| (a) Definitions. The following words and terms, when
used in this chapter, shall have the following meanings, unless the
context clearly indicates otherwise.
(1) Constructed wetlands--Designed and man-made complexes
of saturated substrates, emergent and submergent vegetation, animal
life, and water that simulates natural wetlands. Constructed wetlands
as described in these rules are meant to function exclusively as wastewater
treatment units. They consist of two varieties: submerged flow systems
and free water surface systems. Combinations of these varieties may
also be acceptable methods of treatment. Constructed wetlands are
constructed treatment systems that are inundated or saturated by wastewater
flows at a frequency and duration sufficient to support, and under
normal circumstances do support, a prevalence of flora and fauna typically
adapted for life in saturated or inundated soil conditions, i.e.,
a wetland. Terms that are considered synonymous with constructed wetlands
treatment systems are man-made wetlands, engineered wetlands, artificial
wetlands, rock reed filters, vertical bio-reactor, submerged flow
systems, free water surface systems, artificial marsh, marsh reed
filter, botanical reactor, rooted emergent wetland filters, and microbial
rock plant filters.
(2) Submerged flow--A submerged flow system consists
of a lined basin or channel filled with a granular rock media. The
media supports the growth of both emergent vegetation on the surface
and fixed bio-film on the subsurface. The wastewater flows horizontally,
vertically, and transverses the subsurface of the rock media through
interstices of the media and vegetation root structure. Wastewater
levels are nominally maintained at least six inches below the rock
media surface. Total rock media depth shall not exceed 24 inches.
(3) Free water surface--The free water surface system
consists of a lined basin or channel partially filled with soil or
other media suitable for supporting rooted emergent and/or submergent
vegetation. Wastewater flows over the top of the media and through
the stalks of the emergent and/or submergent vegetation at an average
depth no greater than 18 inches.
(b) General considerations. These guidelines are intended
for an exemplary basis. The criteria for design, construction, and
operation should be based on data collected from operational data
of similar facilities, pilot-plant and bench-scale studies, and/or
proper engineering and scientific investigations which should be submitted
at the time of review.
(1) Algal mat removal. Provisions shall be made for
algal mat removal from primary treated effluent prior to entering
into the wetland units. These provisions may include bar screens,
adjustable inlets, baffles, and other methods as approved by the commission.
(2) Natural wetlands. The commission will prohibit
the use of any land defined as a wetland by the United States Army
Corps of Engineers in 40 Code of Federal Regulations §122.2 and
subject to regulations found in the federal Clean Water Act, §404,
for use in wastewater treatment. Any subsequent construction activity
located in a natural wetland may require a permit from the United
States Army Corps of Engineers.
(3) Typical wetlands vegetation. Suggested flora for
constructed wetlands in the State of Texas, include the following.
(A) Emergent aquatic vegetation such as Typha spp.
(cattails), Scirpus spp. (bulrush), Sagittaria spp. (arrowhead), Phragmites
spp. (reeds), Juncus spp. (rushes), Eleocharis spp. (spikerush), caladium
spp. (elephant ear), or other acceptable species may be used.
(B) Floating aquatic vegetation such as Lemna spp.
(duckweed), Hydrocotyle umbellata (water pennywort), Limnobium spongia
(frogbit), Nymphaea spp. (water lily), Wolffia spp. (water meal),
or other acceptable species may be used.
(C) The use of indigenous plants is strongly recommended
provided that these species have been proven suitable for use in wastewater
treatment. Procurement of these seed plants from natural wetlands
should ensure the natural wetlands are not significantly impacted.
(D) The use of all harmful or potentially harmful wetlands
plants and organisms, as described in 31 TAC §§57.111 -
57.118 (concerning Potentially Harmful Fish, Shellfish, or Aquatic
Plants) and 31 TAC §§57.251 - 57.258 (concerning Introduction
of Fish, Shellfish, and Aquatic Plants), must first be approved by
the Texas Parks and Wildlife Department.
(4) Allowed uses. Constructed wetlands can be used
as a:
(A) secondary treatment unit; or
(B) advanced secondary treatment unit.
(5) Primary treatment. All systems shall be preceded
by primary treatment. Systems may be preceded by secondary treatment.
Primary treatment can include septic tanks, Imhoff tanks, facultative
lagoons, aerated lagoons, stabilization ponds, and any other treatment
process which removes the settleable solids and floating material.
The design of these pretreatment units shall conform with applicable
state design criteria.
(6) Liners. When required in the facility's permit
or by the commission, basins shall be lined with an impermeable liner,
either soil or synthetic, as described in subparagraphs (A) and (B)
of this paragraph.
(A) Soil.
(i) All placed clay or in-situ soils used for basin
liners shall be certified by adequate geotechnical test results. For
all in-situ soils, the design engineer shall present adequate soil
borings information which ensures the homogeneousness of the selected
soil. Placed clay or in-situ soils shall have a measured permeability
of less than 10-7 cm/sec. and/or the
following characteristics:
(I) more than 30% passing a #200 mesh sieve;
(II) liquid limit greater than 30%;
(III) plasticity index greater than 15;
(IV) no clods larger than two inches;
(V) minimum compacted thickness of two feet for placed
clay liners and four feet for in-situ soils.
(ii) All placed clay liners shall be installed according
to the following criteria. However, when using in-situ soils for the
required liner, only the upper six inches should be reworked as follows:
(I) maximum loose lift of eight inches, six inches
compacted;
(II) minimum compaction effort of 95% Standard Proctor
(ASTM D-698);
(III) liners shall be keyed into the existing in-situ
soils.
