By Jorge Navarro
Summary: The degree of water contamination in México is often a matter of location and economic status. With available sources continuously under scrutiny, the search is always present to find a better alternative for all residents. As such, the idea of POU/RO units may have found its calling.
In the industrialized world, point-of-use (POU) units are used mainly to enhance the water quality supplied to homes. The water is supplied 24 hours a day and the quality usually complies with local water quality standards.
Often, water availability—rather than quality—is the more important factor in Latin America. A large percentage of the population doesn’t receive their water from a municipal water supply, and those who do will have it available for only a few hours per day. Most cities have rationed their water supply because almost half of municipal water is wasted through leaks in distribution system piping when it’s pressurized.
Detecting the problem
When the water service is cut off in a region, the system may be under a slight vacuum and groundwater may be siphoned into the piping if proper backflow prevention design isn’t employed or maintained. This groundwater is usually heavily contaminated as the sewage mains may be in worse condition than the municipal water mains and may leach into the drinking water system.
This contaminated water is the main vector for diseases such as cholera (bacteria Vibrio cholerae), typhoid (bacteria Salmonella tiphy), shigellosis (bacteria Shigella dysenteriae), giardiasis (protozoan Giardia lamblia), amoebiasis or amoebic dysentery (protozoan Entamoeba histolytica), cryptosporidiosis (protozoan Cryptosporidium parvum), rotavirus (virus causing diarrhea), etc.
Warmer waters than found in the United States also contribute to an increase in the significance of the microorganism problem. This likely recontamination of México’s municipal drinking water—either in the distribution system and/or cisterns and storage tanks—requires disinfection of the water at the point of use to assure consumers receive the highest quality water possible from the tap.
The process of pathogen inactivation includes filtration to remove suspended solids followed by disinfection with free chlorine, ozone, chlorine dioxide, chloramines or UV light. Some of the oxidation byproducts are trihalomethanes (THM) such as chloroform, bromodichloromethane, dibromochloromethane and bromoform. In some regions in Latin America, iodine is used and can form iodoform.
In México, the turbidity requirement is less than 5 nephelometric turbidity units (NTUs) coinciding with the World Health Organization’s requirements. As of Jan. 1, 2002, the U.S. National Primary Drinking Water Regulation will require that turbidity may never exceed 1 NTU and must not exceed 0.3 NTU in 95 percent of daily samples in any month.
It’s understood that higher turbidity:
- Interferes with disinfection
- Inhibits maintenance of an effective disinfectant agent throughout the distribution system, or
- Interferes with microbiological measurement.
According to a model developed by Mark LeChevallier, research director for American Water Works Service Co., Voorhees, N.J., an increase in turbidity from 1 NTU to 10 NTUs in the surface water, with a constant dose of chlorine, could result in an eight-fold decrease in disinfection.
Addressing the need
Among reasons POU units are needed in Latin America are:
- To improve water quality in systems not pressurized all the time,
- To minimize exposure to water disinfectant by-products (DBPs),
- To improve bad tasting drinking water, and
- To assure untreated or partially treat waters that may still need the removal of some contaminants such as arsenic, fluoride and others afford some sense of added protection to consumers.
The POU system may include a booster pump, sediment filtration, carbon filtration, reverse osmosis (RO) membrane, pneumatic tank, faucet and a shutoff valve. Post-carbon filtration may not be as helpful unless bacteriostatic materials are used because warmer waters increase the potential for microorganism growth in the cartridge.
There are some considerations to take into account when selecting or installing a POU unit in México.
Home piping in México
Normally, municipal water comes from underground mains to a box in front of the house where the water meter is located. In some areas, the water pressure is so low that the water supply is interrupted. To ensure the possibility of obtaining water, residences in these areas have cisterns or tanks on the roof to store water whenever it’s available.
Homeowners with higher income may have underground cisterns with chlorinating and hydropneumatic systems to keep a pressurized water supply during the day. Middle class homes have a cistern with a pump supplying water to an elevated storage tank (on the roof) where the water comes down by gravity to service the home. On lower income homes, they may have an elevated storage tank or a direct supply with no storage.
