Water treatment: Introduction

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Water treatment
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Treatment process:

  1. Pretreatment:

In pretreatment, biological contaminants, chemicals, and other materials are removed from water. The first step in that process is screening, which removes large debris such as sticks and trash from the water to be treated. Screening is generally used when purifying surface water such as that from lakes and rivers. Surface water presents a greater risk of having been polluted with large amounts of contaminants. Pretreatment may include the addition of chemicals to control the growth of bacteria in pipes and tanks (prechlorination) and a stage that incorporates sand filtration, which helps suspended solids settle to the bottom of a storage tank.

Preconditioning, in which water with high mineral content (hard water) is treated with sodium carbonate (soda ash), is also part of the pretreatment process. During that step, sodium carbonate is added to the water to force out calcium carbonate, which is one of the main components in shells of marine life and is an active ingredient in agricultural lime. Preconditioning ensures that hard water, which leaves mineral deposits behind that can clog pipes, is altered to achieve the same consistency as soft water.

Prechlorination, which is often the final step of pretreatment and a standard practice in many parts of the world, has been questioned by scientists. During the prechlorination process, chlorine is applied to raw water that may contain high concentrations of natural organic matter. This organic matter reacts with chlorine during the disinfection process and can result in the formation of disinfection by-products (DBPs), such as trihalomethanes, haloacetic acids, chlorite, and bromate. Exposure to DBPs in drinking water can lead to health issues. Worries stem from the practice’s possible association with stomach and bladder cancer and the hazards of releasing chlorine into the environment.

        1. Other purification steps:

After pretreatment, chemical treatment and refinement can occur. That process includes coagulation, a step in which chemicals are added that cause small particles suspended in the water to clump together. Flocculation follows, which mixes the water with large paddles so that coagulated particles can be brought together into larger clumps (or “floc”) that slowly settle on the bottom of the tank or basin.

After the majority of the suspended particles have settled, water exits the flocculation basin and then enters a sedimentation basin. Sedimentation basins move treated waters along through the purification process while allowing remaining particles to settle. Sludge forms that appear on the floor of the tank are removed and treated. From that basin, water is moved to the next step, filtration, which removes the remaining suspended particles and unsettled floc in addition to many microorganisms and algae.

Disinfection is the final step in water purification. During that step, harmful microbes, such as bacteria, viruses, and protozoa, are killed through the addition of disinfectant chemicals. Disinfection usually involves a form of chlorine, especially chloramines or chlorine dioxide. Chlorine is a toxic gas, resulting in some danger from release associated with its use. To avoid those risks, some water treatment plants use ozone, ultraviolet radiation, or hydrogen peroxide disinfection instead of chlorine. Other purification methodologies include ultrafiltration for specific dissolved substances, ion exchange to remove metal ions, and fluoridation to prevent tooth decay.

In certain areas of the world that do not have access to water treatment plants, alternative methods of purification must be used. Those methods include boiling, granular activated-carbon filtering, distillation, reverse osmosis, and direct contact membrane distillation.

        1. Industrial water purification:

In addition to drinking and domestic uses, industries also consume significant amounts of water. Chemical, petroleum, food processing, and textile industries, for example, require water for manufacturing, processing, heating, cooling, washing, rinsing, and other applications. Such industrial systems require treated water, and the lack of appropriate purification can lead to issues such as scaling, corrosion, deposition, bacterial growth within piping or processing equipment, and poor product quality. In addition to conventional water treatment processes, industrial water purification may also involve specialized techniques such as electrodeionization, ion exchange, membrane systems, ozone treatment, evaporation, and ultraviolet irradiation. Technologies selection depends upon the raw water quality and the intended industrial use.

        1. Saline water purification:

The vast majority of communities rely on freshwater resources for drinking and domestic water supplies. However, with shrinking freshwater reserves and rising water demands complicated by natural factors such as droughts, floods, and climate change impacts, several countries have begun to utilize oceans and inland seas as alternative water sources. Desalination technologies that remove salts and minerals from seawater are emerging to produce potable water suitable for drinking and domestic purposes. Reverse osmosis, vacuum distillation, multistage flash distillation, freeze-thaw, and electrodialysis are gaining importance for saltwater purification. Such processes usually involve higher energy consumption and are comparatively more expensive than conventional freshwater treatment processes. Numerous efforts are under way to make desalination methods cost-effective and economically viable.

Fig 5.3.1. A simple water treatment process

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