627 17 Sizing of FWS Wetlands A performance-based procedure is unavoidable for FWS wet- lands because they address too many applications with vary- ing degrees of source strength and pretreatment. In general, the targets are also variable because of the differences in reg- ulatory requirements among countries, states, and discharge recipients. Loading specications, whether as pollutant kilo- grams per wetland hectare per year or as wetland hectares per person equivalent, have not, and, in general, cannot be developed to deal with the spectrum of situations created by FWS diversity of applications. It is not an exaggeration to say that each FWS wetland is unique. Design of FWS wetlands may be roughly divided into two categories: sizing calculations and physical specica- tions. Sizing requires characterization of the incoming water and regional meteorology as well as the goals of wetland treatment, as discussed in Chapter 16. Here, a comprehen- sive sizing strategy is presented, based on the information assembled. Chapter 18 will deal with physical consider- ations, including the number of cells, layout, depth, bathym- etry, soils and plants, structures, and lining. It is recognized that wetlands are almost never stand-alone treatment devices, but rather form part of a treatment train. Other components may be mechanical, such as clariers or lters, or more natu- ral, such as settling basins or lagoons. More than one type of wetland may be involved at the same site—FWS, VF, and HSSF. This chapter focuses on only the FWS component of treatment systems. Based on the performance-based sizing algorithm intro- duced in Chapter 15, the key features described in this chap- ter are the following: Parameter selection and calculation 6 . Select rate constants and seasonality. (Select plant community type.) 7. Select DTD (hydraulic) efciency P-value. (Select compartmentalization.) 8. Adjust area till goals are all met. Constraint checks and iteration 9. Set estimated growth cycle. 10. Check biogeochemical cycles for consistency. 11. Check chemical constraints. 12. Check loading graph for risk assessment. 13. Adjust for seasonality. Repeat this procedure as needed to meet design goals. The complexity of wetland behavior and, hence, of the sizing calculation is such that a single equation for wetland area cannot be written. At this point in the evolution of the treatment of wetland design, the sizing procedure must move out of the realm of the pocket calculator and into the world of spreadsheet computations. The single formula, black-box approach will not sufce. 17.1 POLLUTANT REDUCTIONS AND PERFORMANCE COMPUTATIONS The information collected is utilized as a basis to forecast the area needed to achieve the goals of the project. The pro- cedure outlined here is for a steady-ow situation. Different ows may need to be considered to deal with daily or sea- sonal peaking. Different seasons may need to be considered, so that the “bottleneck” period of the year is identied and analyzed. Many literature sources provide single equations into which anyone may insert numbers to compute a wetland area. Although such an option would be very convenient, it is not realistic except for a few highly specialized cases. Accordingly, the procedure given here explores the effect of changing wetland area (or equivalently, detention time) on the forecasted efuent concentrations of contaminants. It is presumed that the designer will conduct the design calcula- tions via a computer spreadsheet so that design changes may be easily explored. Wetland hydrology is rst determined as a necessary precur- sor to area calculations. Annualized calculations are considered rst; the effects of season will be considered later in the chapter. WATER BUDGET The annual water budget forms the basis for understand- ing pollutant reductions and the area required. Assuming a constant water level, the ows out of the wetland may be computed from the inow and meteorological data. Some inltration is also assumed here if leakage is present in the wetland. The relevant equations are QQAPETI oi () (17.1) qq PETI oi () (17.2) where wetland area, m evapotranspiratio 2 A ET nn rate, m/d infiltration rate, m/d preci I P ppitation rate, m/d hydraulic loading rateq ,, m/d flow rate, m /d 3 Q © 2009 by Taylor & Francis Group, LLC 628 Treatment Wetlands The inlet hydraulic loading will be increased by rainfall (≈0.5–1.5 m/yr) and decreased by evapotranspiration (ET) (≈ 0.5–1.5 m/yr). However, there may be seasonal imbalances. These amounts are important if the wetland is to have a very low hydraulic loading or, correspondingly, a long detention time. The ratio a (P ET)/q i is the atmospheric augmentation. Evapotranspiration (rain) has two effects: lengthening (shortening) of detention time and concentration (dilution) of dissolved constituents. The use of an average ow rate com- pensates for altered detention time but not for dilution or concentration. The fractional error in a rst-order model prediction of concentration due to ow averaging is approxi- mately equal to a, for a −0.5. Thus, if 25% of the inow evaporates, use of a rst-order model with average ow pre- dicts concentrations 25% lower than required by the mass balance. If rain adds 25% to the ow, use of a rst-order model predicts concentrations 25% higher. A possibly important feature of ET is that the transpira- tion carries water into the root zone (see Part I, Chapter 4). Therefore, if the pollutant mass balances are done on surface water, then the transpiration component is the same as inl- tration: it carries materials into the soil. For a fully vegetated wetland, somewhat more than half of ET is attributable