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    Activated Carbon: Filtration, Contactor or Both?


    By Greg Reyneke, CWS-VI

    As I chat with dealers around the country, I have sadly come to realize that many don’t understand the dynamics of what they’re selling. All water treatment technologies involve a physical change to the water that they are processing. The nature of the change truly defines the technology.

    Subtractive treatment

    The most commonly understood and frequently used type of water treatment technology is subtractive; something is removed from water for health or aesthetic reasons. Contaminants are physically removed from the water using a physical separation or adsorptive process. Within the subtractive realm, filters generally comprise depth, barrier or membrane types:

    · Depth filters use physical media in granular form of varying sizes and densities to physically filter contaminants.

    · Barrier filters use an organic, synthetic or metallic screen that provides a barrier to passage of contaminants, based on their physical size.

    · Membrane separators typically use a semipermeable membrane wrapped around a central core, allowing selective passage of water and contaminants, depending on the size, molecular weight and even electrical charge of the contaminant(s). Ultrafiltration, nanofiltration and reverse osmosis purifiers are types of membrane separators.

    Reverse osmosis separation is an extremely effective form of water purification. Contaminants are removed from a typical residential RO purifier through the combined technologies of physical filtration, absorption, adsorption and membrane separation. Multiple technologies are used to ensure maximum removal of contaminants from the water. Prefiltersin a typical RO purifier are designed to protect the membrane from oxidants and physically large contaminants that would prematurely foul the membrane pores. While protected by the prefilters, the semi-permeable RO membrane works to reject as many contaminants as possible from the water. The membrane operates under pressure from the mu- nicipal supply or with the assistance of a pressure-boosting pump. As a crossflow filter, the membrane allows passage of purified water through its pores, while a waste stream flows to drain. Backpressure from the drain restrictor creates sufficient pressure against the membrane to allow for rapid processing while continuously cleaning the membrane. This purification process is quite slow in residential systems, with a typical purifier being able to purify approximately 0.05 gpm. Purified water is typically stored in a pneumatically charged tank that allows for repressurization and accumulation in a sanitary environment.

    Manufacturers of residential RO purifiers make substantial claims to remove contaminants. These claims are required by most states to be verified by an independent certifying/testing body, such as the National Sanitation Foundation (NSF), which has established standards of performance. NSF/ANSI Standard 58 establishes the minimum requirements for the certification of POU reverse osmosis systems designed to reduce contaminants that may be present in public or private drinking water. The scope of NSF/ANSI 58 includes material safety, structural integrity, TDS reduction and other optional contaminant reduction claims. The most common optional claims addressed by the standard include cyst reduction, hexavalent and trivalent chromium reduction, arsenic reduction, nitrate/nitrite reduction and cadmium and lead reduction. Consumers seeking RO purifiers are looking for water that is as functionally close to pure as possible. This water is often described as empty water to describe its high level of purity (see Table 1 for typical performance).

    Exchange technology

    Exchange technology removes one contaminantfrom the water while replacing it with an alternate. Both natural and synthetic materials can act as an ion exchanger and depending on the functional matrix of the ion exchange media, various positively and negatively charged contaminants can be removed from water. A water softener is an example of ion-exchange technology.

    Additive technology

    Additive technologies change the nature of water through addition of chemicals to the water. Various things are typically added to water, such as pH neutralizers, ions, phosphate sequesterants, colorants and flavorings. While the types and quantities of additives will vary from application to application, the effect is to add something to the water that will change its character.

    Water ionizers are a type of additive water treatment tech- nology, whereby they add ions to water through the process of electrolysis to create both acidic and alkaline streams of water. They do not filter or purify the water to any appreciable degree. The residential water ionizer will accept municipal water and then pass it through an ionizing chamber, which comprises a number of metallic electrodes (usually titanium or platinum). When energized, the electrodes will initiate an electrolysis reaction where the water yields hydrogen (H+) and hydroxyl (OH-) to the remaining water. The unstable hydroxyl radicals then quickly become hydroxides in the water. Sodium chloride is sometimes added to the feedwater to potentiate the reaction and develop hypochlorous acid and caustic soda as byproducts. The electrodes are separated within the chamber by semiperme- able membranes that isolate the streams of acidic and alkaline water as they are created. Electronic controls allow the user to vary the electric energy input to the chamber, thereby changing the relative strength of the electrolysis reaction. These devices typically process water at a rate of one gpm and require no ad- ditional water storage or repressurization apparatus (see Table 2 for typical performance).

