SECTION 317.15. Appendix G--General Guidelines for the Design of Constructed Wetlands Units for Use in Municipal Wastewater Treatment

(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.

(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.

Attached Graphic

(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.

Attached Graphic

(ii) Imhoff tank or clarification as primary treatment method.

Attached Graphic

(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.

Attached Graphic

(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.

Source Note: The provisions of this §317.15 adopted to be effective November 26, 2015, 40 TexReg 8341