to transpiration. Transpiration is typically about one half to two thirds of ET (Kadlec, 2006c): TETE ET()A (17.3) where evaporation, m/d transpiration, m/d E T AAtranspiration fraction of , dimensionlET eess The pollutant mass balances will be conducted on a cells- in-series basis (see Figure 17.1). Results of the overall water mass balance are apportioned to the cells according to the chosen number of TIS. For the rst unit in the series: QQ APETI 11 in () (17.4) where area of tank number 1, m flow ra 1 2 1 A Q tte out of tank 1, m /d 3 Flows are thus computed sequentially, from inlet to outlet, for the number of tanks chosen (PTIS). The input data requirements for water mass balances are 1. Inow (Q in ) 2. Rain (P) 3. Evapotranspiration (ET) 4. Inltration (I) 5. Area (A) 6. Apparent number of tanks in series (P-value) An example is detailed in Table17.1. This hypothetical example is congured to include small rainfall, considerable ET, and some inltration; in other words, a leaky arid region system. The net loss of incoming water is 41%, of which 44% is ET and 56% is inltration. POLLUTANT MASS BALANCES The TIS model is then carried forward via a sequential cal- culation of pollutant concentrations for each “tank” in the chosen hydraulic model (Figure 17.1). A rst-order areal model with rate constant k is selected with necessary wetland background concentration C*. A volumetric rst-order model may also be chosen for which k Ehk V . The pollutant mass balance for the rst of the wet- land segments, designated by subscript “1” for steady-state, FIGURE 17.1 Conceptual TIS model for pollutant reduction. Tank 1 PET Q 1 C 1 Q n–1 C n–1 Q 1 C 1 kAC 1 Removal Conversion Burial Infiltration I 0.5T kAC* Return Flux Tank 2 Tank N © 2009 by Taylor & Francis Group, LLC Sizing of FWS Wetlands 629 nonuniform ow is QC Q C I AC ET AC kA C 11 11 11 11 ()( )( ) ( in in A CC*) (17.5) In this simple version, rainfall has been assumed to have zero pollutant concentration, but it is easy to add an atmospheric input of the pollutant if it exists. Inltration is assumed to occur at the outlet concentration. Transpiration ow of the contaminant has been included. Combining Equation 17.4 with Equation 17.5 gives the concentration exiting the hypo- thetical segment number one: C QC k A C QETAIAkA 1 1 1111 iin (*) () ()(A )) (17.6) Or C qC kC qETIk 1 1 in in (*) ()A (17.7) Note that the hydraulic loading rates in Equation 17.7 are the individual tank loading rates, not the overall system load- ing rates. This computation is then repeated sequentially for the remaining segments, using, in each case, the outlet concentrations and ows from the preceding unit. The wet- land outlet concentration is that exiting the nal hypotheti- cal segment. Outgoing pollutant loads are calculated as the product of the volumetric outow (m 3 /d) and the outow concentration (kg/m 3 ). The additional input data requirements for the pollutant mass balances are 7. Input concentration (C in ) 8. Background concentration (C*) 9. Rate coefcient (k) 10. Transpiration fraction (A) The hypothetical example presented for the water budget is extended to illustrate these calculations, continuing the choice of P 3 TIS. Phosphorus is chosen as the pollutant of interest, entering the wetland at 2.00 mg/L. The value of C* is chosen to be low, 0.01 mg/L. The rate constant is selected to be the median shown for phosphorus in FWS systems in Table 10.11, k 10 m/yr. Because the transpiration ux is presumed to draw phosphorus into the root zone, the frac- tion of ET, that is, transpiration, must be selected, and in the example it is picked as A 0.5. The computed phosphorus concentration in the surface outow water is C o 0.62 mg/L, or a concentration reduction of 69% (Table 17.2). However, the existence of ET losses and inltration creates a different result for mass removal: 82% of the phosphorus entering does not leave with surface water. If inltration water departs vertically downward without fur- ther treatment, then 15% of the phosphorus mass removal is due to inltration. Thus, it is seen that load reduction and concentration reduction are two different goals, which may lead to different designs. Both these types of design sizing require mass balance computations for water and rate calcu- lations for pollutants. TABLE 17.1 Water Budgets for a FWS Design Example Parameters Flow rate 5,000 m 3 /d Rain 0.05 cm/d ET 0.40 cm/d Inltration 0.50 PTIS 3 Area 24 ha Area/tank 8 ha Depth 0.3 m Porosity 0.95 Volume/tank 22,800 m 3 Inflow Tank 1 Tank 2 Tank 3 Outflo w Total Flow rate (m 3 /d) 5,000 4,320 3,640 2,960 — Rain (m 3 /d) — 40 40 40 120 ET (m 3 /d) — 320 320 320 960 Inltration (m 3 /d) — 400 400 400 1,200 Nominal detention (d) 13.7 5.3 6.3 7.7 19.2 HLR (cm/d) 2.08 5.40 4.55 3.70 — Note: Sequential tank-to-tank calculations are based on Equation 17.4; this example has a com- bined detention time of 19.2 days, whereas averaging the ows leads to 17.2 days; the required input data are shown in bold. © 2009 by Taylor & Francis Group, LLC 630 Treatment Wetlands INTERCONNECTED POLLUTANTS:THE CASE OF NITROGEN Nitrogen species interconvert, thereby linking the mass bal- ances for organic, ammonia, and oxidized nitrogen. It is some- times possible to disconnect these species, as, for instance, in the case of wetlands that receive nitrate but little or no organic or ammonia nitrogen. However, in many cases, it is neces- sary to account for the (internal) production of ammonia from organic sources, i.e., from either the incoming water or the decomposition of wetland necromass, and the internal produc- tion of oxidized nitrogen. A simple, presumed chemistry is ORG-NNH-NNO-NN 4x2 