Servicios personalizados
Inglés (pdf)
Articulo en XML
Referencias del artículo
Enviar articulo por email
Citado por SciELO 
Similares en
SciELO 
Permalink
Introduction
Wastewater is a threat to urban communities. Artificial or constructed wetlands (AW or CW) are systems emulating the same chemical processes that take place in the natural wetlands, in order to remove pollutants, particularly those dissolved in the water, trying to avoid its sending to natural ecosystems.1 Nowadays, artificial wetlands are an excellent option for developing economies urban cities for the depuration of its wastewaters, either coming from small suburbs with scarcity of funds 2,3 or for more affluent communities, since this technology solves the same kind of problems.
In some cases a support media may be included to provide a surface for the formation of microbial biofilms and to maintain the hydrophytes vertical.4 The systems may operate according to its water flow, either vertical or horizontal. Also, water may flow on the surface or some distance under the support media, and the AW are named superficial or sub-superficial. When horizontal and vertical AW are combined, they are called hybrid systems and the main aspect of these new versions is to improve its efficiency reducing the necessary area per equivalent inhabitant, that is one of the limiting factors of these systems.5-7
Depending upon the characteristics of the microbial community present, the system may remove from water heavy metals 8-11 or organic matter 12 and organics toxic.13-15 The aquatic plants play a crucial role thanks to photosynthesis in the daytime 16-18, as well as removing nitrogen.19-21
An international group of researchers has joined efforts to implement these artificial wetlands, both in small communities and small neighbourhoods within big urbanizations. One of the examples was to support communities to build their own systems, as well as to build demonstration facilities located in Mexico City at the end of the XX Century. One is still in a suburb called Coyoacán, in a Vivarium with a horizontal sub-superficial flow, and a second one with a vertical sub-superficial flow at the University City grounds. The second one has been moved to a senior high school belonging to UNAM, also in the University City, changing its flow to horizontal. The present installation has as a main aim to create an environmentally conscious mind among the young students (between 15 al 18 years old), as well as to be a prototype to be followed by other UNAM installations.22
The group has extended the research activities to the South Eastern region of Mexico to serve the population of a small community near the city of Puebla.23 In the same Mexican state, there are some pilot installations in the region of Izúcar de Matamoros.24-25) A hybrid artificial system is to be built in the neighbourhood of San Martín Alchichica, near Izúcar de Matamoros. The community of San Martin Alchichica has provisions for growing up to 2000 inhabitants approximately. The Universidad Tecnológica de Izúcar de Matamoros has been carrying out research with different types of artificial wetlands since 2002.24-25
As the cost of these systems is much lower than the traditional WWTP, the case of Santa María Coatepec El Seco, Puebla, Mexico 23 was taken as example. The additional benefits of using gravity to receive the waste waters reduces or even eliminates the costs not only of equipment but in operation and maintenance. In this case, considering the appropriation of the technology by the community, when their inhabitants participate in its construction, starting-up, and operation through communal work is an added value.
The objective, for the two case studies presented here, is to solve the threats in Mexico City and Havana City due to polluted water, using artificial wetlands as a key point considering the fact that urban neighbourhoods confront the same type of problems than rural communities, such as the two examples mentioned above from the state of Puebla, Mexico, and that are applicable to any country.
Development (methodology)
Case of study: Mexico City
Mexico City is located in what it used to be a series of interconnected lakes.26 As human populations started inhabiting the area, constructions were held to separate salty marshes from the rest of them, on the Nezahualcóyotl time, 14th century (figures 1a-b, 2a).27
Presently, the ecocide caused after the Spanish conquest of the Aztec empire, made a valley of that basin, losing all the ecological characteristics that sustained the life of an estimated population of 10 million people that were quickly decimated by diseases, hunger, and violent death (figure 2b).26-28
A vivid example of this ecocide was presented in the first chapter of a recent book on artificial or constructed wetlands 1 showing that badly treated wastewaters dumped in the remains of the Xochimilco Lake need not really traditional wastewater treatment plants, WWTP, to restore the quality of the water used to refill it, but artificial wetlands, AW.29
On the other hand, drinking water has to be pumped in from the Michoacán and Mexico states to provide it to the population, roughly the same as in the Aztec times before the conquest, nine millions.30 As the volume pumped is insufficient, subterranean water from aquifers and springs is used, creating its over exploitation.31) Consequently, the city is sinking at a very high rate.31
There are areas of Mexico City that have no drinking water facilities and water has to be provided using cistern tank trucks, colloquially known in Mexico as “pipas”.
