BOD Removal

The recommended approach to design for BOD removal in SSF wetlands is the volume-based detention time model, as expressed in Equation 7.13:



= Wetland surface area (ac; m2).


= Average design flow (ac-ft/d; m3/d).


= Influent BOD concentration (mg/L).


= Effluent BOD concentration (mg/L)


= Rate constant = 1.1 d-1 at 20°C.


= Design depth (ft; m).


= Porosity of media (see Table 7.1).

The temperature of the wastewater will affect the rate constant according to Equation 7.14:

Rate constant at temperature T. 1.1 d-1.

Wastewater temperature (°C).

Most operational SSF wetlands in the United States have a treatment zone and operating water depth of 2 ft (0.6 m). A few, 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. A bed 2 ft (0.6 m) deep also requires special operation to induce desirable root penetration to the bottom of the bed.

Subsurface flow wetlands in the United States utilize at least the equivalent of primary treatment as the preliminary treatment prior to the wetland component. This can 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 the entry zone.

where Kj

7.4.2 TSS Removal

The removal of total suspended solids in SSF wetlands is due to physical processes and is only influenced by temperature through the viscosity effects on the flow of water. Because the settling distance for particulate matter is relatively small and the residence time in the wetland is very long, the viscosity effects can be neglected. The removal of TSS in these wetlands is not likely to be the limiting design parameter for sizing the wetland, because TSS removal is very rapid as compared to either BOD or nitrogen.

Most of the solids in domestic, municipal, and many industrial wastewaters are organic in nature and will decompose in time, leaving minimal residues. The equivalent of primary treatment, as with BOD, will provide an acceptable level of preliminary treatment prior to the wetland component for these types of wastewaters. The subsequent decomposition of the remaining solids in the wetland should leave minimal residues and result in minimal clogging. Wetland systems designed for stormwater, combined sewer overflows, and some industrial wastewaters that have high concentrations of inorganic solids may not require primary treatment but should consider use of a settling pond or cell as the first unit in a wetland system to avoid a rapid accumulation of inorganic solids in the wetland.

The removal of TSS in SSF wetlands has been correlated to the hydraulic loading rate (HLR) as shown in Equation 7.15:


HLR = Hydraulic loading rate (cm/d).

The hydraulic loading rate is the flow rate divided by the surface area. Equation 7.15 is valid for HLR values between 0.4 and 75 cm/d. To use Equation 7.15, calculate the HLR by dividing the flow in ac-ft by the area in acres. Then convert the HLR in in./d to cm/d by dividing by 2.54 cm/in.

7.4.3 Nitrogen Removal

Because the water level is maintained below the media surface in SSF wetlands, the rate of atmospheric reaeration is likely to be significantly less than the FWS wetland type; however, as described previously, the roots and rhizomes of the vegetation are believed to have aerobic microsites on their surfaces, and the wastewater as it flows through the bed has repeated opportunities for contact with these aerobic sites in an otherwise anaerobic environment. As a result, conditions for nitrification and denitrification are present in the same reactor. Both of these biological nitrification and denitrification reactions are temperature dependent, and the rate of oxygen transfer to the plant roots may vary somewhat with the season.


Performance Comparison for Vegetated and Unvegetated Cells at Subsurface Flow Wetlands in Santee, California


Performance Comparison for Vegetated and Unvegetated Cells at Subsurface Flow Wetlands in Santee, California

Bed Condition

Root Penetration (in.)

Effluent BOD

















No vegetation





Note: HRT = 6 d; primary effluent applied: BOD = 118 mg/L, TSS = 57 mg/L, NH3 = 25 mg/L; depth = 2.5 ft.

Note: HRT = 6 d; primary effluent applied: BOD = 118 mg/L, TSS = 57 mg/L, NH3 = 25 mg/L; depth = 2.5 ft.

Source: Gersberg, R.M. et al., Water Res, 20, 363-367, 1985. With permission.

The major carbon sources supporting denitrification are the dead and decaying roots and rhizomes, the other organic detritus, and the residual wastewater BOD. These carbon sources are probably more limited for SSF wetlands, during initial operations, as compared to the FWS case because most of the plant litter collects on top of the bed. After a few years of litter build-up and decay, both types of wetlands may have comparable carbon sources for support of denitrification.

Because a major source of oxygen in the SSF case is the plant roots, it is absolutely essential to ensure that the root system penetrates to the full design depth of the bed. Any water that flows beneath the root zone is in a completely anaerobic environment, and nitrification will not occur except by diffusion into the upper layers. This response is illustrated by the data in Table 7.5, where removal of ammonia can be directly correlated with the depth of penetration by the plant roots. The beds containing Typha (root penetration about 40% of the bed depth) achieved only 32% ammonia removal as compared to the Scirpus beds, which achieved 94% removal and had complete root penetration.

Many existing SSF systems in the United States were designed with the assumption that regardless of the plant species selected the roots would somehow automatically grow to the bottom of the bed and supply all of the necessary oxygen. This has not occurred, and many of these systems cannot meet their discharge limits for ammonia. This problem can be avoided in the future if proper care is taken during design and operation of the system. The root depths listed in Table 7.5 for Santee, California, probably represent the maximum potential depth for the plant species listed because Santee has a warm climate with a continuous growing season and the applied wastewater contains sufficient nutrients. This suggests that the design depth of the bed should not be greater than the potential root depth of the plant intended for use, if oxygen is required for ammonia removal.


