Historically, water sanitation has not been a priority for any sector of society in Mexico, and substantial technical and ecological problems exist in this country's wastewater treatment systems. Constructed wetlands (CWs) have proven to be an exceptional alternative, particularly for rural areas in developing countries. This paper identifies the status of research on CWs in Mexico, and discusses the possibilities for their use. Our review showed that interest in CWs in Mexico is growing exponentially, particularly in academic institutions. Consequently, published documents are mostly on experimental wetlands, although there are a few experienced groups devoted to producing technology and providing training needed to apply CWs. CWs are generally used for domestic wastewater treatment, disregarding other pollution sources such as agriculture and industry. Rural communities have the most potential to obtain and apply this technology, but unfortunately their degree of use of these systems is still very low. The current status of research and application of CWs leads to a few options discussed in this paper to promote their use in Mexico, taking into account that the success of these alternatives can only be achieved by partnering with governments, water treatment companies, non-governmental organizations, academic institutions and rural communities.

Introduction

Historic perspective on water quality and sanitation in Mexico

Water quality and sanitation are not a political priority in Mexico today nor have they been in the past. The Olmeca culture built the first hydraulic infrastructures for drainage, water supply, and irrigation purposes. They constructed drain pipes, irrigation channels and dams, and exploited natural wetlands for their crops (Martínez-Ruiz & Murillo-Licea, 2012). Mexican pre-Hispanic populations had the skills to construct artificial water bodies in the shape of lagoons built with dams for several purposes, which included agriculture, animal breeding, aquatic plant cultivation, and navigation. Examples of these are the Tula lagoon, in the state of Hidalgo, and the Amanalco lagoon, in the state of Mexico (Rojas-Rabiela, 2012). However, information about whether the pre-Hispanic people used hydraulic infrastructures for water sanitation purposes is scarcely known. In Mesoamerica, it was very common to collect sewage water in underground drains, which were mainly used to collect rainwater and domestic sewage. For example, the City of Zempoala (Central Veracruz), the capital of the Totonaca culture in the postclassical period (between 900–1521 b.c.), had an urban drainage system to send wastewater from buildings to fields of crops for irrigation (Rojas-Rabiela, 2012). It was not until the second half of the 19th century that revolutionary ideas of sanitation, hygiene and urban modernization were taken into account as part of the economic development (Olivares & Sandoval, 2008). The Sanitary Code of 1891 stipulated that it was mandatory to clean pipe lines and that it was forbidden to discharge wastewater into aqueducts (Olivares & Sandoval, 2008). In the 19th century and until the 1980s, economic development in relation to water issues was focused on building hydraulic infrastructure to supply more water to the growing population (Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT), 2009). This emphasis on water development and neglect of the wastewater problem has resulted in the fact that water quality and the provision of wastewater sanitation services are among the most serious water management problems in Mexico. Both of these problems have been addressed by many researchers over the last decade (Cotler, 2004; Torjada et al., 2004; Carabias & Landa, 2005; Jiménez & Marín, 2005; Aboites et al., 2008; Olivares & Sandoval, 2008; Cotler & Caire, 2009; Cotler, 2010; Jiménez et al., 2010; Lahera, 2010).

Water quality in Mexico today

The lack of attention on water quality issues is the reason why Mexico was ranked 106th out of 122 countries in 1992 in terms of good water quality (United Nations (UN), 2003). The National Monitoring Network of CONAGUA (National Water Commission) has been monitoring the water quality of surface water since 1974 (CONAGUA, 2008). The number of monitored sites is not representative of the total number of surface bodies of water. According to CONAGUA (2010, 2012), there are 70 lakes, 125 lagoons, 149 rivers and 667 large dams in Mexico, and yet the monitoring network only comprises 10% of lakes, 52% of lagoons, 34% of rivers and 2% of large dams. According to CONAGUA (2010), the average of the three parameters used to monitor water quality showed that 86.5% of the sampled sites were beyond an acceptable level. It is fair to acknowledge that the three parameters used (chemical oxygen demand (COD), biochemical oxygen demand (BOD5), and total suspended solids (TSS)) are insufficient to adequately assess water quality.