The figure 3 presents some data from the official Instituto Nacional de Estadística y Geografía, INEGI32, that although are almost 20 years old only depict the huge problem. It is clear that the Tlalpan municipality or county (alcaldía in Spanish) suffers from this problem. It is precisely the place that traditionally had most springs and rivers. Unfortunately, during these five centuries due to the logging and invasion of this land located in the southern part of Mexico City to build households, especially for low and very low income people, the area is no longer receiving the rain water and storing it in the subsoil. These households have no drinking water network due to the geographical conditions of the Ajusco area, with an altitude of 3,930 m (deep slopes from the Sierra Ajusco-Chichinautzin volcanic mountain ranges, figure 2b).26
Tlalpan inhabitants dispose of their wastewaters directly to the ground, that is from volcanic origin, giving place to the mixing of the wastewaters with the subterranean aquifers polluting all the water sources.32 INEGI statistics 32 indicate that, for the particular case of Tlalpan is the third place in problems of sanitation. The sad first two places are for Alvaro Obregón and Magdalena Contreras municipalities (alcaldías), west of Tlalpan (figure 4).
The last but not least important problem of Mexico City Metropolitan Area, MCMA is that it is located in a seismic area.33-36 Very slight earthquakes occur every day that only the devices of the Mexico’s National Seismological Service can appreciate. As the whole water network for drinking water and for wastewaters is subterranean the joint surfaces of the piping systems become leaky and release the vital liquid in the case of drinking water piping, and the not so clean wastewaters too. Sadly, these leaks cannot be easily observed. The losses of drinking water due to these leaks are quite considerable. Some estimates indicate that they might be up to 30% of the total flow.30
Fig. 3 Deficit of drinking water in Mexico City households: Tlalpan municipality with 822 households receiving drinking water from street hydrants, 11,288 from cistern tank trucks or “pipas”, and non depicted is the hugely reduced number of households that have a well or take it from a creek since there no rivers anymore and even less lakes.37
Fig 4. Number of households with deficit of sewage systems: The Tlalpan municipality has 1,652 households dumping wastewaters to creases or cracks (grietas in Spanish), 791 just put outside of their homes in the street, and 110 to former bed rivers or bed lakes.37)
Considering the solution of using artificial wetlands to avoid polluting aquifers with wastewaters of these 2,553 households in the next paragraphs there are some chemical data to support it.
Solutions for the case of mexico city´s tlalpan municipality with drinking water supply deficit and water sewage treatment systems deficit
As mentioned in an international presentation: 29
“First of all, awareness of the problem is the first step to be considered. The majority of the inhabitants of Mexico City are not conscious that they are all part of the problem but also of the solution. Most of them simply expect to receive drinking water from the local government, preferably free of charge and of good quality. Also, the majority completely ignore the costs associated to bring water to the Mexico City Metropolitan Area, MCMA. These costs are not only monetary but ecological and social since several basins in the states of Michoacán and Mexico have being deprived of their clean water.30 ... On the other side, the water of the rivers that originally were to be discharged to the Pacific Ocean is now used in the MCMA, loaded with enormous amounts of pollutants ranging from organic matter from households but also with toxic pollutants from the industrial large area located in this former basin, plus those of hospitals, and other numerous services, and sent to the state of Hidalgo where it has been used to irrigate large cultivation areas, the so called Valley of Mixquiahuala, and then finally discharged in the Gulf of Mexico.31”
The problem is certainly hard. Drinking water should be provided at prices in accordance with the effort to obtain it but, at the same time, being a public and human right to have clean water it is rather difficult to establish just prices for those persons that can pay it and to those that cannot. Also, if the population acquires the sensitivity to understand this issue, the inhabitants of Mexico City Metropolitan Area will be able to use this vital liquid in a sensible manner. Besides, the minimization of wastewaters dumping and its mixing with the rain water has become a very important issue, particularly with climate change phenomena that increase the intensity of rainfall in Mexico City due to meteorological problems in both coasts, the Pacific Ocean one and the Gulf of Mexico and the Atlantic one (Caribbean Sea) requiring that the whole population be aware of the effects of the use of chemical products to enhance body and clothing visual characteristics (shampoos, lotions, rinsing liquids, etc.), or to have in the refrigerator or in the kitchen food items for a very long time that should contain preservation chemical compounds.
