Article 9. Natural Treatment
9VAC25-790-870. Conventional alternatives.
Article 9
Natural Treatment
A conventional land treatment system utilizes a secondary process for pretreatment of sewage followed by irrigation, overland flow, or infiltration-percolation (or combination thereof) methods for applying the treated effluent to an approved site. Other natural treatment alternatives such as aquatic ponds and constructed wetlands may provide conventional sewage treatment. Reuse of treated effluents that meet the quality standards established by the DEQ for the reclamation and reuse of wastewater will be governed by DEQ permitting programs. However, the sewage treatment process that produces the reclaimed water will remain subject to evaluation by the department as prescribed by this chapter.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
Historical Notes
Former 12VAC5-581-930 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-870, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.
9VAC25-790-880. Land treatment.
A. Site specific information shall be submitted with the preliminary proposal in accordance with this chapter and standards contained in this chapter.
Land treatment systems shall have adequate land for pretreatment facilities, storage reservoirs, administrative and laboratory buildings, and buffer zones, as well as the application sites (field area). The availability of this land should be determined prior to any detailed site evaluation. Site availability information should be obtained concerning:
1. Availability for acquisition or acceptable control.
2. Present and future land use.
3. Public acceptance.
B. Site design. Conformance to local land use zoning and planning should be resolved between the local government and the owner. Adjacent owners should be contacted by the applicant to establish whether significant opposition to the proposed location, or locations, exists. Concerns of adjacent landowners will be considered in the evaluation of site suitability. Public meetings may be scheduled either during or after the evaluation of final design documents so that the department can discuss the technical issues concerning the system design through public participation procedures. Public hearings may be held as part of the certificate/permit issuance procedures.
1. The estimated established site size should be calculated using a typical maximum annual loading depth of 36 inches for slow rate systems and a maximum depth of 72 inches per year for high rate systems to compute the field area size. In addition, the buffer zone area should be estimated using a typical distance of 200 feet from the extremities of the field areas to adjacent property lines. This total estimated site area should be available and permission obtained to gain access to the site for field investigations.
2. When investigating a potential site for application of wastewater, there are some limiting factors, including topography, soils, and vegetative growth (crop), which shall be evaluated early to determine site suitability for a land treatment system. This evaluation should be made in two phases: a preliminary phase and a field investigation phase.
3. The preliminary phase of site evaluations should include the identification of the proposed location of the land treatment system on a recent U.S.G.S. topographic map (7.5 minute quadrangle) or acceptable reproduction or facsimile thereof. A property line survey map should also be available for use in identifying the site location or locations.
4. The 100-year flood elevation should be identified and the proposed pretreatment unit processes should be roughly located in relation to elevation.
5. Preliminary soils information should include a soil site suitability map and include information to identify soil textures, grades, drainage, erosion potential, suitability for certain crops, etc. Information on soil characteristics may be available from either the National Resources Conservation Service (NRS) Office, the local Cooperative Extension Service Agent, or the Soil and Water Conservation Nutrient Management Specialist.
6. The field area available for effluent application may be estimated using typical criteria based on topography and soil characteristics. Field areas should be delineated on topographic maps of the proposed land treatment site.
7. The land treatment system design consultant should arrange a Preliminary Engineering Conference (PEC), as described in this chapter, as a final step in the preliminary phase of the site evaluation. The requirements for soil borings and backhoe pits as needed to study soils should be established at the PEC. A site visit should be scheduled at the PEC that involves the appropriate regulatory personnel and the owner and design consultant.
8. The land treatment system design consultant may not wish to conduct detailed field investigations of site topography, hydrology and soil characteristics prior to the site visit by regulatory personnel and their advisors. However, the proposed locations of field areas and pretreatment units should be established and identified during the site visit. The location of any existing soil borings, backhoe pits, springs, wells, etc., should also be identified during the site visit. Soil borings and backhoe pits may be excavated prior to, during and following the site visit as required. The requirements for soil permeability and hydraulic conductivity testing should be developed either during or shortly after the site visit.
9. Applicants for development of all land treatment systems shall be required to submit at least the minimum required information as required for the appropriate certificate/permit to be issued.
