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Part II Implementation © 2009 by Taylor & Francis Group, LLC 573 15 Evolution of Sizing Methods The purpose of Part II of Treatment Wetlands, Second Edi- tion, is to provide information on how to design, construct, and operate a wetland for the purpose of water quality improvement. It is recognized that wetlands are almost never stand-alone treatment devices but rather form part of a treat- ment train. Other components may be mechanical, such as clariers or lters, or more natural, such as settling basins or lagoons. More than one type of wetland may be involved at the same site—FWS, VF, HSSF, or biosolids systems. In this chapter, historical sizing methods are reviewed and placed in perspective. Design of treatment wetlands may be roughly divided into two categories: sizing calculations and physical speci- cations. Sizing procedures may in turn be predicated on the particular application, whether the purpose is for treatment of ows intended for surface water discharge, or for conditioning water for groundwater discharge, or for retaining and treat- ing stormwater runoff. Further, local regulations may require compliance with specic numerical standards (performance- based) or may simply require certain design specications according to prescriptive criteria (technology-based). The physical aspects of treatment wetland design seem deceptively simple. Indeed, the basic elements have been on record for over a hundred years. From Brian Mackney (Australia) via Hans Brix (1994d), an excerpt from an essay written by Nemo to the head of the Hornsby Literary Institute in 1904 reads: Anyone who has a little ground about his house can dispose of his dirty water as follows: Dig up a plot of ground thoroughly to a depth of fteen to eighteen inches. Cut a channel leading from the kitchen and washhouse into the highest side of the plot and let all the dirty water drain into it. Plant the plot with plants that grow rapidly and require a great deal of water such as Arum Lilies, for instance. The dirty water will be all absorbed by the roots of the plants and a most luxuriant garden will be produced which will defy the hottest weather and will always be green and beautiful. By this means a curse will be transformed into a blessing. Twenty or thirty feet square properly worked would be enough for any ordinary family. Of course, Nemo did not conceive of the difculties of scale- up from a single household to all the wastewater from a city of 100,000 population. For even modest-sized systems, there is a need to consider the system layout, the number of indi- vidual cells and their arrangement, and how to move water through a large wetland complex. Questions of hydraulic and vegetation efciency have important economic consequences. Therefore, the subsequent chapters consider the engineering of the wetland. The rst documented use of a wetland within a deliber- ately engineered treatment vessel appears to belong to Cleo- phas Monjeau (1901), as shown in Figure 15.1. Claims for this United States patent (issued in 1901) include distributed vertical ow, a uctuating water level, and aeration of the wastewater. This would be considered a cutting- edge wetland design even in the 21st century. Design of wetland systems may be either performance- based or technology-based, depending upon whether the per- formance expectations are explicit in an operating permit or implicit in a set of statutory design specications. Large sys- tems are often in the former group, whereas small on-site and urban stormwater systems are in the latter. The primary focus of this chapter is performance-based design. 15.1 HISTORICAL PERSPECTIVES Much of treatment wetland sizing has evolved along very simplistic lines, mostly adapted from civil and sanitary engi- neering. However, it is become increasingly apparent over the last 20 years that treatment wetlands are much more bio- logically complex than treatment processes used in the sani- tary engineering eld, and simplifying assumptions applied to activated sludge, trickling lter, and pond systems do not adequately describe the internal biogeochemical processes that govern treatment performance in wetlands. Historically, mechanistic wetland models have been of little utility because of calibration difculties. General rules of thumb for area requirements are used to good avail in conceptual design. Input–output regressions allow calcu- lations of outlet concentrations for specied systems, and scatter plots are available to examine intersystem variabil- ity. Several variants of rst-order models are now in use. Virtually all of these existing techniques fail to address sig- nicant aspects of wetland performance, including effects of hydraulics, meteorology, interactions of biota and treatment, supply-side constraints, and seasonal and stochastic variabil- ity. Soil accretion and the sustainable storage and dilution of irreducible constituents are other aspects not covered by existing design methods. There is also great danger of using empirical relations outside the envelope for which they were developed, and of unintentionally exceeding the capabili- ties of the fundamental wetland processes. Proper use of this imperfect set of design tools is a current design challenge. Three principal themes have been prevalent in the his- tory of constructed treatment wetland design: consideration of the pollutant and hydraulic loadings, rst-order removal models, and regression equations. First, some general history and concepts of wetland design are discussed, and then some specics for the principal variants of the technology. © 2009 by Taylor & Francis Group, LLC 574 Treatment Wetlands FIRST-ORDER MODELING In the mid-1980s, simple rst-order