The manner in which society develops is contributing to the deterioration of water quality, because wastewater increasingly contains pathogens of human and animal origin, as well as drugs discharged by the human body through urine and feces (Aboites et al., 2008). This is quite a challenge for human health because surface water (streams, rivers, lakes) supplies 63% of the water needed for total consumption. According to the type of pollutants, 36% of rivers, lakes and dams are mainly polluted by organic matter, nutrients (phosphorus and nitrogen), and pathogens, and less frequently by metals and organic compounds (SEMARNAT, 2008). The most polluted Mexican rivers – the Grijalva, Papaloapan, San Juan, Pánuco, Blanco, Balsas, and Lerma-Santiago – have been steadily presenting this situation over the past two decades (Jiménez et al., 2010; see also CONAGUA, 2011).

Information on water quality in lakes is not available or updated, but it has been reported that eutrophic conditions are present in some lakes (Chapala and Pátzcuaro in Jalisco and Michoacán states) due to the intrusion of sewage, and that they contain metal levels beyond the accepted limits, as well as fish with dioxins (De Anda & Shear, 2001; De Anda & Maniak, 2007; Bravo-Inclán et al., 2008).

As for underground water, its history is not so different from that of surface water. Mexico has 653 aquifers, 106 of which are overexploited, 15 present marine intrusion, and 31 are saline due to overexploitation (CONAGUA, 2014). In addition, water quality information and programs that protect aquifers are scarce (Aboites et al., 2008), despite the fact that underground water is the main water source for all uses in the Northern Pacific, Northern and Central areas of Mexico (CONAGUA, 2013). Other aquifers are polluted with naturally occurring fluoride, iron, manganese, and arsenic, which are released by rocks in higher concentrations than the limits established in the standards, especially in Northern and Central Mexico (SEMARNAT, 2008; Moreno-Vázquez et al., 2010). Wastewater and fecal pollution have also been filtered in some aquifers, mainly those located in Central Mexico (Moreno-Vázquez et al., 2010).

Mexican coasts are the final receptacle of most basins, and thus receive all of the accumulated discharges from upstream. In Mexico, the Pacific coast and the Gulf of California receive 32 principal rivers, while the Gulf of Mexico and the Caribbean Sea receive 16 main rivers that triple the total runoff into the Pacific and Gulf of California (CONAGUA, 2008). Caso & Garrido (2010) estimated the potential impact of continental drainage on the marine eco-regions based on the following variables: treated wastewater per basin, potential sedimentation of dams, non-point source pollution of agrochemicals, and water quality (COD, BOD5, TSS). They observed a strong potential effect of continental drainage on the Gulfs of Mexico and California. In the coastal zone of Veracruz and Tabasco, the values reached the categories ‘very high’ and ‘high’. High values were found to the south of the Gulf of California and on the Guatemalan border. Medium and low values prevailed in the rest of the Pacific coasts.

Discharges accumulated from upstream produce an excess of nutrients, whose slow movement in coastal lagoons and stratification in the water column give rise to a hypoxia zone with oxygen concentrations below 2 mg/l, where no aerobic organism can live and which also affects biogeochemical cycles. A few lagoons in Mexico, such as Nichupté (Cancún), are hypoxic due to excessive organic matter and algal bloom (Rabalais, 2004). Potential areas of hypoxia in Mexico are the Grijalva-Usumacinta system in the Gulf of Mexico (Day et al., 2004; Rabalais, 2004); the adjacent areas of the Tonalá river basin in Veracruz, Tabasco and Chiapas; the outlets of the Cazones, Tenixtepec, Tecolutla, Arroyo Blanco and Bobos (Nautla) rivers; and the inlets of the Papaloapan, Coatzacoalcos, Jamapa, Tonalá and La Antigua rivers, in Veracruz (Caso & Garrido, 2010).

In Mexico, water monitoring efforts are centralized in the most populated and industrialized areas (Moreno-Vázquez et al., 2010), because CONAGUA monitors municipal and industrial water pollution sources. However, rural and agricultural areas require attention because of their environmental impact. Cotler & Lura (2010) determined the vulnerability of some regions caused by agriculture, as non-point source pollution, considering the amounts of pesticides and fertilizers used in each municipality, based on the agricultural census of 2008 (Instituto Nacional de Estadística y Geografía (INEGI) data). The authors observed high values for potential contamination by sources spread through basins in the North Pacific, Central Mexico, and the Gulf of Mexico, which may reflect the effect caused by chemical fertilizers in 35–55% of the total surface of the basins, of which 45–60% are farming areas.