C. Site features. The soil at a potential site should be identified in terms of its absorption capacity and crop production classification, which is a function of physical and chemical characteristics. Important physical characteristics include texture, structure and soil depth. Chemical characteristics that may be important include pH, ion exchange capacity, nutrient levels, and organic fraction. The absorption capacity of a soil may be directly related to soil texture and structure. Soil color may provide an indication of the movement of moisture through soil. Hydraulic conductivity may be estimated from in-field tests using acceptable infiltrometer devices. In addition, the absorption characteristics of a soil may be related to its hydraulic conductivity as measured by both in situ and laboratory tests using acceptable procedures (Table 9). The conductivity tests should be conducted in the most restrictive layer within the depth affected by the land application system. Soil productivity and nutrient management characteristics are discussed in the Virginia Pollution Abatement Permit Regulation (9VAC25-32).
1. Soil evaluation for a land treatment system should follow a systematic approach of selecting proper locations for borings or excavations based on topographic position, slopes and drainage. The physical characteristics of site soils should then be verified by an acceptable number of recorded observations that include soil depth to horizon changes, restrictive layers and parent material, color, texture and structure, for borings or excavations to a minimum depth of five feet.
2. If the soil characteristics differ substantially between borings or excavations, without a logical technical reason for the variation, then additional boring and excavation locations should be studied to identify the nature and extent of the changes in soil patterns throughout the proposed site.
3. The soil characteristics of the proposed site should be described by a qualified technical specialist knowledgeable in the principles of soil science, agronomy, and nutrient management. The long-term impact of land application of the treated effluent on site soils and vegetation or crops must be evaluated by the land treatment system design consultant. Certain minimum soil depths are required for approval of a land application site. The minimum required depth for field areas will depend on the type of land application system as well as the soil characteristics.
4. Representative soil samples shall be collected for each major soil type identified by the field investigation and analyzed for certain parameters in accordance with this chapter.
5. Detailed information on the geologic conditions of the proposed site shall be provided by a geologist or other technical specialist, or specialists, knowledgeable in geohydrologic principles.
a. Detailed information on the site hydrology and groundwater shall be provided by a geologist, hydrologist or other technical specialist, or specialists, knowledgeable in hydrologic principles and ground water hydrology.
b. The depth to the permanent ground water table below the site shall be determined. The location, depth and extent of perched water tables as well as the estimated seasonal fluctuations shall be established. The effect of the permanent and seasonal water tables on performance of the particular land treatment system shall be evaluated by the design consultant.
c. The characteristics of ground water movement under the proposed site should be established and evaluated using piezometer installations or other acceptable methods. The potential impact of the land treatment system on aquifer hydraulics and water quality shall be predicted through the use of modeling and appropriate monitoring devices.
d. The present and planned uses of the aquifer(s) identified as affected by the land treatment system should be determined by the consultant.
D. Land treatment methods. The following methods, or combinations thereof, as regulated by the appropriate permit or certificate, are considered conventional technology in accordance with this chapter:
1. Irrigation - slow rate. Wastewater may be applied by spraying, flooding, or ridge and furrow methods. Irrigation methods are designed not to discharge to surface waters.
2. Rapid infiltration. Wastewater may be applied by spreading and spraying. The system shall be designed to meet all certificate/permit requirements and groundwater standards.
3. Overland flow. This method of wastewater renovation is best suited for soils with low permeability. Generally, a permit or certificate for a discharge to surface waters must be issued.
E. Other alternatives. Natural treatment systems such as aquatic ponds, constructed wetlands and biological/plant filters and other aquatic plant systems are somewhat related to land treatment technology. Natural treatment involves the use of plants in a constructed but relatively natural environment for the purpose of achieving treatment objectives. The major difference between nonconventional natural and conventional treatment systems is that conventional systems typically use a highly managed and controlled environment for the rapid treatment of the wastewater. In contrast, nonconventional natural systems use a comparatively unmanaged environment in which treatment occurs at a slower rate.
1. The use of natural treatment as a part of a land treatment system may take several forms including ponds called "Aquatic Processing Units" (APU). Floating plants such as water hyacinths and duckweed are often used in APU treatment.