lll (17.8) In a simplied version of analysis, uptake and return from biomass is not included. The effects of the biogeochemical cycle on nitrogen will be explored in a subsequent part of the design process. The three mass balances then become linked, and the tank equations are: QC Q C I AC ETAC k A 11 11 11 1OinO,in O OO ()( )( )(A CCC OO * 1 ) (17.9) QC Q C I AC ETAC kA A11 11 11 1 AinA,in A A ()( )( ) ( A CCC kACC AAOOO111 ** )( ) (17.10) QC Q C I AC ETAC kA 11 11 11 1 NinN,in N N N ()( )( ) ( A CCC kACC NNAAA * 111 * )( ) (17.11) where organic N concentration, mg/L amm O A C C oonia N concentration, mg/L oxidized N co N C nncentration, mg/L organic N rate coeffic O k iient, m/d ammonia N rate coefficient, m/ A k dd oxidized N rate coefficient, m/d N k and the subscript “in” denotes parameters associated with the inuent ow for each nitrogen form. These may be rearranged to solve the outlet concentra- tions for each tank: C QC kAC QETAIAkA O in O,in O * O 1 1 1111 ¤ ¦ ¥ ³ µ ()A ´´ (17.12) C QC k AC k C C QETA A in A,in A A * OO1 O 1 1 11 () () * A IIA k A 11 ¤ ¦ ¥ ³ µ ´ A (17.13) C QC k AC k C C QETA N in N,in N N A A A * 1 11 11 * () ()A IIA k A 11 ¤ ¦ ¥ ³ µ ´ N (17.14) Equation 17.12 is a direct analog of Equation 17.7 for an unspecied generic pollutant. Equations 17.13 and 17.14 contain extra production terms from ammonication in the ammonia balance—and nitrication in the oxidized nitrogen balance. The three must be solved sequentially—Equation 17.12 followed by Equations 17.13 and 17.14. DESIGN PARAMETERS:SOURCES OF INFORMATION The P-k-C* design model is the basis for this sizing analysis; it is therefore necessary to select values of these three param- eters for all pollutants of concern. Background Concentrations Wetland systems are dominated by plants (autotrophs), which act as primary producers of biomass. However, wetlands also include communities of microbes (heterotrophs) and higher animals, which act as grazers and reduce plant biomass. Most wetlands support more producers than consumers, resulting in a net surplus of plant biomass. This excess material is TABLE 17.2 Phosphorus Budgets for a FWS Design Example Parameters C i 2.00 mg/L C* 0.01 mg/L k 10 m/yr k 0.0274 m/d Area/tank 80,000 m 2 Theta factor 0.5 Inflow Tank 1 Tank 2 Tank 3 Outflo w Removed Reduction Concentration (mg/L) 2.00 1.42 0.96 0.62 — 69% Load in surface water (kgP/yr) 3,650 — — 666 2,984 82% Load inltrated (kgP/yr) — 207 140 90 437 15% Load stored (kgP/yr) — — — — 2,547 70% Note: This example is based on the water budget shown in Table 1 7.1; sequential tank-to-tank calculations are based on Equation 17.6; the required input data are shown in bold; the shaded cells represent potential design targets. © 2009 by Taylor & Francis Group, LLC Sizing of FWS Wetlands 631 typically buried as peat or exported from the wetland (Mitsch and Gosselink, 1993). The net export results in an internal release of particulate and dissolved biomass to the water col- umn, which is measured as nonzero levels of biochemical oxygen demand (BOD), total suspended solids (TSS), total nitrogen (TN), and TP. These wetland background con- centrations are typically denoted by the term C*. Enriched wetland ecosystems (such as those treating wastewater) are likely to produce higher background concentrations than oli- gotrophic wetlands. These elevated background concentra- tions are largely due to increased biomass cycling resulting from the higher levels of nutrients and organic carbon in the wastewater. Even land-locked wetland basins, which only receive water inputs through precipitation, will have nonzero background concentrations. Consequently, many pollutants are not reduced to zero in treatment wetlands—including BOD, TSS, organic nitro- gen, and phosphorus. However, it is important to distinguish between artifacts of data tting and real wetland processes. Short-circuiting can lead to high values of data-t C* for heavily loaded systems, but these high C* can be dealt with by improving the hydraulics. Independent of hydraulics, the wetland can manufacture water-phase organics and solids, and cycle nutrients into and out of the water body. The chap- ters of Part I contain estimates of the C*-values for the com- mon pollutants, and many literature sources provide ranges of background concentrations (U.S. EPA, 1999; IWA Specialist Group on Use of Macrophytes in Water Pollution Control, 2000; U.S. EPA, 2000a; Wallace and Knight, 2006; Crites et al. , 2006). A summary is given in Table 17.3. S ome individual exotic chemicals are foreign to typi- cal wetland environments and are not expected to exhibit background concentrations. Examples include halogenated hydrocarbons and pesticides. Number of Tanks to Be Used in the Model The performance of the wetland depends on the number of tanks (TIS) selected—very strongly if the design is to approach wetland background concentrations or high degrees of removal (see Chapter 6). If a high degree of removal is required, it will necessitate a very large wetland with poor hydraulics, or a smaller wetland with good hydraulics. Figure 17.2 illustrates this high degree of sensitivity in the region of low P for large reductions. This chart quanties the fact that a tiny fraction of the water following a fast short- circuit will carry enough unreacted material to the outlet to make 99% reduction almost impossible to achieve. Low P numbers represent ow patterns that, by virtue of either fast forward mixing or velocity proles with high-speed ele- ments, carry fractions of unreacted material directly to the outlet. The P-value is somewhat at the discretion of the designer. More cells and greater length-to-width ratios