R

FIGURE 7.6 Schematic diagram of nitrification filter bed.

sand filters normally operate with a hydraulic loading of less than 5 gal/ft2-d (0.2 m/d). Gravel is used in the NFB to increase the hydraulic conductivity of the media and permit much higher hydraulic loading rates on the system. The hydraulic loading (with a 3:1 recycle ratio) is about 100 gal/ft2-d (4 m/d) on one of the operational NFB systems at Benton, Kentucky.

The design procedure for the NFB is based on nitrification experience with trickling filter and RBC attached growth concepts where the removal capability is related to the specific surface area available for development of the attached growth nitrifying organisms (USEPA, 1993). Several conditions are required for successful nitrification performance:

• Exposure to the atmosphere or to an oxygen source must be sufficient to maintain aerobic conditions in the attached film of nitrifying organisms.

• The surface must be moist at all times to sustain organism activity at optimum rates.

• Alkalinity must be sufficient to support the nitrification reactions (=10 g alkalinity per 1 g ammonia).

Equation 7.24 can be used to determine the specific surface area (Av) required to achieve a particular effluent ammonia (Ce) at the bottom of the NFB:

TABLE 7.9

Specific Surface Area for a Variety of Media Types

Specific Surface

Specific Surface

Media Type

(in.)

(mm)

(ft2/ft3)

(m2/m3)

Ratio

ks (m/d)a

Medium sand

0.12

3

270

886

40

1

Pea gravel

0.57

14.5

85

280

28

104

Gravel

1.0

25

21

69

40

105

Gravel

4.0

102

12

39

48

106

Plastic media, random pack

1.0

25

85

280

90

107

Plastic media, random pack

2.0

50

48

157

93

108

Plastic media, random pack

3.5

89

38

125

95

108

a Maximum potential hydraulic conductivity; NFB design should utilize a small fraction of this value to ensure unsaturated flow.

a Maximum potential hydraulic conductivity; NFB design should utilize a small fraction of this value to ensure unsaturated flow.

where

A = Specific surface area (m2/kg NH4d; 1 m2/kg NH4/d x 4.882 = ft2/lb-d).

Ce = Desired NFB effluent ammonia concentration (mg/L).

Kt = Temperature-dependent coefficient: [1(1.048)(T-20)] at 10°C+, and [0.626(1.15)(T-10)] at 1-10°C.

Equation 7.24 is based on curve fitting of performance data from attached growth nitrification reactors, and the units involved are not dimensionally compatible. It will still, however, give a reasonably accurate estimate of the specific surface area required to achieve effluent ammonia levels in the range of 0 to 6 mg/L. Equation 7.24 has been verified in a recent full-scale application at Mandeville, Louisiana (Reed et al., 2003).

Information on the specific surface area per unit volume for a number of potential media types is presented in Table 7.9. The specific surface area available for the natural sand and gravel media types tends to increase as the potential hydraulic conductivity decreases. The plastic media listed in Table 7.9 are spherical in shape with a variety of internal members to increase the available surface area per unit. These have a very high specific surface and an equally high potential hydraulic conductivity. Rigid corrugated plastic media and flexible hanging plastic sheets are also available. These plastic media are commonly used in trickling filter units designed for nitrification and could be used in the same capacity as a nitrification component for a wetland system. A relatively small container filled with this plastic media has been proposed as a nitrification component for small-scale wetland systems.

The natural sand and fine gravel media do not drain quickly, and it is usually necessary to design for intermittent wet and dry cycles to allow a portion of the bed to drain and restore aerobic conditions. The coarse gravel and plastic media can be exposed to continuous hydraulic loading (at a reasonable rate) and still maintain aerobic conditions in the media. It is also necessary to keep the media surfaces completely wet at all times to ensure optimum responses from the nitrifying organisms. The minimum hydraulic loading, for this purpose, on the plastic media is in the range of 590 to 1757 gpd/ft2 (24 to 72 m3/d-m2) of bed surface area. The typical hydraulic loading on an intermittent sand filter bed is 0.75 to 15 gpd/ft2 (0.03 to 0.06 m/d). Recycle may not be necessary as long as wetting of the media is complete and sufficient oxygen is present in the profile. In the case of sand and fine gravel systems, a larger bed area, divided into cells, is provided to allow for intermittent hydraulic loading and drainage periods. Assuming one half of the system is draining at any one time, the pumping rate would have to be 2Q as compared to 1 Q for a continuously operated bed.