The lack of interest in avoiding contamination of bodies of water by discharges of industrial wastewater is similar to the indifference shown toward their pollution by agriculture and domestic wastewater. Most of the industries still discharge wastewater directly into watercourses (Gómez, 2012). In 2002, the industries with the largest volume of wastewater discharges in Mexico were aquaculture (40%), sugar cane (27%), petroleum (7%), services (6%), chemicals (4%), cellulose and paper (3%), agricultural (2%), alimentary (2%), and beer (1%), whereas mining, textile, viniculture, coffee, and tannery discharged less than 1% (CONAGUA, 2004). The sugar cane industry was the biggest producer of organic materials, and the petroleum and chemical industry generated the highest environmental impacts (Carabias & Landa, 2005).

The Mexican standard that establishes the maximum limits allowed of wastewater discharges into natural watercourses (NOM-001-SEMARNAT, 1996) does not include enough water quality criteria to cover multiple water uses and protect aquatic life. The three physicochemical parameters (BOD5, COD and TSS) are not sufficient or relevant enough to evaluate water quality in an integral manner (Hansen & Van Afferden, 2005). Therefore, no diagnosis of the exposures and risks of toxic pollutants in the country is available (Hansen & Corso, 2011). Toxic, persistent and bioaccumulative substances were used in the past to counter vector diseases and plagues, without knowing their impact on fauna and human health, or their environmental effects. At least the most dangerous substances, called persistent organic pollutants, must be monitored because Mexico signed the Stockholm Convention, which was created to protect human health and the environment from this type of pollutant (Fernández-Bremauntz et al., 2004).

Since water pollution is already contributing to lower water availability per capita in Mexico, sanitation should be a national priority in order to decrease pollution impairing water quality.

Sanitation in Mexico today

Sanitation has not been a top priority for the Mexican government, since potable water supply, street lighting, solid waste management, and expansion of the sewer system have taken precedence (De Anda & Shear, 2008). The sewer network covers 96.3% of the urban population, but only 67.7% of rural areas (CONAGUA, 2014). However, only 91.7% of the volume generated by municipal wastewater is collected by the sewer network (data from CONAGUA, 2013), and not all of this water is sent to a wastewater treatment plant. In fact, just 50.1% of the collected municipal wastewater and 28.8% from industries collected by sewers is sent to a treatment plant; the remaining portion is discharged directly into natural watercourses, such as canyons and crevices. Conversely, according to CONAGUA (2008) and INEGI (2008), 70–80% of the supplied public freshwater is transformed into wastewater. Only the states of Aguascalientes and Nuevo Leon treat over 90% of their collected water (CONAGUA, 2009).

The 2,287 municipal treatment plants operating in Mexico in 2013 treated 50.2% of the nation's sewage collected by the sewer network (CONAGUA, 2014). According to the data collected by CONAGUA 2007, it is known that 64% of the treatment plants discharge the treated water into natural watercourses, although this water still contains pathogens and suspended solids (Parr et al., 2000). Consequently, water discharge by treatment plants frequently exceeds the maximum limits established by Mexican standard NOM-001-SEMARNAT-1996 (Bunge, 2010), and these treatment systems are not always designed for nutrient removal (Rivas et al., 2011), thus polluting and eutrophicating the recipient watercourses. Despite an increase in the number of treatment plants, the total volume of municipal wastewater treated is still low (Zurita-Martínez et al., 2012). Their lack of operation could be related to their excessive mechanization, instrumentation, and automation, together with a high demand for electricity, which increases their maintenance costs and operation complexity; also their technical personnel are not always adequately trained (Lahera, 2010). Moreover, some wastewater treatment plants are not even adapted to the climate and physical conditions of the place where they are established (Bunge, 2010).

There are 15 principal different processes for municipal wastewater treatment (CONAGUA, 2010). Half of the treated volume corresponds to activated sludges (CONAGUA, 2014), but this method uses a high electrical energy input for aeration, it requires qualified personnel for maintenance, and it is common that no appropriate place is assigned for the final disposal of the resultant sludges (Lahera, 2010). Stabilization ponds treat 13.43% of the total treated volume (CONAGUA, 2014), but can be a source of unpleasant odors and mosquito breeding, which can pose health hazards to the nearby population (Ensink et al., 2007). Both systems have been widely used, mainly because of their simplicity and low operation cost, even though they discharge high quantities of suspended solids (Rivas et al., 2011). The rest of the methods used treat less than 29.25% of the total water volume (CONAGUA, 2014).

According to the Special Program for Water Science and Technology (Programa Especial de Ciencia y Tecnología en Materia de Agua) (Gómez, 2012), the water quality and sanitation challenges that Mexico is currently facing do not only require an increase in the investment to treat water, but also need integral management. A search is being carried out for integral water treatment technologies that can be used and managed by rural communities, which would benefit by reusing treated water to irrigate their crops, thus improving their agricultural production. Water treatment technologies based on natural systems are also needed to help decentralize CONAGUA, which cannot keep up with the demand. Low cost technologies that benefit from natural processes, such as microorganisms that clean water, must be applied.