2. Constructed wetlands are defined as areas where the wastewater surface is controlled near (subsurface flow) or above (free water surface) a soil or media surface for long enough each year to maintain saturated conditions and the growth of related vegetation such as cattails, rushes, and reeds.
3. Constructed wetlands must provide for groundwater protection and may be used to provide additional treatment to primary, secondary, or highly treated effluents prior to final discharge.
4. Natural (existing) wetlands are considered as state waters and any discharge to them shall be regulated in accordance with an issued discharge permit or certificate.
F. Features. Biological treatment that will produce an effluent either with a maximum BOD5 of 60 mg/l or less, or be of such quality that can be adequately disinfected, if necessary, shall be provided prior to natural treatment, including use of conventional unit operations prior to the land application of treated effluent and advanced treatment prior to reuse.
Disinfection may be required following or prior to land application and other natural treatment. If spray irrigation equipment is utilized, adequate aerosol management including pre-disinfection shall be provided.
Buffer zones around field areas shall be provided in accordance with the monitored maximum microbiological content of the applied effluent as follows, with no reduction in required minimum distances to water sources and channels:
Fecal Coliform Count(1) | Minimum Buffer Distance, Feet |
200 or less | 200(2) |
23 or less | 50(3) |
2.2 or less | None, but no application during occupation of field area(3) |
Notes: | |
(1)Exceeded by no more than 10% or less of samples tested. | |
(2)No public use of field areas. | |
(3)Transient public use may occur after a three-hour drying period following application. | |
1. The owner shall provide sufficient holding time to store all flow during periods either when crop nutrient uptake is limited or nonexistent, the ground is frozen, surface saturation occurs during wet weather, the ground is covered with snow, or the irrigation site or field areas cannot otherwise be operated. The total volume of holding required shall be based on the storage necessary to provide for climatic conditions and the nutrient management requirements of the field area crop. Operational storage necessary for system maintenance shall be provided. Climatic holding periods shall be based on the most adverse conditions of freezing and precipitation, as taken from accurate recorded historical data that are available for the local area (in no case less than 25 years). The storage volume shall be sufficient to prevent any unpermitted discharges to state waters.
2. A minimum holding period of 120 days shall be required when climatic data is not available. System backup storage shall be determined by the complexity of the entire treatment system. An increase or reduction of minimum storage may be considered on a case-by-case basis based on adequate documentation of agronomic crop production and nutrient utilization.
3. The depth of the volume containment for total storage requirements shall be measured above any minimum depth requirements for maintenance.
4. The owner shall provide a minimum reserve area equivalent in size to 25% of the design field area. Additional reserve area may be required as evaluated by the division, if the general conditions of the field area are deemed marginal or in proximity of critical areas or waters. The reserve area shall be capable of being used as a functional area within 30 days of notice.
5. Some allowance for a reduced reserve shall be allowed if additional storage is provided or if there is an alternate treatment mode (e.g., discharge) that can be utilized by the facility.
6. Design criteria for treatment or storage ponds shall be in accordance with this chapter and standards contained in this chapter. In addition, the following requirements shall be met:
a. A minimum operational water depth shall be maintained.
b. Provisions shall be made to allow complete drainage of the pond for maintenance.
c. Duplicate pumps shall be provided if necessary to transport pond flows, with the capacity of each pump sized to handle the maximum rate of flow plus an allowance to deplete stored volumes.
d. Disinfection may be provided either upstream from ponds, or the pond effluent may require disinfection.
e. When chlorination is utilized to disinfect pumped flows, the detention time of the holding pond chlorination facilities shall provide a minimum of 30 minutes of contact time, based on the maximum design pumping rate in accordance with this chapter and standards contained in this chapter.
G. Design loadings. Loading rates shall be based on the most critical value as determined by the liquid and nutrient application rates, or total application amounts for other constituents (such as boron, salts, pH-alkalinity, copper or sodium, etc.), present in such concentrations as could produce pollution of either the soil, cover crop, or water quality. Total weekly application (precipitation plus liquid loading rate) shall not exceed two times the design loading rate. This higher than conventional loading rate shall be used only to balance seasonal water deficits, and groundwater quality standards shall not be exceeded unless a variance to the violated standard has been approved by the department.
