Denitrification

Equation 7.16 to Equation 7.21 only account for conversion of ammonia to nitrate and predict the area required for a given level of conversion. When actual removal of nitrogen is a project requirement, it is necessary to consider the denitrification requirements and size the wetland accordingly. In the general case, most of the nitrate produced in a SSF wetland will be denitrified and removed within the area provided for nitrification and without supplemental carbon sources. FWS wetlands can be more effective for nitrate removal than the SSF type because of the greater availability of carbon from the plant detritus, at least during the first few years of operation. Even though the SSF wetland has more surface area for biological responses, it is likely that the availability of carbon in the system limits the denitrification rate so that SSF and FWS wetlands perform in a comparable manner. The recommended design model for estimating nitrate removal via den-itrification is provided by Equation 7.22 and Equation 7.23:

where

Ce = Effluent nitrate-nitrogen concentration (mg/L).

C0 = Influent nitrate-nitrogen concentration (mg/L).

CeIC0 = expC-KT) As = QMCJC0)IKTyn

Kt = Temperature-dependent rate constant (d-1) = 0 d-1 at 0°C, and

1.00(1.15)(T-20) d-1 at 1°C+. n = Porosity of the wetland (see Table 7.1 for typical values). t = Hydraulic residence time (d). y = Depth of water in the wetland (ft; m). Q = Average flow through the wetland (ac-ft/d; m3/d).

The influent nitrate concentration (C0) used in Equation 7.22 or Equation 7.23 is the amount of ammonia oxidized, as calculated in Equation 7.17. Because Equation 7.17 determines the ammonia remaining after nitrification in the SSF wetland, it can be conservatively assumed that the difference (C0 - Ce) is available as nitrate nitrogen. The rate of denitrification between 0°C and 1°C can be determined by interpolation. For practical purposes, denitrification is insignificant at these temperatures. It must be remembered that Equation 7.22 and Equation 7.23 are only applicable for nitrate nitrogen that is present in the wetland system.

Because the SSF wetland is generally anoxic but also has aerobic sites on the surfaces of the roots and rhizomes, it is possible to obtain both nitrification and denitrification in the same reactor volume. Equation 7.23 gives the wetland surface area required for denitrification. This denitrification area is not in addition to the area required for nitrification as determined with Equation 7.18; it is usually less than or equal to the results from Equation 7.18, depending on the input level of nitrate in the untreated wastewater and the water temperature.

7.4.3.3 Total Nitrogen

When denitrification is required, a discharge limit on total nitrogen (TN) usually exists. The TN in the SSF wetland effluent is the sum of the results from Equation 7.17 and Equation 7.22. The determination of the area required to produce a specific effluent TN value is an iterative procedure using Equation 7.17 and Equation 7.22:

1. Assume a value for residual ammonia (Ce) and solve Equation 7.18 for the area required for nitrification. Determine the HRT for that system.

2. Assume that (C0 - Ce) is the nitrate produced by Equation 7.17 and use this value as the influent (C0) in Equation 7.23. Determine effluent nitrate using Equation 7.22.

3. The effluent TN is the sum of the Ce values from Equation 7.17 and Equation 7.22. If that TN value does not match the required TN, another iteration of the calculations is necessary.

7.4.4 Aspect Ratio

The aspect ratio is the ratio of the length-to-width of the normally rectangular SSF beds. The early SSF systems had large aspect ratios and influent clogging, and surfacing of water occurred when little attention was paid to the hydraulics (Reed et al., 1995; USEPA, 1993). At Mesquite, Nevada, a SSF wetlands was successfully designed with an aspect ratio of 0.25:1 (Lekven et al., 1993). Current thinking is that the aspect ratio should be between 0.25:1 and 4:1.

7.5 DESIGN ELEMENTS OF SUBSURFACE FLOW WETLANDS

The design elements for SSF wetlands include pretreatment, media, vegetation, and inlet and outlet structures.

