DI Filtration

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Direct Ion Exchange (DI)

Water Softening (IE removal of Calcium)

Ion exchange can be defined as the reversible interchange of ions between a solid and a liquid phase in which there is no permanent change in the structure of the solid. Typically, in water softening by ion exchange, the water containing the hardness is passed through a column containing the ion-exchange material. The hardness in the water exchanges with an ion from the ion-exchange material. Generally, the ion exchanged with the hardness is sodium. Calcium or magnesium is removed from the water and replaced by an equivalent amount of sodium, that is, two sodium atoms for each cation. The alkalinity remains unchanged. The exchange results in essentially 100 percent removal of the hardness from the water until the ion-exchange material is reached. When the ion-exchange material becomes saturated, no hardness will be removed. At this point breakthrough is said to have occurred between the hardness and the bed.

The ion exchange material can be either naturally occurring clays, called zeolites, or synthetically made resins. There are several manufacturers for synthetic resins. The resins or zeolites are characterized by the amount of hardness that they will remove per volume of resin material and by the amount of salt required to regenerate the resin. People who work in the water treatment industry often work in units of hardness per gallon of water (gr/gal). It is useful to remember that 1 gr/gal equals 17.1 mg/L.

Mixed Bed Media In A DI Filter

It works by exchanging hydrogen ions for cationic and hydroxyl ions for anionic contaminants in the feedwater. The deionization resins are tiny spherical plastic beads through which the feedwater passes. After a while the impurities replace all of the hydrogen and hydroxyl groups in the resin, and it has to be replaced or regenerated.

Activated Carbon Filtration

Activated carbon is widely used in water filtration systems. Carbon is very effective at improving taste, removing chlorine and many other contaminents. A drawback to Activated Carbon is that its high surface area and the Carbon itself promote bacterial growth. Pseudemonas Bacteria has been found to grow in many carbon filters and activated carbon systems.

Activated carbon (AC) filtration is most effective in removing organic contaminants from water. Organic substances are composed of two basic elements, carbon and hydrogen. Because organic chemicals are often responsible for taste, odor, and color problems, AC filtration can generally be used to improve aesthetically objectional water. AC filtration will also remove chlorine.

Water contaminants that can be reduced to acceptable standards by activated carbon filtration.

(Water Quality Association, 1989)

Primary Drinking Water Standards
Contaminant mg/L max

Inorganic Contaminants

Organic Arsenic Complexes 0.05
Organic Chromium Complexes 0.05
Mercury (Hg_2) Inorganic 0.05
Organic Mercury Complexes 0.002

Organic Contaminants

Benzene 0.005
Endrin 0.0002
Lindane 0.004
Methoxychlor 0.1
1,2-dichloroethane 0.005
1,1-dichloroethylene 0.007
1,1,1-trichloroethane 0.200
Total Trihalomethanes (TTHMs) 0.10
Toxaphene 0.005
Trichloroethylene 0.005
2,4-D 0.1
2,4,5-TP (Silvex) 0.01
Para-dichlorobenzene 0.075

Secondary Drinking Water Standards
Contaminant **SMCL

Color 15 color units
Foaming Agents (MBAS) 0.5 mg/L
Odor 3 threshold
or number

Activated carbon filtration does remove some organic chemicals that can be harmful if present in quantities above the EPA Health Advisory Level 'HAL'. Included in this category are trihalomethanes 'THM', pesticides, industrial solvents 'halogenated hydrocarbons',

polychlorinated biphenyls 'PCBs', and polycyclic aromatic hydrocarbons 'PAHs'

THMs are a byproduct of the chlorination process that most public drinking water systems use for disinfection. Chloroform is the primary THM of concern. EPA does not allow public systems to have more than 100 parts per billion 'ppb' of THMs in their treated water. Some municipal systems have had difficulty in meeting this standard.

· What activated carbon does NOT remove

Similar to other types of water treatment, activated carbon filtration is effective for some contaminants and not effective for others. activated carbon filtration does not remove microbes, sodium, nitrates, fluoride, and hardness. Lead and other heavy metals are removed only by a very specific type of activated carbon filter. Unless the manufacturer states that its product will remove heavy metals, the consumer should assume that the activated carbon filter is not effective in removing them.

· The Activated Carbon Filtration Process

Activated carbon works by attracting and holding certain chemicals as water passes through it. activated carbon is a highly porous material; therefore, it has an extremely high surface area for contaminant adsorption. The equivalent surface area of 1 pound of activated carbon ranges from 60 to 150 acres.

