335 7 Subsurface and Vertical Flow Constructed Wetlands Subsurface flow (SSF) wetlands consist of shallow basins or channels with a seepage barrier and inlet and outlet structures. The bed is filled with porous media and vegetation is planted in the media. The water flow is horizontal in the SSF wetland and is designed to be maintained below the upper surface of the media, hence the title subsurface flow . In the United States, the most common medium is gravel, but sand and soil have been used in Europe. The media depth and the water depth in these wetlands have ranged from 1 ft (0.3 m) to 3 ft (0.9 m) in operational systems in the United States. The design flow for most of these systems in the United States is less than 50,000 gpd (189 m 3 /d). The largest system in the United States (Crowley, LA) has a design flow of 3.5 mgd (13,000 m 3 /d) (Reed et al., 1995). A schematic of a typical SSF wetland is shown in Figure 7.1. 7.1 HYDRAULICS OF SUBSURFACE FLOW WETLANDS Darcy’s law, as defined by Equation 7.1, describes the flow regime in a porous media and is generally accepted for the design of SSF wetlands using soils and gravels as the bed media. A higher level of turbulent flow may occur in beds using very coarse rock, in which case Ergun’s equation is more appropriate. Darcy’s law is not strictly applicable to subsurface flow wetlands because of physical limitations in the actual system. It assumes laminar flow conditions, but turbulent flow may occur in very coarse gravels when the design utilizes a high hydraulic gradient. Darcy’s law also assumes that the flow in the system is constant and uniform, but in reality the flow may vary due to precipitation, evaporation, and seepage, and local short-circuiting of flow may occur due to unequal porosity or poor construction. If small- to moderate-sized gravel is used as the media, if the system is properly constructed to minimize short-circuiting, if the system is designed to depend on a minimal hydraulic gradient, and if the gains and losses of water are recognized, then Darcy’s law can provide a reason- able approximation of the hydraulic conditions in a SSF wetland: v = k s s DK804X_C007.fm Page 335 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 336 Natural Wastewater Treatment Systems Because v = Q / Wy then Q = k s A c s (7.1) where v = Darcy’s velocity, the apparent flow velocity through the entire cross- sectional area of the bed (ft/d; m/d). k s = Hydraulic conductivity of a unit area of the wetland perpendicular to the flow direction (ft 3 /ft 2 ·d; m 3 /m 2 ·d). s = Hydraulic gradient, or slope, of the water surface in the flow system (ft/ft; m/m). Q =Average flow through the wetland (ft 3 /d; m 3 /d) = [ Q in + Q out ]/2. W =Width of the SSF wetland cell (ft; m). y =Average depth of water in the wetland (ft; m). A c =Total cross-sectional area perpendicular to the flow (ft 2 ; m 2 ). The resistance to flow in the SSF wetland is caused primarily by the gravel media. Over the longer term, the spread of plant roots in the bed and the accu- mulation of nondegradable residues in the gravel pore spaces will also add resistance. The energy required to overcome this resistance is provided by the head differential between the water surface at the inlet and the outlet of the wetland. Some of this differential can be provided by constructing the wetland with a sloping bottom. The preferred approach is to construct the bottom with sufficient slope to allow complete drainage when needed and to provide outlet structures that allow adjustment of the water level to compensate for the resistance that may increase with time. The aspect ratio (length-to-width) selected for a SSF FIGURE 7.1 Schematic of a subsurface flow constructed wetland. Water Surface Liner Native Soil Gravel Bed DK804X_C007.fm Page 336 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC Subsurface and Vertical Flow Constructed Wetlands 337 wetland also strongly influences the hydraulic regime as the resistance to flow increases as the length increases. Reed et al. (1995) developed a model that can be used to estimate the minimum acceptable width of a SSF wetland channel. It is possible by substitution and rearrangement of terms to develop an equation for determining the acceptable minimum width of the SSF wetland cell that is compatible with the hydraulic gradient selected for design: W = (1/ y )[( Q A )( A s )/( m )( k s )] 0.5 (7.2) where W =Width of the SSF wetland cell (ft; m). y =Average depth of water in the wetland (ft; m). Q A =Average flow through the wetland (ft 3 /d; m 3 /d). A s = Design surface area of the wetland (ft 2 ; m 2 ). m = Portion of available hydraulic gradient used to provide the necessary head, as a decimal. k s = Hydraulic conductivity of the media used (ft 3 /ft 2 /d; m 3 /m 2 /d). The m value in Equation 7.2 typically ranges from 5 to 20% of the potential head available. When using Equation 7.2 for design it is recommended that not more than one third of the effective hydraulic conductivity ( k s ) be used in the calculation and that the m value not exceed 20% to provide a large safety factor against potential clogging and other contingencies not defined at the time of design. Typical characteristics for media (medium gravel is most commonly used in the United States) with the potential for use in SSF wetlands are given in Table 7.1. For large projects, it is recommended that the hydraulic conductivity ( k s ) be directly measured with a sample of the media to be used in the field or laboratory prior to final design. A permeameter is the standard laboratory device, but it is not well suited to the coarser gravels and rocks often used in these systems. A TABLE 7.1 Typical Media Characteristics for Subsurface Flow Wetlands Media Type Effective Size (D 10 ) (mm) Porosity ( n ) (%) Hydraulic Conductivity ( k s ) (ft/d) Coarse sand 2 28–32 328–3280 Gravelly sand 8 30–35 1640–16,400 Fine gravel 16 35–38 3280–32,800 Medium gravel 32 36–40 32,800–164,000 Coarse rock 128 38–45 164,000–820,000 Note: ft/d × 0.305 = m/d. DK804X_C007.fm Page 337 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 338 Natural Wastewater Treatment Systems permeameter trough that has been used successfully to measure the effective hydraulic conductivity of a range of gravel sizes is shown in Figure 7.2. The total length of the trough is about 16.4 ft (5 m), with perforated plates located about 1.5 ft (0.5 m) from each end. The space between the perforated plates is filled with the media to be tested. The manometers are used to observe the water level inside the permeameter, and they are spaced about 9 ft (3 m) apart. Jacks or wedges are used to slightly raise the head end of the trough above the datum. Water flow into the trough is adjusted until the gravel media is flooded but without free water on the surface. The discharge ( Q ) is measured in a calibrated container and timed with a stopwatch. The cross-sectional flow area ( A c ) is estimated by noting the depth of the water as it leaves the perforated plate at the end of the trough and multiplying that value by the width of the trough. The hydraulic gradient ( s ) for each test is ( y 1 – y 2 )/ x (dimensions are shown on Figure 7.2). It is then possible to calculate the hydraulic conductivity because the other parameters in Equation 7.2 have all been measured. The Reynolds number should also be calculated for each test to ensure that the assumption of laminar flow was valid. The porosity ( n ) of the media to be used in the SSF wetland should also be measured prior to final system design. This can be measured in the laboratory using a standard American Society for Testing and Materials (ASTM) procedure. An estimate is possible in the field by using a large container with a known volume. The container is filled with the media to be tested, and construction activity is simulated by some compaction or lifting and dropping the container. The container is then filled to a specified mark with a measured volume of water. The volume of water added defines the volume of voids ( V v ). Because the total volume ( V t ) is known, it is possible to calculate the porosity ( n ): FIGURE 7.2 Permeameter trough for measuring hydraulic conductivity of subsurface flow media. Perforated plate Inflow Test gravel Monometers Calibrated container Perforated plate Outflow y x y Datum DK804X_C007.fm Page 338 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC Subsurface and Vertical Flow Constructed Wetlands 339 n = V v / V t (100) (7.3) Many existing SSF wetlands were designed with a high aspect ratio (length- to-width ratio of 10:1 or more) to ensure plug flow in the system. Such high aspect ratios are unnecessary and have induced surface flow on these systems because the available hydraulic gradient is inadequate to maintain the intended subsurface flow. Some surface flow will occur on all SSF wetlands in response to major storm events, but