See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/248412862 Feasibility of using constructed treatment wetlands for municipal wastewater treatment in the Bogotá Savannah, Colombia ARTICLE in ECOLOGICAL ENGINEERING · JULY 2009 Impact Factor: 2.58 · DOI: 10.1016/j.ecoleng.2009.03.017 CITATIONS READS 26 77 2 AUTHORS: Mauricio E. Arias Mark Brown Harvard University University of Florida 19 PUBLICATIONS 136 CITATIONS 115 PUBLICATIONS 3,675 CITATIONS SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately. SEE PROFILE Available from: Mauricio E. Arias Retrieved on: 24 January 2016 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Ecological Engineering 35 (2009) 1070–1078 Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng Feasibility of using constructed treatment wetlands for municipal wastewater treatment in the Bogotá Savannah, Colombia Mauricio E. Arias ∗ , Mark T. Brown Howard T. Odum Center for Wetlands, Department of Environmental Engineering Sciences, University of Florida, United States a r t i c l e i n f o Article history: Received 18 July 2008 Received in revised form 27 February 2009 Accepted 24 March 2009 Keywords: Municipal wastewater Emergy Developing countries Appropriate technology a b s t r a c t The main water bodies in the Bogotá Savannah have been seriously polluted due to the mismanagement of domestic, agricultural, and industrial wastewater. While there are a number of wastewater treatment facilities in the region, most do not function properly. There is a great need for inexpensive and sustainable wastewater treatment systems that are not technologically sophisticated and that do not require intensive management. The main goal of this study was to quantify the performance and sustainability of treatment wetlands and existing wastewater treatment systems in this region. Using data from the literature, a treatment wetland model was developed, which focused on pollutant removal. The modeled performance was compared to a system of waste stabilization ponds and a sequencing batch reactor. The three systems were subject to cost analysis and an emergy evaluation, leading to the assessment of indicators of cost-beneﬁt for comparison. The economic analysis suggested that the net annual cost of the treatment wetland was US$ 14,672, compared to US$ 14,201 for the stabilization ponds and US$ 54,887 for the batch reactor. The emergy evaluations show that the ponds have the lowest annual emergy ﬂow (6.65 + 16 sej/yr), followed by the constructed wetland (2.88E+17 sej/yr) and the batch reactor (8.86E+17 sej/yr). These results were combined to estimate treatment ratios (contaminants removed per lifetime cost, and contaminants removed per total emergy), cost ratios (cost per volume of water, annual cost per capita, and construction cost per capita), and emergy ratios (treatment yield, renewable emergy, lifetime emprice, construction emprice, non-renewable emergy, empower density, environmental loading, total emergy per volume of water, and emergy per capita). © 2009 Elsevier B.V. All rights reserved. 1. Introduction Colombia is in great need of low-cost, low-maintenance wastewater management strategies that take advantage of this country’s climatic conditions and that offer a good investment for a country where capital for infrastructure is scarce. Bogotá, the capital and largest city of Colombia, creates a large demand on the resources of the surrounding rural area, called the Bogotá Savannah. As of the year 2005, there were 27 municipal wastewater treatment plants (WWTPs) in the region, which served nearly 600,000 people. Of the 27 WWTPs, 16 were waste stabilization ponds (WSPs), 7 were activated sludge systems, 3 were anaerobic reactors, and 1 was a sequencing batch reactor (CAR, 2003). Mechanical treatment systems are maintenance- and energyintensive (Tchobanoglous et al., 2003); consequently, their performance is affected when these requirements cannot be properly provided. Thus, it should be clear that in regions where mechanical treatment technologies cannot be effectively maintained, promoting less energy-intensive wastewater (WW) technologies could result in improved water quality, beneﬁting the health, economy, and aesthetics of the region. The objective of this study was to evaluate the performance and sustainability of a model constructed wetland treatment system (CWTS), and to compare this option to conventional systems (WSPs and a batch reactor) currently used for municipal WW treatment in the Bogotá Savannah. The hypothesis of this study is that a CWTS would yield signiﬁcant improvements in pollutant removal in comparison to WSPs; likewise, the CWTS should result in a much lower monetary and resources investment in comparison to a conventional treatment system. 1.1. Previous studies ∗ Corresponding author at: Howard T. Odum Center for Wetlands, University of Florida, Phelps Lab, P.O. Box 116350, Museum Road, Gainesville, FL 32611, United States. Tel.: +1 352 392 2429; fax: +1 352 392 3624. E-mail address: [email protected]ﬂ.edu (M.E. Arias). 