The figure 5 presents a very simple diagram of the transformation of the pollutants present in the wastewaters into innocuous materials. It is clear that the biochemical methods are environmentally friendlier than the chemical ones that need reagents to precipitate the pollutants in order to separate them from the water as solid materials. The organisms, either microbial communities or micro- and macrophytes, are the reagents, the reactors, and the products, and can be efficiently used in this context.34 In figure 6, a scheme of a horizontal sub-surface flow artificial wetland system with Phragmites australis as the macrophyte is shown.34 It is clear that an impermeable membrane (made of a geotextile material or with natural clays) to isolate wastewater from the surrounding environment is required. It also includes a settling tank as a primary treatment. A secondary tank collects treated wastewater (effluent) with a hose that controls the flooding height within the reactor.34 The by-products from primary treatment together with the excess biomass may be further treated obtaining a solid material (compost). These might be used as a fuel, recovering the ashes as stable pollutants to be disposed of in an ecological manner.35 Three prototype artificial wetlands systems have been designed, constructed, started-up, and operated by UNAM authors (figures 7a,b,c).
Fig. 6 Horizontal sub-surface flow artificial wetland system with an impermeable membrane (including a geotextile material) to isolate wastewater from the surrounding environment. It includes a settling tank as a primary treatment, and a secondary tank to collect treated wastewater (effluent) with a hose that controls the flooding height within the reactor 34
Two of them still remain and are a good example of how resilience politics may become useful to overcome the problems of small nuclei of population even in megacities such as Mexico City. Its design follows the chemical characteristics of the wastewaters being received and the desired treated effluents should comply with the official acts.
As an example of the expected values of the wastewaters and treated effluents (Figures 8a,b, Table 1) some data collected from the experimental phase of the subsuperficial descending vertical flow artificial wetland located at UNAM Cultural Zone 23) are presented.
Fig.8a. Behavior of total chemical oxygen demand data from February until October (data are weekly average values for daily measurements) 23
Data show the differences between two hydraulic residence times (4.8 versus 5.4 days) . The interesting feature is the relative homogeneity of the effluent values for chemical oxygen demand: 450 L d-1 with HRT of 4.8 d, obtaining 80% CODtotal removal versus 400 L d-1 with HRT of 5.4 d, and 73% CODtotal removal. The difference observed for the CODtotal influent data may be due to the rainy season, since statistical analyses (p<0.05) do not indicate statistical differences with an average of 77% CODtot removal. In Table 1 some other experimental data for the artificial wetland exemplified are presented, such as temperature, pH value, electric conductivity, total chemical oxygen demand, helminth eggs, fecal coliform organisms, and Salmonella spp. (using three different media).
Although data indicate a good performance, concerning international acts, microbiological data are not considered in the first two Mexican acts, one for wastewater dumped into national sources of water and soils 36, and the second one listed for treated wastewater to be reused.37 For microbiological parameters and helminth eggs these are not considered except for biosolids or sludges.38 Also, the three of them do not consider chemical oxygen demand but only the first one considers biochemical oxygen demand, and when toxic substances are present this value is not reliable. For this reason, in the UNAM experimental analyses only chemical oxygen demand is taken into account.