the pollutant concentrations are proportionally reduced and treatment efficiency is not usually affected. The system should be initially designed for the average design flow and the impact of peak flows and storm events evaluated. The previous recommendation that the design hydraulic gradient be limited to not more than 10% of the potential head has the practical effect of limiting the feasible aspect ratio for the system to relatively low values (<3:1 for beds 2 ft deep; 0.75:1 for beds 1 ft deep). SSF systems in Europe with soil instead of gravel have been constructed with up to 8% slopes to provide an adequate hydraulic gradient, and they have still experienced continuous surface flow due to an inadequate safety factor. 7.2 THERMAL ASPECTS The actual thermal status of a SSF wetland bed can be a very complex situation. Heat gains or losses can occur in the underlying soil, the wastewater flowing through the system, and the atmosphere. Basic thermal mechanisms involved include conduction to or from the ground, conduction to or from the wastewater, conduction and convection to or from the atmosphere, and radiation to or from the atmosphere. It can be shown that energy gains or losses to the ground are a minor component and can therefore be neglected. It is conservative to ignore any energy gains from solar radiation but is appropriate at northern sites where the temperature conditions are most critical. In the southwest, where solar radiation can be very significant on a year-round basis, this factor might be included in the calculations. Convection losses can be significant due to wind action on an open water surface, but this should not be the case for most SSF wetlands where a dense stand of vegetation, a litter layer, and a layer of relatively dry gravel are typically present. These damp out the wind effects on the underlying water in the wetland, and, as a result, convection losses will be relatively minor and can be ignored in the thermal model. The simplified model developed below is therefore based only on conduction losses to the atmosphere and is conservative. This procedure was developed from basic heat-transfer relationships (Chapman, 1974) with the assistance of experts on the topic (Calkins, 1995; Ogden, 1994). The temperature at any point in the SSF wetland can be predicted by comparing the estimated heat losses to the energy available in the system. The losses are assumed to occur via conduction to the atmosphere, and the only energy source available is assumed to be the water flowing through the wetland. As water is cooled, it releases energy, and this energy is defined as the specific heat. The specific DK804X_C007.fm Page 339 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 340 Natural Wastewater Treatment Systems heat of water is the amount of energy that is either stored or released as the temperature is either increased or decreased. The specific heat is dependent on pressure and to a minor degree on temperature. Because atmospheric pressure will prevail at the water surface in the systems discussed in this book, and because the temperature influence is minor, the specific heat is assumed to be a constant for practical purposes. For the calculations in this book, the specific heat is taken as 1.007 BTU/lb·°F (4215 J/kg·°C). The specific heat relationship applies down to the freezing point of water (32°F; 0°C). Water at 32°F will still not freeze until the available latent heat is lost. The latent heat is also assumed to be a constant and equal to 144 BTU/lb (334,944 J/kg). The latent heat is, in effect, the final safety factor, protecting the system against freezing; however, when the temperature drops to 32°F (0°C), freezing is imminent and the system is on the verge of physical failure. To ensure a conservative design, the latent heat is only included as a factor in these calculations when a determination of potential ice depth is made. The available energy in the water flowing through the wetlands is defined by Equation 7.4: q G = c p ( δ )( A s )( y )( n ) (7.4) where q G = Energy gain from water (Btu/°F; J/°C). c p = Specific heat capacity of water (1.007 Btu/lb·°F; 4215 J/kg·°C). δ = Density of water (62.4 lb/ft 3 ; 1000 kg/m 3 ). A s = Surface area of wetland (ft 2 ; m 2 ). y = Depth of water in wetland (ft; m). n = Porosity of wetland media (percent). If it is desired to calculate the daily temperature change of the water as it flows through the wetland, the