March 22, 2017
The designations employed and the presentation of material throughout this publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The ideas and opinions expressed in this publication are those of the authors; they are not necessarily those of UNESCO and do not commit the Organization.
Maiga, Y., von Sperling, M. and Mihelcic, J.R. 2017. Constructed Wetlands. In: J.B. Rose and B. Jiménez-Cisneros, (eds) Global Water Pathogen Project. http://www.waterpathogens.org (J.R. Mihelcic and M.E. Verbyla) (eds) Part 4 Management Of Risk from Excreta and Wastewater) http://www.waterpathogens.org/book/constructed-wetlands Michigan State University, E. Lansing, MI, UNESCO.
Acknowledgements: K.R.L. Young, Project Design editor; Website Design: Agroknow (http://www.agroknow.com)
|Last published: March 22, 2017|
Constructed wetlands are a sanitation technology that utilize natural removal mechanisms provided by plant vegetation, soil, and associated microbial populations. The type of wetland can be distinguished according to criteria such as presence/absence of free water surface, use of rooted emergent aquatic plants (or free floating plants), and direction of flow. The three types of constructed wetlands discussed in this chapter are: 1) horizontal subsurface flow constructed wetlands, 2) horizontal free water surface flow constructed wetlands, and 3) vertical flow constructed wetlands. Constructed wetlands have been used to treat both centralized and on-site wastewater. Primary treatment is recommended when there is a large amount of suspended solids or soluble organic matter (measured as BOD and COD). This can be accomplished by placing sanitation technologies such as screening and grit removal, followed by a septic tank or primary sedimentation, waste stabilization pond, or anaerobic reactor prior to the wetland. All types of pathogens are expected to be removed in a constructed wetland; however, greater pathogen removal is expected to occur in a subsurface wetland. In a free water surface flow wetland one can expect 1 to 2 log10 reduction of pathogens; however, bacteria and virus removal may be < 1 log10 reduction in systems that are heavily planted with vegetation. This is because constructed wetlands typically include vegetation which assists in removing other pollutants such as nitrogen and phosphorus. Therefore, the importance of sunlight exposure in removing viruses and bacteria is minimized in these systems. Removal in a properly designed and operated free water surface flow wetland is reported to be < 1 to 2 log10 for bacteria, < 1 to 2 log10 for viruses, 1 to 2 log10 for protozoa:, and 1 to 2 log10 for helminths. In subsurface flow wetlands, the expected removal is pathogens is reported to be 1 to 3 log10 for bacteria, 1 to 2 log10 for viruses, 2 log10 for protozoa, and 2 log10 for helminths.
Figure 2. Two horizontal subsurface flow constructed wetlands (plants not yet planted) in parallel that treat septic tank effluent produced by a primary school in Jamaica (photo reproduced with permission from Edward Stewart (2005))
Figure 3. Free water surface flow constructed wetland (reprinted with permission of Eawag: Swiss Federal Institute of Aquatic Science and Technology Department Water and Sanitation in Developing Countries (Sandec). Figure from Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph. and Zurbrügg, C., 2014. Compendium of Sanitation Systems and Technologies. 2nd Revised Edition. Swiss Federal Institute of Aquatic Science and Technology (Eawag). Dübendorf, Switzerland)
Figure 4. (a). Free water surface flow constructed wetland in Copacabana, Bolivia (photo reproduced with permission from Stewart Oakley). (b) An experimental water surface flow constructed wetland that does not contain media and is planted with water hyacinths, Guatemala City (photo reproduced with permission from Stewart Oakley). The experimental wetland in 4b is by definition considered a free-floating plant system because it does not contain rooted emergent plants like the free surface wetland pictured in 4a. Water hyacinth is an aquatic plant native to the Amazon basin and can be a problematic invasive species if released to the natural environment outside its native range.