0925-8574/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2009.03.017 Few CWTS have been studied and documented in Colombia, despite the recognized beneﬁts of low-cost, year-long plant growth and bacterial activity in tropical climates (Okurut et al., 1999). Author's personal copy M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 Williams et al. (1999) published the ﬁrst study on the design and initial performance of a small CWTS in Chinchina, Province of Caldas (design ﬂowrate = 45 L/min, total construction area = 4000 m2 ). In addition, several pilot studies have been conducted in Colombian universities (e.g., Lara and Vera, 2005; Paredes, 2005; Castaño, 2005). Nonetheless, no large-scale, municipal WW treatment wetland had been documented to the date of this study. Previous studies have evaluated WW treatment alternatives using similar tools as the ones used in this study (i.e., treatment performance, monetary cost, and emergy). For instance, Nelson (1998) designed and evaluated two subsurface ﬂow (SSF) wetlands in Mexico. He showed that these systems were efﬁcient at removing a number of wastewater constituents, including Biological Oxygen Demand (BOD), Fecal coliform (FC), Total Nitrogen (TN), and Total Phosphorus (TP). The total emergy of these treatment wetlands was reported to be 3.54E+17 solar emjoules (sej) per year (see Section 2.4 for explanation of this unit), which included 3.45E+17 sej/yr of raw wastewater ﬂow, and 8.7E+15 sej/yr of all environmental, construction, and operation ﬂows combined. The emergy required for operation of a comparable conventional packaged plant was 3.7E+15 sej/yr, which was eighteen times greater than the operational emergy of the treatment wetlands. In a different study, Geber and Björklund (2001) compared a conventional WWTP, a conventional plant combined with a CW for tertiary treatment, and a natural wetland in Sweden. The three systems realized similar pollutants reduction, and similar total emergy use per person equivalent (p.e.), ranging from 154 to 180E+12 sej/p.e. Also, a large variation in the ratio of purchased goods to environmental inputs (environmental loading ratio) was found, with a value of 9 for the natural wetland and 3056 for the conventional WWTP. Despite this large difference, the authors concluded that the natural treatment alternatives studied did not fully substitute the purchased emergy with free environmental inputs. Furthermore, Behrend (2007) performed emergy evaluations of seven wastewater treatment alternatives for the state of Georgia (USA). The environmental loading ratio varied greatly for the systems evaluated, with septic tanks having the lowest (383) and a water reclamation facility with lime phosphorus removal having the greatest (101,176). A facility with conventional secondary treatment followed by a treatment wetland fell in the middle of the range of environmental loading ratios with a value of 58,370 (202 without the wastewater input). This facility treated an average of 1.4 m3 /s, and yielded a total emergy of 1.02E+22 (3.56E+17 if wastewater input is excluded from the calculation). 2. Methods 2.1. Study sites Two sites with municipal WWTPs were selected for the evaluations performed during this study. The ﬁrst site was Tabio, located 50 km northwest of Bogotá with a population of 14,000, and an average temperature of 14 ◦ C. This municipality has a WSP designed to treat 20 L/s of WW. The 3.4 ha treatment plant was constructed in 1992, and consists of a screen, a sedimentation tank, an anaerobic basin, and two series of facultative lagoons with two basins each (CAR, 2003). The second site was the municipality of La Calera, located 30 km east of Bogotá. A Sequencing Batch Reactor (SBR) treatment plant was constructed in 2002 with a design ﬂow of 36.5 L/s and a design population of 16,000. The system consists of primary treatment with a manual screen and a sedimentation tank, followed by secondary treatment with two reactor tanks, a sludge digestor, and sludge drying beds (CAR, 2003). Water quality and ﬂow analyses 1071 from these facilities were performed and reported by the local environmental protection agency (Corporación Agrónoma Regional de Cundinamarca, CAR). 