(B) Synthetic. All synthetic liners shall have a minimum
thickness of 30 mils and contain underdrain leak detection which shall
consist of leachate collection and detection systems. Proper installation
of the materials mentioned in subparagraph (A) of this paragraph shall
be described in the project's specifications. The liner material shall
be resistant to or protected from ultraviolet (UV) light degradation.
(7) Flood hazard analysis. The 100-year flood plain
elevation shall be provided. Proposed treatment units which are to
be located within the 100-year flood plain will not be approved for
construction unless protective measures satisfactory to the commission
(such as levees or elevated treatment units) are included in the project
design. If construction inside the 100-year flood plain is necessary,
authorization from the proper coordinating authority must be obtained.
All units must either be three feet above the 100-year flood plain
or have a berm with at least three feet of freeboard above the 100-year
flood plain.
(8) Berms. Berms shall have side slopes of no steeper
than 3:1. Berms shall be lined or constructed of impermeable clay
as described in the preceding section pertaining to soil liners. All
clay berms shall be keyed into the clay liner.
(9) Configuration. Facilities with permitted average
daily flows over 100,000 gallons per day shall conform with the following
configuration standards.
(A) Multiple units. The treatment system shall be divided
into multiple units that can be operated separately. Each unit shall
have the ability to be completely drained.
(B) Parallel trains. Design considerations may include
parallel treatment streams or trains which can be operated independently
of each other.
(C) Length to width ratio. The units shall be designed
to operate as plug flow channels. A proper length to width ratio to
achieve this condition should be considered in the design of each
system.
(D) Switching capability. The design shall allow for
each unit to be taken out of service at any time and its flows routed
to another unit. The treatment system must be capable of treating
the daily average flow with the largest unit out of service.
(E) Wind protection. All free water surface (FWS) systems
shall be situated so as to minimize the adverse effects of the prevailing
winds.
(F) Minimum slope. All systems should maintain a minimum
slope along the bottom of at least 0.075% to facilitate draining.
(10) Flow distribution.
(A) Inlets. All treatment units shall have multiple
inlets (a minimum of three) and provide a method to mitigate erosion
of the media.
(B) Outlets. All treatment units shall have multiple
outlets (a minimum of three). FWS outlets shall be submerged and be
able to exclude floating detrital material and scum.
(C) Water levels. The design should allow inlets and
outlets to be raised and lowered, so that water levels within the
basin can similarly be varied and provide the ability to flood the
beds when necessary.
(D) Basin hydraulic design.
(i) Submerged flow systems (SFS). SFS systems should
be designed to prevent surface ponding of wastewater. The hydraulic
loading of these systems should be limited to the effective hydraulic
capacity of the media in place. This effective hydraulic capacity
will be a function of the clean media's hydraulic capacity reduced
by root intrusion, slime layer, detritus, algae, and other blockages.
(ii) Free water surface systems. FWS systems should
be designed to prevent scour, erosion, and plant damage during peak
flow periods. The hydraulic loading of these systems should be limited
to the open channel carrying capacity of the unit at full growth.
(11) Flow equalization. Flow to the units shall provide
for a uniform environment and growth conducive to wetlands.
(12) Initial vegetation spacing. Plants should be placed
no greater than 66 inches apart (center to center). All plants to
be used should be healthy, insect free, and undamaged. A broad diversity
of plant species within any unit is recommended.
(13) Total suspended solids (TSS) removal. The TSS
removal efficiency of the wetland system is dependent on the quiescence
of the system. However, if the facility is unable to meet its permitted
parameters, alternate means of solids removal must be pursued.
(14) Nitrification. Current wetland technology has
not proven the ability to consistently nitrify typical domestic strength
sewage to meet average permit limitations below 5.0 mg/liter. The
design of any wetland proposed for use in this type situation will
incorporate a separate nitrification process.
(15) Harvesting. Harvesting of dead wetland vegetation
and detritus plant matter is recommended.
(c) Submerged flow system design.
(1) Basic design parameters. SFS wetlands are sized
according to primary and/or secondary treatment efficiency preceding
the units, i.e., fraction of remaining five-day biochemical oxygen
demand (BOD5 ), and the permitted 30-day
average effluent discharge concentration of BOD5 .
The following factors shall be considered in the selection of the
design hydraulic and organic loadings: strength of the influent sewage,
effectiveness of primary and/or secondary treatment, type of media,
ambient wastewater temperature for winter conditions, and treatment
efficiency required.
(A) Rock/media design. The following are minimum requirements
for material specifications of the rock media.
(i) Crushed rock, slag, or similar media should not
contain more than 5.0% by weight of pieces whose longest dimension
is three times its least dimension. The rock media should be free
from thin, elongated, and flat pieces and should be free from clay,
sand, organic material, or dirt. The media should have a Morhs hardness
of at least 5.0.
(ii) Rock media, except for the top planting layer,
should conform to the following size distribution and gradation when
mechanically graded over a vibrating screen with square openings:
(I) passing six-inch sieve--100% by weight;
(II) retained on two-inch sieve--90% to100% by weight;
(III) passing one-inch sieve--less than 0.1% by weight.
(B) Installation of the rock media.
(i) Rock media shall be rinsed or washed to remove
sediment. This washing should be sufficient to remove any significant
amounts of dirt or accumulated debris.
(ii) The proper placement and installation of media
is vital to the success of the system. Undue compaction exerted on
the media's surface, as it is installed and after its installation,
can fracture and consolidate the media. The introduction of foreign
fine particles and fracturing can adversely affect the system's hydraulic
conductivity. Therefore, the following guidelines are recommended.
(I) A layer of smaller rock (0.5 - 1.0 inches) may
be used on the top of the unit to ease planting of the vegetation
and aid in vector control.
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