1. An overall water balance shall be investigated in accordance with one of the following equations based on design criteria:
a. Irrigation or infiltration
design precipitation + effluent applied = evapotranspiration + hydraulic conductivity.
b. Overland flow
design precipitation + effluent applied = evapotranspiration + hydraulic conductivity + runoff.
2. Design precipitation shall be the wettest year for a 10-year period (return frequency of one year in 10). Minimum time period for this analysis should be 25 years. Average monthly distribution (average percentage of the total annual precipitation that occurs in each month) shall be assumed.
3. Design evapotranspiration (monthly) shall be 75% of average monthly pan evaporation values collected at official weather stations within or contiguous to the Commonwealth of Virginia and should be representative (similar geographically and climatological) of the proposed site.
4. Design hydraulic conductivity shall be a given percentage (see Table 9) of respective laboratory and field measurements that yield the rate at which water passes through the soil under presoaked conditions.
The test methodology should be in accordance with current published procedures made available to the department.
TABLE 9. | |
Type of Test | Percent of minimum measured value to be used in design |
i. Saturated Vertical Hydraulic Conductivity | 7 |
ii. Basin Infiltration | 12.5 |
iii. Cylinder Infiltrometers | 3 |
iv. Air Entry Permeameter | 3 |
v. (Other--to be evaluated by the department) | |
5. During periods of application, the applied nitrogen shall be accounted for through (i) crop uptake and harvest; (ii) denitrification; (iii) addition to surface water and ground water, or storage in soil. In winter, site loadings for slow rate systems shall not exceed the hydraulic design for those particular months. Winter application of treated effluent may be provided only (i) to cool season grasses (ii) following three consecutive days of minimum daily temperatures in excess of 25°F and maximum in excess of 40°F.
6. The annual liquid loading depth for plant nitrogen requirements shall be determined by the following equation:
L = N/2.7C
Where:
N = Crop nitrogen uptake, lb/acre/yr.
C = Total nitrogen concentration, mg/l
C = TKN + NO2-N + NO3-N
L = Annual liquid loadings depth, ft/yr.
TKN = Total KJELDAHL nitrogen = organic N + NH3 - N
7. The monthly nitrogen loading rate design should be distributed over the growth cycle of the particular crop, as much as practicable.
8. If other nutrients, organics, or trace elements are present in concentrations critical to either crops, soil, or water quality, then a total mass balance similar to that for nitrogen shall be investigated for each critical element or compound.
9. The land application design average rate shall be determined by the climatic conditions, selected crops, and soil characteristics. However, the maximum application rates in terms of depth of effluent applied to the field area shall be as follows:
a. One-fourth inch per hour.
b. One inch per day.
c. Two inches per week (one inch per week in forest field areas used for year round application).
H. Field area design. Field area is defined as the area of land where renovation of wastewater takes place (area under actual spray or distribution pattern). The field area shall be designed to satisfy the most critical loading parameter (i.e., annual liquid loading depth) according to the following equation:
Field Area (acres) = Q/D*365/(365-S)
Where:
Q = Wastewater flow in (acre-inches/week)
D = Applied depth in inches/week
S = Minimum required storage capacity + annual resting periods during the application season when no waste can be land applied.
1. The minimum storage capacity shall be the average design volume of flow accumulated over a period of 60 days, unless other storage periods are justified by climatic data. It should be noted that the field area equation does not take into consideration the area needed for reserve capacity or future expansion (no less than 25% of design field area).
2. The field area shall be divided into smaller sections for application to allow for rotational use of these sections. Rotational operation shall be designed to provide the maximum resting periods for field areas. The distribution system shall be designed to meet the requirement for alternating application to the field area sections. Minimum resting periods shall be two days, one day and two weeks for irrigation, overland flow and infiltration-percolation, respectively. Maximum wetting period shall not exceed five days, one week, and one day respectively for irrigation, infiltration-percolation, and overland flow, respectively. Resting and wetting periods depend on soil types, climatic conditions, harvesting requirements, etc.