Typically, the effluent from the wetland cell is applied to the NFB to ensure a low BOD concentration in the liquid. Nitrification in the NFB can be expected when the applied water has a BOD/TKN ratio of less than 1.0 and the soluble BOD concentration is less than 12 mg/L (USEPA, 1993). The ratio of soluble to total BOD in typical wetland effluents is about 0.6 to 0.8 (Reed, 1991, 1993).

Equation 7.15 is used to determine the specific surface area required to achieve the necessary effluent ammonia level. The characteristics of an appropriate media are selected from Table 7.9 to determine the volume of media required. Usually, the NFB bed will be 1 to 2 ft (0.3 to 0.6 m) deep and extend the full width of the wetland cell to ensure complete mixing with the wastewater flowing through the wetland. The use of sprinklers, for distribution on top of the NFB, is recommended to provide proper distribution and maximum aeration. In cold climates with extended periods of subfreezing temperatures, an exposed bed with sprinklers, as shown in Figure 7.6, may not be feasible. In this case, the use of plastic media in a protected tank or similar container should be considered. Such a tank would have to be vented to provide the necessary air flow.

A design for a retrofit NFB at an existing wetland has to conform to the existing wetland configuration and effluent water quality conditions. In many cases, the combination of an NFB and a wetland designed for BOD removal may be more cost effective than the much larger area required for a wetland to remove both BOD and ammonia. In this case the wetland is sized for BOD removal to 5 to 10 mg/L; the ammonia removal expected in this wetland is determined with appropriate models, and then the NFB is designed for the balance of ammonia requiring removal. A cost comparison will then show if the NFB combination is more economical than a larger wetland system.

7.10 DESIGN OF ON-SITE SYSTEMS

On-site systems are defined as relatively small facilities serving a single waste-water source or possibly a cluster of residential units in a development. Usually, the on-site system is at the same location as the wastewater source, but in some cases pumping to a remote site is used if suitable soils for in-ground discharge do not exist at the original wastewater source. Preliminary treatment is typically provided by septic tanks or similar devices, but in some cases packaged secondary treatment plants have been used. In most cases, the advantages inherent in the SSF wetland concept (e.g., no insect vectors, subsurface flow so no risk of public contact with the untreated wastewater) favor its use for these on-site systems. The disposal of the final effluent from a wetland is still a project requirement, even in arid climates where evaporation and seepage (if allowed) may account for a large fraction of the wastewater.

Surface discharge and in-ground disposal are the only two alternatives available. In-ground disposal methods are described in Chapter 10 of this book. Surface discharges must meet the applicable state and local discharge requirements; many states and local governments will not permit surface discharges from small on-site systems so this alternative must be explored with the appropriate agencies prior to any design. The site investigation requirements for on-site in-ground disposal are discussed in Chapter 10. The simple percolation test may be marginally adequate for very small systems at single-family dwellings but is not adequate for larger facilities and flows. In these cases, it is necessary to determine the actual hydraulic conductivity of the in situ soils and to determine the ground-water position and gradient to ensure that mounding and system failure will not occur.

Most current criteria for in-ground disposal systems via leach fields, beds, mounds, etc. specify a hydraulic loading rate (gpd/ft) based on the results of the site investigation as modified by prior performance experience. These hydraulic loading rates are based in part on the hydraulic characteristics of the soil and in part on the clogging potential of typical septic tank effluent, because a clogging layer accumulates at the soil/disposal bed interface.

Because the use of a wetland system prior to the disposal step can produce the equivalent of tertiary effluent, the potential for clogging is significantly reduced, and it should be possible to reduce the surface area of the disposal bed or trenches significantly. A disposal bed or trench, after a wetland system can typically be at least one third to one half the "normal" infiltration area because of the improved water quality. It is still essential to measure (or estimate for very small systems) the actual hydraulic conductivity of the receiving soils to validate the size reduction. Heavy clay soils, for example, have limited permeability regardless of the quality of the water applied. In some cases, in-ground disposal on coarse, highly permeable soils is also prevented because the applied wastewater does not have enough time and contact for adequate treatment. The use of a on-site wetland prior to in-ground disposal should alleviate the problem and allow development on such soils.