2.2. Constructed wetland treatment system design The CWTS model was assumed to be at the location of the Tabio WSPs. The hypothetical system consisted of screens, a sedimentation tank, and anaerobic basin, but in place of facultative lagoons, a combination of subsurface ﬂow and surface ﬂow (SF) wetland units were modeled. The raw wastewater quality data for Tabio provided by CAR were used to size the proposed wetland system. The area used in the design determined from the 3.4 hectares utilized by the current plant, minus 2900 m2 occupied by the anaerobic basin, and minus 30% of the extra area that accounted for pretreatment structures, open area, and others. A combination of SF and SSF wetland cells was deemed necessary to provide sufﬁcient treatment with minimal costs. The CWTS model was subject to a sensitivity analysis to determine the effect of the system conﬁguration on the overall feasibility study. This analysis was done by estimating the pollutant removal and the cost for different area distributions between SSF and SF wetlands; the conﬁguration used in this study was found at the point where maximum pollutant removal and minimum cost intersected. Using the estimated wetland areas, the efﬂuent concentration at each wetland unit was calculated using the k–C* model (Kadlec and Knight, 1996): Ce = C ∗ +(Ci − C∗) exp −kA 0.0365 Q (1) where Ce = outlet concentration (mg/L), Ci = inlet concentration (mg/L), C* = background concentration (mg/L), A = wetland area (ha), Q = water ﬂow rate (m3 /day), and k = ﬁrst-order areal rate constant (m/yr). Values of k used were the global averages proposed by Kadlec and Knight (1996), which are based on a large database of operating treatment wetlands in the temperate climate of North America and Europe. Even though treatment wetlands in the tropics could perform more efﬁciently, design parameters for CWTS in Colombia have not been documented, and therefore using the published constants proposed by Kadlec and Knight (1996) is the most conservative approach for this evaluation. The pollutant removal efﬁciencies of the pretreatment and primary treatment units were not modeled, but assumed from typical values found in the literature (Ramírez and Romero, 1997; Tchobanoglous et al., 2003). 2.3. Cost evaluations A cost evaluation was performed for each of the treatment options studied. These evaluations included the major components of construction cost, and annual operation and maintenance (O&M). All costs were calculated for 2003 Colombian pesos, and then converted to US dollars using the average 2003 exchange rate of 2877.55 Colombian pesos per US dollar. Unit costs for the CWTS and the WSPs came from a Colombian construction costing guide (Construdata, 2003), CAR (2003), or otherwise estimated from local market costs. The construction costs for the SBR were extracted from a budget requested by CAR (Aquavip, 2000). Only costs related to the construction of the treatment units were used from this budget. Annual O&M costs for this facility were acquired from CAR documents or estimated from typical local market costs. The net annual cost of treatment was estimated as: Net annual cost = O&M+ Construction lifetime (2) Author's personal copy 1072 M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 Table 1 Description of indicators calculated. Indicator Units Calculation Preference kg C/US$ [Cin ]−[Cout ] lifetime cost ↑ Contaminant (C) removed (e.g. BOD, SS, coliform, TP) per total emergy kg C/sej [Cin ]−[Cout ] total emergy ↑ Cost ratios Unit costs (X) required (net annual, O&M, construction) per m3 of water US$/m3 /yr X design ﬂow ↓ Net annual cost per capita US$/p.e/yr lifetime cost design population ↓ Construction cost per capita US$/p.e construction cost design ﬂow ↓ Unitless WWemergy −system outﬂow purchased goods and services ↑ Unitless environmental resources+WWemergy lifetime cost ↑ Lifetime emprice sej/US$ total emergy lifetime cost ↑ Construction emprice sej/US$ total emergy construction cost ↑ Non-renewable emergy. Unitless goods and purchased services total emergy ↓ Empower density sej/yr/m2 total emergy area ↓ Environmental loading ratio Unitless goods and purchased services environmental resources+WW ↓ Total emergy per m3 sej/m3 total emergy annual WW ﬂow ↓ sej/p.e. total emergy population ↓ Treatment ratios Contaminant (C) removed (e.g. BOD, SS, coliform, TP) per lifetime cost Emergy ratios Treatment yield ratio Renewable emergy Total emergy per inhabitant ↑: Largest ratio is preferred; ↓: smallest option is preferred. where net annual cost, construction, and annual O&M were calculated in 2003 dollars. Lifetime refers to the facility design lifetime, assumed to be 25 years for the three systems. 2.4. Emergy evaluations For the purpose of this study, sustainability was addressed as the appropriate use of resources to provide the service of wastewater treatment. In order to provide numerical quantities to this concept, emergy syntheses of the three wastewater systems were performed. Emergy was deﬁned as “the availability of energy of one kind that is used up in transformations directly and indirectly to make a product or service” (Odum, 1996). Emergy synthesis is an appropriate accounting method for environmental decision making, because it not only accounts for the energy ﬂows driving these systems, but it also recognizes that these energy ﬂows have a quality component that accounts for the hierarchical distribution of energy in systems. This accounting method has been recently used in a number of environmental decision making applications, including regional sustainability (Lei et al., 2008) ecosystem restoration (Ton et al., 1998; Lu et al., 2006), solar power evaluation (Paoli et al., 2008a), small marinas sustainability (Paoli et al., 2008b), and wastewater technology assessment (Geber and Björklund, 2001; Siracusa and La Rosa, 2006). Emergy is typically measured in solar emjoules, which can be estimated from physical ﬂows using three different unit emergy values (UEV): Transformity, speciﬁc emergy, and emergy per unit money (Odum, 1996). Transformity is the energy used in the transformation of available energy, normally expressed in units of emjoules per joule (sej/J). Speciﬁc emergy is the energy used in concentrating certain amount of mass, and it has units of emergy per unit mass (e.g., sej/g). The emergy per unit money is the amount of emergy represented by a unit of currency. This is measured in sej/US$, and it is estimated from the ratio of emergy use to economic product (typically gross domestic product (GDP)) at the national scale. Systems diagrams were created for the three treatment options (Appendix A). These diagrams show the main energy ﬂuxes and storages for a given system and the interactions among them.1 The ﬂows in and out of the systems diagrams were the basic components analyzed in the emergy evaluation tables. Items in the tables for each system were organized in four categories: environmental resources, wastewater, purchased goods and services, and system outﬂows. The ﬁrst category includes sun, wind, and rain. Both water inﬂows (rain and WW) were divided into four components: water chemical potential, organic matter, total phosphorous, and total nitrogen. The third category includes construction materials, and O&M components such as electricity and labor. The fourth category refers mainly to the treated water (including the same four components as for the environmental resources), and evapotranspiration. Data used for the emergy evaluations came from several sources. Mass, energy, and money ﬂows were converted to their respective standard units from data acquired from the CWTS performance analysis, the cost analysis, CAR reports, journal articles, or estimated from typical values observed in the region. UEVs came from Odum (1996), Geber and Björklund (2001), and emergy evaluations compiled at the Center for Environmental Policy at the University of Florida, Gainesville (Brandt-Williams, 2002; Brown and Bardi, 2001; Buranakarn, 1998; McGrane, 1994; Odum et al., 1983, 1987; Vivas, 2004). All UEVs were corrected to the most recent emergy baseline as suggested by Brown and Bardi (2001). Each table includes a calculation of total emergy of environmental resources, total emergy of goods and purchased services, and total emergy. The maximum ﬂow of environmental resources was estimated to be the total emergy for this category to avoid doubledcounting. Emergy of goods and purchased services was calculated as the sum of all the inﬂows from that category; the total system emergy was then calculated as the sum of the total environmental 1 For a description of the diagramming language see Odum (1994). Author's personal copy M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 1073 Table 2 Design parameters, pollutant concentrations, and percent removal at each stage of the CWTS system. All concentrations expressed as mg/L, except for E. coli (CFU/100 mL). resources and the total goods and purchased services. Even though the WW emergy ﬂows were quantiﬁed, these numbers were not included in the total emergy calculation. This exclusion was made because WW was considered a ﬂow-through component, which rather than being an input, it is the element that the entire system is providing a service to. 2.5. Indicators Once the performance, cost, and emergy evaluations were determined, several indicators were calculated to compare the three options (Table 1). These indicators were divided into three categories according to the unit in their denominator: The ﬁrst category was treatment indicators, and their purpose is to show how effective the systems are at removing the pollutants with respect to cost and emergy. Nitrogen was not included because no data were available for the WSPs and the SBR. The second category was cost indicators, which showed how economically expensive the treatment is per unit of water treated and per inhabitant served. The third category was emergy indicators, which show how energetically expensive each alternative was. 3. Results and discussion 3.1. CWTS design and performance Findings from this study indicate that it is feasible to design a CWTS that satisﬁes size restrictions and pollutant removal efﬁciencies similar to existing WSPs in the region. The hypothetical CWTS was composed of the existing pretreatment units and primary anaerobic lagoon, followed by one series of SSF CW units and one series of SF CW units (Table 2). The total area was maintained at 3.4 ha, with 0.4 ha and 1.7 ha for the SSF and the SF