3. The field area or areas shall be adequately enclosed with suitable fencing to prevent access to livestock and the public where necessary. Signs shall be posted at sufficient intervals (100 to 300 feet) around the entire perimeter of field areas to identify the land treatment operation and specify access precautions.
4. A groundwater monitoring system shall be provided in accordance with the permit or certificate requirements. A minimum of one upgradient and two downgradient monitoring wells shall be provided. The well locations, along with typical well construction specifications, shall be submitted with the proposal. Upon installation, the driller's log shall be submitted. Additional monitoring well locations may be required if deemed necessary upon evaluation of monitoring data. The results of any required sampling and testing of groundwater shall be submitted to the department for evaluation in accordance with the operating permit.
5. Representative agriculturally related soil tests are required on crop dependent systems to ensure adequate vegetative cover. The growing and maintaining of a vegetative cover on application sites is a very integral part of the system. The plants prevent soil erosion and utilize nutrients and water. The system design should provide for a proper balance between applied amounts of water and nutrients. The designer may wish to consult with both agronomic and nutrient management specialists on these matters. The design shall address crop and nutrient management.
6. The wastewater application schedule should be worked around the plans for harvesting. A minimum of 30 days shall be required between the last day of application and utilization of all crops. Crops that will be consumed raw by man shall not be grown in land application field areas.
7. Information on the proposed crops and their intended use may be forwarded to the Virginia Department of Agriculture and Consumer Services for evaluation.
I. Low intensity design. The low intensity application or irrigation field area should be as flat as possible with maximum slopes of 5.0% or less. The design of low intensity irrigation of treated effluent shall provide for nutrient management control. When it is necessary to locate field areas on slopes of eight to 12%, special precautions shall be taken to prevent seepage or runoff of sewage effluent to nearby streams. Dikes or terraces can be provided for field areas, together with runoff collection and return pumping equipment. The maximum field area slope should be 12%. The irrigation field area shall be located a minimum distance of 50 feet from all surface waters.
1. Five feet of well-drained loamy soils are preferred. The minimum soil depth to unconsolidated rock should be three feet. The hydraulic conductivity should be between 0.2-6 inches/hour.
2. The minimum depth to the permanent water table should be five feet. The minimum depth to the seasonal water table should be three feet. Where the permanent water table is less than five feet and the seasonal water table is less than three feet, the field area application rate shall be designed to prevent surface saturation. In addition, underdrain and groundwater pumping equipment may be required.
3. The method of applying the liquid to the field shall be designed to best suit prevailing topographic, climatic, and soil conditions. Two methods of application are:
a. Sprinkler systems with low trajectory nozzles or sprinkler heads to uniformly distribute the applied effluent across a specified portion of the field area. Application is to be restricted in high winds that adversely affect the efficiency of distribution and spread aerosol mists beyond the field areas.
b. Ditch irrigation systems that utilize gravity flow of effluent through ditches or furrows, from which effluent percolates into the soil. For uniformity of distribution, the slope of the field area is to be uniform and constant.
4. The height of spray nozzles, pressure at the spray nozzles and spacing of the laterals shall be adequate to provide uniform distribution of the effluent over the field area. The design height and pressure of the spray nozzles shall avoid damage to vegetation and soil.
5. Adequate provisions shall be made to prevent freezing and corrosion of spray nozzles and distribution lines when the system or a section of the system is not in operation.
6. Appropriate vegetation shall be maintained uniformly on all field areas. Usually water tolerant grasses with high nitrogen uptakes are used. Over seeding with cool season grasses may be necessary during the fall season, prior to October 15 of each year. Silviculture sites and reuse irrigation sites may also be used with this type of land treatment.
J. Rapid infiltration. This form of treatment requires the least amount of land. Renovation is achieved by natural, physical, chemical, and biological processes as the applied effluent moves through the soil. Effluent is allowed to infiltrate the soil at a relatively high rate, requiring a field area with coarse grained soils. This system is designed for three main purposes (i) ground water recharge; (ii) recovery of renovated water using wells or underdrains with subsequent reuse, or (iii) discharge and recharge of surface streams by interception of ground water.