Figure 5. Vertical flow wetland run for operation in a downflow hydraulic regime (reprinted with permission of Eawag: Swiss Federal Institute of Aquatic Science and Technology Department Water and Sanitation in Developing Countries (Sandec). Figure from Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph. and Zurbrügg, C., 2014. Compendium of Sanitation Systems and Technologies. 2nd Revised Edition. Swiss Federal Institute of Aquatic Science and Technology (Eawag). Dübendorf, Switzerland)
In the subsurface flow configuration (Figures 1 and 5), water flows through the media and below the surface. The direction of the flow in a subsurface wetland can be either horizontal or vertical. In this case, there is little, if any visible water, especially for the subsurface wetland designed for horizontal flow. As shown in Figure 1, a horizontal subsurface flow constructed wetland is a basin (that can be lined) that consists of porous media such as large gravel or sand. It is also planted with wetland vegetation (Tilley et al., 2014). A vertical downflow subsurface constructed wetland is a planted filter bed (again with porous media) that is typically drained at the bottom (Figure 5). Wastewater is poured or dosed below the surface, or onto the surface, from above using a mechanical dosing system. The water flows vertically down through the filter matrix to the bottom of the basin where it is collected in a drainage pipe.
An important difference between a vertical and a horizontal flow subsurface wetland is not simply the direction of the flow path, but rather the resulting redox environment (Tilley et al., 2014). For example, the downflow hydraulic regime of a vertical subsurface wetland system receiving pulse feedings will promote the presence of aerobic conditions because the media stays unsaturated. For those considering nutrient management, this will promote biological transformation of nitrogen present as ammonia (NH4+/NH3) to nitrate (NO3-). If this downward flow regime is followed by a second vertical flow constructed wetland operated in an upflow hydraulic regime, anoxic conditions can be obtained in the upflow zone because the media in this cell will remain saturated. This is required for further transformation of nitrogen and its subsequent removal from the water phase, from nitrate (NO3-) to nitrogen gas (N2) (Fuchs et al., 2012).
Surface flow wetlands systems consist of plants grown in porous media or sediments and will have the visible presence of water which is typically 0.15 to 0.60 m in depth (Quinonez-Diaz et al., 2001; Mihelcic and Zimmerman, 2014). In these systems, the water surface of the wetland is exposed to the atmosphere which can theoretically provide oxygen to the water and also UV disinfection.
For more information about the design of constructed wetlands, refer to Crites and Tchobanoglous (1998), US EPA (1988), Kadlec and Knight (1996), Vymazal and Kröpfelová (2008), Kadlec and Wallace (2009) and Mihelcic et al. (2009). A review of pathogen fate in constructed wetlands is provided by Wu et al. (2016).
Figure 6 shows the location a constructed wetland in the sanitation service chain. In terms of wastewater treatment, constructed wetlands can be placed at the end of the overall engineered wastewater treatment system. This set of unit processes can include: 1) preliminary treatment with a bar screen and grit chamber that is followed by flow measurement and 2) another treatment unit process such as primary sedimentation basin, septic tank, waste stabilization pond, or anaerobic reactor (von Sperling, 2007).
Figure 6. Locations where constructed wetlands are used within the sanitation service chain
Constructed wetlands have been used to treat on-site and centralized collected wastewater (Decamp and Warren, 2000; Stewart, 2005), septage (Koottatep et al., 2005), greywater (Morel and Diener, 2006), stormwater runoff (Tanner et al., 2005), organic waste streams (Cronk, 1996), agricultural wastewater (Kantawanichkul and Somprasert, 2005), landfill leachate (Headley et al., 2004), acid mine drainage (Batty et al., 2005), and food processing and tannery wastewaters (Burgoon et al., 1999; Calheiros et al., 2007). Pre-treatment is recommended for waste streams such as domestic wastewater when there is a large amount of particulate matter (i.e., suspended solids) or organic matter (i.e., COD, BOD) (Cronk, 1996, Williams et al., 1999). This can be accomplished by placing screening and grit removal, followed by a septic tank or primary sedimentation, waste stabilization pond, or anaerobic reactor prior to the wetland.
The typical inputs for a constructed wetland are shown in Figure 7. Typical concentrations of pathogens in these inputs depend on the treated effluent from previous treatment stages described in other chapters that are specific to other sanitation technologies. Outputs from constructed wetlands can include treated effluents, sediments accumulated on the top of vertical systems, and harvested plants.
Figure 7. Typical inputs and outputs for constructed wetlands
As discussed in the chapter on Waste Stabilization Ponds, pathogens are removed from the wastewater or inactivated by a variety of different mechanisms that occur at different rates. The effectiveness of these removal mechanisms is dependent on a number of environmental, design, and operational factors. These different factors are observed to affect different pathogens in different ways.