Constructed wetlands (CWs) could clearly meet the requirements of the Special Program for Water Science and Technology, but why have they not been widely adopted by Mexican water policy? And why their advantages regarding traditional systems have not been entirely recognized yet? With these questions in mind, we undertook a critical review of the assessment of the use of CWs in Mexico, and the problems and opportunities they face for implementation and wider use. The particular objectives are (1) to emphasize the potential benefits of CWs for rural Mexican communities, (2) to identify the level of knowledge and use of CWs in Mexico, and (3) to recognize the different barriers that stand in the way of their adoption, as well as their positive bases and potential uses.

Methods

The authors undertook a comprehensive search of the literature based on the most important databases located in Mexican universities, such as Universidad Nacional Autónoma de México (UNAM), Universidad Autónoma Metropolitana (UAM), Colegio de Postgraduados (COLPOS), Universidad Veracruzana (UV) and Instituto Politécnico Nacional (IPN), as well as in federal institutions related to water management and planning, such as the Mexican Institute of Water Technology (Instituto Mexicano de Tecnología del Agua – IMTA) and the National Water Commission (CONAGUA), and the ISI Web of Knowledge (www.isiknowledge.com) database. The literature review was complemented by four informal conversational interviews (Fontana & Frey, 2000) with expert informants under the topic ‘challenges faced by CWs for their implementation’ and ‘CW advantages vs mechanized treatments’. In this type of informal interview no previously formulated set of questions is appropriate under emergent field circumstances, because the field worker does not know beforehand what will be important to ask during the interview. Hence these conversations are different for each person consulted.

The benefits of CWs

CWs are used to remove pollutants from municipal, industrial and agricultural wastewater and rainwater. CWs are based on the biological, chemical and physical removal of pollutants from wastewater, aided by aquatic plants and the coexistence of anoxic–aerobic–anaerobic microenvironments that favor the different mechanisms involved in wastewater treatment, principally biodegradation, plant uptake, sorption, and photodegradation. The ability of CWs to remove COD, BOD5, nitrogen compounds, phosphorus, and different microorganisms from wastewater has been studied extensively, resulting in high efficiency levels (Kivaisi, 2001; Ramírez et al., 2005; Vymazal, 2010; Saeed & Sun, 2012). Li et al. (2006) found high efficiencies of COD, BOD5 and TSS removal (91.8, 97% and 100%, respectively) in a pilot-scale CW in northern China. Similarly, Kayranli et al. (2010) obtained mean removal efficiencies of BOD (95.2%), COD (89.1%), suspended solids (SS) (97.2%) and ammonia nitrogen (58.2%) in CWs treating wastewater in Ireland. Nutrients such as total nitrogen and phosphorus can also be removed with a removal efficiency of 97% and 92%, respectively (Rodríguez & Brisson, 2015).

In comparison to conventional systems, CWs use cheaper and less complex technology, which also lowers operation and maintenance costs. According to the informal interview with Biol. Rivas, an expert on CW, from the Mexican Institute of Water Technology (IMTA), the maintenance of mechanized treatment plants (that remove nutrients) for small flows costs 0.23 to 0.29 USD/m3, whereas in a CW it costs 0.03 to 0.05 USD/m3. Even though the expense to build a CW may be the same or even higher than for conventional treatment plants, its maintenance and operation costs are lower. Also, CWs need periodic on-site labor rather than full time attention, they use renewable resources such as solar and kinetic energy, they use local resources, and they are aesthetic and generate biodiversity by attracting local fauna, providing habitat.

Many developing countries in the tropics do not have sufficient and efficient provision, treatment and disposal of water, which compromises sanitation and health. There is a tendency to use wastewater for irrigation, which has caused huge health problems, such as cholera in Peru in 1992, cyclosporiasis in the USA and Canada (from 1995 to 2000) because of raspberries imported from Guatemala, and hepatitis A in the USA in 2003 caused by onions from Mexico (Oakley & Salguero, 2011). Latin American countries need low-cost and low-maintenance wastewater treatment technologies that can be economically and environmentally sustainable, although the trend in recent years is to look for high energy input and expensive systems that also require technical expertise. Tropical countries are given an advantage by their climatic conditions, which are beneficial for the effectiveness of CWs, because the absence of seasonal low temperatures and the narrow temperature variations maintain the wetland microbial activity during the whole year. By contrast, microbial activity is reduced in temperate zones with low temperatures, thus decreasing BOD and nitrogen removal (Katsenovich et al., 2009). In Canada, CWs are implemented on a small scale as part of development projects, but they work in conjunction with sewer systems and wastewater treatment to help protect natural watercourses.

