WATER TREATMENT: WHAT IS IT? AND THE TECHNIQUES USED FOR IT

Water is one of the key substances responsible for the creation and maintenance of life on Earth. Portable Water is an absolute necessity for human life, which at the same time, is getting scarcer over time. We have strived to increase the availability of this essential resource through real-world implementations of scientific advancements. 

There are various ways of water treatment, all of which are used for their specific purposes with their respective advantages and disadvantages.

Carbon Filtration

Granular Activated Carbon Filtration (GAC) is a method of purification that uses activated carbon to purify liquid by the process of adsorption, in which molecules of a liquid are trapped on the surface of a solid or liquid.

Distillation

Distillation is a simple and widely used method for separating mixtures based on differences in the temperature and pressure required to change the phase of components of the mixture. It is most effective when removing inorganic compounds such as metals (iron and lead) and nitrate; hardness (calcium and magnesium); and particulates from a contaminated water supply. The boiling process also kills microorganisms such as bacteria and some viruses. Distilled water has many industrial applications and hence, is more useful in those specific places than in domestic applications.

Some of these uses are:

  • Appliances such as Low Volume Humidifiers and Lead Acid batteries
  • Alcoholic Beverage breweries Laboratory experiments
  • As a suspension medium in Canning factories Raw Materials for Cosmetics


However, all types of distillation require a high amount of Energy per cubic metre due to the fundamental requirement to change the phase of water. Therefore, it is only used when required.

Softening

Involves the exchanging of contaminant ions for Na+ ions.

Demineralization

Involves the exchanging of contaminant ions H+ and OH- ions

Iron or Oxidizing Filtration

Converting Iron, Manganese or Hydrogen Sulphide (H₂S) dissolved in flowing water to their oxides and then filtering them out is called Oxidizing Filtration.

Oxidation by Aeration

Iron is easily oxidized by atmospheric oxygen. Aeration provides the dissolved oxygen needed to convert the iron and manganese from ferrous and Manganous (soluble) forms to insoluble oxidized Ferric and Manganic forms.

Oxidation using Chlorine

Iron and Manganese in water, on application of Chlorine, form Ferric Hydroxide and Manganese Dioxide. These can then be removed by filtration.

Oxidation using Permanganate

Potassium permanganate is typically more effective at oxidizing manganese than aeration or chlorination.

Ozone

Treatment using Ozone (also referred to as Ozonation) is used to easily kill suspended microorganisms and also attack a wide variety of organic compounds in the water using highly reactive Oxygen radicals produced during the process. Ozone is a gas composed of three oxygen atoms (O₃), which is one of the most powerful oxidants. The treatment of water with ozone has a wide range of applications, as it is efficient for disinfection as well as for the degradation of organic and inorganic pollutants. It has a risk of creating Disinfection By-products that are potentially harmful and require further processing.

Sand Filtration

Sand filtration is used for the removal of suspended matter, as well as floating and sinkable particles. The incoming water flows vertically through a fine bed of sand and/or gravel. Particles are removed by way of absorption or physical encapsulation. This is usually used as one of the steps before other filtration processes, or in case the requirements for water filtration levels are not very stringent.

Reverse Osmosis

Reverse Osmosis is a process where water pressure is employed to force water through a Semi- Permeable Membrane (SPM). When water is forced against the reverse osmosis membrane surface, dissolved materials like ions, unwanted molecules and larger particles are repelled, while the water molecules are diffused through the membrane molecule by molecule, forming purer water on the other side. This “reverse osmosis” method is a popular method for the reduction of contaminants in water.

Pre-treatment

The SPM is engineered in such a fashion as to allow only one-way flow through the system. As such, the spiral-wound design does not allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems, the SPM is highly susceptible to fouling (loss of production capacity). Therefore, pre-treatment is a necessity for any reverse osmosis or nanofiltration system.

Prefiltration pH adjustment

If the pH, hardness and alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form.

Membrane assembly

The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. Reverse-osmosis membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fibre. Only a part of the saline feedwater pumped into the membrane assembly passes through the membrane with the salt removed. The remaining “concentrate” flow passes along the saline side of the membrane to flush away the concentrated salt solution. The percentage of desalinated water produced versus the saline water feed flow is known as the “recovery ratio”. This varies with the salinity of the feed water and the system design parameters.

Energy Recovery

Energy recovery can reduce energy consumption by 50% or more. Much of the high-pressure pump input energy can be recovered from the concentrate flow, and the increasing efficiency of energy recovery devices has greatly reduced the energy needs of reverse osmosis desalination. Devices in this role include Turbine(Pelton Wheel), Turbocharger, Pressure Exchanger and Energy Recovery Pump.

Remineralisation and pH adjustment

The desalinated water is stabilized to protect downstream pipelines and storage, usually by adding lime or caustic soda to prevent corrosion of concrete-lined surfaces. Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and corrosion control. Remineralisation may be needed to replace minerals removed from the water by desalination.

Disinfection

Even though RO provides an effective barrier to pathogens, processes like UV treatment and Ozonisation are employed by Water Distribution Services to secure from further effects downstream.

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