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El agua mineral embotellada generalmente contiene niveles más altos de TDS que el agua del grifo.

El total de sólidos disueltos ( TDS ) es una medida del contenido combinado disuelto de todas las sustancias inorgánicas y orgánicas presentes en un líquido en forma suspendida molecular , ionizada o microgranular ( sol coloidal ). Las concentraciones de TDS a menudo se informan en partes por millón (ppm). Las concentraciones de TDS en agua se pueden determinar utilizando un medidor digital. [1]

Generalmente, la definición operativa es que los sólidos deben ser lo suficientemente pequeños para sobrevivir a la filtración a través de un filtro con poros de 2 micrómetros (tamaño nominal o más pequeños). Los sólidos disueltos totales normalmente se analizan solo para los sistemas de agua dulce , ya que la salinidad incluye algunos de los iones que constituyen la definición de TDS. La principal aplicación de TDS es el estudio de la calidad del agua de arroyos , ríos y lagos . Aunque el TDS generalmente no se considera un contaminante primario (por ejemplo, no se considera que esté asociado con efectos en la salud), se usa como una indicación de las características estéticas del agua potable.y como indicador agregado de la presencia de una amplia gama de contaminantes químicos.

Las fuentes primarias de TDS en las aguas receptoras son la escorrentía agrícola y la escorrentía residencial (urbana) , las aguas de montaña ricas en arcilla, la lixiviación de la contaminación del suelo y la descarga de contaminación del agua de fuente puntual de plantas industriales o de tratamiento de aguas residuales . Los componentes químicos más comunes son calcio , fosfatos , nitratos , sodio , potasio y cloruro , que se encuentran en la escorrentía de nutrientes , la escorrentía general de aguas pluviales y la escorrentía de climas nevados donde el deshielo de carreterasse aplican sales. Los productos químicos pueden ser cationes , aniones , moléculas o aglomeraciones en el orden de mil o menos moléculas, siempre que un micro soluble gránulo se forma. Los elementos más exóticos y dañinos de los TDS son los pesticidas que surgen de la escorrentía superficial . Ciertos sólidos disueltos totales que ocurren naturalmente surgen de la meteorización y disolución de rocas y suelos. Estados Unidos ha establecido un estándar secundario de calidad del agua de 500 mg / l para garantizar la palatabilidad del agua potable.

Los sólidos totales disueltos se diferencian de los sólidos totales en suspensión (SST), en que estos últimos no pueden pasar por un tamiz de 2 micrómetros y, sin embargo, están indefinidamente suspendidos en solución. El término sólidos sedimentables se refiere a material de cualquier tamaño que no permanecerá suspendido o disuelto en un tanque de retención que no esté sujeto a movimiento, y excluye tanto TDS como TSS. [2] Los sólidos sedimentables pueden incluir partículas más grandes o moléculas insolubles.

Los sólidos disueltos totales incluyen sólidos tanto volátiles como no volátiles. Los sólidos volátiles son aquellos que pueden pasar fácilmente de un estado sólido a un estado líquido. Los sólidos no volátiles deben calentarse a una temperatura alta, típicamente 550 ° C, para lograr este cambio de estado. Los ejemplos de sustancias no volátiles incluyen sales y azúcares. [3]

Medida [ editar ]

Medidor de TDS utilizado para probar la pureza del agua

Los dos métodos principales para medir el total de sólidos disueltos son el análisis gravimétrico y la conductividad . [4] Los métodos gravimétricos son los más precisos e implican la evaporación del solvente líquido y la medición de la masa de residuos que quedan. Este método es generalmente el mejor, aunque requiere mucho tiempo. Si las sales inorgánicas comprenden la gran mayoría de TDS, los métodos gravimétricos son apropiados.

La conductividad eléctrica o específica del agua está directamente relacionada con la concentración de sólidos ionizados disueltos en el agua. Los iones de los sólidos disueltos en el agua crean la capacidad de que el agua conduzca una corriente eléctrica , que se puede medir con un medidor de conductividad convencional o un medidor de TDS . Cuando se correlaciona con las mediciones de TDS de laboratorio, la conductividad proporciona un valor aproximado para la concentración de TDS , generalmente con una precisión del diez por ciento.