Several approaches are available for designing on-site wetland systems. One of the most prominent is to utilize guidelines issued by the TVA. In an evaluation published by the USEPA (Steiner and Watson, 1993), it was concluded that these TVA guidelines are probably adequate for the design of small-scale systems at single-family dwellings but are deficient for larger flows and for surface discharging systems. The deficiencies relate to the lack of soils investigations for the larger disposal fields, the lack of design criteria for nitrogen removal, and the lack of any temperature dependence, which will affect winter water quality in colder climates. The USEPA evaluation recommended that the design of on-site systems should follow the same procedures used for large-scale systems because the design principles and thermal constraints are the same. As a result, the design procedures found in earlier sections of this chapter should also be used for on-site wetlands.

The USEPA document recommended several simplifying assumptions for the design of wetlands for smaller on-site systems:

• Determine the design flow; 60 gpd (0.23 m/d) is a reasonable assumption for per-capita flow for residential systems. State or local criteria will govern.

• Use a multicompartment septic tank. Use one tank for single-family dwellings; use two or more tanks in series for larger scale (>10,000 gpd) projects. The total volume of the tanks should be at least twice the design daily flow.

• Assume that the BOD5 leaving the septic tanks is a conservative 100 mg/L. Assume that the wetland effluent BOD will not exceed 10 mg/L.

• Use clean, washed gravel as the treatment media in the bed with a size range of 0.5-1 in. (1.25-2.5 cm), with a total depth of 2 ft (0.6 m). For design, assume the "effective" water depth in the bed is 1.8 ft (0.55 m). Reasonable estimates include: hydraulic conductivity (ks) = 5000 ft3/ft2/d (1500 m3/m2/d); porosity = 0.38. If a large number of systems is to be installed using the same materials, field or laboratory testing for hydraulic conductivity (ks) and porosity (n) is recommended.

• Use reeds (Phragmites) as the preferred plant species.

• Estimate the summer and winter water temperatures to be expected in the bed. In the summer and in year-round warm climates, 2°C is reasonable. In cold winter climates, a winter water temperature of 1°C is a reasonable assumption.

• Determine the bed surface area with:

• As a safety factor, use a rate constant K20 that is 75% of the base value (1.104 d-1). So, for the design of small on-site systems, K20 = 0.828 d-1. At 20°C, and with the other factors defined above, this equation reduces to:

Metric: As = 13.31(Q) = m2 (Q in m3/d) U.S. units: As = 4.07(Q) = ft2 (Q in ft3/d)

Metric: As = 30.1(g) = m2 (Q in m3/d) U.S. units: As = 9.2(Q) = ft2 (Q in ft3/d)

• Adjustments for other temperatures, other media types, etc. should use the basic design equations. Adopt an aspect ratio (length-to-width) of 2:1; calculate bed length (L) and width (W) because the surface area was determined above. In the general case, an aspect ratio of 2:1, or less, with a bed depth of 2 ft (0.6 m) will satisfy the Darcy's law constraints on hydraulic design of the bed, so hydraulic calculations are not required. If site conditions will not permit the use of a length-to-width radio of 2:1 for the bed and a 2-ft (0.6-m) bed depth, then hydraulic calculations as described previously will be necessary. This approach will give an HRT of about 2.8 d (at 20°C) in the bed which is more than adequate for BOD removal to 10 mg/L. If nitrogen removal to 10 mg/L is required, the size of the system should be doubled to produce an HRT of about 6 days. Nitrogen removal during the winter months in cold climates may require an HRT of about 10 d. In these cases, heat-loss calculations should be performed to be sure the bed is adequately protected against freezing.

• Construct the bed as a single cell for single-family dwellings. Use multiple cells (at least two) in parallel for larger sized systems. Use clay or a synthetic liner to prevent seepage from the bed.

• Construct the bed with a flat bottom and a perforated effluent manifold at the bottom of the bed. A perforated inlet manifold a few inches above the bottom of the bed is adequate for most small systems. These inlet and outlet zones should use 1- to 2-in. (2.5- to 5-cm) washed rock for a length of about 3 ft (1 m) and for the full depth of the bed.

• The effluent manifold should connect to either a swiveling standpipe or a flexible hose for discharge to allow control of the water level in the bed. The inlet and effluent manifolds should have accessible cleanouts at the surface of the bed.

