WHAT IS ACTIVATED CARBON AND HOW DOES IT REMOVE CONTAMINANTS FROM WATER?

The primary raw material used for activated carbon is any organic material with a high carbon content (coal, wood, peat, coconut shells). Granular activated carbon media is most commonly produced by grinding the raw material, adding a suitable binder to give it hardness, re-compacting and crushing to the correct size. The carbon-based material is converted to activated carbon by thermal decomposition in a furnace using a controlled atmosphere and heat. The resultant product has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place. The walls of the pores provide the surface layer molecules essential for adsorption. Amazingly, one pound of carbon (a quart container) provides a surface area equivalent to six football fields. Physical Adsorption is the primary means by which activated carbon works to remove contaminants from water. The highly porous nature of carbon provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to. The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores. The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution. Those compounds that are more readily adsorbed onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and are neutral or non-polar chemical in nature.

It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate. Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions. This reaction is important to POU treatment because this conversion of chlorine to chloride is the basis for the removal of some common objectionable tastes and odors from drinking water. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution.

HOW DOES A WATER SOFTENER WORK?

Water softeners use synthetic resin beads to remove potentially troublesome “hard” water minerals (minerals are called ions after they have been dissolved in water) from normal tap water in order to decrease equipment maintenance, increase system efficiency and lengthen the service lives of certain water purification system components by eliminating the possibility of developing hard mineral deposition on this equipment. Water heating equipment and water purification membranes are two types of equipment that are susceptible to mineral deposition and subsequent fouling. Hardness mineral free water is referred to as “soft” water. Water softener resin beads contain ion receptor sites which initially hold only sodium ions. As tap water that contains “hardness” ions (calcium and magnesium) passes through the resin, the calcium and magnesium ions are attracted to the beads, and are exchanged for sodium (“soft water ion”). This process is called ION EXCHANGE, since there is not a net removal of material from the water, simply an exchange of two TYPES of ions. This exchange continues until there are no longer any sites at which the exchange can take place. The resin is then considered “exhausted” and must be recharged with sodium ions in order for the unit to continue working to remove hardness ions from the feed water. During the recharge, or regeneration, process, a concentrated sodium brine solution is rinsed through the resin. The resin prefers sodium ions, when present in higher than normal concentrations, over calcium and magnesium ions, so calcium and magnesium ions are bumped off and go down the drain, while the sodium ions fill all the available receptor sites on the water softener resin beads. The resin is then ready for continued service.

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HOW DOES A REVERSE OSMOSIS MEMBRANE WORK?

Reverse Osmosis (RO) is a modern process technology used to purify water by efficient removal of dissolved contaminants. Reverse Osmosis Systems utilize semi-permeable membranes to separate the dissolved contaminants from the processed water. Rather than retaining the separated contaminants, like a particulate filter collects sediment, the reverse osmosis system is designed to continuously flush the contaminants, still in a concentrated, solution state, to drain. “Semi-permeable” refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product. Osmosis is a natural process involving the fluid flow of across a semi-permeable membrane barrier. Consider a tank of pure water with a semi-permeable membrane dividing it into two sides. Pure water in contact with both sides of an ideal semi-permeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides. If a soluble salt is added on one side, the chemical potential of this salt solution is reduced. Osmotic flow from the pure water side across the membrane to the salt solution side will occur until the equilibrium of chemical potential is restored (Figure 1a). In scientific terms, the two sides of the tank have a difference in their “chemical potentials,” and the solution equalizes, by osmosis, its chemical potential throughout the system. Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides is equal to the osmotic pressure. The osmotic pressure is a solution property proportional to the salt concentration and independent of the membrane. With the tank in Figure 1a, the water moves to the salty side of the membrane until equilibrium is achieved. Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium (Figure 1b). Additional pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential. This phenomenon is called reverse osmosis.

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The driving force of the reverse osmosis process is applied pressure. The amount of energy required for osmotic separation is directly related to the salinity of the solution. Thus, more energy is required to produce the same amount of water from solutions with higher concentrations of salt. In the real world, the “salt solution” is the source water to be purified. The HydroMax Reverse Osmosis System utilizes a pressure pump to apply pressure to this salt solution and drives purified water through the membrane while rejecting nearly all of the dissolved contaminants and flushing them to drain. The membrane barrier in a HydroMax system actually is configured into a space saving, convenient, tubular design as pictured below.