7.5.1 Pretreatment

Both FWS and SSF wetlands in the United States utilize at least the equivalent of primary treatment as the preliminary treatment prior to the wetland component. This might be obtained with septic tanks, Imhoff tanks, ponds, conventional primary treatment, or similar systems. The purpose of the preliminary treatment is to reduce the concentration of easily degraded organic solids that otherwise would accumulate in the entry zone of the wetland system and result in clogging, possible odors, and adverse impacts on the plants in that entry zone. A system designed for step feed of untreated wastewater might overcome these problems. A preliminary anaerobic reactor would be useful to reduce the organic and solids content of high-strength industrial wastewaters. Many of the SSF wetland systems in Europe apply screened and degritted wastewater to a wetland bed. This approach results in sludge accumulation, odors, and clogging but is acceptable in remote locations. In some cases, an inlet trench is used for solids deposition and the trench is cleaned periodically.

7.5.2 Media

The SSF wetland bed typically contains up to 2 ft (0.6 m) of the selected media. This is sometimes overlain with a layer of fine gravel that is 3 to 6 in. (76 mm to 150 mm) deep. The fine gravel serves as an initial rooting medium for the vegetation and is maintained in a dry condition during normal operations. If relatively small gravel (<20 mm) is selected for the main treatment layer, a finer top layer is probably not necessary, but the total depth should be slightly increased to ensure a dry zone at the top of the bed. Most operational SSF wetlands in the United States have a treatment zone and operating water depth of 2 ft (0.6 m). A few systems, in warm climates where freezing is not a significant risk, operate with a bed depth of 1 ft (0.3 m). The shallow depth enhances the oxygen transfer potential but requires a greater surface area, and the system is at greater risk of freezing in cold climates. The deep (2 ft or 0.6 m) bed also requires special operation to induce desirable root penetration to the bottom of the bed.

7.5.3 Vegetation

Vegetation for SSF wetlands should be perennial emergent plants such as bulrush, reeds, and cattails. The SSF wetland concept has significantly less potential

FIGURE 7.3 Inlet manifold for subsurface flow wetlands at Hardin, Kentucky.

habitat value as compared to the FWS wetland because the water is below the surface of the SSF media and not directly accessible to birds and animals. The presence of open-water zones within a SSF system negates many of the advantages of the concept and such zones are not normally included in the system plan. Enhancement of habitat values or esthetics is possible via selected plantings around the perimeter of the SSF bed. Because optimum wastewater treatment is the basic purpose of the SSF concept, it is acceptable to plan for a single plant species; based on successful experience in both the United States and Europe, Phragmites offers a number of advantages. A number of SSF wetlands in the southern states were initially planted with attractive flowering species (e.g., Canna lily, iris) for esthetic reasons. These plants have soft tissues that decompose very quickly when the emergent portion dies back in the fall and after even a mild frost. The rapid decomposition has resulted in a measurable increase in BOD and nitrogen leaving the wetland system. In some cases, the system managers have utilized an annual harvest for removal of these plants prior to the seasonal dieback or frosts. In most cases, the problems have been completely avoided by replacing these plants with the more resistant reeds, rushes, or cattails, which do not require an annual harvest. Use of these soft-tissue flowering species is not recommended on future systems, except possibly as a border.

7.5.4 Inlet Distribution

Inlet devices have ranged from open trenches to single-point weir boxes to perforated pipe manifolds. A surface manifold developed by the Tennessee Valley Authority (TVA) uses multiple, adjustable outlet ports (Steiner and Freeman, 1989; Watson et al., 1989). Having the manifold on the surface allows for operational adjustments if differential settlement occurs. An example of a surface manifold at Hardin, Kentucky, is presented in Figure 7.3. Subsurface manifolds

FIGURE 7.4 Adjustable outlet for subsurface flow wetlands.

encased in coarse gravel have also been used successfully. The disadvantage of this type of manifold is the potential for differential settlement and clogging from nuisance animals or solids. The advantage of a subsurface manifold is that the growth of algae on the outlets is avoided and thermal protection is provided.