La relación de TDS y la conductancia específica del agua subterránea se puede aproximar mediante la siguiente ecuación:

TDS = k e EC

where TDS is expressed in mg/L and EC is the electrical conductivity in microsiemens per centimeter at 25 °C. The correlation factor ke varies between 0.55 and 0.8.[5]

Some TDS meters will use this electrical conductivity measurement to then infer the number of parts per million (ppm); 1 ppm indicates 1 mg of dissolved solids per kg of water.[6]

Hydrological simulation[edit]

See also: Hydrological transport model
Pyramid Lake, Nevada receives dissolved solids from the Truckee River.

Hydrologic transport models are used to mathematically analyze movement of TDS within river systems. The most common models address surface runoff, allowing variation in land use type, topography, soil type, vegetative cover, precipitation, and land management practice (e.g. the application rate of a fertilizer). Runoff models have evolved to a good degree of accuracy and permit the evaluation of alternative land management practices upon impacts to stream water quality.

Basin models are used to more comprehensively evaluate total dissolved solids within a catchment basin and dynamically along various stream reaches. The DSSAM model was developed by the U.S. Environmental Protection Agency (EPA).[7] This hydrology transport model is actually based upon the pollutant-loading metric called "Total Maximum Daily Load" (TMDL), which addresses TDS and other specific chemical pollutants. The success of this model contributed to the Agency's broadened commitment to the use of the underlying TMDL protocol in its national policy for management of many river systems in the United States.[8]

Practical implications[edit]

Aquarium at Bristol Zoo, England. Maintenance of filters becomes costly with high TDS.

When measuring water treated with water softeners, high levels of total dissolved solids do not correlate to hard water, as water softeners do not reduce TDS; rather, they replace magnesium and calcium ions, which cause hard water, with an equal charge of sodium or potassium ions, e.g. Ca2+ ⇌ 2 Na+, leaving overall TDS unchanged[9] or even increased. Hard water can cause scale buildup in pipes, valves, and filters, reducing performance and adding to system maintenance costs. These effects can be seen in aquariums, spas, swimming pools, and reverse osmosis water treatment systems. Typically, in these applications, total dissolved solids are tested frequently, and filtration membranes are checked in order to prevent adverse effects.

In the case of hydroponics and aquaculture, TDS is often monitored in order to create a water quality environment favorable for organism productivity. For freshwater oysters, trouts, and other high value seafood, highest productivity and economic returns are achieved by mimicking the TDS and pH levels of each species' native environment. For hydroponic uses, total dissolved solids is considered one of the best indices of nutrient availability for the aquatic plants being grown.

Because the threshold of acceptable aesthetic criteria for human drinking water is 500 mg/l, there is no general concern for odor, taste, and color at a level much lower than is required for harm. A number of studies have been conducted and indicate various species' reactions range from intolerance to outright toxicity due to elevated TDS. The numerical results must be interpreted cautiously, as true toxicity outcomes will relate to specific chemical constituents. Nevertheless, some numerical information is a useful guide to the nature of risks in exposing aquatic organisms or terrestrial animals to high TDS levels. Most aquatic ecosystems involving mixed fish fauna can tolerate TDS levels of 1000 mg/l.[10]

Daphnia magna with eggs

The Fathead minnow (Pimephales promelas), for example, realizes an LD50 concentration of 5600 ppm based upon a 96-hour exposure. LD50 is the concentration required to produce a lethal effect on 50 percent of the exposed population. Daphnia magna, a good example of a primary member of the food chain, is a small planktonic crustacean, about 0.5 mm in length, having an LD50 of about 10,000 ppm TDS for a 96-hour exposure.[11]

Spawning fishes and juveniles appear to be more sensitive to high TDS levels. For example, it was found that concentrations of 350 mg/l TDS reduced spawning of Striped bass (Morone saxatilis) in the San Francisco Bay-Delta region, and that concentrations below 200 mg/l promoted even healthier spawning conditions.[12] In the Truckee River, EPA found that juvenile Lahontan cutthroat trout were subject to higher mortality when exposed to thermal pollution stress combined with high total dissolved solids concentrations.[7]

For terrestrial animals, poultry typically possess a safe upper limit of TDS exposure of approximately 2900 mg/l, whereas dairy cattle are measured to have a safe upper limit of about 7100 mg/l. Research has shown that exposure to TDS is compounded in toxicity when other stressors are present, such as abnormal pH, high turbidity, or reduced dissolved oxygen with the latter stressor acting only in the case of animalia.[13]

In countries with often-unsafe/unclean tap water supplies, the TDS of drinking water is often checked by technicians to gauge how effectively their RO/Water Filtration devices are working. While TDS readings will not give an answer as to the amount of microorganisms present in a sample of water, they can get a good idea as to the efficiency of the filter by how much TDS is present.