basins, respectively. One concern with the design parameters calculated is that some are out of the typical range of design values for these systems (Kadlec and Knight, 1996). The detention times estimated for the SSF and the SF CW were 0.6 and 4.5 days, respectively, and the loading rates were 40 and 10 cm/day. Detention times in SSF and SF are typically 2–4 and 7–10 days respectively, and loading rates are typically 8–30 and 1.5–6.5 cm/day. Nonetheless, there is evidence that CWTS in sub-/tropical weather have performed adequately even with less conservative design parameters that are out of these recommended ranges (e.g., Yang et al., 1995; Greenway and Woolley, 1999). Another concern with this model is the use of the areal rate constants (k), which ideally, should be determined from treatment wetlands operating under similar conditions. However, as it has been pointed out previously in this paper, large-scale treatment wetlands (if existing) have been poorly documented in Colombia and its neighboring countries, and until sufﬁcient data are generated, treatment wetlands in this region should be designed using information and tools established from databases of largescale functional treatment wetlands (most of which are in North America and Europe). A sensitivity analysis (Appendix A) revealed that a ratio of SSF to SF wetland area of 1–4 yielded appropriate removal while maintaining construction costs as low as possible. This sensitivity analysis was a crucial step that had much impact in the other analyses performed in this study. The ﬁnding of this point of trade off between system performance (represented by BOD removal) and resources utilization (represented by gravel cost) was the basis for the conceptual design of this CWTS, and without determining this point, it is likely that the area distribution of between SSF and SF cells would have been arbitrarily set, and hence, the potential that CWTS have to provide good performance with appropriate resources investment would have been underestimated. The CWTS treatment performance was compared with the removal efﬁciencies of the WSPs and the SBR (Table 3), and it was estimated that the proposed system has the potential of achieving overall higher pollutant removal efﬁciency, particularly those of greatest concern, BOD and TSS, and to a lesser extent, nutrients. Removal of E. coli was the only parameter for which the WSPs were shown to achieve a higher efﬁciency than CWTS. Although the difference in performance is not large, this is likely the result of longer detention times in WSPs due to greater storage volume. It should be noted that the CWTS performance estimates are based on results from an empirical model that has been proven accurate for many treatment wetlands (Kadlec and Knight, 1996), but do not repre- Table 3 Estimated percent removal of pollutants for the three treatment systems evaluated. Constituent CWTS WSPs SBR SS BOD E. coli TN NH4 TP 97 92 98 62 62 47 79 86 99.5 – −8 −35 73 79 84 – – 30 Author's personal copy 1074 M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 Table 4 Summary of cost analysis for the three treatment systems evaluated. Item CWTS WSP SBR Construction Land Scrubbing Earth work Liner Gravel Concrete Piping Steel Vegetation Sand Structure and cover Stone work Other Total construction cost 35,447 2697 34,243 28,720 50,477 874 1878 351 281 – – – – 154,968 35,447 2,686 82,949 28,609 – 582 923 351 – – – – – 151,548 9,383 – 18,513 – 7,264 177,679 8,118 76,045 – 7,531 15,759 7,401 9,624 337,318 Annual O&M Electricity Labor Landscaping Water quality analysis 1,251 3,341 557 3,324 1,251 3,341 223 3,324 31,277 6,682 111 3,324 Total annual O&M cost Annual cost, US$/yr 8,473 14,672 8,139 14,201 41,394 54,887 See Appendix A for complete analyses. sent absolute results. Factors such a proper design, construction, and management have a great impact in the performance of any wastewater treatment system. 3.2. Cost analysis A summary of the cost analyses for the three alternatives is presented in Table 4, and detailed cost analyses and calculations are in Appendix B. Construction cost for the CWTS was estimated to be US$ 154,968, with O&M cost of US$ 8473, resulting in a net annual cost of US$ 14,672. The construction cost of the WSP was estimated to be US$ 151,548, with O&M of US$ 8139, resulting in a net annual cost of US$ 14,201. The main differences between these two cost evaluations were the cost for the gravel media used in the CWTS (US$ 50,477 for the 0.4 ha), and the greater amount of excavation required for the WSP, which resulted in an additional US$ 48,706 of earth work. These two major expenses were nearly the same amount, resulting in very similar ﬁnal construction costs for the two systems. The fact that the gravel is the most expensive item for this CWTS implies that the potential of using this material in a large-scale SSF in this region is limited, and minimizing its use can result in great cost reduction