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Feasibility of Using Constructed Treatment Wetlands for Municipal Wastewater Treatment in the Bogotá Savannah, Colombia

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Feasibility of Using Constructed Treatment Wetlands for Municipal Wastewater Treatment in the Bogotá Savannah, Colombia
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Arias, Mauricio E.
Brown, Mark T. ( Mentor )
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Gainesville, Fla.
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University of Florida
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Feasibility of Using Constructed Treatment Wetlands for Municipal
Wastewater Treatment in the Bogota Savannah, Colombia

Mauricio E. Arias


ABSTRACT


The management of wastewater is a serious concern for Colombia, especially in large urban areas such as

Bogoti. The main water bodies in this region have been seriously polluted due to the mismanagement of

domestic, agricultural, and industrial wastewater. While there are some wastewater treatment facilities in the

Bogota Savannah, most do not function properly. There is a great need for inexpensive, sustainable, and

less management-intensive wastewater treatment systems. The main goal of this study was to quantify

the performance and sustainability of constructed treatment wetland systems (CTWS) and compare this type

of treatment with the most commonly used in this region. Using data from the literature, a model was

developed, which focused on pollutant removal. The model was used to design a CTWS, and its performance

was compared to a stabilization ponds system and a sequential batch reactor. The three systems were subjected

to economic cost analysis and an emergy (embodied energy) synthesis, leading to evaluation of several indices

of cost benefit for comparison. Results of the analysis suggested that a constructed wetland system of 22,900

m2 can potentially remove 96.8 % suspended solids, 94.6% biological oxygen demand, 97.8 % total coliform,

63.0% total nitrogen, 64.0% ammonia, and 47.0% total phosphorus. The economic analysis suggested that

the lifetime cost of the CTWS is $15,933, compared to $14,200 for the ponds and $54,887 for the batch reactor.

The emergy evaluations show that the lagoon has the lowest annual emergy of 1.10E+18 sej/yr, followed by

the constructed wetland (1.46E+18 sej/yr) and the batch reactor (2.77E+18 sej/yr). These performance,

monetary, and emergy results were combined to estimate several indicators of efficiency and sustainability.



INTRODUCTION


Every day people become more aware of the importance of maintaining a clean environment. This awareness

has resulted in the development of highly sophisticated technology that promises a reduction of society's impacts

on the environment. However, highly technical systems are expensive, and when money becomes a

significant variable, environmental protection goes down in the list of a society's priorities. This is especially true

in developing countries, where people struggle to fulfill their basic needs such as food, health, and

shelter. Promoting the sustainable development of society without substantially affecting the economy are

ideal solutions for the protection of the environment in developing countries and elsewhere.






The use of constructed treatment wetlands systems to treat wastewater (WW) has grown in the past decades

in different parts of the world. This treatment system has many advantages over conventional WW

treatment techniques including: low cost of construction and maintenance, self-sustainability, and habitat

creation. CTWS have all these advantages while removing some of the water pollutants of most concern.



This is confirmed by the deterioration of the environment and the major health problems resulting from the lack

of good WW treatment practices. Rojas (2005), quoting the World Health Organization (WHO), stated that in

Latin America, a hundred children die everyday due to permanent contact with WW.



Colombia is not exempt from the deficit in WW management occurring in developing countries. In the Proceedings

to the Conference of Ecotechnology Applied to Wastewater (2005), quoting the Ministry of Economical

Development, it is stated that in 1999 only 235 of the 1,089 municipalities in Colombia had some kind of

WW treatment system (PTU 2005.) In addition, only 12% of the 237 treatment plants in the country work

adequately (Rojas 2005.) This statement implies that there is not only a great lack of treatment facilities in

the country, but also that the technology currently used may be inappropriate.



Cundinamarca is a province located in the central region of the Colombian Andes. The more than 8 million

people living in Bogota (nearly 20% of Colombia's population) create industrial and agricultural demands that have

a great impact on the surrounding areas. The region around the city is called the Bogota Savannah and is well

known for its large agricultural, dairy, and industrial activities.



