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Arias & Brown 2009. Eco Eng. Feasibility study of using CWTS for municipal WWT in the Bogota Savannah

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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
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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).
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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
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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)
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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).
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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
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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.
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M.E. Arias, M.T. Brown / Ecological Engineering 35 (2009) 1070–1078
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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.
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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.
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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
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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. Cohen, and two
anonymous reviewers gave valuable comments and suggestions on
this paper.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.ecoleng.2009.03.017.
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