Activated carbon is made of tiny clusters of carbon atoms stacked upon one another. The carbon source is a variety of materials, such as peanut shells or coal. The raw carbon source is slowly heated in the absence of air to produce a high carbon material. The carbon is activated by passing oxidizing gases through the material at extremely high temperatures. The activation process produces the pores that result in such high adsorptive properties.

The adsorption process depends on the following factors: 1' physical properties of the AC, such as pore size distribution and surface area; 2' the chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it; 3' chemical composition and concentration of the contaminant; 4' the temperature and pH of the water; and 5' the flow rate or time exposure of water to activated carbon.

· Physical Properties

Forces of physical attraction or adsorption of contaminants to the pore walls is the most important activated carbon filtration process. The amount and distribution of pores play key roles in determining how well contaminants are filtered. The best filtration occurs when pores are barely large enough to admit the contaminant molecule 'Figure 1'. Because contaminants come in all different sizes, they are attracted differently depending on pore size of the filter. In general activated carbon filters are most effective in removing contaminants that have relatively large molecules 'most organic chemicals'. Type of raw carbon material and its method of activation will affect types of contaminants that are adsorbed. This is largely due to the influence that raw material and activation have on pore size and distribution.

· Chemical Properties

Processes other than physical attraction also affect activated carbon filtration. The filter surface may actually interact chemically with organic molecules. Also electrical forces between the activated carbon surface and some contaminants may result in adsorption or ion exchange. Adsorption, then, is also affected by the chemical nature of the adsorbing surface. The chemical properties of the adsorbing surface are determined to a large extent by the activation process. activated carbon materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants. For example chloroform is adsorbed best by activated carbon that has the least amount of oxygen associated with the pore surfaces. The consumer can't possibly determine the chemical nature of an activated carbon filter. However, this does point out the fact that different types of activated carbon filters will have varying levels of effectiveness in treating different chemicals. The manufacturer should be consulted to determine if their filter will adequately treat the consumer's specific water problem.
· Contaminant Properties

Large organic molecules are most effectively adsorbed by Activated Carbon. A general rule of thumb is that similar materials tend to associate. Organic molecules and activated carbon are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the activated carbon in the filter rather than staying dissolved in a dissimilar material like water. Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores.

Concentration of organic contaminants can affect the adsorption process. A given activated carbon filter may be more effective than another type of activated carbon filter at low contaminant concentrations, but may be less effective than the other filter at high concentrations. This type of behavior has been observed with chloroform removal. The filter manufacturer should be consulted to determine how the filter will perform for specific chemicals at different levels of contamination.

· Water Temperature and pH

Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered many organic chemicals are in a more adsorbable form.

· Exposure Time

The process of adsorption is also influenced by the length of time that the activated carbon is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of activated carbon in the filter and reducing the flow rate of water through the filter.


1989. Recognized treatment techniques for meeting the National Primary Drinking Water Regulations with the application of point-of-use systems. Water Quality Association, Lisle, Il.

1989. Recognized treatment techniques for meeting the National Secondary Drinking Water Regulations with the application of point-of-use systems.

1990. Fit to drink? Consumer Reports. pp. 27-43, January.

Caldron, R. L., and E. W. Mood. 1987. Bacteria colonizing point-of-use, granular activated carbon filters and their relationship to human health. Research Project CR-811904-01-0, Health Effects Research Lab., U.S. EPA, Cincinnati, OH. Reprinted by the Water Quality Association, Lisle, IL.

Culp, G. L. and R. L. Culp. 1974. New concepts in water purification. Van Nostrand Reinhold Co., New York.

Ishizake, C., I. Marti, and M. Ruiz. 1983. Effect of surface characteristics of activated carbon on the adsorption of chloroform from aqueous solution. In M. J. McGuire and I. H. Suffet 'ed.', pp. 95-106. Treatment of water by granular activated carbon. Advances in Chemistry Series. American Chemical Society, Washington, D.C.

Rodale Press Product Testing Department Staff. 1985. Water treatment handbook - A homeowners quide to safer drinking water. Rodale Press Inc., Emmaus, PA.

Taraba, J. L., L. M. Heaton, and T. W. Ilvento. 1990. Using activated carbon filters to treat home drinking water, IP-6. University of Kentucky Cooperative Extension Service, Lexington, KY.

Temple, Barker, and Sloan Inc. Staff. 1983. Point-of-use treatment for compliance with drinking water standards. Reprinted by the Water Quality Association, Lisle, IL.

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