can increase the P-value. As an illustration of the design decision to be made, consider a hypothetical case relative to Figure 17.2. Suppose the wetland is to achieve a 90% reduction. It is possible to consider a one-cell wetland, with a presumptive P 3 that needs 84 m 2 /(m 3 /d). Or, the designer can opt for two cells in a series, each with a presumptive P 3 that needs 68 m 2 /(m 3 /d). The question might be resolved based on the economics, i.e., does the cost savings of 20% area reduction outweigh the added cost of the divider berm and structures? This illus- tration carries a zero background concentration, but the concepts are applicable to any pollutant, provided the reduc- tion fraction to background is used instead of percentage removal. It should be remembered that the designer can control the N-value for the wetland (inert tracer tanks in series) but cannot entirely control the P-value. Pollutants that are mix- tures, which may undergo weathering in the wetland, act to reduce the applicable P-value. Table 6.3 provides some guid- ance on the apparent P-values to be selected relative to tracer N-values. Rate Coefficients In this initial, annualized analysis, the appropriate rate coef- cients are those (shown in the chapters of Part I) as the result of the tting of annual data from existing wetlands. Those are variable across wetlands and years, thus producing frequency distributions of the tted k-values. These are to be found as follows: BOD 5 Chapter 8, Table 8.2 Organic N Chapter 9, Table 9.11 Ammonia N Chapter 9, Tables 9.17 and 9.20 Total Kjeldahl nitrogen (TKN) Chapter 9, Table 9.12 Oxidized N Chapter 9, Table 9.23 Total N Chapter 9, Table 9.14 Total P Chapter 10, Table 10.11 Fecal coliforms Chapter 12, Table 12.3 Conspicuously absent from this list is the common constitu- ent TSS. Some individual system data were presented in • • • • • • • • TABLE 17.3 Summary of Background Concentrations for FWS Wetlands Parameter Lightly Loaded Heavily Loaded BOD 5 (mg/L) 2 10 TSS (mg/L) 2 15 Organic N (mg/L) 1 3 Ammonia N (mg/L) <0.1 <0.1 Oxidized N (mg/L) <0.1 <0.1 Total phosphorus (mg/L) <0.01 0.04 Fecal coliforms (CFU/100 mL) 10–50 100–500 Note: In the case of fecal coliforms, lightly/heavily loaded refers to animal use. © 2009 by Taylor & Francis Group, LLC 632 Treatment Wetlands Chapter 7 and analyzed for rate coefcients. However, incom- ing TSS is often reduced rather quickly, and wetland-efuent TSS results from a balance of generation and resuspension in the FWS wetland. The removal rate coefcients that pertain to the inlet region of the wetland are often quite high, ≈10 m/d (3,650 m/yr), as shown in Figure 7.8. Colloidal materials are an exception to this generality. The design recommendation suggested here is the use of a high-rate coefcient for TSS (perhaps 200 m/yr), unless the design-limiting bottleneck happens to be TSS. In that event, it is strongly suggested that settling tests be conducted on the candidate wetland inuent waters. The frequency distributions of the reference tables are generally quite broad. It is up to the designer to narrow the selection for a given application, either by choosing the pre- ferred degree of risk (blind to modifying factors), or by delv- ing further into the details of existing wetland data sets, and to narrow the selection to wetlands closest to the intended application in terms of operating conditions. Regrettably, there is no modern, published, all-inclusive database to which to turn. The reader is cautioned that older databases such as the NADB (Knight et al., 1993) and the 1994 Danish data- base (in Kadlec and Knight, 1996) have been superseded. Also, old databases are uneven in the quality and quantity of the data presented. There are numerous examples of misin- terpretation in such databases, and it is concluded that their use as a sole source of narrowing the information eld is dan- gerous. Because there are now so many treatment wetlands, it is not feasible to provide detailed data. In this book, the com- promise is to provide analysis, references, and the generic list of sources used herein (see Appendix A). DESIGN SIZING GOALS:LOAD REDUCTION VERSUS CONCENTRATION REDUCTION A key feature of treatment wetlands is the ability to design or manage the system for either concentration reduction or for mass removal, but only one at the expense of the other (Trepel and Palmeri, 2002). Wetland performance, as mod- eled previously, follows the rule of mass action: the removal rate of a pollutant is greater at higher pollutant concentra- tions in the water. The rst-order model assumes a (nearly) direct proportionality: doubling the concentration doubles the removal rate. As a result of this observed behavior, removal rates decrease as water passes through the treatment wet- land, and pollutant concentrations are reduced (Figure 17.3). However, the actual mass of the pollutant that is removed increases with increasing hydraulic loading. Thus, increas- ing hydraulic loads result in more kilograms removed, but at the expense of higher efuent concentrations. This trade-off between removal efciency and load reduction is a key feature of wetland design for nutrient control. These may be quanti- ed via the rst-order model. A simple version for C* 0 and no water loss is Concentration reduction io i ¤ ¦ ¥ ³ µ ´ CC C k P 11 qq P ¤ ¦ ¥ ³ µ ´ (17.15) Load reduction = Input load r Concentration reduction ¤ ¦ ¥ ³ µ ´ ¤ ¦ ¥ ¥ ³ µ ´ ´ qC k Pq P i 11 (17.16) The maximum mass of pollutant that can be removed in a given footprint area results from a hydraulic load so high that little or no concentration reduction is achieved. Under these condi- tions, pollutant concentration is at a maximum everywhere in the wetland, thus maximizing the mass removal rate. If the output load of a pollutant is to be held below some regulatory limit, then that has the same general effect as specifying an outlet concentration. However, because the load of a pollutant in the wetland outow is usually taken to FIGURE 17.2 The effect of the number of tanks on the area required for different degrees of removal. The calculations are for k 15 m/yr and C* 0. 