Another advantage of tropical regions is the vast diversity of aquatic plant species that grow in wetland zones and the high productivity that can be reached in these regions. Local materials, plants and labor can be used to build wetlands. They also have additional benefits, particularly for small communities in developing countries: people can harvest ornamental plants and raw materials from the wetland for handicrafts that can be sold later, or they can reuse the treated water to irrigate crops, parks, and gardens. In fact, treated water can also be used to breed fish and provide water to livestock. These are all bonuses that may encourage the use of this technology (Zurita-Martínez et al., 2011a). However, it has not been widely used in Latin America (Hernández, 2013), including Mexico, despite the potential benefits that CWs have in the tropics.

Publications on CWs in Mexico

Publications on CWs are quite recent in Mexico. The first articles on CWs were published in 1997 (Hernández, 2013). Although there has been an exponential increase in the number of published documents on CWs in Mexico over the last 15 years (Figure 1), the primary objective of most is not to publish their results. Out of the 76 papers on CWs in Mexico that were reviewed, 43% (33) are classified as graduate and postgraduate theses, 29% (22) are peer-reviewed journal articles (Figure 2(a)), 18% (14) are popular science articles, 7% (5) are reports, and only 3% (2) are manuals. By far, most of the documents are written in Spanish, except for the journal articles, 16 of which are written in English and 6 in Spanish. In agreement with the observation made by Hernández (2013), we found that CWs in Mexico are still in the experimental phase, mostly generating basic research science. From the total reviewed documents, 93% (71) were experimental studies, and only 7% (5) actually operated to treat water in order to solve a water quality problem (called ‘consolidated’) (Figure 2(b)).
Fig. 1.

Number of documents on CWs in Mexico 1998–2012.

Fig. 1.

Number of documents on CWs in Mexico 1998–2012.

Fig. 2.

Number of documents on CWs in Mexico, according to (a) type of documents and language (grey: Spanish, black: English), (b) CW level of application, (c) type of water to be treated, (d) purpose of the document.

Fig. 2.

Number of documents on CWs in Mexico, according to (a) type of documents and language (grey: Spanish, black: English), (b) CW level of application, (c) type of water to be treated, (d) purpose of the document.

Domestic wastewater treatment is the main purpose of CWs in Mexico, according to the reviewed documents, because 52% of them were developed to treat domestic wastewater, and the rest treated experimental water or water from industry, domestic and agriculture activities; CWs were built for recreational purposes in only two cases (an artificial lake and canals) (Figure 2(c)). This coincides with the fact that in Mexico much more attention is paid to domestic wastewater monitoring than to the wastewater resulting from agriculture and industry activities. According to the objectives of the reviewed documents on CWs in Mexico, most of them (74%) described the function of the CWs by evaluating their efficiency, or the studies consisted of experimentation with different physical or biological conditions of their elements (Figure 2(d)). Only 26% focused on the design and construction of the CWs (Figure 2(d)).

Certainly, one of the barriers preventing the adoption of CWs is the limited information about them (Zurita-Martínez et al., 2012); another is the outstanding number of non-published academic documents (theses) compared to published articles, and their limited circulation even though they are written in Spanish. In addition to the lack of published scientific papers (Zurita-Martínez et al., 2012), there is little technical information available in Spanish, such as handbooks for the construction and management of CWs for domestic wastewater treatment; we were only able to find two manuals (Durán-Domínguez et al., 2003; Setty, 2007). This shortage of technical information may explain the poor design observed in these systems. Sometimes they are overloaded, exceeding their design capacity, as is the case of some wetlands in Akumal, Quintana Roo (Krekeler et al., 2007). In other cases they can be flooded during the rainy season as commented by Rivas in the informal interview. They can be the source of unpleasant odor emissions, but this is a result of poor CW performance and should be corrected with mulching (Krekeler et al., 2007). Although an expert is not required for the operation of the CWs, the people in charge must be well trained, and sometimes this is lacking (Varma, 2009) because the original trained staff changes over time and the new people in charge do not receive proper instruction.