term A s /t is substituted for A s in Equation 7.4: q G = (c p )(δ)(A s /t)(y)(n) (7.5) where q G is the energy gain during 1 d of flow (Btu/d·°F; J/d·°C), t is the hydraulic residence time in the system (d), and the other terms are as defined previously. The heat losses from the entire SSF wetland can be defined by Equation 7.6: q L = (T 0 – T air )(U)(σ)(A s )(t) (7.6) where q L = Energy lost via conduction at the atmosphere (Btu; J). T 0 =Water temperature entering wetland (°F; °C). T air =Average air temperature during period of concern (°F; °C). U = Heat-transfer coefficient at the surface of the wetland bed (Btu/ft 2 ·hr·°F; W/m 2 ·°C). σ =Time conversion (24 hr/d; 86,400 s/d). A s = Surface area of wetland (ft 2 ; m 2 ). t = Hydraulic residence time in the wetland (d). DK804X_C007.fm Page 340 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC Subsurface and Vertical Flow Constructed Wetlands 341 The T air values in Equation 7.6 can be obtained from local weather records or from the closest weather station to the proposed wetland site. The year with the lowest winter temperatures during the past 20 or 30 years of record is selected as the “design year” for calculation purposes. It is desirable to use an average air temperature over a time period equal to the design hydraulic residence time (HRT) in the wetland for these thermal calculations. If monthly average temperatures for the “design year” are all that is available, they will usually give an acceptable first approximation for calculation purposes. If the results of the thermal calcu- lations suggest that marginally acceptable conditions will prevail then further refinements are necessary for a final system design. The conductance (U) value in Equation 7.6 is the heat-conducting capacity of the wetland profile. It is a combination of the thermal conductivity of each of the major components divided by its thickness as shown in Equation 7.7: U = 1/[(y 1 /k 1 ) +(y 2 /k 2 ) + (y 3 /k 3 ) + (y n /k n )] (7.7) where U = Conductance (Btu/ft 2 ·hr·°F; W/m 2 ·°C). k (1–n) = Conductivity of layers 1 to n (Btu/ft 2 ·hr·°F; W/m·°C). y (1–n) = Thickness of layers 1 to n (ft; m). Values of conductivity for materials that are typically present in SSF wetlands are presented in Table 7.2. The conductivity values of the materials, except the wetland litter layer, are well established and can be found in numerous literature sources. The conductivity for a SSF wetland litter layer is believed to be conser- vative but is less well established than the other values in Table 7.2. TABLE 7.2 Thermal Conductivity of Subsurface Flow Wetland Components Material k (Btu/ft 2 ·hr·°F) k (W/m·°C) Air (no convection) 0.014 0.024 Snow (new, loose) 0.046 0.08 Snow (long-term) 0.133 0.23 Ice (at 0°C) 1.277 2.21 Water (at 0°C) 0.335 0.58 Wetland litter layer 0.029 0.05 Dry (25% moisture) gravel 0.867 1.5 Saturated gravel 1.156 2.0 Dry soil 0.462 0.8 DK804X_C007.fm Page 341 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 342 Natural Wastewater Treatment Systems Example 7.1 Determine the conductance of a SSF wetland bed with the following character- istics: 8-in. litter layer, 6 in. of dry gravel, and 18 in. of saturated gravel. Compare the value to the conductance with a 12-in. layer of snow. Solution 1. Calculate the U value without snow using Equation 7.7: U = 1/[(0.67/0.029) + (0.5/0.867) + (1.5/1.156)] = 0.040 Btu/ft 2 ·hr·°F 2. Calculate the U value with snow: U = 1/[(1/0.133) + (0.67/0.029) + (0.5/0.867) + (1.5/1.156)] = 0.031 Btu/ft 2 ·hr·°F Comment The presence of the snow reduces the heat losses by 23%. Although snow cover is often present in colder climates, it is prudent for design purposes to assume that the snow is not present. The change in temperature due to the heat losses and gains defined by Equation 7.5 and Equation 7.6 can be found by combining the two equations: T c = q L /q G = (T 0 – T air )(U)(σ)(A s )(t)/(c p )(δ)(A s )(y)(n) (7.8) where T c is the temperature change in the wetland (°F; °C), and the other terms are as defined previously. The effluent temperature (T e ) from the wetland is: T e = T 0 – T c (7.9) or T = T 0 – (T 0 – T air )[(U)(σ)(t)/(c p )(δ)(y)(n)] (7.10) The calculation must be performed on a daily basis. The T 0 value is the temper- ature of the water entering the wetland that day, T e is the temperature of the effluent from the wetland segment, and T air is the average daily air temperature during the time period. The average