    The acidic stream is generally used for cleaning and even disinfection of surfaces. In most residential systems though, this acidic stream is discharged as waste. The alkaline stream is generally used for drinking, skin care and cleaning. Many claims are made about the health benefits of drinking this type of water, particularly the elevated pH and negative ORP antioxidant characteristic.



    pH is a measurement of the acidity or alkalinity of a solution. It provides a value on a scale from 0 to 14 where 7 is neutral, less than 7 is acidic and greater tha 

    By Gary Battenberg

    In the water treatment industry, activated carbon (AC) is widely used for taste and odor removal of free chlorine and chloramines (chlorine/ammonia species). Additionally, AC is specified for the reduction/removal of natural and synthetic organics, toxic organic chemicals and color bodies. Activation of carbon is accomplished by acidification of soft base materials such as peat, sawdust or soft wood. Thermal activation of hard base materials (such as hard wood and coal) is accomplished by steam heating the material up to 1,900°F (718.2°C) in a vacuum. These activation processes produce a honeycomb-type structure, whereby the basematerial surface area is greatly increased, which provides the adsorption sites for contaminants removal.

    What is adsorption?

    Adsorption is the physical process in which suspended matter, liquids or gasses adhere to the surface or in the pores of the adsorbent in the absence of a chemical reaction. Adsorption is not the same as absorption, in which one substance is taken into the body of another substance (such as the absorption of water into soil). It is important to understand the difference between these two phenomena and learn to use these terms correctly in our industry.


    Activated carbon has been widely used for dechlorination for many years. Because of its high affinity for chlorine, it is a proven application with excellent performance and projected life expectancy. For example, under proper operating conditions, one cubic foot (7.481 gallons or 28.32 liters) of AC will remove one ppm of free chlorine from one million gallons (3.7 million liters) of water. In point-of-fact, granular activated carbon (GAC) is generally accepted as the best all-around adsorbent available for removal of organic contaminants. In addition to the previously stated applications, others include the removal of tannins, phenols, pesticides, detergents, tri-halomethanes (THMs) and toxic organic compounds.

    Which carbon to use?

    Typically, the industry standard AC for dechlorination and suspended matter is a 12 x 40 mesh (1.650 mm x 0.417 mm) bituminous coal, which requires a backwash rate of 9 gpm (34 L/m) per square foot of bed surface area at 55°F (12.7°C) water temperature for a 35-percent bed expansion; whereas, an 8 x 30 mesh (2.360 mm x 0.589 mm) requires a backwash rate of 16 gpm (60.5 L/m) per square foot of bed surface area at 55°F water temperature for the same bed expansion percentage. Note: Activated carbon manufacturers recommend a minimum of 35-percent bed expansion during backwash to ensure flushing of the suspended solids to waste and reclassification of the carbon to expose new adsorption sites to the influent stream. Higher water temperatures require a higher backwash rate, so use caution when designing an activated carbon system that requires periodic backwashing. It is a good practice to first obtain a comprehensive water analysis from a reputable testing laboratory before consulting with your carbon supplier for assistance where issues of temperature, pH and removal efficiency must be considered. The contaminant type and calculated service flowrate will determine the mesh size and type of the activated carbon to be used, such as 12 x 40, 8 x 30 or 20 x 50 mesh in either bituminous coal, coconut shell, soft wood or other suitable media, such as catalytic carbon and organoclay products.

    Backwash filter

    When designing a backwash filter, it is important to remember that filter size, media type and volume are in direct proportion to the target contaminant and service flowrate. Let’s look at an example of a typical residential application for chlorine removal. For a calculated service flowrate of 5 gpm, a standard 9- x 48-inch-high fiberglass tank containing one cubic foot of 12 x 40 mesh bituminous coal is generally accepted as sufficient to scrub free chlorine from the water supply. Because of its high affinity for chlorine, this size system can operate very efficiently between 10-15 gpm (37.8-56.7 L/m) per square foot. Activated carbon filters should be designed for bed depths of 30-36 inches (762-914 mm) for optimum efficiency of contaminantremoval. For example:

    · A 9-inch (228.6-mm) diameter tank has a square foot (ft2) surface area of 0.440, so one cubic foot of activated carbon provides a 30-inch (762-mm) bed depth. Therefore, 5 gpm divided by 0.440 = 11.36 gpm/ft2.

    · A 10-inch (254-mm) diameter x 54-inch (1,371-mm) high tank has a square-foot surface area of 0.545. The media volume will be 1.5 cubic feet of activated carbon for a 33-inch (838-mm) bed depth. Therefore, 1.5 cubic feet of activated carbon multiplied by 5 gpm, equals 7.5 gpm calculated service flowrate or 13.75 gpm/ft2.

    Where suspended-solids matter is present in the water with chlorine, AC will function as a light sediment filter as well. Over time, however, the carbon may become plugged up due to compaction of the sediment into the pores of the activated carbon granules, which will shorten the optimal service life of the carbon. An effective way to protect the carbon bed is to use a lighter filter media on top of the carbon that will intercept the suspended matter before it migrates to the carbon.