This second artificial wetland was substituted by the third one, due to the construction of buildings for the UNAM graduate programs in the site where it was located. Figure 7c depicts this relatively newer prototype installed in another UNAM institution. Its interesting feature is that it receives wastewaters not only from sanitary facilities but from the laboratories for experimental teaching of chemistry, biology, and physics. Its results are quite interesting as Table 2 shows the effect of the chemicals on wastewater that impair the performance of the wetland to remove coliforms and helminth eggs complying only with its use without direct contact.27
Table 1 Comparison among physical, chemical, and microbiological parameters obtained in the subsuperficial descending vertical flow artificial wetland using the pertinent Mexican acts
| PARAMETER | Average value for the artificial wetland | DOF (1996) [36] (a) | DOF (1997) [37] (b) (c) | DOF (2002) [38] (d) | ||||
|---|---|---|---|---|---|---|---|---|
| Influent | Effluent | Monthly average value | Daily average value | Monthly average value | Daily average value | A, B | ||
| T [°C] | 23.4 | 19.8 | 40 | 40 | NA | NA | NA | |
| Value of pH | 7.31 | 7.14 | NA | NA | NA | NA | NA | |
| Electric conductivity, EC [μmho cm-1] | 2090 | 1926 | NA | NA | NA | NA | NA | |
| Dissolved oxygen, OD [mg/L] | 0.47 | 1.83 | NA | NA | NA | NA | NA | |
| CODtot [mg/L] | 610 | 138 | NA | NA | NA | NA | NA | |
| Helminth eggs [number/L treated wastewater] | 6 | 1 | NA | NA | NA | NA | ≤1, ≤10 [number/g db] | |
| Fecal Coliform Organisms [MPN/100 mL] | 1.01x104 = 10,100 | 2.006x101 = 20 | NA | NA | 240 | 1000 | (1000 [MPN/g db] | |
| 128x103 = 128,000 | Not present | NA | NA | NA | NA | ( 3 [MPN/g db] | ||
| 187x103 = 187,000 | 1.0x102 = 100 | NA | NA | NA | NA | ND | ||
| 186x103 = 186,000 | Not present | NA | NA | NA | NA | ND | ||
ND = Not determined, NA = Not applicable
MPN = Most probable number and CFU = Colonies forming units [39], SS Agar = Salmonella-Shigella Agar, XLD Agar = Xylose-Lysine-Desoxycholate Agar, MacConkey agar (MAC) was the first solid differential media to be formulated which was developed at 20th century by Alfred Theodore MacConkey. MacConkey agar is a selective and differential media used for the isolation and differentiation of non-fastidious gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas
Use in irrigation (rural areas)
Services to people with direct contact
Services to people with indirect or occasional contact
Allowed maximum limit of biosolids and sludges (A: Urban use without direct contact with people during its application; B: Urban use with direct contact with people during its application) (db = dry basis)
Table 2 Comparison among microbiological parameters obtained in the newer subsuperficial horizontal flow artificial wetland using the pertinent Mexican act 37,39 (data average from 12 samples taken along 7 months)
| Use without direct contact with people during its application | Use with direct contact with people during its application | Influent data | Effluent data | |
| Fecal coliform organisms [MPN/100 mL] | 1,000 | 240 | 2375.0±8.300 | 965.0±4.5 |
| Helminth eggs [number/L treated wastewater] | 5 | 1 | 3.925±0.072 | 2.80±0.13 ( 3 |
Concluding remarks for the México City’s example: Tlalpan
The installation’s costs of an artificial wetland system for a household may range from 3-6 times the minimal wages monthly salary.40
These costs, for most family dwellings in Tlalpan, are unaffordable since they cannot save money to build them. In these cases, the government should help them acquiring the materials and the people should give their hand labour to construct the systems so that they really become involved with them. In this way they will appreciate the effort and will start having this awareness of the problem and how to solve it.
The majority of the inhabitants of Mexico City that are not conscious that they are all part of the problem will become part of the solution.