water temperature (T w ) in the SSF wetland is, then: T w = (T 0 – T c )/2 (7.11) This average temperature is compared to the temperature value assumed when the size and the HRT of the wetland were determined with either the biochemical oxygen demand (BOD) or nitrogen removal models. If the two temperatures do not closely correspond, then further iterations of these calculations are necessary until the assumed and calculated temperatures converge. DK804X_C007.fm Page 342 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC Subsurface and Vertical Flow Constructed Wetlands 343 Further refinement of this procedure is possible by including energy gains and losses from solar radiation and conduction to or from the ground. During the winter months, conduction from the ground is likely to represent a small net gain of energy because the soil temperature is likely to be higher than the water temperature in the wetland. The energy input from the ground can be calculated with Equation 7.6; a reasonable U value would be 0.056 Btu/ft 2 ·hr·°F (0.32 W/m 2 ·°C), and a reasonable ground temperature might be 50°F (10°C). The solar gain can be estimated by determining the net daily solar gain for the location of interest from appropriate records. Equation 7.12 can then be used to estimate the heat input from this source. The results from Equation 7.12 should be used with caution. It is possible that much of this solar energy may not actually reach the water in the SSF wetland because the radiation first impacts on the vegetation and litter layer and a possible reflective snow cover, so an adjustment is necessary in Equation 7.12. As indicated previously, it is conservative to neglect any heat input to the wetland from these sources: q solar = (Φ)(A s )(t)(s) (7.12) where q solar = Energy gain from solar radiation (Btu; J). Φ = Solar radiation for site (Btu/ft 2 ·d; J/m 2 ·d). A s = Surface area of wetland (ft; m). t =HRT for the wetland (d). s = Fraction of solar radiation energy that reaches the water in the SSF wetland, typically 0.05 or less. If these additional heat gains are calculated, they should be added to the results from Equation 7.4 or Equation 7.5 and this total used in the denominator of Equation 7.10 to determine the temperature change in the system. If the thermal models for SSF wetlands predict sustained internal water temperature of less than 33.8°F (1°C), a wetland may not be physically capable of winter operations at the site under consideration at the design HRT. Nitrogen removal is likely to be negligible at those temperatures. Constructed wetlands can operate successfully during the winter in most of the northern temperate zone. The thermal models presented in this section should be used to verify the temperature assumptions made when the wetland is sized with the biological models for BOD or nitrogen removal. Several iterations of the calculation procedure may be necessary for the assumed and calculated temperatures to converge. 7.3 PERFORMANCE EXPECTATIONS The performance expectations for SSF constructed wetlands are considered in the following discussion. As with the free water system (FWS; see Chapter 6), process performance depends on design criteria, wastewater characteristics, and operations. Removal mechanisms are described in Chapter 3. DK804X_C007.fm Page 343 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 344 Natural Wastewater Treatment Systems 7.3.1 BOD REMOVAL Performance data for BOD removal are presented in Table 7.3. The removal of BOD appears to be faster and somewhat more reliable with SSF wetlands than for FWS wetlands, partly because the decaying plants are not in the water column, thereby producing slightly less organic matter in the final effluent. 7.3.2 TSS REMOVAL Subsurface flow wetlands are efficient in the removal of suspended solids, with effluent total suspended solids (TSS) levels typically below 10 mg/L. Removal rates are similar to FWS wetlands. 