In a constructed wetland, bacterial, viral, protozoan and helminth pathogens and indicator species are removed or inactivated through a complex combination of physical/chemical and biological (i.e., microbial/phytological) factors (ITRC, 2003; Kuschk et al., 2012). The primary mechanism(s) of removal will be different for free water surface and subsurface flow wetlands. Important for all these removal/inactivation mechanisms is they may occur to a greater extent in different zones of the free surface constructed wetland. For example, the three zones of a free water surface flow wetland are shown in Figure 8. Note that the zones differ in terms of their level of oxygen concentration, depth, and amount of planted vegetation (Mihelcic et al., 2009). All these can impact the physical-chemical and biological processes that control pathogen fate.
Figure 8. Three zones of a free water surface flow wetlands (reprinted from Mihelcic et al., 2009, with permission of Linda D. Phillips)
The most important factor for the removal of viral and bacterial pathogens is sunlight exposure, although other factors such as temperature and pH are also important. Because constructed wetlands include vegetation (which assists in removing other pollutants such as nutrients), the importance of sunlight exposure is minimized in these systems. Thus, one would expect that viral and bacterial pathogens are not removed as well as in a shallow, clear, open pond. Again, as with waste stabilization ponds, sedimentation is believed to be the most important factor for the removal of protozoan pathogens and helminth eggs. Different pathogen types are also not necessarily removed at the same rate by a similar mechanism. For example, it was stated in the Waste Stabilization Pond chapter that viruses are generally more resistant than bacteria to removal from sunlight mediated reactions (Davies-Colley et al., 2005a; Sinton et al., 2002) and differences in their structural and genetic composition impacts the rate of removal (Silverman et al. 2013; Mattle et al. 2014; Kohn et al. 2016).
Important factors expected to influence pathogen removal include mechanical filtration, temperature, adsorption to organic matter, and adhesion to biofilm. Other removal mechanisms include exposure of pathogens to biocides excreted by some wetland plants, the antimicrobial activity of root excretions, and predation by nematodes and protists, (e.g., Cronk, 1996; Gerba et al., 2000; Vymazal, 2005; Kusch et al., 2012; Wu et al., 2016). Table 2 provides a summary of some of the important pathogen removal mechanisms in constructed wetlands and how these mechanisms may differ in a free water surface flow and subsurface flow wetland.
Solar disinfection can probably only take place in a free water surface flow wetland that has some area that is not planted with emergent or floating vegetation. This is because, as stated previously, the presence of vegetation will limit pathogen exposure to sunlight. This is important because solar inactivation is probably the most important mechanism for virus inactivation. As discussed in the Waste Stabilization Pond chapter, the “UV portion of sunlight directly damages pathogen genomes (photo-biological damage)” and “UV and visible wavelengths can react with photosensitizers (such as natural organic matter) or photosensitizer molecules within bacteria (such as NADH/NADPH, flavins, and porphyrins) to produce reactive species that indirectly damage pathogens (photo-oxidative damage). Sunlight is stronger at lower latitudes, higher elevations, and in locations with less cloud cover.” Sunlight is also rapidly attenuated within the first few centimeters of a pond-like system that contains wastewater or natural organic matter (Davies-Colley et al., 2005b). The issue of sunlight exposure in a planted system is especially important because there is a direct relationship between the amount of sunlight that reaches a pathogen and the inactivation rate (Nguyen et al. 2015; Silverman et al. 2015). The inactivation of viruses by sunlight and the influence that different parts of the solar spectrum have on different viral species is discussed by Silverman et al. (2013).