10 100 1,000 10,000 100 , 000 1 10 100 P-Value Area (m 2 /(m 3 /d)) 99.9% 99% 90% 70% 50% © 2009 by Taylor & Francis Group, LLC Sizing of FWS Wetlands 633 be that in the surface water discharge, any reduction in ow through the wetland assists in providing lower output loads as well as higher load reductions. All three of the potential design goals (concentration out, load out, load reduction) may be sensible, depending on the pollutant in question. These are marked as shaded cells in the example in Table 17.2. For instance, a low required outlet concentration can be of use in preventing ammonia toxicity in receiving waters. But if there is a load allocation, as might be the case for a discharge contributing to the total maxi- mum daily load (TMDL) for an impaired water body, then the mass of the contaminant is of more direct interest. Lastly, if there is a desire to maximize the benets of a given wet- land footprint, then the goal should be to dissipate or retain the maximum amount of pollutant in the wetland. Because it is easy to confuse these potentially conicting goals, it is recommended that the designer clearly identify and state the purpose of the design. 17.2 AREA COMPUTATIONS At this point in the development of constructed wetland tech- nology, it would be disingenuous to provide overly simplistic design-calculation procedures. But it is also a mistake to think that adding more factors that may play a signicant role in perfor- mance lends more accuracy or precision to design predictions. GOAL SEEKING:DETERMINATION OF THE REQUIRED WETLAND AREA The fundamental and straightforward technique for deter- mining the area is to adjust that area until the specied crite- rion is met. That is easy if the calculations have been set up on a spreadsheet; the area can then be sequentially and man- ually changed by the user until the criterion is met, or auto- matic searches may be invoked, such as the Solver ™ routine in Microsoft Excel ™ . The hypothetical phosphorus example is continued to illustrate this process. Concentration Criterion A common criterion for phosphorus in the United States is for monthly means to be less than 1.0 mg/L. Realistically, the project owner would not want to encounter exceed- ances very often. For the example, choose the compliance frequency to be 90%. From Table 10.13, the multiplier to contain exceedance at that frequency is 1.94. Therefore, the design target is adjusted downward to 1.00/1.94 0.52 mg/L, which becomes the value to be achieved as the wetland area is varied. In just three manual iterations, the starting guess of 24 ha (C o 0.62 mg/L, Table 17.2) is changed to 27.6 ha (C o 0.52 mg/L). The same result may be obtained using Solver ™ , which provides the answer in an essentially instantaneous search from any plausible starting condition. The exceedance containment multipliers for common constituents may be found as follows: BOD 5 Chapter 8, Table 8.6 Organic N Chapter 9, Table 9.11 Ammonia N Chapter 9, Table 9.21 Oxidized N Chapter 9, Table 9.25 Total N Chapter 9, Table 9.16 Total P Chapter 10, Table 10.13 Fecal coliforms Chapter 12, Table 12.4 (TKN is not in this list because it is not normally regulated.) Maximum Load Criterion The same philosophy of regulation might lead to an annual load limitation on the efuent from the system. An outlet con- centration of 1.0 mg/L at a ow rate of 5,000 m 3 /d (inow rate) implies a maximum annual load of 1,825 kgP/yr leaving the wetland (50% load reduction). A search for the wetland area leads to a value of 10.1 ha. This is very much lower than that for the concentration goal for two reasons. First, the exceed- ance factor is not in play because of the annual character • • • • • • • • 0 20 40 60 80 100 120 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Hydraulic Loading, HLR (m/yr) Concentration Reduction (%) 0 50 100 150 200 250 300 Load Removal (gN/m 2 · yr) Concentration Reduction Load Removal FIGURE 17.3 Concentration reduction and load reduction as a function of hydraulic load for a hypothetical nitrate treatment wetland. Parameters: C i 10 mg/L, k 35 m/yr, C* 0, P 4. © 2009 by Taylor & Francis Group, LLC 634 Treatment Wetlands of the load limit. Secondly, credit is built into the calculation for the loss of water to ET and for wetland inltration. MINIMUM LOAD REDUCTION CRITERION A slightly different regulatory philosophy requires a minimum load reduction. For instance, some wetland systems in South Florida require a minimum of 75% phosphorus load reduction. A search for the wetland area leads to a value of 19.8 ha. MULTIPLE COMPOUNDS OF CONCERN The projected outlet concentrations of all constituents of interest are calculated via the preceding Equation 17.7 (or Equations 17.12–17.14 in the case of nitrogen). All computed outgoing concentrations and loads vary with the selected wetland area. Area is adjusted until the most stringent cri- terion is met. Criteria often include outlet concentration specications, each with an associated allowable frequency of exceedance. But in other cases, outlet pollutant loads may be specied (or load reduction). Loads are easily calculated using the outlet concentrations and ows from the last unit. If the