Volume 40, Number 47, November 20, 2015, TexReg 8329, eff. 12/4/2015