The Different Types of Water Purification Systems

A water filtration system is a great way to get clean, odor free and delicious tasting water. They also help prevent mineral deposits from building up inside your plumbing pipes and appliances.

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Water purification systems are effective against viruses, bacteria and protozoa, however, they do remove healthy minerals. Pump filters such as the Katadyn Hiker or LifeStraw Peak Squeeze offer reliability and longevity, but are larger, heavier and cost more than tablets and gadgets that add chlorine or iodine to your water.

Pre-treatment

A water pre-treatment process is a necessary step in purification. It improves the efficiency and lifespan of the system by removing contaminants that can foul or damage the membranes. This process also reduces the risk of contamination and safety hazards by removing flammable or combustible materials.

Chemical pretreatments include prechlorination, preozonization and adsorption. Chlorine is an effective disinfectant and oxidizer, and can be used to kill bacteria (e.g. Escherichia coli) and some protozoa (Giardia lamblia and Cryptosporidium). However, chlorine in water reacts with natural organic compounds to form potentially harmful chemical by-products, including trihalomethanes and haloacetic acids.

Other pre-treatment methods include screening, filtration and flocculation. Screening removes large particles and sediments, while filtration removes smaller organics and chemicals. Flocculation and coagulation use chemicals to clump together small particles so that they can be more easily removed by filtration. Membrane filters can remove virtually all particles larger than 0.2 microns, which includes bacteria, protozoans and cysts such as Giardia and Cryptosporidia. A water purification plant typically uses a combination of these pre-treatment processes to treat raw water.

Post-treatment

Once the water has been filtered it needs to be disinfected. This is done to deactivate any bacteria or other microorganisms that may still be in the water.

Disinfection processes usually involve adding chlorine, bromine or iodine. While they are effective, they can leave an unpleasant taste and smell in the water. A more effective disinfectant is sodium hypochlorite, which leaves no chemical odor and has no known effect on the environment or human health.

Other disinfection methods used include UV rays and ozonation. These methods can also be used as a tertiary treatment to remove the last remaining contaminants from contaminated water supplies.

Chemicals can also be added to the water to increase its pH level. Lime, soda ash or sodium hydroxide can be used to raise the pH of acidic water. The higher the pH value, the more pure the water is rated as. For best results, it is a good idea to wait until the sun comes out before treating water. The sun will help the chemical react more quickly. The Steri-pen, a battery operated UV light, can be used to kill pathogens in the water in less than a minute, but it is not as effective in colder or darker water.

Reverse Osmosis

Reverse Osmosis is a process that uses pressure to force water molecules through a semipermeable membrane. It removes a variety of contaminants including dissolved salts, metals, and microorganisms such as cryptosporidium and giardia. It is most commonly used in industrial applications.

Osmosis is the natural tendency of a solvent to flow across a semipermeable membrane from the side with higher solute concentration to the side with lower concentration. This movement is driven by a reduction in Gibbs free energy. By applying external pressure, we can reverse this movement and induce osmotic pressure. This is called reverse osmosis.

The RO stage removes a wide range of contaminants from drinking water, and is typically combined with carbon and sediment pre-filters to protect the delicate thin film composite membranes. Most systems also include a storage tank to hold the resulting clean water. Reverse Osmosis water has a very low mineral content and is ideal for aquarists who want to strip all the minerals from their fish’s environment, and then add them back in with a remineralizing filter.

Activated Carbon

Activated carbon is one of the most effective methods for removing organic compounds, allergens and chemicals that can cause illness in your water. Its vast surface area and porosity allow it to adsorb exponentially more contaminants than other traditional filters.

As the water passes through the activated carbon, dissolved impurities are drawn to the surface of its pores by attractive forces. The impurities are then trapped inside the carbon. The adsorption process can remove an array of contaminants, including pesticides, herbicides, chlorine and its dangerous byproduct chloramines. It can also reduce components that result in unwanted tastes and odors.

There are a variety of activated carbon products available to meet your specific needs. They include coal, coconut shell and wood-based activated carbons. Each type is designed for either air or liquid applications. General Carbon can design an adsorber to suit your specific load, contact time and pressure drop requirements.

When choosing an activated carbon, it’s important to look at its iodine number and molasses number. The iodine number measures the volume of the carbon’s pore structure that is capable of adsorbing ionic and polar substances, while the molasses number indicates the maximum capacity of the carbon for decolorization.

Ion Exchange

Ion exchange is a water treatment method that passes a solution through or over ion-exchange resins that contain mobile, or exchangeable, ions. The ions in the solution that have a stronger affinity to the resins are taken up on them, leaving the less-affine contaminants behind. In this way, the ion exchange process removes hardness in water softening applications and all dissolved ions in demineralization. It also removes specific contaminants, such as nitrate, chromate, and arsenic in drinking water, using specialized resins.

Ion exchange resins are made from synthetic organic materials that contain ionic functional groups to which exchangeable ions attach. Once their capacity is exhausted, the resins must be regenerated for re-use.

An ion exchange system can be cheap to run, costing less than $100 per year for a household application. It can be limited by poor water quality, however, because certain contaminants will scale, foul or clog the resin, requiring a pre-treatment process to prevent this. The resins must also be regenerated frequently, since the process only works if the system has constant access to desirable ions.