The system described here should produce an effluent with BOD of <10 mg/L, TSS of <10 mg/L, and TN of <10 mg/L and should therefore be suitable for either surface or in-ground discharge. The excellent water quality should permit a significant reduction in the area required for the disposal field. For example, a typical conventional on-site system for a family of four (300 gpd, 1 m3/d) might include a 1000-gal (4-m3) septic tank and a 500-ft2 (46-m2) infiltration area in a sandy loam soil. Addition of a wetland component with a 6-d HRT would require about 300 ft2 (28 m2) of area. If appropriate credit for the higher level of treatment is allowed, the total area for the wetland cell and the infiltration bed could be less than 500 ft2 (<46 m2).

7.11 VERTICAL-FLOW WETLAND BEDS

In the vertical-flow wetland concept, the wastewater is uniformly applied to the top of the bed, and the effluent is withdrawn via perforated pipes on the bottom, parallel to the long axis of the bed. The concept is based on the work of Seidel (1966) and is in use at several locations in Europe. A system typically consists of two groups, or stages, of vertical-flow cells in series followed by one or more horizontal-flow polishing cells. Each stage of vertical-flow units consists of several individual wetland cells in parallel because wastewater is applied intermittently in rotation. The operational systems in Europe apply either primary effluent (typically from a septic tank) or in some cases untreated raw wastewater.

Typically, the beds are dosed for up to 2 d and then rested for 4 to 8 d. A 2-d wet and 4-d dry cycle (2/4) would require a minimum of three sets of stage I cells; a 2/8 cycle would require at least five cells. The number of stage II cells is one half that of the stage I components, and these are also loaded in rotation.

The main advantage of the concept is the restoration of aerobic conditions during the periodic resting and drying period. This allows removal of BOD and ammonia nitrogen at higher rates than can be achieved in the continuously saturated and generally anaerobic horizontal flow SSF wetland bed. As a result, the vertical-flow beds can be somewhat smaller in area than a comparable SSF wetland designed for the same performance level.

During the dosing period, hydraulic loading on the stage I beds is typically 7.4 gal/ft-d (0.3 m/d) for primary effluent, and double that value for the stage II cells. Such a two-stage system can typically achieve better than 90% BOD and TSS removal. The bed profile contains several layers of various sized granular materials. A typical profile, from the top of the bed, would include:

3 in. (8 cm) coarse sand, planted with Phragmites

4 in. (10 cm) washed medium gravel (12 mm size)

6 in. (15 cm) washed coarse gravel (40 mm size)

Perforated underdrain pipes are laid on the bottom of the cell on about 3-ft (1-m) centers. The upstream end of these pipes extends up to and above the bed surface to create a "chimney" effect and encourage oxygen transfer to the profile. The upper portion of this perforated pipe is contained within a solid pipe jacket to prevent short-circuiting of percolate flow. Additional vertical "chimney" pipes are placed at 6-ft (2-m) centers in the rows between the perforated effluent piping. These vertical pipes are perforated in the bottom layer of gravel and solid from there to the above-surface end.

Insufficient performance data are available for this concept to permit development of a rational design model. The equations below are based on the performance of a system in the United Kingdom with a 2-d wet and 4-d dry cycle.

They can be used with extreme caution (because of the limited database) to estimate the performance of similar systems.

BOD removal, per stage:

Kt = Temperature-dependent rate constant (d-1) = 0.317(1.06)(T-20) d-1.

HLR = Average daily hydraulic loading rate during the dosing cycle (m/d).

Ammonia removal, per stage:

where

Kt = Temperature dependent rate constant (d-1) = 0.1423(1.06)(T-20) d-1.

HLR = Average daily hydraulic loading rate, during the dosing cycle (m/d).

Ordinarily, a higher rate of ammonia removal should be expected in the second stage of a two-stage system; however, in this two-stage system, the rate of ammonia removal per stage is about equal because the BOD loading on the second stage is still higher than desired for optimum nitrification, as discussed previously for the nitrification filter bed (in Section 7.9). This response suggests that further improvements and optimization of the vertical flow concept as used in Europe are desirable. The first stage should be large enough to produce an effluent BOD in the range of 10 to 15 mg/L. The second stage could then be optimized for ammonia removal, and the principal role of SSF wetland used as the third component would be denitrification and final polishing.