as demonstrated with the sensitivity analysis. If land is available and affordable, a SF system would be more economically feasible. Another alternative is to use a different type of media. For instance, pieces of bamboo were used as media Table 5 Summary of emergy ﬂows for the three treatment systems. Item CWTS, E+12sej/yr WSP, E+12sej/yr SBR, E+12sej/yr Environmental inputs Sunlight Wind, kinetic 4 34 4 34 1.4 13 Rain Chemical potential Nitrogen Phosphorus Organic matter Emergy of environmental resources 2951 110 29 10 2951 2951 110 29 10 2951 1077 24 11 4 1077 95,262 179,958 112,128 121,972 179,958 95,262 179,958 112,128 121,972 179,958 173,853 328,424 370,552 100,846 370,552 Goods Pumps Seedlings Liner Steel Concrete PVC Bricks Gravel Sand – 51 7885 – 796 22 – 221,460 – – – 7885 – 531 11 – – – 264 – – 22,344 107,639 64 15 27,975 52,412 Purchased services Electricity Labor Total emergy of goods and purchased services Total emergy 25,210 29,938 285,362 288,313 25,210 29,938 63,575 66,526 630,254 44,290 885,256 886,333 System outputs Evapotranspiration 3078 3078 – Treated water Chemical potential Nitrogen Phosphorus Organic matter 95,135 67,825 59,440 9341 95,135 120,318 151,291 17,386 173,853 262,739 143,598 21,522 Wastewater Chemical potential Nitrogen Phosphorus Organic matter Emergy of wastewater See Appendix B for complete evaluations. Author's personal copy M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 1075 Fig. 1. Normalized treatment indicators for the three wastewater treatment alternatives; 1 (greatest) represents the indicator for the best option. Fig. 2. Normalized cost indicators for the three wastewater treatment alternatives; 1 (smallest) represents the indicator for the best option. for a SSF in Pereira, Colombia, achieving similar results as gravel (Castaño, 2005). The SBR was found to be the most expensive option, with a construction cost of US$ 337,318, O&M of US$ 41,394, and net annual cost of US$ 54,887. The major components of the construction that caused this increased cost were concrete and steel, in addition to several other materials required for the construction of this system and not for the other two. Moreover, O&M cost was much more Fig. 3. Normalized emergy indicators for the three wastewater treatment alternatives; 1 (greatest) represents the indicator for the best option. Author's personal copy 1076 M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 Table 6 Summary of indicators treatment, cost, and emergy indicators. Indicator Units CWTS WSP SBR Preferred Treatment ratios BOD removed per lifetime cost SS removed per lifetime cost E. coli removed per lifetime cost TP removed per lifetime cost BOD removed per total emergy TSS removed per total emergy E. coli removed per total emergy TP removed per total emergy kgBOD/US$ kgSS/US$ #/US$ kgP/US$ kgBOD/sej kgTSS/sej #/sej kgP/sej 14.4 13.5 1.05E+12 0.097 7.31E−13 6.85E−13 5.34E−02 4.94E−14 13.8 11.3 1.10E+12 −0.075 2.94E−012 2.41E−12 2.36E−01 −1.59E−14 2.7 2.0 1.99E+11 0.055 1.68E−13 1.25E−13 1.23E−02 3.42E−15 ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ Cost ratios Lifetime cost per m3 of water Operation cost per m3 of water Construction cost per m3 of water lifetime cost per capita Construction cost per capita US$/m3 /yr US$/m3 /yr US$/m3 US$/p.e/yr US$/p.e 0.0233 0.0134 0.246 1.0 11 0.0225 0.0129 0.240 1.0 10.8 0.0477 0.0360 0.293 3.4 21 ↓ ↓ ↓ ↓ ↓ Emergy ratios Treatment yield ratio Renewable emergy Emergy to construction price Emergy to annual cost Non-renewable emergy Environmental loading ratio Empower density Total emergy to water treated Emergy per inhabitant Unitless Unitless sej/US$ sej/US$ Unitless Unitless sej/yr/m2 sej/m3 sej/p.e. 0.39 0.010 1.86E+12 1.97E+13 0.99 97 1.17E+13 4.57E+11 2.06E+13 0.94 0.044 4.39E+11 4.68E+12 0.96 22 2.70E+12 1.05E+11 4.75E+12 0.12 0.0012 2.63E+12 1.61E+13 0.999 822 9.85E+13 7.70E+11 5.54 + 13 ↑ ↑ ↑ ↑ ↓ ↓ ↓ ↓ ↓ ↑: Largest ratio is preferred; ↓: smallest option is preferred. expensive for this system because of high electricity requirements to run the machinery, and because it was assumed that twice as much labor is required to operate and maintain this facility. One issue not included in the analysis that would greatly favor the CWTS is the design lifetime. This parameter was assumed to be 25 years for the all the options. However, it is claimed that a CWTS can operate for much longer periods, even up to 90 years (Kadlec and Knight, 1996). If a longer design period had been used for the CWTS, the net annual cost would have been lower than the WSPs; for instance, if the CWTS design lifetime was increased to 30 years, the net annual cost would drop down to US$ 12,410, approximately US$ 2000 cheaper than the annual cost of the WSPs. 3.3. Emergy evaluations A summary of the emergy evaluations for the three alternatives is presented in Table 5. The total emergy for the CWTS was 2.88 + 17 sej/yr, 6.65E+16 sej/yr for the WSPs, and 8.86E+17 sej/yr