For horizontal subsurface flow and vertical subsurface flow constructed wetlands, the influence of solar intensity is not existent because the wastewater is primarily contained below the ground surface. A free surface flow constructed wetland which has some open areas without vegetation (e.g., zone 2 in Figure 8 and the front part of the experimental wetland in Figure 4b) can allow for pathogen exposure to sunlight. UV-B (280 -320 nm), UV-A (320-400 nm) and photosynthetically active radiation (PAR > 400 nm) of the solar spectrum are known to be responsible for inactivating indicator microorganisms (Muela et al., 2000; Sinton et al., 2002; Kadir and Nelson, 2014; Silverman and Nelson, 2016). The dominating bactericidal portion of the solar spectrum is the UV-B radiation which causes direct (photo-biological) DNA damage (Jagger, 1985). Muela et al. (2000) in fact have shown the activity of E. coli exposed to UV-B radiation was reduced with exposure times as short as six hours. In contrast, the response of the E. coli to UV-A and PAR was similar and a longer term exposure was required to reduce the activity. Viruses have also been shown to be inactivated by solar radiation, but to a lesser degree than bacteria (Davies-Colley, 1999; Stinton, 2002).
Sunlight action can be enhanced by environmental factors. Environmental factors include the presence of dissolved organic matter and dissolved oxygen. Dissolved organic matter can attenuate light which decreases inactivation rates. The presence of oxygen is also known to influence the impact of sunlight exposure. For example, the survival of thermotolerant coliforms exposed to sunlight was found to be dependent on the presence of oxygen and increased with decreasing oxygen concentration (Benchokroun et al., 2003). In addition, inactivation of E. coli was found to be dependent on dissolved oxygen concentrations and the presence of UV-B wavelength (Kadir and Nelson, 2014). Molecular oxygen promotes solar photo-inactivation mediated by endogenous photosensitizers (Muela et al. 2002). In the presence of oxygen, UV radiation generates intracellular byproducts such as reactive hydroxyl radicals, hydrogen peroxide, and superoxide anion radicals, which can cause oxidative stress. The biological targets of these highly reactive oxygen species are a pathogen’s DNA, RNA, proteins, and lipids (cytoplasmic membrane) (Cabiscol et al., 2000). In addition, reactions mediated by exogenous photosensitizers can induce lesions in the transport chains in water that contains dissolved organic matter (Muela et al., 2002). However, it is reported that though exogenous photosensitizers played a role in inactivation rates of E. faecalis, they did not play an important role in the inactivation of E. coli in similar experiments (Kadir and Nelson, 2014; Nguyen et al. 2015).
Sedimentation plays an important role in the removal of some microorganisms within the wetland (Gray, 2004). Larger organisms such as protozoan (oo)cysts and helminth eggs can settle by gravity in a free water surface flow constructed wetland. For example, Karim et al. (2004) reported that sediments sampled from a free water surface flow constructed wetland accumulated Giardia and Cryptosporidium (oo)cysts at concentrations which were 2-3 orders of magnitude higher than those in the water column. One study performed in a duckweed pond (not the type of constructed wetland primarily discussed in this chapter) (Falabi et al., 2002) reported removal of 98% Giardia, 89% of the Cryptosporidium, 63% of thermotolerant coliforms, and 40% of coliphages.
As discussed in the Waste Stabilization Pond chapter, bacteria and viruses will not settle unless they are attached to larger and/or denser particles. Only a small percentage of viruses are observed attached to particles in waste stabilization ponds, and the particles they have been found attached to, are reported to be too small to settle (Sobsey and Cooper, 1973; Falabi et al., 2002; Characklis et al., 2005; Symonds et al., 2014). Studies in free water surface flow wetlands support observations made in waste stabilization ponds that bacteria and viruses are effectively removed by sedimentation only if they are attached to larger particles or form larger aggregates. Thus, removal of pathogens should correlate with particle removal (Quinonez-Diaz et al., 2001; Chouinard et al., 2014; Wu et al., 2016).
Mechanical filtration has been well described as an appropriate technology for pathogen removal (Maiga et al., 2014; Ushijima et al., 2015). It plays an important role in removal of pathogens in subsurface flow constructed wetlands, particularly by attachment of helminth eggs, larger protozoan (oo)cysts, and bacteria to the filter media (Chouinard et al., 2014). For example, Redder et al. (2010) observed reduction rates of approximately 2 log10 for Cryptosporidium oocysts and Giardia cysts in subsurface flow constructed wetlands. Wand et al. (2007) reported an average attachment of 8×106 bacteria cells per gram of sand in a column study simulating a vertical flow constructed wetland. Lana et al. (2013) investigated planted and unplanted vertical flow subsurface wetlands receiving a mean influent helminth egg concentration of 61 eggs/L of domestic wastewater. In that study, helminth eggs were removed similarly in the planted and non-planted system (97% and 96%, respectively). This suggested that, as expected, mechanical filtration would be the primary removal mechanism of helminths in a subsurface wetland.