Potential Oxygen from Emergent Wetland Vegetation

Root Depth Available Oxygen Available Oxygen

Phragmites 2.0 8.0 4.8

a Available oxygen per unit volume of measured root zone. b Available oxygen per unit surface area of a 2.5-ft-deep bed.

Operational methods for actually achieving the maximum potential root penetration will still be necessary because the plants can obtain all of the necessary moisture and nutrients with the roots in a relatively shallow position. In some European systems, the water level is lowered gradually in the fall of each year to induce deep root penetration. It is claimed that three growing seasons are required to achieve full penetration by Phragmites using this method. Another approach, in cool climates where winter treatment requirements typically require a larger area, is to construct the bed with three parallel cells and only operate two for a month at a time during the warm periods. The roots in the dormant cell should penetrate as the nutrients in the water are consumed. In warm climates, where freezing is not a risk, it is possible to limit the bed depth to 1 ft (0.3 m), which should allow rapid and complete root penetration. The volume of gravel required will be constant regardless of the bed depth, but the surface area required to achieve the same level of treatment will increase as the depth decreases. Nitrification

No consensus has been reached with regard to how much oxygen can be furnished to the root zone in SSF wetlands or regarding the oxygen transfer efficiency of various plant species. It is generally agreed that these emergent plants transmit enough oxygen to their roots to stay alive under normal stress levels, but disagreement arises (as discussed in Chapter 6) over how much oxygen is available at the root surfaces to support biological activity. The oxygen demand from the wastewater BOD and other naturally present organics may utilize most of this available oxygen, but based on the ammonia removals observed at Santee (Table 7.5) there must still be significant oxygen in the root zone to support nitrification.

If the ammonia removals observed at Santee are assumed to be due to biological nitrification, it is possible to calculate the amount of oxygen that should have been available for that purpose, as it requires about 5 g of oxygen to nitrify 1 g of ammonia. The results of these calculations are shown in Table 7.6.

The oxygen available for nitrification per unit of wetland surface area ranged from 2.1 to 5.7 g/m2-d because the depth of root penetration varied with each plant species. These oxygen values are in the published range (4 to 5 g O2 per m2-d); however, the available oxygen, when expressed in terms of the actual root zone of the various plants, is about the same, regardless of the species (average 7.5 g O2 per m3-d). This suggests that, at least for these three species, the oxygen available for nitrification will be about the same so the rate of nitrification is therefore dependent on the depth of the root zone present in the SSF bed. Equation 7.16 defines this relationship:

where KNH is the nitrification rate constant at 20°C (d-1) and rz is the fraction of SSF bed depth occupied by the root zone (decimal).

The Knh value would be 0.4107 with a fully developed root zone and 0.01854 if there were no vegetation on the bed. These values are consistent with performance results observed at several SSF sites evaluated in the United States (Reed, 1993). Independent confirmation of this rate constant is provided by the design model published by Bavor et al. (1986). Bavor's model takes the same form as Equation 7.17 with a rate constant at 20°C of 0.107 d-1 in a gravel bed system where the plant root zone occupied between 50 and 60% of the bed depth.

Having defined the basic rate constant KNH, it is possible to determine the ammonia removal, via nitrification, in a SSF wetland with Equation 7.17 and Equation 7.18:



= Effluent ammonia concentration (mg/L).


= Influent ammonia concentration (mg/L).


= Temperature-dependent rate constant (d-1).


= Hydraulic residence time (d).


= Surface area of wetland (ac; m2).


= Average flow through the wetland (ac-ft/d; m3/d)


= Depth of water in the wetland (ft; m).


= Porosity of the wetland (see Table 7.1).

The temperature dependence of the rate constant KT is given by:

For temperatures below 10°C, it is necessary to solve Equation 7.16 to determine the Knh value. Interpolation can be used for temperatures between 0 and 1°C.

It is unacceptable to assume that the root zone will automatically occupy the entire bed volume, except for relatively shallow (1 ft or 0.3 m) systems using small-sized gravel (20 mm). Deep beds (2 ft or 0.6 m) require the special measures discussed previously to induce and maintain full root penetration. If these special measures are not utilized it would be conservative to assume that the root zone occupies not more than 50% of the bed depth unless measurements show otherwise. It is also unlikely, based on observations at numerous operational systems, that the plant roots will penetrate deeply in the large void spaces occurring when large-size rock (>2 in. or >50 mm) is selected as the bed media.

Equation 7.19 will typically require an HRT of between 6 to 8 d to meet stringent ammonia limits under summer conditions with a fully developed root zone and an even longer period at low winter temperatures. A cost-effective alternative to a large SSF wetland designed for ammonia removal may be the use of a nitrification filter bed (NFB). In that case, the SSF wetland can be designed for BOD removal only, and the relatively compact NFB can be used for ammonia removal. The combination of the SSF wetland and the NFB bed should require less than one half of the total area that would be necessary for a SSF wetland designed for ammonia removal. The NFB bed can also be used to retrofit existing wetland systems. Design details for the NFB concept are presented in a later section of this chapter.

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