Implementation of CWs in Mexico

Despite the technical barriers existing for CW implementation, these systems are being adopted on small and medium scales by municipalities. According to Zurita-Martínez et al. (2012), the number of municipal CWs increased from 0 to 137 between 2000 and 2008. States with a significant number of CWs are Sinaloa (57), Oaxaca (38) and Chihuahua (15). At present there are approximately 69 CWs (CONAGUA, 2013), and another 101 are combined with different methods such as Imhoff tank, sedimentation, and UASB (upflow anaerobic sludge blanket), most of them combined with a septic tank (CONAGUA, 2011). From the 2,342 total treatment plants (CONAGUA, 2013), only 3% are using a CW. In terms of the total treated municipal wastewater volume, CONAGUA uses this method for only 0.56% (CONAGUA, 2010), in a total of 17 states. There is no knowledge of whether this technology is being applied in the other half of the states. The state of Oaxaca accounts for the majority of CWs with 56%.

Since 2003, the Mexican Institute for Water Technology (IMTA), in conjunction with CONAGUA, municipalities of Michoacan and local communities, has obtained funds from the non-profit Gonzalo Río Arronte Foundation to develop six CWs located in Santa Fe de la Laguna (city of Quiroga), Cucuchucho (city of Tzintzuntzán), and the city of Erongarícuaro, as well as two CWs in the city of San Jerónimo Purenchecuaro and San Francisco Uricho, in order to treat municipal wastewater (IMTA, 2007). These CWs are still working. They cover an area between 0.2 and 1.16 ha and handle a flow from 43.2 to 432 m3/d. The treated water has been reused for crop irrigation, aquaculture, production of ornamental plants, and assembly of handicrafts made of the wetland plants. These successful cases have also attracted the attention of schools, and they provide environmental education.

Another academic institution with an experienced group that has developed CW projects, infrastructure and an operation manual over the last decade, is the College of Chemistry at the Universidad Nacional Autónoma de México (UNAM). It has established several CW projects: one in a museum (Universum, Ciudad Universitaria, UNAM) and another in a forest nursery (Viveros de Coyoacán), both in Mexico City. Unfortunately, they are no longer in operation, according to the informal interview with Dr. Carmen Durán Domínguez, an expert on CWs from the College of Chemistry, UNAM. In 2009, a CW was designed by the College of Chemistry, UNAM, under an agreement with the Department of the Environment of Mexico City (Secretaría de Medio Ambiente de la Ciudad de México) to treat wastewater from the Tlacos treatment plant and to clean the water from the artificial lake in a recreational park located in the Bosque de Aragón, Nezahualcóyotl, in the state of Mexico. This CW has an area of 0.813 ha and a flow rate of 32–109 m3/d. One of its beneficial results, in addition to cleaning the lake, is the attraction of 16 migratory bird species. Another wetland was constructed in 2008 in a high school (CCH-Sur, UNAM) in Mexico City. The latter two CWs are still in operation and are used for experiments conducted by the students.

In Akumal, Quintana Roo, CWs have become an important wastewater treatment technology in the tourist resort community. In 1996, the Akumal Ecological Center (Centro Ecológico Akumal – CEA), a non-governmental organization, collaborated with personnel from the University of Florida to design and install a CW in the tourist destination Akumal, on the eastern Yucatan Peninsula. It started with an area of 81.2 m2 and a capacity for 24 people (Whitney et al., 2003). At present it holds over 50 CW systems for domestic and commercial wastewater treatment, ranging from one person to a maximum of 70 people (Zurita-Martínez et al., 2012).

According to an informal interview with Mr. Noé Ramírez Mendoza, a member of the cooperative society in the Ixmiquilpan municipality, state of Hidalgo, a CW was developed in 1995 to treat domestic wastewater from the local community and greywater from Mexico City (this project was originally started by the academy belonging to the Biology major in FES Zaragoza (belonging to the UNAM), along with a non-governmental organization (Servicios de Apoyo para el Desarrollo A. C. – SEDAC) and 16 members of a local family with ethnic origin (‘La Coralilla’ Cooperative Society, hñahñu ethnicity) at the JuliánVillagrán ejido). They began with a 50 m2 wetland, but expanded over the years to 400 m2 of CW on the family's own land. The treated water was initially used to produce Zantedeschia sp. Women used to make floral arrangements and sold the flowers in a local market. At present, most of the treated water is used to produce fish (Oreochromis sp.), which is cooked and sold as part of their restaurant menu. It is worth mentioning that this cooperative is certified by following the sanitary specifications for fish production established by the Mexican standards provided by SENASICA (National Service of Sanitary, Safety and Agro-alimentary Quality). The treated water is also used to irrigate vegetables and it is shared with local livestock producers. Currently, this cooperative has touristic facilities such as an artificial lake. Students from the UNAM and other institutes still participate in it academically, and new projects are kept in mind. This example was so positive and beneficial that the family in charge of the CW stopped emigrating and obtains its income from the CW system. This example is a unique case of the success of a CW, since members of the community took over the project.