Development in the region has created an obvious increase in the demand for water and hence for treatment of

WW. Currently there are 27 municipal WW treatment plants in the region, many of which are stabilization

pond systems (also referred to lagoons). There are a few mechanical treatment facilities in the region,

including activated sludge, anaerobic reactors, and batch reactors. Mechanical treatment requires a high degree

of maintenance, which for the most part has not been done, leading to poor performance of these

expensive technologies. From the foregoing, it should be clear that the region must continue its efforts to improve

its WW treatment systems without increasing dependency on sophisticated WW treatment technologies.

Promoting sustainable wastewater technologies would result in cleaner surface waters, which is a great benefit for

the health, economy, and recreation of the region.



The main purpose of this study was to quantify the performance and sustainability of three WW treatment

systems used to treat municipal WW. This objective was accomplished by: I) developing and using a model

to characterize a CTWS, II) evaluating the monetary costs involved in the establishment and maintenance of

the CTWS, III) evaluating the real wealth of the inputs to these systems using emergy (embodied energy), and

IV) comparing the efficiency and sustainability of CTWS with conventional treatment technologies.


METHODS







Study Sites


The municipality of Tabio is located 50 km northwest of Bogota. It has a population of 14,000, and an

average temperature of 14?C. A stabilization pond system designed to treat 20L/s of WW was constructed in

1992. This plant was built on 3.4 ha, and it consists of a screen, a sedimentation tank, an anaerobic basin, and

two series of facultative ponds with two parallel basins (CAR 2003).



The municipality of La Calera is located 30 km east of Bogota. A WW treatment plant was constructed in this town

in 2002 with a design flow of 36.5 L/s and a design population of 16,000. The system built is a Sequencing

Batch Reactor (SBR) consisting 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). This plant

was chosen as a study site for conventional/mechanical treatment because it is a medium-size facility with a

design flow similar to Tabio's facility, and because it was recently built and hence its performance has not yet

been affected by lack of maintenance.



CTWS Design


The CTWS was assumed to be placed in Tabio where the existing plant is. It would use an existing

screen, sedimentation tank, and anaerobic basin, but facultative lagoons were replaced with a combination

of Subsurface Flow (SSF) and Surface Flow (SF) wetland units.



In order to size the proposed system, influent and effluent water quality parameters were needed. CAR

provided water quality data for the wastewater coming into the existing plant. The goal for the effluent water

quality was 20 mg/L of BOD, and SS, 1000 fecal coliform/ml, and 10 mg/L TN. The fecal coliform goal reflects

the WHO guideline for use of WW for unrestricted irrigation (Blumenthal et al. 2000.) These concentrations do

not reflect the Colombian regulations for discharge from WW treatment plants, since the existing regulation

requires only the removal of 80% of BOD, SS, and fecal coliforms.



Different factors were taken into account for the CTWS system area estimate. One is the fact that the area

was limited to the 3.40 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.

Moreover, considering the pollutants' high loading to be treated in this limited area, it is not likely that just an

SF wetland would have provided sufficient treatment. Likewise, using only a SSF wetland would have resulted

in elevated costs due to the media required.



Once the proper area and combination of CTWS units were found, the effluent concentration at each unit

was calculated using the Kadlec and Knight k-C* Model (1996):







Ce = C* +(Ci - C*)exp .0365Q
0.0365Q)


Where
Ce = outlet concentration, mg/L
Ci = inlet concentration, mg/L
C* = background concentration, mg/L
k = first-order areal rate constant, m/yr
A = wetland area, ha.
Q = water flow rate, m3/d


The pollutant removal efficiencies of the pre-treatment and primary treatment units were not modeled, but
assumed from typical values found in the literature (Ramirez & Romero 1997, Tchobanoglous et al. 2003).


In addition to the area the volume (V), loading rate (q), and hydraulic detention time (HRT) were calculated:





V=A*h


Where h = mean depth.



Q
A

V nV
HRTSF = -, or HRTssF = -
Q Q


Where q= porosity of media.