Again using the material of a previous presentation: 29
In parallel, the social groups in each “colonia” (a division within the political territorial area in Mexico City) should start step by step bringing out a solution, especially those that are lucky to have drinking water and sanitation:
(a) every time there is a leak detected from the subterranean areas where the network systems are located to have the care of disinterring it for avoiding further leakages caused by the telluric daily movements, especially with the drinking water;
(b) for the sewage, every time a leak is detected, to start up the segregation or separation of rain water from sewage with two different types of piping, and with this solution greatly reducing the flow rate and encouraging the reuse of rain water for irrigation of green areas and the filling out again of river beds, channels, lakes, ponds, fountains, etc., for recovering the beauty of the city and tempering again the climate 41; and
(c) the sending of the separated wastewaters to environmentally friendly systems such as rotating biological reactors coupled with artificial wetlands for apartment building complexes whereas for horizontal building complexes and small private households the proposed hybrid systems would be the pertinent solution.6,24,25
Case of study: havana city
The following material was presented in RESURBE III 29 and has not been published:
Presently, around the world 750 millions of people have no access to drinking water enough for daily consumption and 2,500 million cannot enjoy sanitation basic facilities for water used.42 Multiple causes contribute to this situation. Among them, the pollution of aquatic ecosystems that have suffered an ever increasing damage in its quality, mainly due to the discharges of household, industrial, and derived of the agricultural and livestock keeping activities.43
This problem concerns not only to the countries in development but also to those already developed since their companies tend to migrate their enterprises and businesses to those countries considering that pollution protection laws and norms are laxer than those in their home countries and that pollution will never reach them.
For Cuba, this is the present case, being one of the most stringent environmental problems the dumping of wastewaters to water bodies and ecosystems in general either untreated or with deficient treatments.44-46
This fact is clearly observed in some important surface water bodies such as the Almendares river, pertaining to the hydrographic basin of Almendares-Vento, La Habana, Cuba (figures 9a, b).
This basin possesses a total surface of 402.02 km², where the Almendares river is the most important one and reaches an extension of 49.8 km where more than 570,000 inhabitants live. Around 70 polluted sources of different origins discharge their wastewaters to the river, estimating the existence of high levels of chemical pollutants there 47, as well as microbiological sources of pollution. Studies carried out in this ecosystem have shown elevated contents of organic matter, ammonium compounds, phosphates, polycyclic aromatic hydrocarbons and some heavy metals, as a consequence of these discharges.44,46,48 The chemical oxygen demand values, COD, a commonly used parameter to measure global pollutants in water or wastewater, exceed in approximately ten-fold the reference values of the Cuban norms. The phosphates concentrations (0 a 4 ppm) and ammonium compounds (1 a 12 ppm) in some occasions are very near to the limits established. The concentrations of some heavy metals are high, for example, for Pb (II) (2.86-73.29 μg.L-1) and Cr (VI) (0.14-0.20 μg.L-1), and for Cd (II) (0.12-0.81 μg.L-1).44
According to these data it can be said that the main course of the Almendares River shows a critical hygienic - sanitary situation. In Cuba, due to its condition of a country with a developing economy, there is a high deficit of maintenance and attention to the operation of the wastewater treatment facilities. This situation increases its inefficiency for the wastewaters treatment.49
Thus, some options should be considered to solve this urgent problem. That is, low cost and ingenious solutions to appropriately deal with the pollution of the wastewaters. One of these options is precisely the use of artificial or constructed wetlands.
