Journal of Undergraduate Research
Volume 8, Issue 1 - September / October 2006

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

Mauricio E. Arias

ABSTRACT

The management of wastewater is a serious concern for Colombia, especially in large urban areas such as Bogotá. 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 Bogotá 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 Bogotá (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 Bogotá 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 Bogotá. 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 Bogotá. 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):

Where
C_e = outlet concentration, mg/L
C_i = 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:

Where h = mean depth.

Where η= 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:

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 storages 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.

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 (#/100mL).

Cost Analysis

Table 1 Total percent pollutants removal for the three WW treatment systems evaluated
Constituent	Constructed Wetland	Stabilization Ponds	Sequential Batch Reactor
SS	97.0	78.6	73.0
BOD	94.6	85.7	78.7
E. coli	97.8	99.5	84.1
TN	63.0	-	-
NH4	64.0	- 8.39	-
TP	47.0	-34.9	2.9

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
Costs	Units	Constructed Wetland	Stabilization Ponds	Sequential Batch Reactor
Construction Cost	$	186513	151548	337318
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

Figure 3. Stabilization ponds system diagram

Figure 4. SBR system diagram

Table 3. Summary of emergy evaluations
Emergy Type	Units	Constructed Wetland	Lagoon	SBR
Emergy of Environmental Resources	sej/yr	1.76E+15	1.76E+15	6.41E+14
Emergy of Wastewater	sej/yr	1.04E+18	1.04E+18	1.89+18
Emergy of Goods and Purchased Services	sej/yr	4.20E+17	6.35E+16	8.85E+17
Emergy of System Outputs	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	Units	CTWS	Pond	SBR	Preferred
BOD removed per lifetime cost	kgBOD/$	13.6	13.8	2.7	↑
SS removed per lifetime cost	kgSS/$	12.4	11.3	2.0	↑
E. coli removed per lifetime cost	#/$	9.68E+11	1.10E+12	1.99E+11	↑
TP removed per lifetime cost	kgP/$	0.089	-0.075	0.055	↑
BOD removed per total emergy	kgBOD/sej	1.48E-13	1.78E-13	5.35E-14	↑
SS removed per total emergy	kgSS/sej	1.36E-13	1.46E-13	3.99E-14	↑
E. coli removed per total emergy	#/sej	1.06E-02	1.43E-02	3.94E-03	↑
TP removed per total emergy	kgP/sej	1.68E-14	-1.49E-14	3.86E-15	↑
Cost Ratios
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 capita	$/p.e/yr	1.3	1.2	3.4	↓
Construction cost per capita	$/p.e	16	12.6	21	↓
Emergy Ratios
Treatment Yield Ratio	unitless	1.55	5.40	0.43	↑
Renewable emergy	unitless	0.712	0.942	0.681	↑
Construction emprice	sej/$	7.81E+12	7.26E+12	8.23E+12	↑
Lifetime emprice	sej/$	9.14E+13	7.75E+13	5.06E+13	↑
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	↓

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

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

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

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 (Castaño 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 organics 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).

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ACKNOWLEDGEMENTS

Thanks to Dr. Mark T. Brown, the University Scholars Program, Mr. Federico Arias, Corporación Agrónoma Regional de Cundinamarca, Dr. Matt J. Cohen, and Dr. Angela S. Lindner. 

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APPENDICES

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