contaminants are considered singly, then a last step remains: all performances are calculated on the controlling (maximum required) wetland area. As an example of multiple contaminants, the previous phosphorus illustration, along with the additional require- ments to also reduce BOD and total nitrogen, is continued. BOD enters the wetland at 30 mg/L, and total nitrogen, at 30 mg/L. The value of C* is chosen to be 2 mg/L for BOD, and P 1. The value of C* is chosen to be 1.5 mg/L for TN, and P 3. The rate coefcients are selected to be the medians shown for BOD and TN in FWS systems in Tables 8.2 and 9.14, i.e., k BOD 33 m/yr and k TN 13 m/yr. It is assumed that exceedances must be contained at the 90th percentile. The multipliers are 1.56 for BOD 5 (Table 8.6) and 1.55 for TN (Table 9.16). The calculations of the required areas for each of the three contaminants are shown in Table 17.4. The largest area is needed for the reduction of total nitrogen—40 ha for the 5,000-m 3 /d ow. When that area is applied to the calculation of performance for BOD 5 and TP, those pollutants are reduced more than necessary (Table 17.5). This concludes the preliminary calculation of the required wetland area. It may seem that there are no further steps needed, but in fact there is no guarantee that these pre- liminary calculations t into reasonable, known patterns of wetland behavior. It is critical that the bounds of plausible biogeochemical cycles are not exceeded, that the required ancillary chemicals (oxygen, carbon) are present in ample supply, and that the forecasted results are reasonable com- pared to known performance data. Further, the seasonality of the system has yet to be investigated via forecasting. 17.3 CHECKING THE BIOGEOCHEMICAL CYCLES In this phase of design, vegetation in the prospective wet- land and its role in nutrient processing is brought into the picture. The rate coefcient-based analysis has provided rst estimates of the quantities of materials entering and leaving the system, but there has been no allocation of the removals to the various processes that comprise the entire ecosystem function. It is not feasible to go too far in breaking down processes because the knowledge base does not support great detail. Here, the processing of carbon, nitrogen, and phos- phorus are examined via the mass balances used in ecosys- tem analysis in Part I (see Chapter 9, Figure 9.14). In the analysis that follows, two points are important. First, the analysis is based on estimates of what the eco- system will be like. Such estimates cannot be precise, and, consequently, the analysis is “order-of-magnitude” only. The intent is to gain some insight into the relative importance of wetland processes that will likely be operating if the proj- ect is built. Second, the analysis is for an annual period, and hence there remains the issue of seasonality. Seasonal analy- sis proceeds more readily if we can rst establish whether the wetland should be viewed as an agronomic system or a microbial system. TABLE 17.4 Areas Needed for a Three-Parameter Wetland Sizing BOD 5 TN TP Inlet C mg/L 30 20 2.00 Regulatory C mg/L 10.0 5.0 1.00 Containment multiplier mg/L 1.56 1.55 1.94 Target C mg/L 6.41 3.23 0.52 Background C* mg/L 2.0 1.5 0.01 PTIS — 1 3 3 Annual k m/yr 33 13 10.0 Area ha 30.3 40.0 27.6 Hydraulic loading cm/d 1.65 1.25 1.76 Outlet C mg/L 6.41 3.23 0.52 TABLE 17.5 Performance for a Three-Parameter Wetland Sized for TN Reduction BOD 5 TN TP Inlet C mg/L 30 20 2.00 Inlet loading g/m 2 ·yr 137 91 9.1 Regulatory C mg/L 10.0 5.0 1.00 Containment multiplier mg/L 1.56 1.55 2.00 Target C mg/L 6.41 3.23 0.50 Background C* mg/L 2.0 1.5 0.01 PTIS — 1 3 3 Annual k m/yr 33 13 10.0 Area ha 40.0 40.0 40.0 Hydraulic loading cm/d 1.25 1.25 1.25 Outlet C mg/L 5.48 3.23 0.29 © 2009 by Taylor & Francis Group, LLC Sizing of FWS Wetlands 635 C, N, AND P CYCLES The pollutant removals simplistically described in the pre- ceding section take no account of wetland functions; they are based on purely empirical k-values and outlet concentration data. It is therefore prudent to examine the empirical fore- casts and to ascertain whether they comply with an estimated set of wetland processes. Of particular interest are the car- bon, nitrogen, and phosphorus cycles. The carbon cycle involves the growth, death, and decom- position of biological materials, including animals, plants, algae, and microbes. For many wetlands, the standing crop of these organic materials is relatively constant throughout the year, but the proportions of living and dead may vary considerably, as may the physical location (i.e., standing dead or litter). Further, the speed of cycling varies with the nature of the material and the time of the year (see Chapter 3). Fine detritus from microbes cycles rapidly, whereas the decompo- sition of some woody plant parts may take years. However, the important feature of the carbon cycle is the amount of material that does not decompose, for it is this residual that accretes in the ecosystem and forms storage for many pollut- ants, including phosphorus. This storage is relatively perma- nent under appropriate hydroperiod conditions. An approximate assessment of the implied impacts of the carbon cycle can be made on the basis of two numeri- cal characteristics: the speed of the cycle in grams of dry material per square meter per year, and the fraction of the cycled material that does not decompose. The speed of the cycle may also be characterized by the standing crop, in g/m 2 , plus a turnover rate, in number of times per year. The recycled organic fraction contains carbon that is a source of support for denitrication and other heterotrophic