7.11.1 Municipal Systems

In a vertical-flow wetland system, the wetland is divided into a number of distinct beds or cells that operate in parallel. The cells contain about 3 ft (0.9 m) of granular media, which are typically planted with bulrush to maintain the porosity of the bed. The wastewater is uniformly applied to the top of the cell and flows vertically downward through the bed. The effluent is withdrawn via perforated pipes on the bottom, parallel to the long axis of the bed. The beds are intermittently dosed, allowing time for the beds to rest between dosing cycles. Hydraulic loading rates, during dosing, are typically 30 cm per day (7.4 gal/ft2-d) for primary effluent and double that for secondary cells.

The operational vertical-flow wetland systems in Europe apply either primary effluent (typically from a septic tank) or in some cases untreated raw wastewater.

where

TABLE 7.10

Vertical Flow Wetlands Performance for Salem, Oregon3

TABLE 7.10

Vertical Flow Wetlands Performance for Salem, Oregon3

(mg/L)

bod5

10.7

3

TSS

8.4

3

Ammonia nitrogen

13.3

1.3

Nitrate nitrogen

1.0

10.4

a Average for year 2003.

Note: BOD5, biochemical oxygen demand; TSS, total suspended solids.

a Average for year 2003.

Note: BOD5, biochemical oxygen demand; TSS, total suspended solids.

Municipal wastewater applications in North America, in addition to the demonstration project at Salem, Oregon, include Pelee Island, Ontario; Niagara-on-the-Lake, Ontario; and Vineland, Ontario. The experience at the Salem, Oregon, demonstration natural reclamation system with vertical-flow wetlands has been very positive. The system was constructed in 2002. Secondary effluent is flooded onto beds at 4 cycles per day with 1 hour on and 5 hours off. The beds are 3 ft (0.9 m) deep and graded from sand to gravel from top to bottom. The bottom material is gravel for the collection of the underdrainage. The 2003 year results when the beds were loaded at 15.3 in. (39 cm) are shown in Table 7.10. The VF wetlands are shown in Figure 7.7.

7.11.2 Tidal Vertical-Flow Wetlands

Tidal flow wetlands involve cyclic flooding and draining of a media bed (Sun et al., 1999). In 1901, a patent application was made for a tidal flow wetland (Monjeau, 1901). Recent research by David Austin of Living Machines, in Taos, New Mexico, has concentrated on media characteristics, such as ammonium adsorption. The results of the pilot testing of the Living Machines tidal vertical-flow wetlands are presented in Table 7.11. The influent flow was 450 gpd (1.7 m3/d), the wetland area was 96 ft2 (8.9 m2), and the average hydraulic loading rate was 4.7 gal/ft2-d (19 cm/d) (Austin et al., 2003).

7.11.3 Winery Wastewater

Winery wastewater at the EastDell Estates Winery in Ontario, Canada, is pre-treated with a vertical-flow wetland for BOD reduction. The wastewater BOD was reduced by 65% through a septic tank followed by a 96% reduction through the vertical-flow wetlands (Rozema, 2004).

FIGURE 7.7 Salem, Oregon, vertical flow wetlands: (a) recently planted bed showing distribution system, (b) bulrush growth, which matures in 1 year.

7.12 CONSTRUCTION CONSIDERATIONS

Both types of wetlands typically require an impermeable barrier to ensure containment of wastewater and to prevent contamination of groundwater. In some cases, such a barrier may be provided if clay is naturally present or if in situ soils can be compacted to a nearly impermeable state. Chemical treatments, a bentonite layer, and asphalt or membrane liners are also possibilities. In the case of a

TABLE 7.11

Tidal Vertical Flow Wetlands Design and Performance

Value

Design Factor

Influent flow Recycle ratio Flow regime Hydraulic residence time Area of five cells Media depth Media type Performance bod5

Total nitrogen Nitrate nitrogen TKN TSS

Units gal/d

ft2 ft mm x mm

Influent (mg/L)

450 3:1 to 14:1 Downflow flood and drain 24 96 2

9.5 x 2.4 expanded shale Effluent (mg/L)

Note: BOD5, biochemical oxygen demand; TKN, total Kjeldahl nitrogen; TSS, total suspended solids.