Cost Evaluations

A cost evaluation was performed for each of the treatment options studied. These evaluations included the
major components of construction cost, and operation and maintenance (O&M). All costs were calculated for
2003 Colombian pesos, and then converted to U.S. dollars using the average exchange rate for that year.






Most unit costs for the CTWS and the lagoon came from a Colombian construction costing guide (Construdata

2003), from CAR, or estimated when necessary. The construction costs for the SBR were extracted from a

budget requested by CAR in 2000 (Aquavip 2000). Only costs related to the construction of the treatment units

were used from this budget. O&M costs for this facility were acquired from CAR documents or assumed

when necessary.


Moreover, a lifetime cost of treatment was estimated:




Construction
Lifetime.cost = O&M +
lifetime



Where lifetime cost, construction, and O&M are in 2003 dollars. Lifetime refers to the facility design lifetime,

assumed to be 25 years for the three systems.


Emergy Evaluations


Prior to the numerical evaluation, it was necessary to draw the systems diagrams for the three options studied.

These diagrams show the main energy fluxes and storage for a given system and the interactions among them.

The flows in and out of the systems diagrams were the basic components analyzed in the emergy evaluation tables.


Items in the evaluation 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 evapo-transpiration.


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 CTWS performance analysis, the cost analysis,

CAR reports, journal articles, or assumed when needed. Transformities, specific emergies, and emergies per

unit money came from Environmental Accounting, Handbook of Emergy Evaluation, journal articles, and

other emergy evaluations compiled at the Center for Environmental Policy at the University of Florida,

Gainesville (Odum et al. 1983, Odum et al. 1987).


Each table includes a calculation of the total emergy of environmental resources, total emergy of goods

and purchased services, and total emergy. The total emergy of environmental resources is the sum of the

maximum empowers of each of the groups of elements from a common source. For instance, sunlight, wind, and





rain come from the same geobiospheric baseline (Odum 1996), and all the constituents of WW belong to the

same source. The emergy of goods and purchased services is just the sum of all the inflows within that category,

and the total emergy is the sum of the total environmental resources and the total goods and purchased services.



Indicators


Once the performance, cost, and emergy evaluations were done, several ratios or indicators were calculated in

order to compare the main results found for the three options studied. These indicators were divided into

three categories according to the unit in their denominator. The first 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 water quality data was available for TN for the lagoons and the SBR. The second 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 in essence show how

emergetically expensive each alternative was. See Appendix 1 for a definition of each indicator.



Results


CTWS Design


The proposed system is composed of the existing pretreatment units and primary anaerobic lagoon, followed by

one series of SSF and one series of SF. The total area was maintained at 3.4 ha, 2.2 of which are for the SSF and

the SF basins. The detail design parameters and pollutant concentrations for this system are shown in Figure 1.

The total system performance is compared with the same type of data for the ponds and the SBR in Table 1.



The distribution of the available area between the SSF and SF CTWS was subject to a sensitivity analysis.

Initially, the total area available was split equally between the two series. However, a 1.1 ha SSF resulted in a

large amount of gravel used, which would lead to a great impact in the cost and emergy evaluations. Therefore,

the area of SSF was optimized so that the effluent concentrations of SS and BOD still met the goals set.

In comparison with the initial set up, the final SSF unit area of 0.7 ha yielded savings of $45,040 and 2.0E+17 sej/yr.






Anaerobic Lagoon

Area - 2900 m2
Deplh= 1.8 m.
Detention tine = 3 days
Loading rate = 60 cm1day
%
influnTl Rc rr alx
ss 323 65

BOD 362 50

Go 2.5E+07 64

Tm 40.5 20

NHI 24.3 20

TP 4.81 20


SSF CTW

Area - 7000 m-'
Depth = 0.6 m
Detention time = 0.97 days
Loading rale = 25 n il d.N
%
Lrdulucrl Remos, ral
SS 113 86.8

BOD 181 80.2

Co 9.DE+06 65,2
TN 32.4 24.7

NIL 19.5 31.4

TP 3.85 12.4


Figure 1. Design parameters, pollutant concentrations, and percent removal at each stage of the

CTWS system. All concentrations expressed as mg/L, except for E. Coli (#/lOOmL).