processes. The burial fraction leads to sediment buildup and storage of nitrogen, phosphorus, and other trace contaminants that par- tition to organics. Typical values for the vegetative standing crop, turnover time, speed, and burial fraction are given in Table 17.6. In general, the algal and microbial standing phytomass is small compared to the vegetative phytomass. However, turnover times are also small, so the annualized rate of uptake and burial for this component of the phytomass may be estimated to be 50–100% of the vegetative annual rates. Simply stated, the amounts of N and P processed by the big green biomass are not very much larger than those processed by the nearly invisible microbes and algae. This nondisparity has been characterized as the “buckets and teacups” analogy set forth by Richardson et al. (1986). The term nutrient poor would reect very low nutrient status, with TP < 20 µg/L and NH 4 -N 0.2 mg/L. Nutrient- moderate wetlands would have TP < 200 µg/L, with NH 4 - N < 1.0 mg/L. Nutrient-rich wetlands would have TP ≈ 1.0 mg/L, with NH 4 -N ≈ 5.0 mg/L. Very-nutrient-rich systems would have TP 5.0 mg/L, with NH 4 -N 10.0 mg/L. These denitions do not correspond to the equivalents for aquatic water bodies, where the dominant vegetation is plankton. In the treatment wetland context, bacterial and algal materials are of comparable importance with above- and belowground macrophytic vegetation. The magnitude of the biogeochemical cycle increases with increasing nutrient availability—up to some presumptive limit enforced by availability of space and sunlight. This fer- tilizer response is not well-quantied for treatment wetlands and, therefore, cannot be used directly in design. However, rough estimates are of value in assessing the potential impor- tance of the biogeochemical cycle—particularly to check that the empirical design calculations do not imply unreasonable ecosystem functions. One technique for making such checks is to graphically link a presumed cycle to the mass balance calculations avail- able from the preliminary sizing step. The biogeochemistry check, thus, is implemented via linked ecosystem mass bal- ances, one each for C, N, and P. These do not replace, nor can they substitute, the water column mass balances used in k-rate removal calculations. In an annualized mass balance storage calculation, the following equations may be used: GDB (17.17) GX DX BX GDB (17.18) BX GX BG B (17.19) where biomass burial, g/m d biomass deco 2 B D mmposition, g/m d biomass growth, g/m d 2 2 G X fraction of the pollutant in each biomass compartment fraction of pollutant uptakeB that is stored The Carbon Cycle The nomenclature for biomass processes in treatment wet- lands is a bit confusing. The growth, death, and decomposi- tion processes are referred to as part of the wetland carbon cycle, but more than carbon is involved. However, most veg- etation and other wetland organisms are about 40% carbon; so, either dry biomass or carbon serves to track the amount of the material involved. Carbon itself is withdrawn from atmospheric sources as carbon dioxide for photosynthesis. Likewise, it is returned to the atmosphere as methane from anaerobic mechanisms, or carbon dioxide from oxidative processes (respiration included). The ability to estimate nutrient cycling rests upon our knowledge of the biomass pools in the wetlands and their changes. Guidelines are shown in Table 17.6. According to Equations 17.17–17.19, on an annual basis, the important esti- mation quantities are Annual growth rate, g/m 2 ·yr (standing crop phytomass necromass biomass times turnover per year) Annual burial fraction (undecomposable residual fraction) © 2009 by Taylor & Francis Group, LLC 636 Treatment Wetlands An assumption is made that the nutrients taken up, but not buried as accretion, are returned to the water column of the FWS wetland. For nitrogen, this is the maximum estimate, as microbial processes in abovewater tissues can transfer nitro- gen to the atmosphere without entering the water. These phy- tomass quantities, together with phytomass nutrient content (percentage or mg/kg), allow checks on the empirical removal calculations. For purposes of design, for order-of-magnitude checks on the calculated nutrient removals, the total growth rate and burial fraction are assumed based on the strength of the water to be treated. This provides a rough annual estimate of the biomass cycle (Figure 17.4). Note that the magnitude of this cycle and the nutrient contents are a function of the degree of fertilization of the wetland (see Chapter 3). The wetland carbon cycle is also critical to observe per- formance as it relates to sediment oxygen demand and to the carbon supply for denitrication. The implied supply con- straints of this carbon cycle are examined in the constraint check section of this chapter. The Phosphorus Cycle For phosphorus, the calculated removal is represented as a large uptake (GX G )—in major part balanced by the return of soluble phosphorus from tissue decomposition (DX D ). For TABLE 17.6 Estimates of Wetland Nutrient Cycling in Biomass Nutrient Poor Nutrient Moderate Nutrient Rich Very Nutrient Rich Organic matter Standing crop gDW/m 2 150 500 3,000 10,000 Turnover time times/year 2 4 3 2 Cycling rate (G)gDW/m 2 ·yr 300 2,000 9,000 20,000 Burial fraction (b)% 2 10 20 25 Accretion (B)gDW/m 2 ·yr 6 200 1,800 5,000 Phosphorus Tissue content mgP/kg 1,000 2,000 3,000 4,000 Uptake gP/m 2 ·yr 0.30 4.0 27.0 80 Return gP/m 2 ·yr 0.29 3.6 21.6 60 Accretion gP/m 2 ·yr 0.01 0.4 5.4 20 Nitrogen Tissue content %dw 1.0 1.5 2.0 2.5 Uptake gN/m 2 ·yr 3.00 30.0 180 500 Return gN/m 2 ·yr 2.94 27.0 144 