1. Five feet of sand or loamy sand is preferred. Soil grain size should be greater than.05 mm in size. The hydraulic conductivity should be greater than two inches/hour.
2. The permanent ground water table shall be a minimum of 15 feet below the land surface. With this method, a recharge mound is not uncommon and shall be properly evaluated by the consultant. A minimum distance of 10 feet should be maintained between the land surface and the apex of the recharge mound (during a worse-case situation). Lesser depths may be acceptable where under drainage is provided.
3. Spreading and spraying are the two main application techniques that are suitable for infiltration-percolation.
4. Design application rates will vary according to the site area, soil, geology, and hydrology characteristics.
5. The buffer distances from extremities of field areas to private wells should be at least 400 feet.
K. Overland flow. Renovation of wastewater is accomplished by physical, chemical, and biological means as applied effluent flows through vegetation on a relatively impermeable sloped surface. Wastewater is sprayed or flooded over the upper reaches of the slope and a percentage of the treated water is collected as runoff at the bottom of the slope, with the remainder lost to evapotranspiration and percolation. Overland systems should be capable of producing effluent at or below secondary level; however, additional treatment units may be needed to achieve the permitted effluent limitations.
1. Soils should have minimal infiltration capacity, such as heavy clays, clay loams or soils underlain by impermeable lenses. The restrictive layers in the soil should be between one to two feet from the surface to maintain adequate vegetation. The hydraulic conductivity should be less than 0.2 inches/hour. Field area slopes shall be less than 8.0%. Monitoring wells shall be provided.
2. Renovated water shall be collected at the toe of the slope in cut off ditches or by similar means and channeled to a monitoring point and disinfected as required.
3. The effluent application method should achieve a sheet flow pattern that will produce maximum contact between the applied wastewater and the soil medium. This can be accomplished by lateral distribution methods, low pressure sprays and moderate to high pressure impact sprinklers discharging onto porous pads or aprons designed to distribute the applied flow while preventing erosion. Maximum application rates in terms of depth of effluent should be less than 10 inches per week.
4. Perennial field area vegetation shall be required. Hydrophilic or water tolerant grasses are usually grown with this type of system.
L. Alternative design. Information submitted for approval of other natural treatment systems and reuse alternatives shall include performance data obtained from either full-scale systems similar to the proposed design, or pilot studies conducted over a testing period exceeding one year, to a period of two years, based on test results.
Special consideration should be given to the following factors in planning and design of natural systems:
1. Many aquatic plants are sensitive to cold temperatures and may require the use of a protected environment or operation on a seasonal basis. Some plants may be considered unacceptable for use and their growth must be controlled.
2. Control of insects, particularly mosquitoes, is normally required for constructed wetlands and aquatic plant systems. The use of mosquito-eating fish and water depth adjustments are recommended.
3. Some constituents which may be present in wastewaters, particularly those having high industrial loads, are toxic to many aquatic plants. Therefore, tests should be conducted to identify possible toxics prior to selection of the aquatic plant species.
4. Natural systems utilize a higher life form of less diversity than found in more conventional biological treatment systems. This lack of biological diversity may reduce treatment performance. Constructed wetland and aquatic plant systems could be more susceptible to long term process upsets. Therefore, the effects of fluctuations in climate and wastewater characteristics is extremely important in the design of natural systems.
5. Some aquatic plant and animal species have the potential to create a nuisance condition if inadvertently released to natural waterways. Federal, state and local restrictions on the use of certain aquatic plants and animals shall be considered.
6. Harvesting and the use or disposal of aquatic plants should result in removal of organics, solids and nutrients such as nitrogen and phosphorous from the APU effluent. Management of residual matter shall be in accordance with this chapter and standards contained in this chapter.
Statutory Authority
§ 62.1-44.15 of the Code of Virginia.
Historical Notes
Former 12VAC5-581-940 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-880, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008; Volume 39, Issue 5, eff. November 23, 2022.
9VAC25-790-890. Constructed wetlands.
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.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
Historical Notes
Former 12VAC5-581-950 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-890, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.