Dr. Victor Miguel Ponce
| DESIGN OF OXIDATION POND AND CONSTRUCTED WETLAND FOR THE CITY OF TLAXIACO, OAXACA, MEXICO |
|
PURPOSE The objective of the proposed research is the environmental cleanup of the Tlaxiaco River, near Tlaxiaco, Oaxaca, Mexico. The study will lead to a system design that can be readily considered for implementation by the municipality of Tlaxiaco. The participants are students and faculty from SDSU and the Instituto Tecnologico de Oaxaca (ITO). SDSU and ITO have an ongoing cooperative agreement, signed in March 2000, with the objectives of "student and faculty exchange and ... the combined development of research projects which have a scientific and/or technological purpose." An intrinsic component of the project is the securing of additional funding to implement the selected project design, which will sought by both cooperating parties, SDSU and ITO. BACKGROUND In May 1999, Dr. Victor M. Ponce, was invited to lecture at the Centro Interdisciplinario de Investigacion para el Desarrollo Integral Regional (CIIDIR), Oaxaca, a unit of the Instituto Politecnico Nacional (IPN), Mexico's comprehensive institution of technological education. Subsequently, in November 1999, Dr. Ponce was invited to lecture at the Instituto Tecnologico de Oaxaca, where he met the faculty and students of that institution and visited its facilities. A fact-finding field trip to the Mixteca Alta region with a team of CIIDIR and ITO researchers was part of the November visit. From these two visits, Dr. Ponce gained first-hand knowledge of Oaxaca's reality and needs, both academic and socioeconomic. Upon his return to the United States, he was instrumental in the execution of a cooperative agreement between SDSU and ITO. The agreement was signed by SDSU President Stephen Weber on February 14, 2000, and by ITO Director Jose Luis Sosa on March 9, 2000. On August 16-23, 2000, Dr. Ponce visited Oaxaca and held meetings with his counterparts, Dr. Oliverio Gonzalez Alafita, ITO professor; Lic. Juana Yolanda Lopez, research professor at IPN/CIIDIR-Oaxaca; and Fernando Eli Ortiz, professor at IPN/ESIME-Culhuacan. A special meeting with ITO graduate students was called, where the following students presented their ongoing thesis topics at various stages of development: Maria de Jesus Gil Gallegos, Oscar Reyes Valadez and Fernando Perez Mendoza. This visit sharpened the principal investigator's awareness of the need for continued international institutional cooperation between SDSU and ITO, to benefit both institutions of higher learning. ITO's current interest in research and development in the Mixteca Alta region, which includes the municipality of Tlaxiaco, was particularly highlighted in the August meetings. This proposal is an intrinsic component of the ongoing cooperative research between SDSU and ITO, with focus on the Mixteca Alta region. Dr. Ponce's counterpart at ITO is Dr. Oliverio Gonzalez Alafita. Dr. Alafita has agreed to cooperate to bring the proposed work to a successful conclusion. An ITO graduate student will be selected to work as a counterpart to the SDSU student. SIGNIFICANCE The state of Oaxaca is located in the southern part of Mexico, surrounded on all sides by the states of Guerrero, Puebla, Veracruz and Chiapas, and the Pacific Ocean to the South. Oaxaca stands among the less economically developed regions of Mexico, a situation that may be attributed to its difficult topography, which greatly impairs transportation and communications. Moreover, the state is home to several native ethnic groups (Mixtecos, Zapotecos, Mazatecos, Chinantecos, Chatinos, Amusgos, Mijes, Guaves) who proudly continue to practice their cultural traditions, including language, music, and dances. The Mixteca Alta region, situated in the northwest portion of the state between the districts of Teposcolula and Tlaxiaco, is particularly important because of its fragility; i.e., a predominantly semiarid region with a history of ecological abuse, including deforestation, overgrazing, overcultivation, and soil erosion. The city of Tlaxiaco is the seat of the district of Tlaxiaco, one of 25 municipalities of the state. Tlaxiaco is an important center of local commerce, and is located in the center of the so-called Mixteca Alta region, one of the less economically developed areas of the state. Like many other towns in Oaxaca, Tlaxiaco discharges its sewage waste directly into the Tlaxiaco River, without any treatment. Over the years, this situation has led to the pollution of the river, which is now, for all practical purposes, an open sewer. Unlike other towns, the situation in Tlaxiaco is exacerbated by a relatively large population, which with about 25,000 people, is the fifth largest town in the state. The pollution of the Rio Mixteco requires immediate attention if the unsightly and unsanitary conditions in Tlaxiaco are to be remedied. However, the city can ill afford conventional sewage treatment, with its heavy reliance on machinery and energy. A workable alternative is to provide a low-technology solution, patterned after the works of nature, to minimize the cost of construction, operation, and maintenance. A solution consisting of a combination oxidation pond/constructed wetland in series is envisioned, with the aim of reducing the influent BOD to secondary treatment levels. The cleanup of the Tlaxiaco River can provide an example to other municipalities in Oaxaca, leading to replication and consequent improvement of the quality of life of other midsize communities in the state. METHODOLOGY The methodology for the project consists of the following steps:
The next step is to identify the alternative locations where the new wastewater facility can be constructed. It is envisioned that the facility can be constructed in an area close to the Tlaxiaco River, downstream of the city of Tlaxiaco, where wastewater will concentrate and be available for treatment. Several alternatives will have to be weighed with regard to suitability, convenience and economics. The final selection will be made after proper consultation with local authorities and cognizant agencies. The system envisioned in this project consists of a combination oxidation pond/constructed wetland in series. Properly designed and operated, this system should be capable of effecting BODL removal to secondary levels, i.e., from 300 ppm influent load to less than 30 ppm effluent. OXIDATION POND An oxidation pond is a widely used alternative for treatment of domestic wastewater. Its major advantage is its low cost and simplicity of operation. It is capable of achieving good reduction of wastewater BODL as well as reduction in nitrogen and phosphorous and destruction of pathogens. It is a particularly useful method of wastewater treatment in developing regions such as Oaxaca, where resources for the construction and operation of conventional treatment systems are not available. An oxidation pond is any lagoon or pond system in which there are both phototrophic and heterotrophic microorganisms. The combined metabolic activities of algae (or cyanobacteria) and organic degrading bacteria is required. An aerobic oxidation pond is preferred because it minimizes objectionable odors. This type of pond is generally quite shallow, has adequate mixing to prevent stratification, and has a sufficient balance of phototrophic over heterotrophic activity so that dissolved oxygen is present throughout the pond. Depths of 0.3 to 1.2 m are typical of aerobic oxidation ponds. Wastewater of a certain BODL concentration flows into the pond. Phototrophs within the pond grow when solar energy enters the pond. When carrying out photosynthesis, they reduce CO2 carbon into phototrophic cell carbon and produce O2 from water to gain the electrons. The oxygen so produced is used by heterotrophic bacteria to oxidize the wastewater BODL, thus effecting the destruction of organic waste. With the proper balance of algal growth to produce the oxygen and the organic destruction that is dependent upon the oxygen, the oxidation pond effects good BODL removal, while maintaining aerobic conditions which help prevent objectionable odors. The detention time must be sufficient to achieve the degree of BODL removal desired. Moreover, the algal growth rate (for oxygen production) and the heterotrophic bacterial growth (for BOD destruction) must remain in balance. The success of the oxidation pond as an effective design for BODL removal depends on the size of the pond relative to the wastewater load, the available solar energy, and on the maintenance of the required balance between phototrophs and heterotrophs. Thus, a careful design that takes into account the local conditions (wastewater loads, insolation, available land) is required. CONSTRUCTED WETLAND The second component of the system design for the cleanup of the Rio Mixteco is a constructed wetland. Increasingly, this type of wastewater treatment is being used as an alternative to high-cost conventional systems, particularly for small communities. The most well known successful constructed wetland was built in 1978 in Arcata, California, a city of about 25,000, i.e., similar in size to Tlaxiaco. A constructed wetland is an engineered basin, consisting of several rectangular channelized trenches or cells built in parallel, where wastewater flows through natural vegetation under a specified gradient. They are typically shallow (less than 0.6 m) bodies of water, where dense stands of water-tolerant species such as catails, bulrushes, or reeds are grown. The organic matter is retained and taken up by the vegetation, typically effecting cleanup of the wastewater to secondary treatment levels. Factors such as influent load, velocity, flow rate, detention time, and frequency of inundation have a major bearing on the physical and chemical properties of the wetland substrate. These properties in turn influence the character and health of the wetland ecosystem, as reflected by species composition and richness, primary productivity, organic deposition, flux, and nutrient cycling. The major costs and energy requirements for constructed wetlands are associated with pumping or transmission of wastewater to the site, minor earthwork, and land costs. In the case of Tlaxiaco, pumping is minimized due to the wetland's proposed location, downstream of a natural water course. It is expected that wastewater will flow mostly by gravity to the site. A barrier layer to limit percolation to groundwater and for containment in case of flooding may be required. The performance of the constructed wetland is dependent upon the system hydrology. Precipitation, infiltration, evapotranspiration, hydraulic loading rate, and water depth can affect the removal of organics, nutrients, and trace elements. A hydrologic budget will be prepared as part of the design. The wetland can significantly reduce BODL suspended solids (SS), and nitrogen (N), as well as metals, trace organics, and pathogens. The basic mechanisms include sedimentation, chemical precipitation and adsorption, and microbial interactions with BODL, SS, and N, as well as some uptake by the vegetation. For example, the effluent concentration at Arcata was less than 20 ppm of BODL. There are limits to the technology of using constructed wetlands for high BODL wastewater treatment. Constructed wetlands are best used for polishing secondary effluent. In the case of Tlaxiaco, a system of oxidation pond/constructed wetland in series is the most effective solution to optimize the performance of the overall system and effect secondary level treatment. The combined system has minimum energy requirements and can be readily implemented without the need for specialized high technology. Monitoring is required to check performance. For this purpose, a small laboratory will be established on the site, to monitor water quality parameters such as BODL, SS, and N. The facilities of the neighboring Instituto Tecnologico de Tlaxiaco, a unit of the system of technical institutes of Mexico (of which the ITO is a part), will be used as a resource for lab technicians and other specialized manpower needs.
|
Map of the city of Tlaxiaco, Oaxaca, Mexico.
Aerospace | Civil | Electrical | Mechanical | Search