models of pollutant removal in wetlands made their appearance in the treatment wetland literature (U.S. EPA, 1983b; Boon, 1985; Kadlec, 1985; Reed et al., 1988). This was, in part, a natural exten- sion of models that had earlier been developed for waste stabilization ponds (Marais and Shaw, 1961; Marais, 1974; Thirumurthi, 1974). Some of these early models were founded on shaky ground because they borrowed extensively from nonwetland results and were not well-calibrated to actual treatment wetland data. Over the past two decades, consid- erable progress in understanding the behavior of treatment wetlands has allowed major improvements in the way that rst-order models are constructed and implemented. It is the premise of this book that rst-order modeling remains one of the most effective wetland design tools but that other infor- mation can and should also be brought to bear on design. The information in Part I sets forth the many processes affecting pollutant removal in wetlands. Nearly all of the compo- nent processes are rst-order within some range of constraints that are often met within wetland ecosystems. Therefore, it is not surprising that longitudinal proles are virtually all describ- able by such a model, provided proper account is taken of background concentrations and the actual ow patterns observed in treatment wetlands. Further, calibrated rst-order models are capable of adequately representing the trends in time series of pollutant concentrations exiting wetland systems. However, it is also the case that the many varieties of treatment wetlands, together with the variations of a given type, should not be expected to produce identical results. Plant varieties and their fractional coverage, media depth, aspect ratio, com- partmentalization, and environmental factors vary from wet- land to wetland. A distribution of k-values is to be expected, and has been observed for all the common pollutants (see Part I). Intersystem variability is thus a fact of life for wet- land systems that are intimately connected to their climate and surrounding environment. It is also true that, without exception, efuent con- centrations exhibit random variation. Environmental and biological factors drive behavior on hour-to-hour and day-to-day time scales. Any model capable of describ- ing such rapid phenomena would be quite complex, and in fact no such models currently exist. Therefore, effects such as those of wind, rain, and animal invasions must be dealt with through an understanding of the intrasystem variability present in the performance of a treatment wet- land. Because such random effects are a large proportion of the wetland response, that variability must be understood, quantied, and included in design. Deterministic relations, either a rate-constant model or a loading relationship, will produce only the general trend of outlet concentrations and not the random variability. Pr e vious Criticism of First-Order Modeling Reliance on rst-order modeling, although nearly universally accepted, has had one severe critic. Because that criticism appears in a U.S. government publication advocating a spe- cic treatment wetland design procedure, it is necessary to examine this lone voice of opposition in more detail. U.S. EPA (2000a) contended that “ … the development of a rigorous model of the process, or parts of it, has not been achieved as is true with many of the wastewater treatment processes designed today.” This statement is no longer correct. Credible treatment wetland models exist at all the levels that they do for activated sludge processes, which are the most advanced in terms of modeling. The work of McBride and Tanner (2000), Langer- graber (2001), Wynn and Liehr (2001), Howell et al. (2005) and others has carried wetland modeling to an advanced level of detail for municipal wastewater treatment. Stormwa- ter wetland models abound (Fitz et al., 1996; Moustafa and Hamrick, 2002; Munson et al., 2002; Walker and Kadlec, 2005). However, to our knowledge, these complex models are not often in use for purposes of design, the exception being the dynamic model for stormwater treatment areas (DMSTA) (Walker and Kadlec, 2005). FIGURE 15.1 1901 U.S. patent for a treatment wetland system. (From U.S. Patent 681,884.) Fig. 1. Vegetation Fig. 2. Fig. 3. B C d 1 d w D F x x x CC C P P P f f 1 f D F A © 2009 by Taylor & Francis Group, LLC Evolution of Sizing Methods 575 Indeed, U.S. EPA (2000a) went on to conclude that the design should be based on loading, an approach that: has been used for many decades by environmental engi- neers in the design of highly complex unit processes, includ- ing the activated sludge process and waste stabilization ponds. Only when a carefully designed series of iterative studies have been conducted, and data based on quality-controlled specications have been analyzed, can rigorous models be provided for use in wetland system design. It is almost unbelievable that this statement should have been made on the heels of a carefully designed series of iterative studies sponsored by U.S. EPA for the purpose of develop- ment of design equations (George et al., 1998) for treatment wetland systems. A compromise position is the use of highly aggregated models that contain major features of the observed behav- ior of treatment wetlands. That compromise is a rst-order model of some sort, possibly with temperature- or season- dependent parameters. A primary message of the U.S. EPA (2000a) is that such rst-order removal models should not be used, because these are prominently absent from that manual. That is paradoxical, because the wetland publications of all of the major contributors/authors advocate the use of rst- order removal models (Reed et al., 1988; Crites, 