Artificial or constructed wetlands, as mentioned in the preceding paragraphs are those that simulate the operation of the natural wetlands. These engineered systems are designed and constructed to use the natural processes implicated in the symbiotic interactions between living organisms and its abiotic environment for recycling the so called wastes or pollutants present in the water, that for those organisms (micro- and macro-phytes, microbial communities) are really resources. As it was said at the beginning of this document, these systems were created to take advantage of the processes occurring in nature but in a more controlled environment.4,50,51
Microorganisms are a fundamental part of the correct functioning of these systems, since these are the limiting step for the efficient removal of pollutants: they contribute to the degradation of the organic matter and to the removal and transformation of the nitrogen and phosphorus compounds present in the wastewaters.17,52,53,54 Among the various advantages of this technology are the very low costs of construction (even more if they are built with collective work, known in Mexico as tequio or tequiotl, a Nahuatl or Aztec word for collective tasks to improve the community) starting-up, operation, and maintenance, compared with the conventional physical, chemical, and biological systems. It is an effective and safe technology for the treatment and recycling of water especially if they are properly operated and maintained. It is simple and highly efficient operation warrants the removal of pollutants.54,55,56,57,58 These are aspects that strengthen its use as an option for the treatment of wastewaters especially where land is not expensive.59
Additionally, they can be constructed in small communities or by the neighbours of localities immersed in the big cities where traditional wastewaters treatment facilities are not feasible such as the case of many big cities in the world.60,61 They can also be built in situ where wastewaters are being produced and maintenance may be carried out by personnel relatively untrained,61,62,63 Besides, the environmental impact, such as noise, odour, etc., is very low and they are quite well integrated to the environment. Furthermore, there has been a strong approach to improve its performance to reduce the total area needed for its construction, one of its main limitations, especially in the urban areas. One of these strategies is directed towards bio-augmentation. Bio-augmentation or “seeding” consists in la addition of highly specialized and concentrated microbial populations of pure strains or consortia, to a specific contaminated site.64,65
This technique is considered suitable for those sites that do not possess enough microbial cells or when the native population does not present the metabolic capabilities and necessary characteristics for the proper interaction with the compounds to be removed. 65 The most common options used with this strategy are the addition of pre-adapted pure bacterial strains, the addition of pre-adapted consortia, the introduction of genetically modified bacteria, and the incorporation of genes relevant to the efficient interaction with the specific pollutant, through a vector, to be transferred to the indigenous microorganisms.66) In spite of the many elements to be considered and that some contradictory results from different research groups are to be found, bio-augmentation represents one of the best options to improve the operation of the artificial wetlands 65, mainly due to the specialization and selectivity of the inoculated microorganisms since process efficiency depends upon the biological agents.67 Thus, the search for competent microorganisms for its application in each specific site or given pollutant is probably one of the interesting approaches in the present moment to enhance the use of artificial wetlands.65
This is precisely one of the subjects that the Group for Microbial Treatment of Wastewaters of the Faculty of Biology at the Universidad de La Habana (Grupo de Tratamiento Microbiano de Aguas Residuales de la Facultad de Biología). It is done in cooperation with the UNAM Facultad de Química. The main objective is to study the internal functioning of the artificial wetlands to be applicable in communities, as well as enterprises, and other needed areas such as mine sites, etc.
One of the results reached was the isolation and selection of bacterial strains, some with the potentiality to remove organic matter, ammonium, phosphates, and others with the capacity to remove heavy metals such as cadmium, chrome, mercury, and lead.9,10,12,13 With these selected bacteria consortia were designed to bio-augment the metals removal efficiency in lab scale reactors simulating artificial wetlands resulting in an increased removal of the selected pollutants as well as the removal of organic matter, nitrogen, and phosphorus. These results may be used to reduce the surface area needed in heavily built households in big cities improving its operation as is the case of the Almendares river basin. Near the Almendares river there are some identified communities that require treatment systems for its wastewaters. Most of them comprise around 2000 inhabitants. Thus, the application of these environmentally friendly approaches is just what it is needed. As a surplus of the treatment system based on artificial wetlands, the treated effluents might be used not only for the sanitary facilities but also for the now so popular urban agriculture. Besides, the self-employment among the community may become a status of pride to be cooperating to improve the quality of the river basin and even to use it for touristic purposes. All these actions constitute resilient responses of the inhabitants towards its living standards improvement (figures 10, 11).
Fig. 10 Metalo-tolerant microorganisms isolated from the Almendares River to enhance artificial wetlands performance 9,10,12,13
Concluding remarks for the case study of havana city
The subsequent bioaugmentation with the consortium of CWs at laboratory scale allowed 100%, greater than 70 and 55% removal of organic matter, ammonium, and phosphate, respectively.68) Also removals over 95, 80 and 50% for lead, mercury and chromium, respectively, were obtained.
Conclusions
The use of artificial or constructed wetlands for rural communities as well as for suburban areas where nowadays the construction of sewage systems is not only too costly but full of problems associated with already built streets and avenues, as well as type of soil and other construction constrains, is a very promising solution.
Authors hope that this approach becomes a useful tool for sanitation solutions not only in Mexico and Cuba but all over the world
