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RULE §317.15Appendix 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.


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