1994; Reed et al., 1995; Kadlec and Knight, 1996; Kemp and George, 1997; Crites and Tchobanoglous, 1998; Campbell and Ogden, 1999). In recent years, rst-order models have been reafrmed as a useful tool in wetland design. Rousseau et al. (2004) performed a review of model-based design of horizontal subsurface- ow treatment wetlands, and concluded: At present, the state-of-the-art k-C* model seems to be the best available design tool if the designer makes sure that all the assumptions are fullled and if he is aware of the pitfalls in the model. Crites et al. (2006) reiterate the basic features of both areal and volumetric rst-order models for free water surface (FWS) wetlands, and acknowledge that they do not directly account for the complex reactions and interactions that occur in wetlands. Crites et al. (2006) go on to state: Such an approach is the best that can be done with the cur- rently available database and understanding of wetland processes. In this book, the rst-order model is retained as the primary tool in the design process. However, the loading approach is also retained, as a means of accounting for intersystem variability. LOADING SPECIFICATIONS An early source of guidance utilized by wetland analysts was the methods used by wastewater stabilization pond design- ers. A common method was the prescription of a specied areal-loading rate for biochemical oxygen demand (BOD), on the order of 40 kg/haãd to achieve an outow concentra- tion of 30 mg/L (Reed et al., 1988). BOD loading is still the recommended design method for ponds, although with modications, that acknowledge temperature effects (Shilton and Mara, 2005; Shilton, 2005). At 3C, the recommended loading rate is 40 kg/haãd, which is the winter condition for cold climate systems. As the minimum winter temperature goes up, say, to 20C for a tropical climate, the allowable loading goes up as well to about 250 kg/haãd. Reed et al. (1988) suggested using the BOD-loading rate to the wetland as a constraint, not for design sizing, with an upper limit of 100 kg/haãd. It is perhaps coincidental that U.S. EPA (2000a) concluded that the allowable design loading to a fully veg- etated FWS wetland should be 40 kg/haãd, identical to that for temperate pond design. The areal-loading procedure may easily be converted to a population equivalence (PE), if the person equivalence of the particular wastewater is known. For instance, the per-person BOD generation rate in the United States is approximately 63 g/PEãd (U.S. EPA, 2002c) and in Austria is specied as 60 g/PEãd (Haberl et al., 1998). In the United States, BOD generation is also taken to be 60 g/PEãd, but a septic tank (or other primary treatment device) is virtually always used as pretreatment for domestic wastes, and therefore 40 g/PEãd is used as the inuent to a wetland (Wallace and Knight, 2006). So, for example, if a design specication is 5 m 2 /PE for a HSSF wetland treating primary efuent, then the equivalent loading specication is 40/5 8 g/m 2 ãd 80 kg/haãd. To compare rst-order calculations to loading criteria, the hydraulic loading to the wetland must be known. The rate- constant approach separates the effect of inlet concentration from that of ow rate. The water ow per person equivalent is to some extent independent of the amounts of pollutants gen- erated. Water usage in a typical household varies from 50 to 200 L/PEãd, depending on whether it is situated in an arid or wet location, or in a developed or developing country. Small communities have slightly greater water usage per person. The relationship between rate-constant specication and areal-loading specication is straightforward, but not unique: the rate approach is strength (concentration) dependent and the loading method is not. The connection for a pollutant with zero background is CC k q C kC qC oi i i i Ô Ư Ơ à Ô Ư Ơ à Đ â ă ă ả á ã exp exp ãã (15.1) where C C i o inlet concentration, mg/L outlet conce nntration, mg/L rate constant, m/d hydrau k q llic loading, m/d and qC i 2 pollutant loading, g/m d â 2009 by Taylor & Francis Group, LLC 576 Treatment Wetlands It is clear from Equation 15.1 that the outlet concentration depends not only on inlet pollutant loading but also on inlet concentration. Suppose it is required that C o 25 mg/L, and that k 0.1 m/d. For C i 100 mg/L, the allowable hydraulic loading is q 0.072 m/d. The corresponding pollutant loading is 7.2 g/m 2 ãd. However, if the inlet concentration is C i 200 mg/L, the rate model suggests that the allowable hydraulic load- ing is q 0.048 m/d and the corresponding pollutant load- ing is 9.6 g/m 2 ãd. Thus, the rate approach gives different area requirements, depending on waste strength. To further complicate the matter, we might consider that the two cases differed only in water use, and that the same number of pop- ulation equivalents (same mass load) was involved. Suppose PE 100 and the per capita generation was 50 g/PEãd, for a total of 5,000 g/d. For the weak strength of 100 mg/L (more water usage), the area required would be 5,000/7.2 694 m 2 or 6.9 m 2 /PE. For the stronger strength of 200 mg/L (less water usage), the area required would be 5,000/9.6 520 m 2 or 5.2 m 2 /PE. Thus, the loading method and the rate method are not equivalent. REGRESSION EQUATIONS The principal variables that determine the outlet concentration (C o ) are the hydraulic loading (q), or the equivalent detention time (h/q), and the inlet concentration (C i ). Other inuences include temperature, solar radiation, and pH. There is, there- fore, the possibility of simply postulating a functional rela- tion, such as a product-power law, and regressing intersystem wetland inputoutput data to t that equation. In so doing, some of the power of intrawetland data is