The removal of pathogens in a subsurface constructed wetland also depends on the characteristics of the filter bed (e.g., nature of filter media, grain size). In fact, it has been reported that wetlands constructed with peat media removed a larger amount of Salmonella than a wetland constructed with sand media (Pundsack et al., 2001). Ushijima et al. (2013) used a filter bed of fine soil (1-4 mm diameter) in a horizontal subsurface flow wetland and reported removal of 5 log10 units for E. coli and 3 log10 units for MS2 phage, while coarse soil could not remove these microorganisms. In contrast, Redder et al. (2010) showed 2 log10 removal of protozoan parasites (Cryptosporidium oocysts and Giardia cysts) that was dependent of different filter media types (washed sand and clay of different particle sizes) in horizontal and vertical flow subsurface wetlands. This level of removal of protozoan pathogens is supported by other studies (e.g., Hagendorf et al., 2002; Caccio et al., 2013). Redder et al. (2010) also showed that media type and size can have an effect on attachment of some, but not all, pathogens to bed media. Information on how media type impacts microbial fate and transport in subsurface environments can be found elsewhere (e.g., Yates et al., 1988).
The presence of vegetation in constructed wetlands can improve the removal of pathogens by mechanical filtration and adsorption processes. The root-substrate complex and associated biofilm in planted constructed wetlands are reported to have the capacity for mechanical filtration and adsorption of pathogens (Morsy et al., 2007). Viral particles can be removed by adsorption to plant surfaces and roots (Quinonez-Diaz et al., 2001; Nokes et al., 2003); however, adsorption to sediment and suspended particles (dead algal and bacterial cells) may also be important (Nokes et al., 2003). Some bacteria could also attach to plant roots contributing to their removal (Wu et al., 2016). For example, Kadlec and Wallace (2009) reported that in wetland environment, parts of submerged plants and their associated biofilms form “sticky traps” for particles. These biofilms are believed capable of trapping a considerable number of organisms (Stott and Tanner, 2005). Biofilms that occur around rhizosphere have also been shown to support development of bacterial populations that have antibiotic activities which may contribute to pathogen removal (Broadbent et al., 1971). Webber and Legge (2008) have suggested that the biofilms present in a constructed wetland may also facilitate the retention of pathogens through attachment and harboring grazing protozoa in and on the surface of the biofilm region. On the other hand, shading by vegetation will reduce exposure of pathogens to UV light and prevent heating of the water by sunlight, thus decreasing the rate of inactivation (Quinonez-Diaz et al., 2001; Morsy et al., 2007).
Vymazal (2005) demonstrated through a review of sixty studies that free water surface flow constructed wetlands that contained emergent vegetation were more effective in removal of bacteria than systems that were not planted. They reported 95 to 99% removal of E. coli and 80 to 95% removal of fecal streptococci in free water surface flow wetlands planted with emergent vegetation. They discussed that planted systems can enhance the presence of oxygen and the plants may produce excretions that have antimicrobial properties (Neori et al., 2000).
Previously, Hench et al. (2003) observed greater reduction of thermotolerant coliforms, enterococci, Salmonella, Shigella, Yersinia and coliphage populations in planted subsurface wetlands compared to those lacking vegetation. In this case, the plant roots reach into the subsurface treatment area. In addition, Hill and Sobsey (2001) found that the presence of plants in a horizontal subsurface flow constructed wetland improved the removal of Salmonella when compared to an unplanted unit. However, for vertical flow constructed subsurface wetlands, no significant difference in the removal efficiency was observed of thermotolerant coliforms, E. coli, somatic coliphages, and F-specific bacteriophages for planted and unplanted systems (Torrens et al., 2009; Lana et al, 2013).
As stated in the Waste Stabilization Pond chapter, the most important physical-chemical factors for pathogen inactivation are pH, temperature, and dissolved oxygen. Most bacterial pathogens are vulnerable to high pH, with Vibrio spp. being an exception (Mezrioui et al., 1995). The sanitizing effect of free ammonia (NH3), which becomes more available at higher pH, is even more effective at higher temperatures (Decrey et al., 2014; Emmoth et al., 2011; Burge et al., 1983).