7.3.3 NITROGEN REMOVAL Although the SSF system at Santee, California, was able to remove 86% of the nitrogen from primary effluent, other SSF systems have reported removals of from 20 to 70%. When detention times exceed 6 to 7 d, an effluent total nitrogen concentration of about 10 mg/L can be expected, assuming a 20- to 25-mg/L influent nitrogen concentration. If the applied wastewater has been nitrified (using extended aeration, overland flow, or recirculating sand filters), the removal of nitrate through denitrification can be accomplished with detention times of 2 to 4 d. TABLE 7.3 Total BOD Removal Observed in Subsurface Flow Wetlands Location Pretreatment Influent Effluent Removal (%) Nominal Detention Time (d) Benton, Kentucky a Oxidation pond 23 8 65 5 Mesquite, Nevada b Oxidation pond 78 25 68 3.3 Santee, California c Primary 118 1.7 88 6 Sydney, Australia d Secondary 33 4.6 86 7 a Full-scale operation from March 1988 to November 1988, operated at 80 mm/d (Watson et al., 1989). b Full-scale operation, January 1994 to January 1995. c Pilot-scale operation in 1984, operated at 50 mm/d (Gersberg et al., 1985). d Pilot-scale operation at Richmond, New South Wales, near Sydney, Australia, operated at 40 mm/d from December 1985 to February 1986 (Bavor et al., 1986). DK804X_C007.fm Page 344 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC [...]... Taylor & Francis Group, LLC DK804X_C0 07. fm Page 364 Friday, July 1, 2005 3:46 PM 364 Natural Wastewater Treatment Systems 7. 10 DESIGN OF ON-SITE SYSTEMS On-site systems are defined as relatively small facilities serving a single wastewater 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... 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 © 2006 by Taylor & Francis Group, LLC DK804X_C0 07. fm Page 358 Friday, July 1, 2005 3:46 PM 358 Natural Wastewater Treatment Systems TABLE 7. 7 Design... provided by Equation 7. 22 and Equation 7. 23: Ce/C0 = exp(–KTt) (7. 22) As = Qln(Ce/C0)/KTyn (7. 23) where As = Surface area of wetland (ac; m2) Ce = Effluent nitrate-nitrogen concentration (mg/L) C0 = Influent nitrate-nitrogen concentration (mg/L) © 2006 by Taylor & Francis Group, LLC DK804X_C0 07. fm Page 352 Friday, July 1, 2005 3:46 PM 352 Natural Wastewater Treatment Systems KT = Temperature-dependent rate... followed by a 96% reduction through the vertical-flow wetlands (Rozema, 2004) © 2006 by Taylor & Francis Group, LLC DK804X_C0 07. fm Page 370 Friday, July 1, 2005 3:46 PM 370 Natural Wastewater Treatment Systems (a) (b) 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... DK804X_C0 07. fm Page 348 Friday, July 1, 2005 3:46 PM 348 Natural Wastewater Treatment Systems TABLE 7. 5 Performance Comparison for Vegetated and Unvegetated Cells at Subsurface Flow Wetlands in Santee, California Bed Condition Scirpus Phragmites Typha No vegetation Root Penetration (in.) Effluent BOD (mg/L) Effluent TSS (mg/L) Effluent NH3 (mg/L) 30 5.3 3 .7 1.5 >24 22.3 7. 9 5.4 12 30.4 5.5 17. 7 0 36.4... This phenomenon is shown in Figure 7. 5 The improvement in treatment between 1996, when continuous flow was practiced, and 19 97, when the sequential fill/drain operation was initiated, is shown in Table 7. 8 (Reed and Giarrusso, 1999) © 2006 by Taylor & Francis Group, LLC DK804X_C0 07. fm Page 360 Friday, July 1, 2005 3:46 PM 360 Natural Wastewater Treatment Systems TABLE 7. 8 Constituent Loadings and Removals... 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... (USEPA, 2000) 7. 7 POTENTIAL APPLICATIONS The applications for SSF wetlands are many and expanding Municipal wastewater 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... (Section 7. 8) illustrates one approach Other approaches are described under the section on vertical flow wetlands (Section 7. 11) © 2006 by Taylor & Francis Group, LLC DK804X_C0 07. fm Page 356 Friday, July 1, 2005 3:46 PM 356 Natural Wastewater Treatment Systems 7. 6.2 RECIPROCATING (ALTERNATING) DOSING (TVA) Researchers at the TVA developed and patented a “reciprocating” dosing of SSF wetlands in which the wastewater. .. 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: Av = [ 271 3 – 1115(Ce) + 204(Ce)2 – 12(Ce)3]/KT © 2006 by Taylor & Francis Group, LLC (7. 24) DK804X_C0 07. fm Page 362 Friday, July 1, 2005 3:46 PM 362 Natural Wastewater Treatment Systems TABLE 7. 9 Specific Surface Area for a Variety . (ac-ft/d; m 3 /d). The influent nitrate concentration (C 0 ) used in Equation 7. 22 or Equation 7. 23 is the amount of ammonia oxidized, as calculated in Equation 7. 17. Because Equa- tion 7. 17 determines. Tennessee (personal commu- nication). DK804X_C0 07. fm Page 345 Friday, July 1, 2005 3:46 PM © 2006 by Taylor & Francis Group, LLC 346 Natural Wastewater Treatment Systems 7. 4 DESIGN OF SSF WETLANDS Subsurface. snow using Equation 7. 7: U = 1/[(0. 67/ 0.029) + (0.5/0.8 67) + (1.5/1.156)] = 0.040 Btu/ft 2 ·hr·°F 2. Calculate the U value with snow: U = 1/[(1/0.133) + (0. 67/ 0.029) + (0.5/0.8 67) + (1.5/1.156)]