lost, because such regression formulae may not represent the observed longitu- dinal proles for a specic wetland. However, there can be no doubt that an individual data set can be t with a reasonable guess at a tting formula, because there are typically three or more tting parameters. For example, Kadlec and Knight (1996) set forth linear regressions and power law regressions for different common pollutants. A few examples are: Gumbricht (1993a), total nitrogen leaving a single FWS SAV system (R 2 0.92): CC o i nom 163 078 17 .ln()T (15.2) where C C i o inlet concentration, mg/L outlet conce nntration, mg/L nominal detention time, nom T d Kadlec and Knight (1996), nitrate leaving several FWS sys- tems of different types (R 2 0.35): CCq oi 0 093 0 474 0 745 . (15.3) where q hydraulic loading rate, cm/d Tunỗsiper et al. (2006), ammonia leaving one square meter HSSF mesocosms with various plants (R 2 0.67): () CC C qC io i i pH Ô Ư Ơ à 0 371 0 0048 0 080 2 2886 (15.4) The operational data and multiple linear regression models are shown in Figure 15.2, along with the rate-constant model. Tunỗsiper et al. (2006) concluded that the multiple linear model was better, but that is not supported by the sum of the squared errors derived from data and models (SSQE 0.30 for rate- constant model, and SSQE 1.98 for multiple linear model). Great care must be exercised in dening the variable ranges of such regression models. The potential extremes of 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Time (days) Ammonia N (mg/L) Data Rate Constant Model Multiple Linear Model FIGURE 15.2 Time series of ammonia concentrations exiting HSSF mesocosms, together with rate constant and multiple linear regression models. (From Tunỗsiper et al. (2006) Water Science and Technology 53(12): 111120. Reprinted with permission.) â 2009 by Taylor & Francis Group, LLC Evolution of Sizing Methods 577 variables can produce unintended anomalies that are a poten- tial trap for designers. For instance, the Gumbricht (1993a) regression (Equation 15.2) produces negative concentrations for long-detention times, a fact that might go unnoticed by a potential user of this equation set. The Kadlec and Knight (1996) regression (Equation 15.3) suggests that C o increases with decreasing C i for C i 1.0 mg/L, which is a highly improbable circumstance. The Tunçsiper et al. (2006) regres- sion (Equation 15.4) shows that removal goes up with inlet concentration and, in fact, for C i 10 mg/L, more than 100% removal is forecast. That C i is higher than the calibration range, but an unsuspecting designer could easily misapply Equation 15.4 to that condition. These example regressions contain 3, 3, and 4 param- eters, respectively. Any possibility of generalization, and use in design, is hindered by the need to predict all of those coef- cients in a new situation. Consequently, regression equa- tions have not found use in treatment wetland design. 15.2 FREE WATER SURFACE WETLANDS Early FWS treatment wetlands were often natural wetlands employed to improve water quality. Although those ecosys- tems were preexisting, there was no issue of sizing, but rather an issue of how much water could be put into the wetland. Recipes existed, and continue to exist, covering the appropri- ate hydraulic loading to natural systems (e.g., for Florida). In contrast, FWS constructed wetland design sizing for continu- ous ows has historically been virtually entirely performance based. Unfortunately, there has been considerable misinter- pretation and misuse of these simplistic models. One of the problems has been unsupported pronouncements of model parameters. When plug ow and C* 0 are presumed, there is only one parameter remaining, which is here designated as k 1 (or k V1 ). For instance, there is an oft-repeated value of k V1 0.678 d −1 in U.S. literature for BOD reduction in FWS wetlands at 20°C (Reed et al., 1995; Water Environment Federation, 2001; Crites et al., 2006). This value is intended for use in an exponential (plug ow) model with a zero back- ground concentration. Although no basis for this number has ever been presented, it is now possible to place it in the per- spective of calibrations for large numbers of FWS systems. The suggested value, for a system 40 cm deep, corresponds to k 100 m/year. This ranks at the 98th percentile of k 1 - values for 386 wetland-years of data for FWS wetlands (Figure 15.3). From Figure 15.3, it is clear that the expo- nential plug-ow model with C* 0 has the wrong shape compared to intersystem data, and therefore a different cali- bration k V cannot x it. Most of the time this particular single- parameter model will overestimate removals, as well as the model providing incorrect trends. 15.3 STORMWATER WETLANDS Stormwater wetlands are also FWS systems, but these differ markedly in design philosophy. Early in the brief history of urban stormwater wetlands, data was collected from a variety of constructed and natural systems, and the central perfor- mance tendency of these treatment wetlands was determined (Strecker et al., 1992; Schueler, 1992). The time-variable nature of stormwater wetlands means that the information was and is quite sparse. Source characterization is difcult and often the result of semiquantitative rules for the amounts of water and pollutants that are washed off of various types of watersheds. Given this meager performance base, it is not surpris- ing that the earliest sizing rules were empirical. One method involves the specication of the wetland area compared to the contributing watershed area, the wetland-to-watershed area ratio (WWAR). This is a scaling rule, comparable to those in use for continuous-ow FWS wetlands. The implied per- formance of a particular WWAR is inferred