Texas Administrative Code
Title 30 - ENVIRONMENTAL QUALITY
Part 1 - TEXAS COMMISSION ON ENVIRONMENTAL QUALITY
Chapter 317 - DESIGN CRITERIA PRIOR TO 2008
Section 317.15 - Appendix G-General Guidelines for the Design of Constructed Wetlands Units for Use in Municipal Wastewater Treatment
Universal Citation: 30 TX Admin Code § 317.15
Current through Reg. 49, No. 52; December 27, 2024
(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% to 100% 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.
(II) Media should be gently put in place,
avoiding excessive dropping, jostling, and abusive handling.
(III) Heavy machinery should not be allowed
on the surface of the media after final placement. If machinery is allowed on
the surface, all tire ruts should be smoothed over to prevent ponding in
ruts.
(IV) Provisions should be
made prior to planting to provide water and nutrients to the plants if the
system start-up will be delayed.
(2) Organic loadings. The following tables
present typical ranges for detention time within the system in days. Each
detention time represents combinations of different classes of secondary and
advanced secondary treatment and different effluent parameters. Design
engineers may submit sufficient operating data for similar installations,
and/or actual field conditions to justify their efficiency calculations. These
times represent the theoretical detention time of wastewater within the basin.
Therefore, the amount of detention volume available is equal to the basin's
volume multiplied by the average porosity of the media. Evapotranspiration and
precipitation should also be considered when calculating detention time. The
tables are based upon an average effective porosity media of 32%, and an
average wastewater treatment plant influent BOD5 of 200
mg/liter.
(A) Secondary and advanced
secondary treatment. The detention times in Table Number 1 are based on the
fractional BOD5 remaining in the wetland system's
influent and the permitted effluent limits. For permitted effluent
BOD5 concentration and removal efficiencies that fall
between the listed quantities, linear interpolation is permissible. Table
Number 1 is based on the following assumptions:
(i) ambient winter conditions wastewater
temperature of 7.5 degrees Centigrade (45.5 degrees Fahrenheit); and
(ii) an average wastewater treatment plant
influent BOD5 of 200 mg/liter. If the wastewater winter
temperature is lower than that indicated above, detention times must be
modified.
(B) Advanced secondary treatment following
pond systems only. The detention time is based on the assumption that the
treatment facility is composed of a facultative lagoon followed by two
stabilization ponds, each sized according to the current state design criteria
found in this chapter. For applications where pond effluent is to be polished
to meet an effluent BOD5 concentration of 30 mg/liter, a
minimum of one-day detention time through the wetland system will be
required.
(3) Oxygen
loadings. Since SFS should function in an aerobic environment, the wastewater
dissolved oxygen level is critical. Surface area needed to maintain sufficient
oxygen transfer through developed plant roots shall be designed based on
approved and acceptable engineering methods.
(d) Free water surface system design.
(1) Basic design parameters. FWS wetlands are
sized according to primary and/or secondary treatment efficiency, i.e.,
fraction of remaining BOD5, and the permitted 30-day
average effluent discharge concentration of BOD5. The
following factors are 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.
(2) Organic loading. The following tables
present typical ranges for detention time within the wetland system in days.
Each detention time represents combinations of different classes of primary and
secondary treatment and the different effluent parameters. Design engineers may
submit sufficient operating data for similar installations, and/or actual field
conditions to justify their efficiency calculations for the wetland system. The
tables are based on the following assumptions: specific surface area of the
media (stems, roots, detritus, etc. 15.7 m2/m3; ambient winter conditions
wastewater temperature of 7.5 degrees Centigrade (45.5 degrees Fahrenheit); and
an average wastewater treatment plant influent BOD5 of
200 mg/liter.
(A) Secondary treatment. These
detention times are based on the type and efficiency of the primary treatment
unit which precedes the FWS wetlands.
(i)
Septic tank or facultative pond as primary treatment method.
(ii)
Imhoff tank or clarification as primary treatment method.
(B) Advanced secondary treatment. The
detention times given in Table Number 4 are based on the fraction of
BOD5 remaining after secondary treatment. Table Number 4
assumes a wastewater treatment plant influent BOD5 of
200 mg/liter. For percentages that fall between the listed quantities, linear
interpolation is permissible.
(C)
Vector control. The presence of mosquitos and other vectors has been associated
with open water. Since the FWS systems will have open water surfaces, vector
control must be a priority. Vector control mechanisms using natural controlling
agents such as introduction of Gambusia spp. (mosquito fish) have been proven
effective. However, if the predatory fish are used to control vectors,
provisions must be made within the basin for designated open water areas so the
fish can surface for oxygen. At least 20% of the basin's surface should be open
to the atmosphere. Other methods of vector control may be considered. However,
the introduction of chemicals (such as pesticides) should be carefully
evaluated so that there are no adverse effects on vegetation or on effluent
water quality.
Disclaimer: These regulations may not be the most recent version. Texas may have more current or accurate information. We make no warranties or guarantees about the accuracy, completeness, or adequacy of the information contained on this site or the information linked to on the state site. Please check official sources.
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