375 Accretion gN/m 2 ·yr 0.06 3.0 36 125 Note: Bacterial and algal cycling are included in these values. Source: For rst two categories, data from Davis (1994) In Everglades: The Ecosystem and Its Restoration. Davis and Ogden (Eds.), St. Lucie Press, Delray Beach, Florida, pp. 357–378. For the second two, data from Kadlec (1997a) Ecological Engineering 8(2): 145–172. FIGURE 17.4 Estimated annual biomass cycle in a FWS treatment wetland for a rich nutrient condition. Note that the standing stock and turnover refers to above- and belowground material, and to macrophytes, algae, invertebrates, and microbes. Water Biomass 9,000 g/m 2 · yr 9,000 g/m 2 · yr 2,250 g/m 2 · yr 6,750 g/m 2 · yr Standing stock: 3,000 g/m 2 · yr Turnovers per year: 3.0 Turnover rate: 9,000 g/m 2 · yr Burial fraction: 0.250 Bulk density: 0.100 g/cm 3 Accretion: 2.25 cm/yr Necromass Soil Air © 2009 by Taylor & Francis Group, LLC [...]... • • • TSS BOD5 Organic N Ammonia N TKN Oxidized N Total N Total P Fecal coliforms Chapter 7, Figure 7.19 Chapter 8, Figure 8.9 Chapter 9, Figure 9 .17 Chapter 9, Figure 9.37 Chapter 9, Figure 9.23 Chapter 9, Figure 9.51 Chapter 9, Figure 9.29 Chapter 10, Figure 10 .17 Chapter 12, Figure 12.11 640 Treatment Wetlands TABLE 17. 8 Supply and Demand Constraints for the Example Annual Flux BOD removed SOD equivalent... • BOD5 Chapter 8, Table 8.4 Ammonia N Chapter 9, Table 9.29 TKN Chapter 9, Table 9.16 Oxidized N Chapter 9, Table 9.40 Total N Chapter 9, Table 9.20 Total P Chapter 10, Table 10.12 Fecal coliforms Chapter 12, Table 12.5 Note: Data are presently not adequate to determine a temperature coefficient for either TSS reduction or ammonification © 2009 by Taylor & Francis Group, LLC As discussed in Chapter. .. extensive data from treatment wetlands that this percentage ranges from ≈500 to 5,000 mgP/dry kg, or from 0.05–0.5% dry weight A somewhat narrower range is shown in Table 17. 6 because extremely nutrient-poor wetlands are uncommon in treatment applications It is unlikely that higher removals can be sustained via accretion The corresponding accretion rates are 0.01–20 gP/m2·yr The values in Table 17. 6 are provided... processes produce organic nitrogen The nitrogen content in accreting sediments is known from extensive data from treatment wetlands to range from ≈1.0–2.5% dry weight (Table 17. 6) A lower value would be associated with nutrient-poor wetlands, a higher with nutrient-rich systems Again, because the k-rate calculations are independent of the cycle calculation, there is one degree of freedom, which for nitrogen... average total nitrogen loading to these wetlands was 130 gN/m2·yr, just over the putative limit of 120 for agronomic wetlands; the ammonia loading was a bit lower at 90 gN/m2·yr It is noteworthy that January remains the “bottleneck” month, with a rate coefficient that is about 644 Treatment Wetlands TABLE 17. 9 Apparent First-Order Rate Constants for Ammonia Removal in FWS Treatment Systems in Cold Climates... Reduction Example No Storage Controlled Release Uniform Release Wetland Pond Wetland Pond Wetland 4,000 20.0 2,000–7,800 11.3 4,000 20.0 2,600–9,400 9.7 4,000 20.0 8,000 11.3 17. 0 2.0 17. 0 2.0 17. 0 2.0 17. 0 2.0 17. 0 2.0 17. 0 2.0 17. 0 2.0 m/yr m/yr 14.9 3.9 9.6 3.7 14.9 3.9 9.6 3.7 14.9 3.9 9.6 3.7 14.9 3.9 Storage Working depth days of flow m 0 0.25 87 2.0 — 0.25 138 2.0 — 0.25 184 2.0 — 0.25 Max month... another This example suggests that wetlands should be used in winter if hydraulically practical, because land and liner costs typically will not outweigh earth-moving costs Wetland Systems with Storage There are considerable numbers of treatment wetlands that operate seasonally, with off-season storage (Table 17. 11) Many of these are preexisting lagoon systems with wetlands added, so storage was already... allowance for random variability The degree of storage pond treatment is as predicted by procedures in the lagoon literature (U.S EPA, 1983c; Crites et al., 2006), and the treatment in the wetland is computed using the P-k-C* model with P 3, C* 0, and a temperature dependent k value, and with k20 0.05 m/d and 1.09 The pretreatment facilities, upstream of wetlands and storage lagoons, are presumed to produce... Option 1, year-round wetland operation, uses a wetland size that meets winter requirements The flow and storage time series for the various options are shown in Figures 17. 15 and 17. 16 Associated effluent ammonia concentrations are summarized in Figure 17. 17 The ranking of alternatives indicates an increasing footprint (total area) as storage alternatives move from Options 2 to 3 to 4 (Table 17. 10) Option... calculated by mass balance © 2009 by Taylor & Francis Group, LLC 638 Treatment Wetlands Denitrification Uptake Nitrification Uptake Death gN/m yr gN/m yr Ammonification Release Burial FIGURE 17. 6 Estimated annual nitrogen cycle in a FWS treatment wetland for a nutrient-rich condition Flows, concentrations, and loadings leaving the wetland are from k-rate forecasts The cycle turnover and tissue–N concentrations . NH 4 -N 0.2 mg/L. Nutrient- moderate wetlands would have TP < 200 µg/L, with NH 4 - N < 1.0 mg/L. Nutrient-rich wetlands would have TP ≈ 1.0 mg/L, with NH 4 -N ≈ 5.0 mg/L. Very-nutrient-rich. follows: TSS Chapter 7, Figure 7.19 BOD 5 Chapter 8, Figure 8.9 Organic N Chapter 9, Figure 9 .17 Ammonia N Chapter 9, Figure 9.37 TKN Chapter 9, Figure 9.23 Oxidized N Chapter 9, Figure 9.51 Total N Chapter. 2,600–9,400 4,000 8,000 Inlet C mg/L 20.0 20.0 11.3 20.0 9.7 20.0 11.3 Max temperature °C 17. 0 17. 0 17. 0 17. 0 17. 0 17. 0 17. 0 Min temperature °C 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Max k m/yr 14.9 9.6 14.9 9.6