| (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.
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