Cost Analysis


Table 1

Total percent pollutants removal for the three WW treatment systems evaluated


Constituent Constructed Wetland

SS 97.0


Stabilization Ponds Sequential Batch Reactor


BOD


E. coli 97.8

TN 63.0


Cost Analysis


The complete cost tables are presented in Appendix 2. Construction cost for the CTWS was estimated to be

$160489, O&M cost was $8473, and the total annual cost was $14893. The pond was found to be cheaper, with

an annual cost of $13164. SBR was found to be the most expensive option, with an annual cost of $54887. Table

2 shows the summary of the cost evaluations for these three options.



Table 2

Summary of Cost Analysis


Units Constructed Stabilization Sequential
Wetland Ponds Batch Reactor


SF CTW

Area 14770 m2
Depth = 0.45 m
Detention lime = 3.9 days
Liading, rate = 12 cm/day
%
LIlrctlit lliuiirn Rcirmoa
SS 14.9 9.7 35.3

BOD 35.9 19.5 45.7

c 3.1E+06 5.4E+05 82.7
TN 24A 15.0 38-5

NH 13.3 8.75 34.4

TP 337 2.55 24.4


Costs






Construction Cost $ 186513 151548


Operation Cost $/yr 8473 8139 41394


Lifetime Cost $/yr 15933 14201 54887


Emergy Evaluations


The detailed evaluations including all the calculations are attached in Appendixes 5, 6, and 7. The diagrams for

the CTWS, the lagoons, and the SBR are shown in Figures 2, 3, and 4, respectively. From these diagrams, it

was found that the total emergy for the CTWS is 1.46E+18, 1.10E+18 for the pond, and 2.77E+18 sej/y for the

SBR. The major results of the three emergy evaluations are summarized in Table 3.


Figure 2. Constructed wetland system diagram


337318









































Figure 3. Stabilization ponds system diagram






Figure 4. SBR system diagram


Table 3.

Summary of emergy evaluations


Constructed
Emergy Type Units Wettand Lagoon SBR
Wetland


Emergy of Environmental

Resources


Emergy of Wastewater


Emergy of Goods and

Purchased Services


Emergy of System Outputs


sej/yr 1.76E+15 1.76E+15 6.41E+14



sej/yr 1.04E+18 1.04E+18 1.89+18



sej/yr 4.20E+17 6.35E+16 8.85E+17


sej/yr 3.83E+17


6.92E+17 1.51E+18


Total Emergy


sej/yr 1.46E+18 1.10E+18 2.77E+18


Indicators


Table 4 presents a summary of all the indicators calculated for the three systems evaluated. To make

comparison easier, the same results are shown in a series of graphs. Figure 5 shows the results for the

treatment indicators; Figure 6 shows the results for the cost indicators. In Figures 6, 7, and 8, a value of 1

represents the ratio for the best option, and the values for the other options represent the normalized deviation

from the best option (e.g., 2 means twice the value for the best option).