The lack of government involvement in CWs

According to Zurita-Martínez et al. (2011b), rural communities are generally provided the least assistance to cover sewage services because the government gives priority to denser populations. Federal programs prioritize urban areas (with more than 2,500 inhabitants) to access federal funds for water services. Although there is a federal program entitled Prossapys (Program for the Construction and Retrofitting of Drinking Water and Sanitary Systems in Rural Areas – Programa para la Construcción y Rehabilitación de Sistemas de Agua Potable y Saneamiento en Zonas Rurales), most of those funds give priority to drinking water and sewage systems rather than wastewater treatment.

Based on a review of the 2012 investment in water infrastructure for rural areas, the states of Chiapas and Veracruz did not invest in sanitation any of the 179.2 million and 372.2 million Mexican pesos that were spent respectively on water infrastructure (CONAGUA, 2013). In a review of the 2012 investment in water and sanitation provision in rural areas, it was found that 60.4% of the funds were used for tap water, 28.9% for sewerage systems, and just 4.2% were invested in sanitation. However, in rural areas the provision of sanitation is mainly limited to latrines. According to Guerrero et al. (2006), latrines are deposits where human excreta are stored. They are a temporary solution and have huge disadvantages, consisting of possible water and aquifer pollution, bad odor, proliferation of transmitting fauna and deposit saturation. Since the governments give almost no economic support for sanitation in rural areas, most experiences with CWs in these zones are promoted by academic institutions.

Abandonment of CWs

Most CWs in Mexico are pilots and comprise short projects created by academic institutions, which are funded only for the experimental phase or at most to start up the project; these CWs are later abandoned for years, as commented by Rivas in the informal interview. Another reason why CWs are abandoned is that the changes of 6-year federal governments and secretariats bring about changes in personnel and projects, so the follow-up of CW projects is not achieved; such is the case of Viveros de Coyoacán, where according to the informal interview with Dr. Durán Domínguez, the interest in maintaining and operating this wetland infrastructure was lost due to the 6-year cycle change in SEMARNAT and the Mexico City Government. Another important cause of CW abandonment is that the money needed to provide maintenance is not taken into consideration in the projects, so there is no vision for the future of the CWs. No maintenance or monitoring was performed in 5 years for a CW in Akumal, Quintana Roo. The flow increased more than 300% from around 0.76 m3/day in 1998 to more than 2.27 m3/day in December 2001 (Whitney et al., 2003).

Social participation, an alternative to promote CWs in Mexico

Over recent decades, mostly people from environmental and human right fields have comprised civic groups to exert pressure on water and sanitation. These groups include the Advisory Boards of the State Water Commissions and Operating Departments, Social Accountability Committees in Federal Programs, and Water and Sanitation Citizen Observatories (Palacios-Moreno, 2013). Citizens involved in the latter supervise and monitor water issues. The Water and Sanitation Citizen Observatory, created by citizens interested in water problems at their localities, is a non-institutional group that collectively generates background knowledge, skills and activities aimed at the improvement of local water problems (Dominguez et al., 2013).

Another path for social participation in sanitation is for the local community to be involved in the construction, start-up, maintenance and operation of sanitation infrastructure. However, this option is less common, structured or stable. Economic benefits from CWs are important in developing countries to encourage the community to maintain treatment wetlands (Belmont et al., 2004). Some incentives can be economic returns to communities when harvesting for biogas production, animal feed, fiber for paper making, and compost (Lakshman, 1987). But in some cases, when the CWs benefits only fall on a few members of the community, this can create envy and thus social problems, as commented by Rivas in the informal interview. In Pátzcuaro, Michoacán, the local community was involved in the building, operation and maintenance of the CWs (http://www.imta.mx). Setty (2007) also worked with the Colonia 5 de mayo community in San Cristobal de las Casas, Chiapas, but the CW project there was apparently abandoned. The CW in Ixmiquilpan, Hidalgo, is another case where the local people took over the treatment system, but sadly in Mexico most people do not believe that sanitation is a priority and leave this issue to the government. There is a generalized lack of ownership by the communities benefited by the CW, mainly because people do not entirely trust the utility of CWs, and they take little responsibility for CW management, thus leading to mismanagement. There is also a shortage of staff or a passive involvement in the planning and management of CW systems.

