Virginia Law

A. Design. These unit operations typically consist of inundated or saturated media supporting flora and fauna typically found in natural wetlands. Two basic designs are referred to as submerged flow systems (SFS) and free water surface systems (FWS). Terms that are also considered synonymous with these systems include (i) rock-plant filters; (ii) marsh-reed filters; (iii) microbial rock-plant filters; and (iv) artificial wetland bio-reactors.

1. The design of constructed wetlands is considered nonconventional technology. Design loading values shall be established in accordance with the type of treatment proposed, established performance data, and site specific features. The use of indigenous wetland flora is recommended provided that those species proposed have been evaluated as suitable for such use by technical experts qualified to make such judgements. Certain flora and fauna may be restricted for use in constructed wetlands.

2. All constructed wetlands shall be preceded by pretreatment of sewage, established as at least equivalent to primary treatment in accordance with this chapter and standards contained in this chapter. Constructed wetlands may be preceded by secondary or better treatment when used for effluent polishing, nutrient reduction, or advanced treatment.

3. The design of individual constructed wetlands shall provide the appropriate features specified for pond treatment systems in accordance with this chapter. Required detention times may vary from one day to 20 days or more, in accordance with the type of pretreatment and the issued permit or certificate effluent limitations.

4. The following factors shall be considered in the selection of the design hydraulic and organic loadings: strength of the influent sewage, effectiveness of primary or secondary treatment, type of media, ambient wastewater temperature for winter conditions, and treatment efficiency required.

5. For design flows of 0.1 mgd or more, the treatment system shall be divided into multiple units that can be operated separately. Each unit shall have the ability to be sufficiently drained for operational maintenance. Design considerations may include parallel treatment streams or trains that can be operated independently of each other.

6. The constructed wetland units shall be designed to operate with plug flow type hydraulics. A proper length to width ratio to achieve this condition should be considered in the design of each system. The inlet design shall provide for proper distribution of the influent.

7. All treatment units shall be provided with outlets that can withdraw flow at various depths (a minimum of three). FWS outlets shall be submerged and be able to exclude floating detrital material and scum.

8. 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.

9. All FWS systems shall be situated so as to minimize the adverse effects of the prevailing winds.

10. All systems should maintain a minimum slope along the bottom of at least 0.075% to facilitate draining.

11. Constructed wetland design should allow inlet and outlet depth levels to be raised and lowered in order to (i) vary water levels within the unit basin; (ii) provide the ability to flood the media surface when necessary; and (iii) to drain the unit basin sufficiently for maintenance.

B. Features.

1. SFS systems should be designed to prevent uncontrolled surface ponding of wastewater. Design flow depths exceeding 24 inches shall be justified by evaluation of adequate performance data. The hydraulic loading of these systems should be limited to the effective hydraulic capacity of the media in place. The effective hydraulic capacity will be a function of the clean media's hydraulic capacity reduced by root intrusion, biological slime layer, detritus, algae, and other blockages. Hydraulic loadings exceeding one gallon per day per square feet of total surface area shall be substantiated by evaluation of adequate performance data.

2. FWS systems should be designed to prevent scour, erosion, and plant damage during peak flow periods. Design flow depths exceeding 12 inches shall be justified by an evaluation of adequate performance data. The hydraulic loading of these systems should be limited to the open channel carrying capacity of the unit at full growth. Design organic loadings exceeding 10 pounds of influent BOD₅ per day per acre of surface area shall be substantiated by evaluation of adequate performance data.

3. The flow pattern and depth shall provide for a uniform environment and growth conducive to wetlands.

4. 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. Harvesting of dead wetland vegetation and detritus plant matter is recommended.

5. The following specifications shall be considered as minimum requirements for material specifications of constructed wetlands rock media:

a. 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 Mohs hardness of at least 5.0.

b. 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:

(1) Passing six-inch sieve—100% by weight;

(2) Retained on two-inch sieve—90-100% by weight;

(3) Passing one-inch sieve—<0.1% by weight.

c. 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. 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:

(1) 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.

(2) Media should be uniformly placed avoiding compaction.

(3) Compacting operations should not be allowed on the surface of the media after final placement.

(4) Depressions shall be leveled and smoothed over to prevent ponding.

(5) Provisions should be made prior to planting to provide water and nutrients to the plants if the system start-up will be delayed.

6. Other media specifications shall be in accordance with filtration standards as provided in this chapter.

C. Performance.

1. The total suspended solids (TSS) removal efficiency of the constructed wetland units is dependent on the quiescence of the flow through the units. However, if the facility is unable to meet its permitted parameters, alternate means of solids removal must be pursued.

2. Current constructed wetland technology has not demonstrated the ability to consistently nitrify typical domestic strength sewage influent to meet average flow permit limitations below 5 mg/l of ammonia. The design of any constructed wetland to achieve a permit or certificate effluent limitation of 5 mg/l, or less, of ammonia, shall consider the use of a separate nitrification process.

3. The performance of constructed wetlands is a function of the primary or secondary treatment efficiency preceding the units, i.e., fraction of remaining BOD₅ and TSS.