from the perfor- mance data from operating systems similar to WWAR. FIGURE 15.3 A presumptive model line on the BOD-loading graph for an inlet concentration of 50 mg/L. The model is the rst-order volumetric with k V1 0.678 d −1 and with C* 0, but truncated at BOD 5 mg/L per literature recommendation. A free water depth of 45 cm is presumed for the model. © 2009 by Taylor & Francis Group, LLC 578 Treatment Wetlands A second approach relies on the capture and detention of a specied amount of runoff, which is then subject to treat- ment during the interevent period. For instance, the antici- pated runoff from a rain event of specied return frequency may be used to set the wetland water volume. Runoff from larger events is partially bypassed or runs quickly through the wetland. Again, the implied degree of treatment is that asso- ciated with holding the water during the interim periods. A third approach involves adapting the continuous ow rate-constant method to event-driven systems. The use of the k-C* model, with average ow rates, was suggested by Kadlec and Knight (1996) and by Wong and Geiger (1997). A number of calibrations for stormwater systems were provided in the summary of Carleton et al. (2001). More recently, the concept has been extended to the P-k-C* model by Wong et al. (2006). A drawback of the use of P-k-C* calculations for event- driven wetlands is that rate constants for continuous-ow situations do not necessarily carry over to the averaged transient-ow situation (Kadlec, 2001a). As a result, dynamic modeling is to be preferred. However, the complexity of dynamic modeling is daunting, partly because of the need to construct input time sequences for a signicant period of operation. Nonetheless, for large and costly projects, dynamic simulations may be warranted. Currently, these are in use for the design of the phosphorus removal wetlands in South Florida (Walker and Kadlec, 2005). 15.4 HORIZONTAL SUBSURFACE FLOW WETLANDS Over the last 50 years, multiple variations of horizontal sub- surface ow (HSSF) wetlands have been developed, and in more recent years, HSSF wetland technology has made sig- nicant advances. As a result, there are a large number of ref- erence documents on HSSF wetlands that contain outmoded concepts and should no longer be used for design purposes. This section provides an overview of HSSF wetland design and how the technology has evolved. THE ROOT-ZONE METHOD In 1952, a German scientist, Kathe Seidel, began investigat- ing the water purication capabilities of bulrush (Schoeno- plectus lacustris) grown in articial-rooting environments. In the early 1960s, in collaboration with Seidel, Reinhold Kickuth at the University of Göttingen, Germany, developed a wetland treatment process known as the root-zone method (Wurzelraumentsorgung) (Brix, 1994d). Root-zone method wetlands are comprised of a soil media (typically clay loam to sandy clay), and calcium, iron, or aluminum additives were sometimes proposed as additives to the media to increase phosphorus adsorption (Kickuth, 1977). The soil was seen as a reactive media, a concept that has resurfaced with HSSF wetlands using expanded shale and clay aggregates (see Chapter 10). Although no longer considered a modern design concept, many wetlands designed on the principles of the root-zone method exist in Europe. In most of the root-zone method wetlands, the exclusive use of the common reed (Phragmites australis) was based on the theory that root formation would increase the hydraulic conductivity of the soil matrix from approximately 10 −5 m/s to 10 −3 m/s within two to four years (Cooper and Boon, 1987). The intended hydraulic conductivity of the soil (10 −3 m/s) required many of these wetlands to be considerably wider than their length, and the beds were often tipped at a slope of 1% or greater to increase the gradient in the (mistaken) belief that the tipped beds would force subsurface ow through the beds (Figure 15.4). After a few years, the collaboration between Seidel and Kickuth ended for personal reasons. Both scientists and their respective schools became rivals. The information produced by the two groups during this period was often conict- ing, and resulted in confusion among practicing wastewater engineers and the governing regulatory authorities (Börner et al., 1998). By the early 1980s, most wetlands in Germany were being constructed on the root-zone method, although examples of the MPIP (Seidel) system were constructed in St. Bohaire, France (Liénard et al., 1990), and Oaklands Park, United Kingdom (Burka and Lawrence, 1990). EVOLUTION OF HSSF WETLAND DESIGN IN EUROPE In 1984, water utilities in the United Kingdom began to investigate the root-zone method as an alternative for small treatment works (Cooper and Boon, 1987). An effort was made to coordinate research efforts with Denmark, Germany, FIGURE 15.4 Example of an early root-zone method HSSF wetland. (From Cooper and Boon (1987). In Aquatic Plants for Water Treat- ment and Resource Recovery. Reddy and Smith (Eds.), Magnolia Publishing, Orlando, Florida, pp. 153–174. Reprinted with permission.) Phragmites " ! © 2009 by Taylor & Francis Group, LLC Evolution of Sizing Methods 579 France, Spain, and the Netherlands. Early treatment wetlands in Denmark and the United Kingdom had designs based on the root-zone method principles. The claim of hydraulic- conductivity improvement from 10 −5 m/s to 10 −3 m/s did not hold true. Instead, the hydraulic conductivity either remained stable or decreased with time, remaining on the order of 10 −5 m/s (Brix and Schierup, 1989a; Netter and Bischofsberger, 1990; Findlater et al., 1990; Coombes, 1990; Bucksteeg, 1990). Because the anticipated increase in hydraulic conductiv- ity did not