Table 4

Summary of indicators treatment, cost, and emergy indicators


Treatment Ratios

BOD removed per lifetime cost


SS removed per lifetime cost


E. coil removed per lifetime cost


TP removed per lifetime cost


BOD removed per total emergy


SS removed per total emergy


E. coil removed per total emergy


TP removed per total emergy


Units

kgBOD/$


kgSS/$


CTWS

13.6


12.4


9.68E+1 1


kgP/$


0.089


kgBOD/sej 1.48E-13


kgSS/sej


#/sej


kgP/sej


1.36E-13


1.06E-02


1.68E-14


Cost Ratios


Preferred




1


1.10E+12


-0.075


1.78E-13


1.46E-13


1.43E-02


-1.49E-14


1.99E+11 T


5.35E-14


3.99E-14


3.94E-03


3.86E-15 T





Lifetime cost per m3 of water $/m3/yr 0.0253 0.0225 0.0477

Operation cost per m3 of water $/m3/yr 0.0134 0.0129 0.0360

Construction cost per m3 of water $/m3 0.2957 0.2403 0.2930

Lifetime cost per capital $/p.e/yr 1.3 1.2 3.4

Construction cost per capital $/p.e 16 12.6 21

Emergy Ratios

Treatment Yield Ratio unitless 1.55 5.40 0.43 1

Renewable emergy unitless 0.712 0.942 0.681 1

Construction emprice sej/$ 7.81E+12 7.26E+12 8.23E+12 T

Lifetime emprice sej/$ 9.14E+13 7.75E+13 5.06E+13 1

Non-renewable emergy unitless 0.29 0.06 0.32

Environmental loading ratio unitless 0.405 0.061 0.47

Empower density sej/yr/m2 5.91E+13 4.48E+13 3.08E+14




BOD removed per lifetime cost
TP removed per total eme-uy t- . S removed r er lifetime cost
0-
B. coi remved vt totl emegy ~ d t " ' . d emvd0r ibiiccs


SS removed per total


i~CWl
I/ I-c-
_-- removed per lifetime cost --- Pond
lemergy I-- SBR I


BOD removed per otau


Figure 5. Normalized treatment indicators. 1 represents the indicator for the best option.






Lifetime cost per m3 of water


Construction cost per capital











liretimerc cost per


Operation cost per m3 of water





- Pond
--SBR


Figure 6. Normalized cost indicators. 1 represents the indicator for the best option.


Trcatmnt Yield Ratio
1la


lifetime cmpricc


renewablc emncrgy







Pond
SBR


cunrikuiron cmprnit



Figure 7. Normalized emergy indicators. 1 represents the indicator for the best option.





~~ier~y
8.


Em~o~w deiflity Environmen l fading mratio


Figure 8. Normalized emergy indicators. 1 represents the indicator for the best option.




DISCUSSION



CTWS Design


The results showed that it is feasible to design a CTWS system in an area of similar dimensions to the ones used

to site stabilization ponds in this region. Moreover, the existing lagoons could be upgraded to CTWS systems

once existing facilities reach their design flow or lifetime. A significant part of the existing plants (pretreatment

and primary treatment) could be utilized as part of a CTWS system. The only concern with the design

parameters calculated is that some of them fall out of the typical range of design values for these systems (Kadlec

& Knight 1996). 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 CTWS in sub-/tropical weather

have performed adequately even with designs parameters out of these recommended ranges (Yang et al.

1995, Greenway & Woolley 1999).



With regard to the CTWS performance, the model showed that the goal of an effluent with 20 mg/L of BOD and

SS can be achieved. However, the goal of 1000/100 ml fecal coliform is not likely to be achieved based on the E.

coli concentration in the effluent. It should be noticed that the goal proposed reflected WHO recommendations for

the most stringent type of irrigation, which includes sports fields, public parks, and crops eaten raw (Blumenthal

et al. 2000). Although the effluents from WW facilities in this region may be reused in such a way, it is also

likely that these effluents qualify for a less stringent irrigation category such as industrial crops and pastures. For

this type of irrigation, the original WHO guidelines do not set any limit, although recommendations to

these guidelines propose 105/100mL fecal coliform, which is a concentration in the same order of magnitude as

the one calculated for E. coli in the CTWS effluent.



In comparison to the performance of the lagoons and the SBR, the CTWS is expected to achieve better





overall pollutant removal efficiency. Not only did the CTWS shows a better performance in removing constituents

of typical concern (BOD and SS), but also it addressed nutrients, which seems to be an issue not controlled with

the existing treatment systems. Bacterial removal is the only exception to the overall best performance of the

CTWS. For this parameter, the lagoons achieve a higher efficiency. Although the difference in performance is

not large, one of the reasons affecting this may be the higher detention time due to greater storage volume in

the lagoons.



Cost Analysis


In short, the CTWS system is more expensive than the lagoons, but the costs of these two options are nearly half

the cost of the SBR. The difference in costs between the CTWS and the lagoons is mainly caused by the gravel

media used in the SSF unit ($81,152 for the 0.7 ha). The fact that the gravel is the most expensive item implies

that the potential of using SSF in a large system is limited, and minimizing its use can result in great cost

reduction as demonstrated with a sensitivity analysis. If land is available and not expensive, a system with mainly

SF would be more economically feasible. Other alternatives to the gravel issue would be to use a different type

of media. For instance, pieces of bamboo were used as media for a SSF in Pereira, Colombia, achieving similar

results as gravel (Castaho 2005).