According to Zurita-Martínez et al. (2011b), 23.5% of Mexicans live in rural localities. A rural locality is defined in population terms, comprising of 100 to 2,500 inhabitants. Only 0.2% of these rural localities have a water treatment system. This means that the potential of CWs to address the needs of rural populations in terms of water treatment is being lost. Consequently, 45,400 localities are potential CWs users. The states with the most rural localities are Veracruz, Chiapas, and Oaxaca. Rural communities have the most opportunities to adopt and follow up these systems.

Concluding remarks and future perspectives

Even though historically water sanitation in Mexico has not been a priority for any sector of society, the tradition of five decades of research and use, their proven economic and ecological advantages over conventional methods, and their increasing use worldwide, have proven that CWs are an exceptional alternative for water treatment, particularly for rural areas in developing countries. Even though many barriers block the way for CWs to be adopted in Mexico, they can also be seen as opportunities. Research on CWs in the country has been conducted for two decades, but published documents still are mostly experimental and there is a shortage of manuals with information to plan, construct, maintain and monitor CWs. It is very important to produce manuals, because other countries’ guidelines, which could be copied, may not be directly transferable to Mexico's environments and conditions. The good news is that academia has shown a clear growing interest in this technology, and, although they are few, Mexico has some experienced groups that have worked with CWs for the past two decades. They have created technology, provided training, and developed and evaluated projects on this subject.

On the other hand, technical problems derived from poorly designed CWs and a lack of trained personnel could somehow be solved if established groups with experience in CWs were to provide guidance, although this may exceed their workload. A good option could be to promote private consultancy companies that build water treatment systems to also apply this kind of technology. Academia, which is the principal promoter of CWs in Mexico, should also be interested in sharing its studies with governments and even selling its technology to water treatment firms and non-governmental organizations, to be applied in projects. It should also consider partnering with the governments, which should contribute financial aid, particularly in rural communities. Approximately 99.8% of rural areas that do not have any water treatment system are potential users, as well as at least half of the Mexican states, where apparently no CWs are being used. Federal programs and state programs that financially facilitate the construction of wetlands are needed, but they must also take into account proper funding for the management of CWs and eventual counseling, seeing that currently there is no vision for the future of CWs. It is therefore urgent for governments to assign more monetary resources to water sanitation.

CWs are mainly used to treat domestic wastewater; however, agriculture and industries also pollute water and are mostly ignored. As previously mentioned, basins in the North Pacific, Central Mexico, and the Gulf of Mexico are threatened the most by water pollution caused by agriculture. On the other hand, a large percentage of industries still discharge wastewater directly into watercourses. It is urgent to monitor the quality of this wastewater. Also, a legal framework must be created to handle wastewater, and a manner to maintain surveillance of compliance with laws and standards must also be found. Part of the solution to water pollution from agriculture and industries could be for the law to impose mandatory water treatment of their effluents, and to force them to be in charge of their management. The implementation of programs to provide technology and funding for farmers is also needed.

Given the shortage of personnel and money to maintain the CWs, particularly in rural areas, there is an urgent need for local communities to become involved, for the people to trust in the utility of CWs and make this technology their own for their benefit. At present there have been some examples, albeit very few, where, in addition to locals being in charge of the maintenance and monitoring of CWs, they also receive economic income, which has encouraged people to own their community CW.

Regarding the main question this research posed, namely whether CWs are a solution to water quality issues in Mexico, we can conclude that they are, based on the deep analysis of literature and interviews. However, the achievement of such a goal can be accomplished to the extent governments, water treatment firms, non-governmental organizations, academic institutions and rural communities work together to run well designed projects for CWs. This means that there should be ad hoc projects rather than standardized projects, replicated disregarding local circumstances. Experts’ participation is essential throughout the planning process, because not only is the construction phase of CWs critical, but also their operation, maintenance and monitoring. Perhaps, these three last phases have been the main barriers for CWs to be a more widely adopted technology.

Acknowledgements

The authors wish to express their gratitude to Armando Rivas Hernández, María del Carmen Durán Domínguez de Bazúa, Rojas Rabiela Teresa, and Noé Ramírez Mendoza for the interviews. Funding for this work was provided by the Mexican National Council for Science and Technology (CONACYT) through post-doctoral Scholarships 168443 and 336959. The authors thank El Colegio de Veracruz College for its support. Finally, thanks to Xochitl Ponce Wainer for the English revision.

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