The piping may be buried under the floor (ceramic tile) or in the walls (concrete or bricks and cement), or it goes outdoors through the internal yards or “patios.” Otherwise, it may be installed in the ventilation ducts or through skylights.
Standards & water quality
In México, potable water standards are regulated (see Table 1)
Known as México, D.F., for Distrito Federal or Federal District, some neighborhoods located in the west, northwest and southwest parts of the city such as Las Lomas Chapultepec, Bosques de las Lomas, Polanco, Condado de Saavedra and Pedregal de San Ángel have good buying power, a reliable water supply and average total hardness (TH) is <150 parts per million (ppm)—or milligrams per liter (mg/L)—and with total dissolved solids (TDS) is <250 ppm. During the rainy season, they may get a higher turbidity than normal.
In the downtown area, the average TH is <400 ppm with a TDS <600 ppm. In the eastern part of the city, TH is <600 ppm with a TDS <1,000 ppm.
In the areas surounding the Federal District, there are different qualities as the water usually comes from wells and the entity managing the water supply (Comisión del Agua) mixes water from different sources. TH fluctuates from 300-to-600 ppm and the TDS from 400-to-1,000 ppm.
Monterrey, Nuevo León
The higher income neighborhoods are Colonia Del Valle, Valle Oriente and Valle Alto. They are located in the eastern part of the city. Their water quality has a TH of <350 ppm with a TDS <600 ppm. Middle class homes are located in the western part of the city and they have similar water quality. The worst water quality is in the northern part of the city in the neighborhoods of Apodaca and Escobedo. These are industrial zones or neigborhoods with lower economical resources, and contain TH of <1,700 ppm with a TDS <3,500 ppm.
Some higher income neighborhoods are Villa Universitaria with excellent water quality, Jardín del Bosque with high TH, and other neighborhoods such as Puerta de Hierro, Ciudad Bugambilias, El Palomar and Santa Anita, which have good water quality in hardness and TDS, but contain fluoride. Middle- and low-income neighborhoods such as El Rosario, Atlas and San Pedro get their water from the Chapala Lake and have a TDS from 600-to-700 ppm.
The higher income neighborhoods are La Ceiba, La Hacienda, Monte Cristo and Buenavista and they’re located in the northern part of the city. Middle income homes are to the west and east. Throughout the Yucatán Peninsula, the water comes from an aquifer and wells in the region have similar qualities with TH <500 ppm and TDS <700 ppm.
There is a need to remove suspended solids and microorganisms in the water to lower fluoride and arsenic content of the water and to lower TDS and other contaminants in some areas of México that may give rise to complaints from consumers. A well-designed POU/RO system backed up by excellent service may provide the best solution for all these needs.
The author would like to thank Ernesto Castro, International Marketing Development Manager, Osmonics Inc., for additional information and translation of this article. Castro can be reached at (262) 238-4400, (262) 238-4402 (fax) or email: [email protected].
- Mexican Norms for Drinking Water Quality NOM-127-SSA1-1994.
- Mexican Norms for Bottling Purified Water NOM-041-SSA1-1993.
- Mexican Norms for water for human use and consumption, Domestic Water Treatment Equipments NOM-180-SSA1-1998.
- Catalog of Mexican Norms, website: http://www.secofi..gob.mx
- WHO Guidelines for Drinking Water Quality, 1993, website: www.who.int/water_sanitation_health/GDWQ/index.html
- USEPA’s Primary National Drinking Water Standards, website: http://www.usepa.gov/safewater
About the author
Jorge Navarro has worked in water treatment for 34 years with Industrias Mass, S.A. de C.V. The company has been manufacturing equipment and systems for the Mexican water treatment market for 35 years in residential, comercial and industrial markets throughout the country. Navarro works in the technical department supporting Industrias Mass’ sales efforts. He can be reached at +52-5397-9800, +52-5361-6525 (fax) or e-mail: [email protected].