7.5.5 Outlet Collection

Outlet collection should incorporate a manifold to avoid short-circuiting to a single outlet. A subsurface manifold is recommended to ensure the flow path is through the media. An adjustable outlet weir or swivel elbow allows control of the hydraulic gradient, as shown in Figure 7.4.

7.6 ALTERNATIVE APPLICATION STRATEGIES

Most SSF wetlands have been designed for continuous-flow applications. The lack of oxygen transfer, noted by Reed et al. (1995; USEPA, 1993) as the principal limitation of nitrification in SSF wetlands, led to researchers trying batch flow, rapid drainage of SSF beds, and reciprocating wetlands to get more oxygen into the wastewater.

7.6.1 Batch Flow

A number of modes of batch flow have been attempted. The case study of SSF wetlands at Minoa, New York (Section 7.8) illustrates one approach. Other approaches are described under the section on vertical flow wetlands (Section 7.11).

7.6.2 Reciprocating (Alternating) Dosing (TVA)

Researchers at the TVA developed and patented a "reciprocating" dosing of SSF wetlands in which the wastewater is quickly drained from one wetland cell and pumped into a second parallel cell (Behrends et al., 1996). The draining and filling occur within 2 hr, and then the process is reversed; the second cell is drained quickly and the first cell is refilled. The reciprocating flow process is repeated continuously, with a small amount of influent continually being added to the first cell and a fraction of the wastewater continually being withdrawn from the second cell as system effluent (USEPA, 2000). The reciprocating two-cell system was compared to a conventional two-cell system for 6 months in side-by-side testing in late 1995 and early 1996 at Benton, Tennessee. Operation of both two-cell pairs in the reciprocating mode has continued since May of 1996. Comparing conventional operation to the reciprocating mode, the reciprocating mode has produced significantly lower effluent BOD and ammonia nitrogen (USEPA, 2000).

7.7 POTENTIAL APPLICATIONS

The applications for SSF wetlands are many and expanding. Municipal waste-water examples are numerous, onsite wetlands are widely used, and a variety of industrial wastewaters have been treated. Some examples are presented here.

7.7.1 Domestic Wastewater

In the majority of cases, the utilization of SSF wetlands is preferred over the FWS type for on-site systems treating domestic wastewaters. This is because of the advantages of the SSF approach, which excludes mosquitoes and other insect vectors and eliminates risks of personal contact or exposure with the wastewater being treated. In northern climates, the additional thermal protection provided by the SSF concept is also an advantage. The design of these systems should follow the recommendations given in Section 7.6, supplemented as required. If nitrogen removal is a project requirement, the use of either Phragmites or Scirpus as the system vegetation is recommended. If stringent nitrogen limits prevail, the use of a compact recirculating NFB bed with plastic media should be considered to minimize the total area of the wetland (see Section 6.8 for details). In locations with relatively warm winter conditions, a 1-ft deep bed with Typha would also be suitable, but such a bed would require twice the surface area as compared to a 2-ft deep Phragmites or Scirpus bed. If nitrogen removal is not required, then the use of ornamental plants or shrubs is acceptable. In these cases, a layer of suitable mulch on the bed surface will enhance plant growth. The use of at least two parallel wetland cells is recommended, except for the smallest applications at single-family dwellings.

7.7.2 Landfill Leachate

The HRT in the "coarse" gravel cell at Tompkins County, New York, was estimated to be about 15 d. The total HRT in the two SF cells at Broome County, New York, at the estimated leachate flow of about 260 gal/d (1 m3/d) is calculated to be about 22 d. At these long detention times, the expected removal of BOD and ammonia should have been much greater than indicated by the results in Table 6.14. The poor performance observed for BOD removal at both of these systems is believed to be due to insufficient phosphorus in the untreated leachate to support the necessary biological reactions. The phosphorus concentration was only 0.15 mg/L at Tompkins County and was not measured at the Broome County site. This very low phosphorus level is insufficient to effectively remove the BOD loading applied, regardless of the detention time provided in the system. There appears to be sufficient quantities of nitrogen and other essential micro-nutrients to support BOD and ammonia removal. Treatment optimization at these two landfills and possibly at many others would require regular additions of at least supplemental phosphorus.