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WHAT DOES ULTRAVIOLET DO FOR THE PURIFIED WATER?

Ultraviolet light is part of the light spectrum, which is classified into three wavelength ranges:

  • UV-C, from 100 nanometers (nm) to 280nm

  • UV-B, from 280nm to 315nm

  • UV-A, from 315nm to 400nm

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UV-C light is germicidal - i.e., it deactivates the DNA of bacteria, viruses and other pathogens and thus destroys their ability to multiply and cause disease. Specifically, UV-C light causes damage to the nucleic acid of microorganisms by forming covalent bonds between certain adjacent bases in the DNA. The formation of such bonds prevent the DNA from being unzipped for replication, and the organism is unable to reproduce. In fact, when the organism tries to replicate, it dies.

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Ultraviolet technology is a non-chemical approach to disinfection. In this method of disinfection, nothing is added to the water, which makes this process simple, inexpensive and requires very little maintenance. Ultraviolet purifiers utilize germicidal lamps that are designed and calculated to produce a certain dosage of ultraviolet (usually at least 16,000 microwatt seconds per square centimeter but many units actually have a much higher dosage.) The principle of design is based on a product of time and intensity - you must have a certain amount of both for a successful design. In the real world, Ultraviolet treatment technology is very useful and effictive at controlling bacteria in bulk flows of water being processed in order to help prevent active biological passage into downstream processes or storage. Ultraviolet is, however, not designed to eliminate all possibilities of downstream biological growth or contamination and other additional methods of biological control, such as chlorination, are necessary for potable water storage and distribution type water systems.

WHY DOES THE PURIFIED WATER NEED TO BE REMINERALIZED WHEN BEING SUPPLIED FOR POTABLE WATER USE?

All natural water contains a small amount of dissolved carbon dioxide. Gasses readily pass through semi-permeable membranes and when water is processed by reverse osmosis, the carbon dioxide passes through the membrane with the purified water. Since most of the alkalinity (natural pH buffering ions dissolved in tap water) are removed by the reverse osmosis process, the excess carbon dioxide causes the pH of the purified water to become acidic (low pH). Acidic water, especially water containing a low amount of dissolved ions, like reverse osmosis water, will tend to corrode and dissolve metallic piping components. When reverse osmosis water is used in typical domestic water piping systems composed of copper or iron piping materials, it is wise to temper the aggressive nature of the purified water so that the piping systems’ integrity can be maintained and so that excess dissolved metals do not end up in water being used for drinking. It is also commonly desired to replace a small amount of “good” mineral into the purified water in order to improve the palatability of the water for drinking and to aid in soap removal during washing and bathing. In order to correct the purified water chemistry, the HydroMax system uses calcite (crushed, pure limestone) filtration media to neutralize the excess carbon dioxide and raise the pH of the purified water. The calcite media dissolves very slowly in the purified water flowing through the remineralization filters, releasing a small amount of calcium. Fresh calcite media will need to be added to the remineralization filters periodically to replace that which dissolves. Potable water can also require a chemical addition in order to increase the pH and alkalinity of the purified water in order to help prevent water distribution system component and piping corrosion.

WHEN DOES CHLORINE NEED TO BE ADDED TO PURIFIED DRINKING WATER WATER?

Even though the HydroMax system uses multiple and various technologies to purify the water, it is necessary to provide a chlorine residual in the finished water in order to prevent biological recontamination during water storage and during distribution through piping systems to the locations where the water is to be used or consumed. In the United States, the Safe Drinking Water Act legislates that all public water supplies contain adequate chlorine concentrations to prevent the presence of coliform bacteria. Chlorine destroys nearly all types of biological life forms by oxidation. Direct contact with a strong oxidizer will cause the cell walls of a bacteria to rupture, effectively deactivating the organism. The HydroMax system injects sodium hypochlorite solution (bleach), a common, inexpensive source of chlorine, into the purified water in order to create legal, active bacteria free potable water.