Water classification[edit]

[14]Water can be classified by the level of total dissolved solids (TDS) in the water:

  • Fresh water: TDS is less than 1,000 ppm
  • Brackish water: TDS = 1,000 to 10,000 ppm
  • Saline water: TDS = 10,000 to 35,000 ppm
  • Hypersaline:TDS greater than 35,000 ppm

Drinking water generally has a TDS below 500 ppm. Higher TDS Fresh Water is drinkable but taste may be objectionable.

See also[edit]

  • Acid rain
  • Surface runoff
  • Regarding meters:
    • EC meter
    • pH meter
    • Salinometer

References[edit]

  1. ^ "What Is The Acceptable Total Dissolved Solids (TDS) Level In Drinking Water?". The Berkey. Retrieved 2020-02-22.
  2. ^ DeZuane, John (1997). Handbook of Drinking Water Quality (2nd ed.). John Wiley and Sons. ISBN 0-471-28789-X.
  3. ^ Wetzel, R. G. (2001). Limnology: Lake and river ecosystems. San Diego: Academic Press.
  4. ^ "Total Dissolved Solids (TDS): EPA Method 160.1 (Gravimetric, Dried at 180 deg. C)". Washington, D.C.: U.S. Environmental Protection Agency (EPA). 1999-11-16. Archived from the original on 2016-02-23.
  5. ^ Atekwanaa, Eliot A.; Atekwanaa, Estella A.; Roweb, Rebecca S.; Werkema Jr., D. Dale; Legalld, Franklyn D. (2004). "The relationship of total dissolved solids measurements to bulk electrical conductivity in an aquifer contaminated with hydrocarbon" (PDF). Journal of Applied Geophysics. Elsevier. 56 (4): 281–294. Bibcode:2004JAG....56..281A. doi:10.1016/j.jappgeo.2004.08.003. Retrieved 15 February 2016.
  6. ^ "Frequently Asked Questions". Archived from the original on 2017-06-18. Retrieved 23 May 2017.CS1 maint: unfit URL (link)
  7. ^ a b C.M. Hogan, Marc Papineau et al. Development of a dynamic water quality simulation model for the Truckee River, Earth Metrics Inc., Environmental Protection Agency Technology Series, Washington D.C. (1987)
  8. ^ EPA. "Guidance for Water Quality-Based Decisions: The TMDL Process." Doc. No. EPA 440/4-91-001. April 1991.
  9. ^ W. Adam Sigler, Jim Bauder. "TDS Fact Sheet". Montana State University. Archived from the original on 2015-04-29. Retrieved 23 January 2015.
  10. ^ Boyd, Claude E. (1999). Water Quality: An Introduction. The Netherlands: Kluwer Academic Publishers Group. ISBN 0-7923-7853-9.
  11. ^ Position Paper on Total Dissolved Solids, State of Iowa, IAC 567 61.3 (2)g et sequitur updated March 27, 2003
  12. ^ Kaiser Engineers, California, Final Report to the State of California, San Francisco Bay-Delta Water Quality Control Program, State of California, Sacramento, CA (1969)
  13. ^ Hogan, C. Michael; Patmore, Leda C.; Seidman, Harry (August 1973). "Statistical Prediction of Dynamic Thermal Equilibrium Temperatures using Standard Meteorological Data Bases". EPA. Retrieved 2016-02-15. Cite journal requires |journal= (help) Environmental Protection Technology Series. Document No. EPA-660/2-73-003.
  14. ^ https://www.usgs.gov/special-topic/water-science-school/science/saline-water-and-salinity?qt-science_center_objects=0#qt-science_center_objects