In addition to the absence of gravel, the lagoons resulted in overall less cost than the CTWS to a minor

extent because no seedlings or harvesting are needed. Yet, the cost of earth work is much greater for the

lagoons, contributing 55% of the total cost of construction. The SBR was found to be the most expensive

option, with a construction cost of $337,318, O&M of $413,934, and total annual cost of $54,887. The

major components of the construction that cause this increased cost are concrete and steel, plus the fact that

there are several other materials required for the construction of this system and not for the other two.

Moreover, O&M cost is much more expensive for this system because there is a great electricity consumption to

run the machinery and because it was assumed that twice as much operation is needed for this facility.



One issue that was not addressed as part of the analysis, but would greatly favor the CTWS system, is the

design lifetime. This parameter was assumed to be 25 years for the all the options. However, it is claimed that

a CTWS system can operate for much longer periods, even up to 90 years (Kadlec & Knight 1996). If a longer

design period had been used for the CTWS, the total annual cost would have been lower than the lagoons. In fact,

if the CTWS design lifetime is increased to 30 years, the annual cost would drop to $12,410.



Emergy Evaluations


In general,the trend in emergy is similar to the trend in cost, with the lagoons having the lowest total emergy

and the SBR the highest. Although the major emergy contribution to the SBR was expected to come from goods

and purchased services, renewable emergy (environmental resources and wastewater) demonstrated to be

the largest one. This occurred mainly because the design flow of this system (36.5 L/s) is larger than the flow for

the other two systems (20 L/s), increasing greatly the amount of organic and nutrients into the system.








In a similar way to the cost analysis, gravel played a major role in the high emergy value of goods and

purchased services of the CTWS (nearly 80% of the emergy on this category was contributed by the gravel).

As mentioned previously, a small decrease in area of SSF had a great impact in the emergy value of gravel.

Another factor that could potentially decrease the gravel emergy contribution as well as the contribution from

the other goods (seedlings, geomembrane, concrete, and piping) is an increase in the design lifetime.



Indicators


The comparison of all the treatment ratios suggests that the CTWS realizes the best overall performance per

emergy and money invested as demonstrated by the results of the treatment indicators. Even though the

lagoons have the best results for five of these indicators, the fact that this system does not remove

phosphorous affects its overall performance. Moreover, the CAR water quality report shows a negative

performance for NH4 in this pond, which would have a further negative impact in this system's overall

performance. All of the SBR indicators were relatively far from the best ratios not because of low performance,

but because of higher cost and higher emergy.



Cost indicators revealed a more defined distribution in the sense that the lagoons resulted in the best option for all

of the five ratios calculated, followed closely by the CTWS, 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 m3 of water (1.47), mainly

due to this plant's larger design flowrate.



The emergy indicators suggest that the lagoons are the best overall option from a sustainability point of view.

The lower emergy of goods and purchased services of this system is responsible for this system having much

better results for several of these emergy ratios. Conversely, the CTWS system appeared closer to the SBR ratio

than to the lagoons for four indicators: treatment yield ratio, renewable emergy, non-renewable emergy,

and environmental loading ratio. Exceptions to this trend were the results for the lifetime and the

construction emprice, where the CTWS and the SBR, respectively, appeared to have the most emergy per

money invested. The CTWS also resulted in a similar empower density to the lagoons (5.93E+13 versus 4.51E

+13 sej/yr/m2), while the SBR has empower density nearly six times higher than the other two (3.08E+14 sej/

yr/m2).






ACKNOWLEDGEMENTS


Thanks to Dr. Mark T. Brown, the University Scholars Program, Mr. Federico Arias, Corporaci6n Agr6noma

Regional de Cundinamarca, Dr. Matt J. Cohen, and Dr. Angela S. Lindner.








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