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Municipal water treatment.

The municipal water supplied by public systems to supply the population undergoes a process of collection, transportation, treatment, storage, and distribution of the liquid. The first stage in this process is the collection of water, which may come from surface sources, such as rivers and lakes, or from subway sources. It must be considered that water in its natural state may contain a variety of potential contaminants, ranging from microbiological pollutants to chemicals and sediments. Rivers and lakes may be exposed to contamination from human activities, such as agriculture and industry, making the water susceptible to the presence of chemical and organic contaminants. Groundwater sources, although often cleaner, are not without risk, as they may contain excess minerals or have inadequate pH levels for use in everyday activities by the population. This is why municipal water must pass through a water treatment plant that is capable of providing safe water for general use. This is achieved through coagulation, flocculation, sedimentation, filtration and disinfection processes.

Municipal water catchment

As mentioned above, municipal water comes from a variety of sources where contamination is commonly present. The water is collected from rivers, lakes, ground water, etc. The water is extracted by pump systems that carry the fluid through the various stages of the water treatment plant. Before reaching any other stage, the water passes through screens of different sizes that retain sediments and solids of considerable size. It works as a first barrier that makes the following purification steps more efficient.

Coagulation

The water reaches tanks where solids, sediments and particles that could not be retained by the screen system remain. In coagulation for municipal water, chemicals such as aluminum sulfate and ferric chloride are added to the raw water. Coagulants work by neutralizing the electrical charge of the particles present in the water, as they usually have negative charges causing them to repel each other and remain in suspension. The coagulating agent forms positively charged ions, leading to neutralization and the formation of floccules or clots of the suspended particles.

Sedimentation

Once the clots or flocs are formed, they rise to the surface of the water, removing contaminating elements such as algae and plankton. As the particles accumulate in a single clump, their size and weight becomes greater, allowing them to reach the bottom of sedimentation tanks by gravity until they accumulate and form sediment sludge. The municipal water without sediment or particles remains in the upper part of the tank with greater clarity and ready to pass to the following processes of potabilization.

Filtration

Although a considerable amount of contaminants are removed in the collection and sedimentation process, the water may contain fine particles that could not be sedimented, so the water is treated by filtration in porous media. These fine particles may be so small that they tend to remain suspended in the water due to their low settling velocity. Even when chemical coagulants are used to group these particles into larger clumps, some fine particles may escape the sedimentation process and remain in the liquid phase of the water.

Activated Carbon

Activated carbon is a purifier thanks to its chemical adsorption; however, it is not used in all water treatment plants in Mexico, unlike water treatment plants in the United States and Europe. Its application in water treatment plants helps eliminate undesirable substances, including pesticides, chlorine disinfection by-products and organic compounds. It also improves water clarity, odor, taste, reduces turbidity and removes suspended particles.

Disinfection

Finally, municipal water must be disinfected because, although the above purification processes clarify the water, this does not mean that it is free of viruses and bacteria. For the elimination of microorganisms, chlorine must be applied, which through its oxidizing qualities disinfects municipal water. The hypochlorite ion (OCl-) and hypochlorous acid ion (HClO) can penetrate the cells of microorganisms and react with components such as proteins and nucleic acids, effectively inactivating the microorganisms and preventing their ability to reproduce and cause disease.

Municipal water supplies

Granular media

Filter media use a porous matrix composed of solid particles, such as sand, gravel, zeolite, among others. When the fluid passes through this matrix, the larger solid particles are trapped due to the reduction of the space between the particles in the media. This is due to the sieving action and the force of gravity. The smaller particles pass through the media, while the larger particles are retained on the surface or in the structure of the filter media.

The relationship between the average particle diameter of the filter media and its ability to retain solids is governed by a fundamental principle in filtration science. The smaller the average particle diameter of the media, the greater the efficiency in retaining solid particles in the fluid passing through it.

Activated mineral carbon

The adsorption process on activated carbon is based on intermolecular forces, such as Van der Waals forces and the formation of weak chemical bonds. Particles and organic compounds in the water are attracted to the surface of the activated carbon and adhere to it due to these forces. This involves a transfer of the pollutants from the water to the activated carbon solid, which reduces the concentration of pollutants in the water.

Activated carbon is highly effective in removing organic contaminants, volatile organic compounds, pesticides and undesirable flavors/odors. However, over time, as the carbon becomes saturated with contaminants, its adsorption capacity decreases and it needs to be regenerated or replaced.

Benefits of municipal water treatment

  • Public health care: Municipal water purification removes pathogenic microorganisms, chemical contaminants and other hazardous substances from the water, significantly reducing the risk of disease.
  • Access to quality water: Municipal water purification ensures that communities have consistent access to high quality water that meets safety and quality standards.
  • Improved quality of life: Access to safe, quality potable municipal water improves people's quality of life by facilitating personal hygiene, sanitation and food preparation.

Other sources.

CONAGUA: What is a water treatment plant?

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