occur, the HSSF beds did not have enough cross- sectional area to allow for subsurface ow. As a result, water would pond at the inlet and ow across the surface of the bed. Thus, the hydraulically failed mode of operation for these wet- lands was overland ow—essentially a FWS wetland with a very shallow water depth. As it has turned out, there is not a large difference in treatment performance between FWS and HSSF wetlands, so the overland ow mode of operation has often been tolerated because it provides an acceptable level of treat- ment, even though the systems do not function as intended. However, some of these systems were constructed with sloping beds (top and bottom surfaces both sloped). When overland ow occurred, the water would channelize, further reducing treatment performance. These experiences led to the creation of design standards for subsurface-ow wetlands based on coarser bed materials and beds with at upper sur- faces (only bottom surface sloped) (WRc, 1989). Other root-zone method predictions also turned out to be inaccurate. The actual oxygen transfer from the plant root systems was much lower than anticipated (Brix and Schierup, 1990). In addition, actual BOD removal was slower (observed rate removal coefcient of 0.10 m/d versus the predicted 0.19 m/d), resulting in a trend towards larger bed areas (5 m 2 /PE versus 2.2 m 2 /PE) (EC/EWPCA Emergent Hydrophyte Treat- ment Systems Expert Contact Group and Water Research Centre, 1990). Regional differences in HSSF wetland designs still exist. The practice in Germany is to use relatively ne sand (0.2 mm to 1.0 mm) (Gesellschaft zur Förderung der Abwassertechnik d.V [GFA], 1998) because it is thought to be the best com- promise between available surface area for biolm growth, suitability as a rooting media, and hydraulic conductivity (Geller, 1996). Because they are designed in accordance with Darcy’s law, these beds are much wider than they are long. This design tactic minimizes the organic loading across the inlet cross-sectional area. German authorities report that the decomposition of most organic substances is essentially com- plete within 1 to 2 m of the inlet zone (Geller, 1997). Figure 15.5 is a schematic representation of a HSSF wetland in Germany (Geller et al., 1990). The HSSF wetland systems in Austria, the Czech Republic, and the United Kingdom are usually designed with the media of similar diameter. Austrian standards recommend the use of a gravel media between 4 and 8 mm (ÖNORM B 2505, 1997). HSSF wetlands in the Czech Republic tend to use sand and gravel less than 20 mm (Vymazal, 1996). In the United King- dom, bed materials of 3 to 6 mm and 5 to 10 mm have been recommended (EC/EWPCA Emergent Hydrophyte Treat- ment Systems Expert Contact Group and Water Research Centre, 1990). HSSF wetlands using coarser materials are often constructed with a length-to-width (L:W) ratio of greater than 1.0. For example, the average L:W ratio was 1.76:1 for 28 HSSF wetlands in the Czech Republic (Vymazal, 1996). Wetland systems throughout Scandinavia are typically dimensioned using EC/EWPCA criteria. In Norway, the use of light-expanded clay aggregate (LECA) has been used as an alternative to gravel (Jenssen et al., 1994b). Assessment of the performance of these wetlands indicates that LECA has a high-phosphorus sorption capacity (Jenssen et al., 1996) compared to natural aggregates. As the technology has gained acceptance in Scandinavia, LECA remains one of the preferred media materials because of its availability and its ability to bind phosphorus (Jenssen et al., 2002). Phos- phorus removal in LECA wetlands occurs through chemical Shaft Regulation o f water-level in the beds Inlet drainage pipe (bottom) Inlet drainage pipe (bottom) Aboveground inlet 50 m to IDM Maisach-river Aboveground inlet Main distribution From settling-basin 20 m Outlet drainage pipe (bottom) Outlet drainage pipe (bottom) Dyke Bed with alkali-fine-grained soil Bed with alkali-coarse-grained soil FIGURE 15.5 HSSF wetland in Germerswang, Germany. (From Cooper and Findlater (1990) Constructed Wetlands in Water Pollution Control. Pergamon Press, New York. Reprinted with permission.) © 2009 by Taylor & Francis Group, LLC 580 Treatment Wetlands adsorption to the surface of the media (Zhu et al., 1997). When the sorption sites are exhausted, phosphorus removal ceases. Studies in Sweden indicate that these adsorption sites are primarily associated with iron and aluminum oxides present on the surface of the particle, and are a function of the parent material and manufacturing process (Johansson, 1997). Materials with similar surface characteristics also dis- play comparable phosphorus removal capabilities (Brooks et al., 2000). Phosphorus sorption capacities of various SSF aggregates are discussed in detail in Chapter 10. The HSSF wetlands are recognized as being effective for BOD removal. However, due to the low-oxygen trans- fer rates in HSSF wetlands, nitrication is limited. Vertical ow wetlands that are capable of nitrication are being used increasingly in Europe (Cooper et al., 1997). Combinations of vertical ow and horizontal ow wetlands are being devel- oped (Platzer, 1996; Cooper, 1999). These hybrid systems are designed with the goal of nitrication (in the vertical ow wetland) and denitrication (in the horizontal ow wetland). EVOLUTION OF HSSF WETLAND D ESIGN IN NORTH AMERICA In the United States between 1972 and 1976, Frederic Spangler and colleagues studied articial wetlands (Spangler et al., 1976b) in conjunction with a study of the assimilation capabilities of natural wetlands. The earliest articial marsh cells contained wire frames to support the emergent wetland plants, but this was quickly abandoned in favor of gravel media (19 to 