Source: Austin, D. et al., in Proceedings of WEFTEC 2003, Water Environment Federation, Los Angeles, CA, October 11-15, 2003.

wetland treating landfill leachate, a double liner with leak detection may be required by some regulatory agencies.

The bottom surface must be level from side to side for the entire length of the wetland bed. Both types of wetlands may have a slight uniform slope to ensure drainage, but as described previously the bottom slope should not be designed to provide the necessary hydraulic conditions for flow in the system. The necessary hydraulic gradient and water level control in each wetland cell are provided by an adjustable outlet device. The bottom of the wetland, during the final grading operations, should be compacted to a degree similar to that used for highway subgrades. The purpose is to maintain the design surface during subsequent construction activities. Several constructed wetland systems, both SSF and FWS types, have been found with significant flow short-circuiting due to inadequate grade control during system construction. A particular concern for the SSF type is trucks delivering the gravel media. The ruts from just a few of these vehicles can induce permanent short-circuiting in the completed system. Construction traffic should not be permitted on the cell bottom during wet weather conditions.

The membrane liner, if used, is placed directly on the completed cell bottom. The SSF media can be placed directly on heavy-duty liner materials. In the case of FWS wetlands, a layer of reserved topsoil is placed on top of the liner to serve as the rooting medium for the vegetation.

The selection of SSF media type is critical to the successful performance of the system. Unwashed crushed stone has been used in a large number of existing projects. Truck delivery of such material during construction can lead to problems due to segregation of fines in the truck during transit and then deposition of all of the fine material in a single spot when the load is dumped. This can result in a number of small blockages in the flow path and internal short-circuiting in the system. Washed stone or gravel is preferred. Coarse aggregates for concrete construction are commonly available throughout the United States and would be suitable for construction of SSF wetland systems.

The dikes and berms for the wetland cells are constructed in the same manner as those for lagoons and similar water impoundments. For large-scale systems, the top of the berm should be wide enough for small trucks and maintenance equipment. Each cell in the system must have a ramp into the cell to permit access for maintenance vehicles.

7.12.1 Vegetation Establishment

Establishing vegetation at an appropriate density is a critical requirement for construction of both types of wetland systems. Local plants are already adapted to the regional environment and are preferred, if available. Several commercial nurseries are also capable of providing the plant stock for large projects. Planting densities are discussed in Section 6.2; the closer the initial spacing, the sooner the system will be at full density. Most of the species will propagate from seed, and aerial seeding might be considered for large-scale projects. Plant development from seed takes significant time and requires very careful water control, and seed consumption by birds can be a problem. The quickest and most reliable approach is to transplant rhizomes of the vegetation of choice in the prepared treatment bed.

Each rhizome cutting should have at least one bud or preferably a growing shoot and is planted with one end about 2 in. (5 cm) below the surface of the medium with the bud or shoot exposed to the atmosphere, above the saturated media. Planting of seeds or rhizomes can occur in the spring after the last frost; rhizome material can also be planted in the fall. The bed is flooded and the water level maintained at the soil or media surface for at least 6 weeks or until significant new growth has developed and emerged. At this stage, the wetland can be placed in full operation as long as the water level is not above the tops of the new plant growth. If freshwater is used during the incubation period, the use of some supplemental fertilizer is desirable to accelerate plant growth.

The design of very large systems might consider planting the vegetation in parallel bands, with the long axis of the band perpendicular to the flow direction.

TABLE 7.12

Construction Costs for Subsurface Flow Constructed Wetlands

TABLE 7.12

Construction Costs for Subsurface Flow Constructed Wetlands

Design Flow

Area

Construction Cost

Location

(gpd)

(ac)

($/ac)

La Siesta, Hobbs, New Mexico

5000

0.11

198,900

Howe, Indiana

6000

0.14

221,700

McNeil, Arkansas

15,000

0.39

263,300

Santa Fe Opera, New Mexico

17,000

0.15

374,000

Phillips H.S. Bear Creek, Alabama

20,000

0.50

94,600

Carville, Louisiana

100,000

0.57

234,700

Benton, Louisiana

310,000

1.19

294,100

Mesquite, Nevada

400,000

4.8

130,800

Carlisle, Arkansas

860,000

1.09

379,500

Note: Costs updated to June 1998.