Temperature has been shown to play an important role in the reduction of enteric bacteria and viruses in constructed subsurface wetlands (Quinonez-Diaz et al., 2001; Winward et al., 2008). In Germany, Ulrich et al. (2005) found that higher wastewater temperatures in the summer improved pathogen removal performance by approximately 1 log10 unit. Weber and Legge (2008) reported that increased temperature will increase predator activity of grazing protozoa. Subsurface wetlands may be cooler than a horizontal subsurface flow constructed wetland during summer months; however, they may provide higher temperatures during low temperature winters in some parts of the world. In some specific situations, there may even be a need to reduce organic loading during the winter (Gray, 2004).
Constructed wetlands are known to support diverse biota (Vymazal et al., 2001) which includes nematodes, rotifers, and protozoa that can prey on pathogens. Free-living ciliated protozoa and copepods can be important predators in the removal of Cryptosporidium oocysts (Stott et al., 2001) and bacteria (Wand et al., 2007). Furthermore, the ciliate Paramecium was reported to consume over 100 E. coli per hour (Decamp and Warren, 1998) and another study estimated the grazing rates in the gravel media of a subsurface wetland to be in the order of 49 bacteria/ciliate-hour (Decamp et al., 1999). Morsy et al. (2007) also reported an average Cryptosporidium oocysts predation rate by wetland ciliates of 10 oocysts/ciliate-hour. Protozoa grazing is also reported to have an impact on bacterial community structure in soil microcosms (Ronn et al., 2002).
The performance of constructed wetlands on pathogen removal depends on a synergistic effect of many environmental, design, and operational factors. However, pathogen and indicator species removal in constructed wetlands is expected to be primarily influenced by the hydraulic loading rate (HLR) (Brix et al., 2003) and thus the hydraulic retention time (HRT) (Vymazal, 2005). In free water surface flow constructed wetlands, longer HRTs are reported to increase pathogen inactivation (e.g., Garcia et al., 2003; Ulrich et al., 2005). This is because of increased exposure to sunlight in zones where there is not extensive plant coverage and more time is allowed for sedimentation, adsorption to organic matter, predation, and the effect of toxins from plants (Diaz et al., 2010).
The HRT depends on the flowrate and the presence of plants, characteristics of the porous media for a subsurface wetland, and the desired water depth for a free water surface flow wetland. In addition, short-circuiting of wastewater flow can lead to lower treatment efficiency because the actual treatment residence time may be less than the theoretical hydraulic retention time. Ulrich et al. (2005) indicated that issues of clogging of inlet pipes in horizontal and vertical flow subsurface constructed wetlands lead to hydraulic short circuiting. Finally, Wu et al. (2016) reported that hydraulic overloading could reduce the removal efficiency of thermotolerant coliforms because of decreased ability of the coliforms to adsorb to the biofilm.
The presence of local animals (domesticated and wild) may introduce pathogens and indicators to a free water surface flow wetland (e.g., E. coli and Salmonella). Moreover, free water surface flow wetlands can be sources of some (but not all) nuisance mosquitoes, some of which have a public health implication (Culex quinquefasciatus) (IWM and IDRCI, 2010). Wetland design and operation can be directly integrate with mosquito control (Russell, 1999; Sarneckis, 2002) if that is an important management objective. It has also been shown that intense precipitation events can dilute microorganism concentrations which will lower removal rates and consequently a wetland’s overall performance (Ulrich et al., 2005).
Key factors and strategies that could enhance pathogen removal in different types of constructed wetlands are presented in Table 3.
Pathogen removal in constructed wetlands is influenced by a number of factors including engineering, environmental, and operation and maintenance practices. Factors reported to have the greatest influence on pathogen removal are hydraulic retention time (HRT), hydraulic loading rate (HLR), and the presence of plants (e.g., Brix et al., 2003; Vymazal, 2005). The concept of HRT does not apply for unsaturated media, such as vertical flow subsurface wetlands operated with intermittent feeding (i.e., the wastewater is not occupying all the void spaces). Accordingly, the Figures 9 and 10 were constructed using only data collected for horizontal subsurface flow and free water surface flow constructed wetlands.