7.7.3 Cheese Processing Wastewater

A subsurface flow wetlands with supplemental aeration has been constructed for Eichten Cheese near Center City, Minnesota. The treatment system consists of a septic tank, a SSF wetland, and an infiltration bed. The forced-air aeration system improved the SSF wetland BOD reduction performance from 17 to 94%. The aeration system consisted of a blower and a perforated aeration tubing system (Wallace, 2001).

7.7.4 Airport Deicing Fluids Treatment

Glycol is used at airports to deice the wings of airplanes. Runoff of stormwater with glycol in it is an environmental problem that SSF wetlands can help to solve. SSF wetlands are appropriate because open water is not acceptable near airport runways and close-growing vegetation can be used. SSF wetlands have been used to treat deicing fluids at Edmonton and Toronto, Canada; at Airborne Express Airport in Wilmington, Ohio; and at Heathrow Airport in London (Richter et al., 2003; Karrh et al., 2001). A SSF wetlands was designed for glycol treatment at Westover Air Reserve Base in western Massachusetts, and the design criteria are presented in Table 7.7. The expected BOD removal for the system was 90% (Karrh et al., 2001).

7.8 CASE STUDY: MINOA, NEW YORK

The Village of Minoa, New York, near Syracuse, has a three-cell SSF constructed wetland. The conceptual design was prepared by Sherwood C. Reed in 1994. The treatment capacity of the 1.1-ac (0.45-ha) wetland as constructed was

TABLE 7.7

Design Criteria for Subsurface Flow Wetlands Treating Deicing Runoff

Value

Parameter

Design flow (mgd) 0.1

Peak wetlands flow (mgd) 0.4

System residence time (d) 2.2

Wetlands residence time (d) 1.85

Bed length (ft) 212

Bed width (ft) 110

Length-to-width ratio 1.9

Bed bottom slope 0.0001

Media porosity 0.47

Inlet/outlet width (ft) 10

Notes

Flow constrained by limited bed area Flow constrained by limited bed area

Volume/design flow Constrained by site Distance perpendicular to the flow Distance in the direction of flow

Allows for bed drainage Material should have <1% fines High for most gravels Rip-rap-sized material Distribution and collection trenches

Source: Karrh, J.D. et al., in Wetlands and Remediation II: Proceedings of the Second International Conference on Wetlands and Remediation, Nehring, K.W. and Brauning, S.E., Eds., Battelle, Columbus, OH, 2001. With permission.

130,000 gal/d (454 m3/d) with a hydraulic residence time of 2.4 d (Reed and Giarrusso, 1999). BOD reduction of the primary effluent to 30 mg/L was the design objective. The three cells were constructed at different elevations, and piping was provided to allow either series or parallel operation. Each cell was provided with water level controls, drainage, and internal sampling wells. The slope of each bed was 1%, but the media surface was level so the depth of water varied from 1.6 ft (0.5 m) at the inlet to 3 ft (0.9 m) at the outlet. The top 4 in. (100 mm) of the bed was 0.25-in. (0.6-mm) pea gravel and served as the rooting medium for the plants. The treatment zone in the bed used 1.5-in. (40-mm) washed and screened coarse gravel, obtained as crushed stone from a local source. The cell bottoms were lined with a 60-mil high-density polyethylene (HDPE) liner. Each cell was also divided by a longitudinal barrier so the influence on performance of two different plant species (Phragmites and Scirpus) could be evaluated.

Start-up occurred in June 1995 using secondary effluent from the trickling filters. In January 1996, the operation was switched to the primary effluent. The effluent BOD value during the spring and summer of 1996 was only 84 mg/L,

TIME (hours)

FIGURE 7.5 Sulfate reduction at Minoa, New York, subsurface flow wetlands.

which was unacceptable. In addition, objectionable sulfide odors were noted at the outlet structures of the cell, and a black "sludge-like" substance was observed accumulating in the gravel void spaces in each of the wetland cells. Suggested remedial actions, including reduction in the loading rate, dilution of the influent with secondary effluent, and chemical oxidants, were tried without success. The operator, Steve Giarrusso, began to operate the cells by sequentially draining and refilling the cells on a regular basis. BOD removal, which had averaged 44% in 1996, increased to 95% in 1997.