for the SBR. This trend in emergy is similar to the trend in monetary cost, with WSPs having the lowest total emergy and the SBR the highest. This is indeed the trend that was expected, which follows the pattern of goods and services required to treat wastewater in this facilities. The major inputs to the WSPs were found to be electricity and labor, which were also inputs to the CWTS. In addition to these two inputs, gravel played a major role in the high emergy value of goods and purchased services of the CWTS (nearly 80% of the total emergy on this category). This large emergy contribution from the gravel implies that the construction of SSF wetlands with this material is not preferred if appropriate use of resources is considered. Moreover, the considerable larger emergy of the SBR was mainly due to the greater electricity consumption, which was responsible for 71% of the total emergy in goods and purchased services. Fig. 4. Normalized emergy indicators for the three wastewater treatment alternatives; 1 (smallest) represents the indicator for the best option. Author's personal copy M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 1077 Table 7 Comparison of emergy ﬂows and indicators from previous studies. Item 3 Flow, m /day Environmental resources, sej/yr Purchased goods and services, sej/yr Total emergy, sej/yr BOD removed per total emergy, kg/sej/yr TP removed per total emergy, kg/sej/yr TN removed per total emergy, kg/sej/yr Non-renewable emergy, % Environmental loading ratio Empower density, sej/m2 /yr Total Emergy per m3 Total Emergy per p.e. CWTS, this study NW, Swedena CWTS, Swedena CWTS, Mexicob , c CWTS, USAd , c 1728 2.95E+15 2.85E+17 2.88E+17 7.31E−13 4.94E−15 5.53E−14 98.98 97 1.17E+13 4.57E+11 2.06E+13 429 1.25E+16 1.17E+17 1.29E+17 1.00E−13 4.26E−15 1.67E−14 90.4 9 6.00E+11 8.26E+11 1.7E+14 6384 1.26E+16 1.79E+18 1.80E+18 7.69E−14 4.20E−15 1.32E−14 99.3 141 8.20E+12 7.72E+11 1.81E+14 0.5 4.80E+14 8.07E+15 8.55E+15 2.69E−15 1.28E−16 7.71E−16 94.4 17 1.71E+14 4.88E+13 2.38E+14 120455 1.75E+17 3.54E+19 3.56E+19 – – – 99.5 202 2.44E+13 8.09E+11 1.12E+10 CWTS, constructed wetland treatment system; NW, natural wetland. a From Geber and Björklund (2001). b Estimated from Nelson (1998). c Estimated from Behrend (2007) using his estimate of 100 gal/day/p.e. of wastewater. d Wastewater emergy input reported in these studies was not included in the calculations. In comparison to previous emergy evaluations of CWTSs, the system evaluated in this study falls within the range of values previously estimated (Table 7). The total emergy from the system in Tabio was found to be greater than the natural wetland evaluated by Geber and Björklund (2001), but smaller than the combined conventional and wetlands system that these authors evaluated; this is indeed the position where the CWTS proposed in this study was expected to be. The systems evaluated by Nelson (1998) yielded a total emergy value almost two orders of magnitude lower than the system from this study, difference which is rather small considering that the systems studied by Nelson were nearly three orders of magnitude smaller than the CWTS of this study (50 m2 versus 24,670 m2 ). 3.4. Indicators A total of 21indicators were calculated for the three WW treatment systems (Table 6). To make comparison easier, the same results are shown in a series of graphs normalized to the best option for each indicator. Fig. 1 shows the results for the treatment indicators; Fig. 2 shows the results for the cost indicators, and Figs. 3 and 4 show the results for the emergy indicators. The comparison of all the treatment ratios suggests that the CWTS had the best overall performance per emergy and money invested. For the treatment indicators evaluated, the CWTS showed to be the best option for four categories, and even though the WSP system was the best option for the greatest number of treatment indicators (5), the fact that it did not demonstrate phosphorous or nitrogen removal compromises its viability when these constituents are of particular concern. However, if the nutrients in the treated water were to purposely contribute to the needs of agricultural practices in the region, then WSP would be an adequate treatment solution. Cost indicators revealed a more deﬁned distribution in the sense that the WSPs resulted in the best option for all of the ﬁve ratios calculated, followed closely by the CWTS, and last the SBR. The only indicator for which the SBR yielded similar results as the other two systems was for the construction cost per cubic meter of water (1.47), mainly due to this plant’s higher design ﬂow. The emergy indicators suggest that the WSPs are the best overall option from an energetic point of view. The low emergy of goods and purchased services of this system is the reason the WSPs gave better results for several of the emergy indicators. Conversely, the CWTS appeared closer to the SBR ratio than to the WSPs’ for 4 indicators (treatment yield ratio, renewable emergy, non-renewable emergy, and environmental loading ratio). Exceptions to this trend were the results for the emergy to construction price and the emergy to annual cost, where the CWTS and the SBR, respectively, appeared to have the most emergy per money invested. A comparison of the indicators calculated for the CWTS in this study and other treatment wetlands previously documented is shown in Table 7. With regard to the treatment indicators compared (kg BOD/TN/TP per total emergy), the 7.31E−13 kgBOD/sej/yr yielded by the CWTS in Tabio fell in the range of values for the natural wetland and the CWTS evaluated by Geber and Björklund (2001), whereas the ratios of TN and TP to total emergy were slightly smaller than the treatment wetlands in Sweden. In the case of the CWTS, the difference resulted from the additional 6131 kg of TP and 7862 kg of TN that this system removed annually, whereas in the case of the natural wetland, the difference was due mainly to the smaller total emergy from this system. The treatment indicators from the CWTS studied by Nelson (1998) were considerably smaller than the other because of the much smaller mass pollutant load this system treated. With regard to the non-renewable emergy fraction and the environmental loading ratio, the CWTS from this study fell on the lower end of a trend of increasing indicators with ﬂow, which was observed for the municipal CWTSs, while the natural wetland in Sweden (Geber and Björklund, 2001) and the CWTS in Mexico (Nelson, 1998) realized considerably lower values than the municipal CWTSs. The empower density of the CWTS from this study was placed in the middle of the range of values compared, while the total emergy per volume of wastewater and total emergy per inhabitant of this system were placed lowest and second lowest. These comparably low values resulted partially from the design ﬂow and/or population of this system, which were larger than for the systems that yielded similar total emergy ﬂows. 3.5. Limitations The authors acknowledged that this study was limited in a number of ways. First, this study compared two existing plants versus a hypothetical example. It is arguable that making this comparison neglected a number of uncertainties, including changes during detailed design, appropriate construction, treatment performance, and operation. However, the absence of documentation on existing municipal CWTS at the time of this study impeded a more desirable comparison among existing systems. Second, a more robust water quality dataset, with more frequent sampling and a larger number of constituents, would have decreased the uncertainty associated Author's personal copy 1078 M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078 with the limited monthly analyses used. However, existing water quality reports from CAR were used, and performing an intensive water quality analysis for the facilities evaluated was beyond the scope of this study. A third limitation of this study was the use of transformities for the emergy evaluations. The most appropriate analysis would have included transformities generated from emergy evaluations performed in this region to ensure that emergy ﬂows calculated reﬂect the local environment, production, and market. Yet, no other emergy evaluations have been performed and documented in this region, which made the use of other documented transformities the most appropriate alternative for this study. 4. Conclusions This study found that using constructed wetlands to treat municipal wastewater in the Bogotá Savannah is a promising option that should be considered. This recommendation was supported by analysis of three important variables in wastewater treatment technology selection: performance, cost, and resources utilization. A CWTS demonstrated the best performance for investment (i.e., treatment indicators), and the most value for the investment (i.e., emergy to lifetime price). When increased pollutant removal performance is needed (especially nutrients), CWTS should be used instead of WSPs. When land area is available at a reasonable price and wastewater ﬂows are not very large (i.e., rural areas), CWTS should be used instead of conventional treatment facilities that municipalities cannot afford to maintain. The methodology used in this study considered essential aspects for the selection of wastewater treatment technologies in rural areas in developing countries (performance, cost, and resources utilization), and hence it presented a tool that should be used when selecting appropriate technologies that promote sustainable waste management in this region. Acknowledgements Financial support was provided by the University of Florida Scholars Program. Corporación Agrónoma Regional de Cundinamarca provided much of the data synthesized in this evaluation. Wes Ingwersen, Dr. Lynn Saunders, Dr. Matt J. 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