Note: Costs updated to June 1998.

Source: Crites, R.W. and Ogden, M., in Proceedings of WEFTEC 1998, Water Environment Federation, Orlando, FL, October 3-7, 1998.

Each band would commence operation with relatively dense vegetation, and the spaces between bands can be filled in over the long term. If cost constraints are an issue, it is advantageous to put about 75% of the vegetation stock in the last half of the cell and 25% in the first half.

7.13 OPERATION AND MAINTENANCE

Constructed wetlands provide passive treatment and therefore require minimal operating labor. The issues requiring operational attention in SSF and vertical-flow wetlands are maintenance of inlet and outlet manifolds and monitoring of water quality. The vegetation, once established, requires very little attention, unless it is attacked by predatory animals. Water level control usually is maintained by the outlet device and may be modified seasonally.

7.14 COSTS

The cost elements for constructed wetlands are described in Chapter 6. Additionally, for SSF wetlands and vertical-flow wetlands bed media are a significant cost item. Construction costs for SSF wetlands are summarized in Table 7.12. The cost for media depends on local gravel costs and the cost for hauling. Three SSF systems with detailed media costs are presented Table 7.13.

TABLE 7.13

Costs of Media for Subsurface Flow Constructed Wetlands

Location and Gravel Size

Mesquite, Nevada 3/8 to 1 in. Carville, Louisiana

3/4-in. top layer 1/2- to 3-in. bed Ten Stones, Vermont 3/8-in. top layer 3/4- to 1-in. bed

Gravel Depth (ft)

2.67

Gravel Quantity

4308

806 3226

806 3226

8.40

20.75 15.45

19.17 9.18

43,800

18,103 53,952

15,451 29,615

Source: Adapted from WEF, Natural Systems for Wastewater Treatment, 2nd ed., Manual of Practice FD-16, Water Environment Federation, Alexandria, VA, 2001.

7.15 TROUBLESHOOTING

Troubleshooting in SSF and VF wetlands may be necessary to address:

• Hydraulic problems due to clogging

• Water quality problems due to metal sulfide precipitation

Historical problems with surfacing of water in SSF wetlands can usually be traced to inadequate design of head loss through the media and outlet devices being placed too high and without the ability to be lowered. These systems often have an excessively high aspect ratio, which exacerbates the problem by concentrating the applied solids in the first 10% of the bed length. If organic solids are the problem, either resting or drying of the bed or applications of hydrogen peroxide can be used. Hydrogen peroxide seemed to overcome organic clogging at Mesquite, New Mexico (Hanson et al., 2001). Metal sulfide problems, such as caused by anaerobic conditions at Minoa, New York, can be overcome by batch dosing and draining of the beds. Other water quality problems, such as inadequate ammonia removal, can be overcome by using nitrifying filter beds (Section 7.9) or converting to vertical-flow wetlands (Section 7.11).

REFERENCES

Askew, G.L., Hines, M.W., and Reed S.C. (1994). Constructed wetland and recirculating gravel filter system, full scale demonstration and testing, in Proceedings of the Seventh International Symposium on Individual and Small Community Sewage Systems, Atlanta, GA, American Society of Agricultural Engineers, St Joseph, MI, 84-91.

Austin, D., Lohan, E., and Verson, E. (2003). Nitrification and denitrification in a tidal vertical flow wetland pilot, in Proceedings of WEFTEC 2003, Water Environment Federation, Los Angeles, CA, October 11-15, 2003.

Bavor, H.J., Roser, D.J., Fisher, P.J., and Smalls, I.C. (1986). Joint Study on Sewage Treatment Using Shallow Lagoon-Aquatic Plant Systems, Water Research Laboratory, Hawkesbury Agricultural College, Richmond, NSW, Australia.

Baybrook, T. et al. (1998). Cold Climate Performance of Sub-Surface Flow Constructed Wetlands for Wastewater Treatment, paper presented at the Annual Technology Symposium, Water Environment Association of Ontario, Toronto.

Behrends, L.L., Sikora, F.J. Coonrod, H.S. Bailey, E., and McDonald, C. (1996). Reciprocating subsurface flow constructed wetlands for removing ammonia, nitrate, and chemical oxygen demand: potential for treating domestic, industrial and agricultural wastewater, Proc. WEFTEC 1996, 5, 251-263.

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