25 mm) covered with smaller pea gravel. These early gravel-lled marsh cells could be operated with the water above or below the gravel surface, although subsurface ow was identied as the preferred mode of operation. In 1973, Seidel obtained a U.S. patent (Seidel, 1973), and her concepts were more widely introduced in the United States (Wolverton, 1987a). By 1981, B.C. (Billy) Wolverton and his colleagues at the National Aeronautics and Space Administration (NASA), who had been researching oat- ing-plant treatment systems, began to focus on subsurface- ow wetlands (Wolverton, 1983). These rock–reed lters were very long and narrow and used a coarse gravel media of crushed railroad ballast (Wolverton, 1987b). Although these HSSF wetlands were similar to the horizontal stages in the Max Planck Institute Process (MPIP) systems, they operated without the preceding vertical ow wetland cells. Figure 15.6 is a schematic representation of an early rock–reed lter for a single-family home (Wolverton and Wolverton, 2001). Richard Gersberg and colleagues began to study nitro- gen transformations in pilot-scale subsurface-ow wetlands (Gersberg et al., 1983) and larger demonstration-scale wet- lands (Gersberg et al., 1984) in Santee, California. Early studies treated nitried secondary efuent, although primary efuents were used in later studies. Subsequent work focused on pathogen reduction in subsurface-ow wetlands (Gersberg et al., 1987). In 1988 the U.S. Environmental Protection Agency pub- lished a design manual on constructed wetlands that outlined design procedures for subsurface-ow wetlands (U.S. EPA, 1988b). This manual included a rst-order plug-ow equa- tion for BOD reduction with a temperature- and surface-area- adjustable removal rate. Media sizes ranged from a d 10 of 1 to 8 mm, with associated hydraulic conductivities of 4.9 r 10 −3 to 5.8 r10 −3 m/s. The 1988 manual was replaced with updated manuals in 1993 (U.S. EPA, 1993c) and 2000 (U.S. EPA, 2000a). The Tennessee Valley Authority (TVA) began demonstra- tion of subsurface-ow constructed wetlands in 1986, based on design information obtained from the work of Kickuth, Wolverton, and Gersberg (Steiner et al., 1987). This led to the development of a guide for small system and single- family home wetland system design in 1991, which was Elephant ears Roses Cattails Septic tank Plastic liner with rock filter Reeds Purified wastewater enters lake FIGURE 15.6 Early rock–reed lter concept for treatment of wastewater from a single-family home. (From Wolverton and Wolverton (2001) Growing Clean Water. Wolverton Environmental Services, Picayune, Mississippi. Reprinted with permission.) © 2009 by Taylor & Francis Group, LLC Evolution of Sizing Methods 581 revised and updated in 1993 (Steiner and Watson, 1993). A schematic representation from this early guide is shown in Figure 15.7. The sizing criterion was effectively 11 m 2 /PE. The TVA guidelines recommended a gravel media of 3 to 6 mm, pre- sumed to have hydraulic conductivity of 3 r 10 −3 m/s, be used to account for clogging in the gravel bed. This guideline also recommended that the cross-sectional organic loading be lim- ited to less than 244 g BOD/m 2 ·d to minimize the potential for bed clogging and associated overland ow. In 1993, the U.S. Environmental Protection Agency pub- lished a technology assessment of subsurface-ow wetlands (U.S. EPA, 1993f) that compared the design methods of Wolverton and TVA. This report recommended a rst-order plug-ow equation for BOD reduction, corrected for temper- ature. Media sizes of 12 to 25 mm with a presumed hydraulic conductivity of 1.2 r 10 −2 m/d and an L:W ratio of 2:1 were recommended. Also in 1993, Region Six of the U.S. Environ- mental Protection Agency published design guidelines that originally recommended 50- to 250-mm rock media (U.S. EPA, 1993c); this recommendation was later revised to be in conformance with the 1993 Technology Assessment (U.S. EPA, 1993f). In 1995, Sherwood Reed and his colleagues published design recommendations for subsurface-ow wetlands (Reed et al., 1995), which essentially expanded on an earlier work (Reed et al., 1988). Reed’s work included thermal design, a rst-order plug-ow equation for BOD and ammonia removal, and an empirical relationship for total suspended solids (TSS) removal. Recommended media sizes ranged from a d 10 of 2 to 128 mm, with presumed hydraulic con- ductivities between 1.2 r 10 −3 m/s and 2.8 m/s. Reed’s work also recommended that no more than 20% of the clean-bed hydraulic conductivity be used for design purposes. In 1996, Robert Knight and Robert Kadlec published design recommendations based on an areal-based, rst-order equation set with nonzero background concentrations for BOD, total suspended solids (TSS), ammonia, nitrate, total nitrogen, phosphorus, and fecal coliform (Kadlec and Knight, 1996). This approach was later reported in an international guideline (IWA Specialist Group on Use of Macrophytes in Water Pollution Control, 2000). Kadlec and Knight provided methods for determining hydraulic conductivity based on the grain size of the media, with the recommendation that only 10% of the clean-bed hydraulic conductivity be used for design purposes. Methods to calculate the water surface prole within the wetland were also provided. These methods were applied in a forensic anal- ysis of a HSSF wetland with inlet ponding problems (Kadlec and Watson, 1993). In 2000, the U.S. Environmental Protection Agency published a new design manual on constructed wetlands &'*'%"'%&'#"&'%('*' " *%'&&'%(')" +%#&&'*'#' .%&' +&%&#$ &') )&#%'&, .%&' #"'"&%) *'%$%## "%&(&#"' &&"'#''#!#'.%&' '##"&%)*'%"$%#)!#% -')'%'!"''' &"( %(&&%(&( +$ "'"'.%&' ,%##'&#'&!%&$ "'&#%!"&!'!#"'%) %! # # "$+& $%#&&&'$ *$(%+' *'%'%%#!'