In 1997, the typical sequence consisted of opening the drain for cell 1 on a Tuesday morning while the full flow continued to enter cell 1. After 24 hr, the drain to cell 1 was closed and the drain to cell 2 was opened. The next day, the drain for cell 2 was closed and the drain for cell 3 was opened. On Friday, the drain for cell 3 was closed, the drain for cell 1 was opened, and the cycle was repeated. Cells 1 and 2 took 4 to 5 hr to drain to their lowest levels and 24 hr to refill, so for about 20 hr the media was exposed to aerobic conditions.

The wastewater contained about 50 mg/L of sulfate, and, after a few hours of conventional loading, the water in the cell became anaerobic and the sulfates were reduced to sulfides. During a 90-hr test conducted by Clarkson University, the sulfates were found to be reduced to near zero until the draining and reaeration restored the aerobic conditions and stopped the reduction of sulfates. This phenomenon is shown in Figure 7.5. The improvement in treatment between 1996, when continuous flow was practiced, and 1997, when the sequential fill/drain operation was initiated, is shown in Table 7.8 (Reed and Giarrusso, 1999).

TABLE 7.8

TABLE 7.8

 Constituent Loading (lb/acd) Removal in 1996 (lb/acd) Removal in 1997 (lb/acd) Improvement from 1996 to 1997 (%) bod5 160 65 107 64 COD 305 121 273 125 TSS 75 59 73 24 NH3-N 16 1.1 1.9 75 TP 3.9 1.7 1.8 5

Note: BOD, biochemical oxygen demand; COD, chemical oxygen demand; TSS, total suspended solids; NH3-N, nitrogen ammonia; TP, total phosphorus.

Note: BOD, biochemical oxygen demand; COD, chemical oxygen demand; TSS, total suspended solids; NH3-N, nitrogen ammonia; TP, total phosphorus.

Source: Reed, S.C. and Giarrusso, S., in Proceedings of WEFTEC 1999, Water Environment Federation, New Orleans, LA, October 9-13, 1999.

7.9 NITRIFICATION FILTER BED

The nitrification filter bed (NFB) concept was developed by Sherwood C. Reed as a retrofit for existing wetland systems having difficulty meeting their ammonia discharge limits. It has been used successfully for both FWS and SSF wetland systems. As shown in Figure 7.6, it consists of a vertical-flow gravel filter bed on top of the existing SSF or FWS wetland bed. In the latter case, the fine-gravel NFB is supported by a layer of coarse gravel to maintain aerobic conditions in the NFB.

The NFB unit can be located at the head of the wetland channel or near the end. In either case, the wetland effluent is pumped to the top of the NFB and uniformly distributed. The inlet location has advantages in that the nitrified percolate will mix with the influent wastewater. The resulting denitrification will remove nitrogen from the system, further reduce the BOD, and recover some of the alkalinity consumed during the nitrification step. Locating the NFB near the end of the wetland cell will produce the desired level of nitrification but there is insufficient time for significant denitrification so most of the nitrate produced will pass out of the system with the effluent. Pumping capacity and power costs will be higher for the inlet location, particularly for retrofit of long, narrow wetland channels. A U-shaped wetland channel with the inlet adjacent to the outlet would retain the advantages of denitrification and minimize the pumping requirements.

The NFB is similar in concept to the familiar recirculating sand filter (see Chapter 10), which has been used successfully for many years to polish and nitrify septic tank effluent (Crites and Tchobanoglous, 1998). These recirculating gravel gravel

Recycle

SF bed

r Fine gravel r- Sprinkler

Influent

FIGURE 7.6 Schematic diagram of nitrification filter bed.

r Fine gravel r- Sprinkler

Influent

FWS bed

0 0