Figure 9. Pathogen removal as a function of hydraulic residence time (HRT) in (a) horizontal subsurface flow constructed wetland and (b) free water surface flow constructed wetland
Figure 10. Influence of hydraulic loading rate on pathogen removal in constructed wetlands
The effect of hydraulic retention time (HRT) on pathogen removal is shown in Figures 9a and 9b. These two figures show HRT has an influence on pathogen removal (bacteria, viruses, helminths and protozoan) in horizontal subsurface (Figure 9a) and free water surface flow (Figure 9b) constructed wetlands. The impact appears more pronounced for the helminth and protozoan data (as suggested by information provided previously in Table 3).
Figure 9a shows that for subsurface wetlands, one could expect 1 to 3 log10 reduction in pathogens with an HRT of ≤ 10 days. With an HRT of 10 to 15 days, one might be able to achieve one additional log10 reduction in protozoa concentration. Figure 9b shows free water surface flow wetlands are expected to achieve a wide range of removal of bacteria and viruses that can range from less than 1 log10 reduction to up to 2 or 3 log10 reduction. This supports the conclusions of others (e.g., Nguyen et al., 2015; Silverman et al., 2015) that sunlight inactivation is very important for virus and bacteria reductions and this mechanism may be limited in a free water surface flow constructed wetland that contains emergent plants because of shading of the water by the vegetation. The observations in Figure 9a and 9b are also supported by Wu et al. (2016) who showed that horizontal subsurface flow constructed wetlands typically have a better capacity to remove pathogens than a planted free water surface flow constructed wetlands.
Figure 10 shows the impact of hydraulic loading rate (HLR) on pathogen removal. For this figure the data is not sorted by wetland type. Note that contrary to reports by Brix et al. (2003) and others, the data we assembled indicated that pathogen was not influenced by increasing HLR.
Constructed wetlands can be used to treat on-site or centralized wastewater. They are typically placed after other sanitation technologies that may include some type of treatment for solids. Even with this pretreatment step, over time, some of the pathogens entering a free water surface flow constructed wetland will accumulate in wetland’s bottom sediments. Sedimentation does not necessarily result in the inactivation of pathogens and pathogens may remain viable in sediments. The most durable and persistence pathogens that survive best in sediments are helminths and protozoans. Thus, as discussed in the Waste Stabilization Pond chapter, the recommended treatment method for sediments excavated from a constructed wetland used to treat municipal wastewater should be either burial in a nearby field (to limit human contact), or dewatering in an open field or a constructed sludge drying bed. There the materials should be stored in the sunlight and out of the excessive rain for a minimum of one year prior to reuse. Greenhouse solar drying beds may be constructed to help speed up the process by desiccating the material and raising temperatures during the day. The addition of seeds from wetland plants such as Ludwigia spp. can also help accelerate dewatering (Oakley et al., 2012). Lime can also be added to sediments to stabilize the materials and increase the pH which is known to inactivate pathogens.
Constructed wetlands are a sanitation technology that have not typically been designed for pathogen removal, but instead, have been designed to remove other water quality constituents such as suspended solids, organic matter (BOD/COD) and nutrients (nitrogen and phosphorus). Table 4 provides a summary of typical pathogen removal efficiencies that can be expected in constructed wetlands. The data summarized in this table was based on constructed wetlands that had hydraulic retention times of primarily ≤ 10 days and hydraulic loadings rates of ≤ 8 cm/day. All types of pathogens (i.e., bacteria, viruses, protozoan and helminths) are expected to be removed in a constructed wetland; however, Table 4 shows that one would expect greater pathogen removal in a subsurface wetland. Figure 9a, Figure 9b, Figure 10, and Table 4 indicate that the efficiency of pathogen removal in full-scale constructed wetland systems can be highly variable, and depends on a number of other factors discussed in Tables 2 and 3). One reason for this is because constructed wetlands include free-floating, subsurface, or planted emergent vegetation (which assists in removing pathogens, but especially other pollutants such as nutrients); thus, the importance of sunlight exposure is minimized in a free water surface flow wetland.