.%&' $&&&"'#'&#" '%#($%#%' $$!" %&'#",*'% ) *'" &%( '+ &*) &'"$$& #'"#"%''"&''"# &'*'% "'&#" &&'%(')" +%#&&'& '%#("#'%$%#%' $$ & +%#%) #)%*''#$&# "'"!( ,& &$ "'*')%'+##%"!"' *' "$ "'&&(&%& $"'%"%%#*,*'%" )"'( +&$&"'#'&# #*#%$&&&"'#"#'%$%#%'$$*%'&% &"'# %". &! %'#'#&(&*'#")"'#" &$''"& #"&'%('*' "& '&#"%"( ' '%#(#(''"'#"'#" *&'*'%%#!!#&' +&! %(% #!!("'&"#!&*%'%'#" '%'!"'&+&'!&% $%# ! &'*'%/#*&"'#'#"&'%('*' "%#!&$''"#% #'%'+$#$%!%+'%'!"'&+&'!%'*&'*'%&)" + &'%('!#"'$ "'&*%!%##%"&!&"! %'#"&%#*"#%"!'% &"$# ('"'& #"&'%('*' "&$%#)&!$ -')" #*#&' *&'*'%'%'!"'*"#!$%*'#")"'#" &+&'!& FIGURE 15.7 HSSF wetland schematic from TVA wetland design manual. (From Steiner and Watson (1993) General Design, Construction, and Operation Guidelines: Constructed Wetlands Wastewater Treatment Systems for Small Users Including Individual Residences. Second Edition, TVA/WM-93/10, Tennessee Valley Authority Resource Group Water Management: Chattanooga, Tennessee.) © 2009 by Taylor & Francis Group, LLC [...]... tool 15. 6 COMMON DESIGN MISUNDERSTANDINGS AREA- AND VOLUME-BASED RATES Chapter 6 provides a discussion of the two forms of the P-k-C* model: (C C*) (Ci C*) 1 ky Pq P P 1 A k y PhF B 1 kV y P P C (15. 7) where C Ci C* hF k kV P q y concentration at fractional distance y, g/m 3 / inlet concentration, g/m 3 background concentration, g/m 3 wetland free-water depth, m first-order areal constant, m/d first-order... (1990) Constructed Wetlands in Water Pollution Control Pergamon Press, New York Reprinted with permission.) © 2009 by Taylor & Francis Group, LLC 584 Treatment Wetlands Filter bed Reed 200 DN 150 Inlet Elimination bed DN 150 1000 Outlet FIGURE 15. 10 Max Planck Institute Process wetland treatment system (profile view) (From Börner et al (1998) In Constructed Wetlands for Wastewater Treatment in Europe... for reed bed treatment systems, WRc: Swindon, United Kingdom 590 Hyde H.C., Ross R.S., Demgen F (Eds.) (1984) Technology Assessment of Wetlands for Municipal Wastewater Treatment, EPA/600/ 2-8 4 /154 ; NTIS No 8 5-1 06896 Reed S.C., Crites R., Middlebrooks E.J (1988) Natural Systems for Waste Management and Treatment 1st ed., McGraw-Hill, New York U.S EPA (1983b) Design principles for wetland treatment systems,... (2000a) Constructed wetlands treatment of municipal wastewaters, EPA 625/R-99/010, U.S EPA Office of Research and Development: Washington, D.C U.S EPA (2000c) Guiding principles for constructed treatment wetlands: Providing water quality and wildlife habitat, EPA 843/B-00/003, U.S EPA Office of Wetlands, Oceans, and Watersheds Wallace S.D., Knight R.L (2006) Small-scale constructed wetland treatment systems:... wetland wastewater treatment facilities guidance, Water-001-NPD Korkusuz E.A (2005) Manual of Practice on Constructed Wetlands for Wastewater Treatment and Reuse in Mediterranean Countries, Added Value Knowledge Report No 5 (INCOCT-200 3-5 02453), Agbar Foundation and MED-REUNET (Mediterranean Network on Wastewater Reclamation and Reuse) LEC (2000) Draft guidelines for constructed stormwater wetlands, Report... data set So, not only is the single-parameter model not particularly suited to HSSF wetlands, but the available calibration estimates do not achieve good data fits, either 15. 5 VERTICAL FLOW WETLANDS Vertical flow (VF) wetlands are a newer type of treatment wetland, having come into widespread use in the mid-1990s Several variations of the technology exist; pulse-loaded systems (most commonly implemented... plant systems for municipal wastewater treatment, EPA 625/ 1-8 8/022, U.S EPA Office of Water: Cincinnati, Ohio U.S EPA (1993f) Subsurface flow constructed wetlands for wastewater treatment: A technology assessment, EPA 832/R93/001, U.S EPA Office of Water Water Environment Federation (1990) Natural Systems for Wastewater Treatment (WEF Manual of Practice FD-16) Chapter 13: Wetland Systems 1st ed., Water... greatly reduced 15. 8 A PERFORMANCE-BASED SIZING ALGORITHM The methods that are used in treatment wetland sizing vary from type-to-type Rules of thumb are valuable in preliminary assessment, but final decisions must be predicated either on performance requirements or upon regulatory prescriptions Here, the performance-based approach is outlined, and that is the primary approach for FWS wetlands, which... systems, EPA 600/ 2-8 3/026, Hammer D.A., Kadlec R.H (Eds.), National Technical Information Service U.S EPA (1985) Freshwater Wetlands for Wastewater Management Handbook, EPA/904/ 9-8 5/135, Atlanta, Georgia U.S EPA (1987) Report on the use of wetlands for municipal wastewater treatment and disposal, EPA 430/0 9-8 8/005, U.S EPA Office of Water U.S EPA (1988b) Design manual: Constructed wetlands and aquatic... ) 4.3( NH 4 N in NH 4 N out ) (15. 6) where A BOD in BODout IOTR NH 4 -N in NH 4 -N out Q bed area, m 2 inlet BOD, mg/L outlet BOD, mg/L implied oxygen transfer rate, gO/m 2 d inlet NH 4 -N, mg/L outlet NH 4 -N, mg/L flow rate, m 3 /d Cooper (1999) recognized that this approximation does not allow for BOD removal by settlement/filtration or denitrification, nor for NH4-N lost through (a) plant uptake, . Group, LLC 590 Treatment Wetlands Hyde H.C., Ross R.S., Demgen F. (Eds.) (1984) Technology Assess- ment of Wetlands for Municipal Wastewater Treatment, EPA/600/ 2-8 4 /154 ; NTIS No. 8 5-1 06896. Reed. of those coef- cients in a new situation. Consequently, regression equa- tions have not found use in treatment wetland design. 15. 2 FREE WATER SURFACE WETLANDS Early FWS treatment wetlands were. ranks at the 98th percentile of k 1 - values for 386 wetland-years of data for FWS wetlands (Figure 15. 3). From Figure 15. 3, it is clear that the expo- nential plug-ow model with C* 0 has the
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