    This is what is known as a filter cap and is typically a 5- to 8-inch (127- to 203-mm) layer of FilterAG, which has an apparent density of 25 pounds/ft3. Dry activated carbon has an apparent density of 34 pounds/ft3. The sediment that is captured by the filter cap creates a filter cake that accumulates during the service run and is easily backwashed out of the tank every couple of weeks. Because of the difference in apparent densities, the medias remain stratified, which creates an effective dual-function filter in a single tank. Note: Wet carbon achieves an apparent density of 58-62 pounds, which is much higher than that of the Filter AG at approximately 40 pounds wet.

    When chloramine is the target contaminant for removal/reduction, the typical service flowrate for the previously mentioned 9- and 10-inch diameter tanks drops significantly to 2.6 gpm and 3.3 gpm (9.8 and 12.4 L/m) respectively. The reason is because chloramine removal requires more contact time with carbon to be effectively removed or reduced. The flowrate drops from 10-15 gpm/ft2 for chlorine to 6 gpm/ft2 for chloramine. To achieve the same 5-gpm service flowrate, the tank size would be 13- x 54-inch with 2.5 cubic feet of activated carbon and a 16- x 65-inch (406.4- x 1,651-cm) tank with 4.5 cubic feet of activated carbon for 7.5-8 gpm (28.3-30.2 L/m) service flowrate.


    An activated carbon contactor array is typically designed for tertiary (third-stage) treatment of a water supply where organic and toxic contaminant removal is required. The pretreatment ensures the influent stream into the contactor(s) is free of iron, oil and suspended matter that would shorten the life of the AC. Contactors do not use a backwashing control valve but rather are a simple down-flow configuration that utilizes a tank adapter with inlet and outlet connections. Contactors can be configured for parallel or series operation. Residential applications are generally configured for series operation where a leading and lag tanks are plumbed together with a sample valve at the inlet and outlet of the leading tank and another on the outlet of the lag tank. The purpose is to detect the breakthrough of the contamination from the leading tank, which indicates the replacement of that contactor is needed. The lag tank is rotated to the lead position and the new contactor is placed in the lag position. This allows for full utilization of the activated carbon.

    A parallel contactor configuration is recommended where the required flowrate is not feasible by using larger tanks because of spatial constraints. In this case, two smaller tanks, fitted with a flow-balanced supply and return manifold, split the flowrate equally, which in turn yields the effluent quality required. The effluent from the first parallel contactors then enter a second parallel set of contactors, which provides the same level of efficiency as the series contactor configuration. The sample valves will be located at the same locations as the series contactors to detect breakthrough and indicate rotation, as is the case for the series array. Tech tip: Since contactors don’t require a freeboard for backwash, the tanks can be filled to full capacity. This will increase both the media volume and the service flowrate. For the nine-inch diameter tank, the media load increases to 1.5/ft3 and the 10-inch diameter tank media load increases to 2.0 /ft3.

    Organics and toxic organics require even more contact time than chlorine or chloramine. In the case for the 9-inch tank, organic removal would slow to 1.4 gpm (5.2 L/m) per cubic foot and 1.7 gpm (6.4 L/m) per cubic foot for the 10-inch tank. Toxic organic removal would drop further to 0.9 gpm (3.4 L/m) per cubic foot for the 9-inch tank and 1.1 gpm (4.1 L/m) per cubic foot for the 10-inch tank. The slowdown of the flowrates is what is known as empty bed contact time (EBCT). EBCT is equal to the volume of the empty bed divided by the flowrate. One cubic foot of activated carbon is equal to 7.48-gallons (28.3-liters) capacity (conversion factor) divided by the flowrate of the filter. For example: 2 cubic feet = 7.481 x 2 / 1.6 gpm = 9.35 minutes EBCT.

    For a reference, the following EBCT times can be used for estimating the tank diameter and activated carbon bed volumes. Again, consult your carbon supplier and obtain a comprehensive water analysis to find out how many contaminants are competing for adsorption sites on the activated carbon.

    Free chlorine: 1 -2 minutes / 10 – 15 gpm/ft2

    Chloramines: 3 – 4 minutes / 6 gpm/ft2

    Organics: 5 – 6 minutes / 1.0 – 1.4 gpm/ft2

    Toxic organics: 8 – 10 minutes / 0.7 – 0.9 gpm/ft2

    As you can see, activated carbon is a proven workhorse when it is properly applied to treating contaminated water supplies. As water contamination issues become more serious and urgent remediation is needed, look to activated carbon as your first line of attack to provide an effective removal/reduction of those contaminants with excellent results.

    Environmental compliance

    One final point to keep in mind when working with contamination remediation is to contact the local Environment Health Department (EHD) and the state US EPA office for approved methods of disposal and/or regeneration of spent activated carbon. Simply dumping contaminated carbon back into the environment is not acceptable and is subject to fines and penal detention. Protecting water resources is just as important as providing consistent and reliable performance of water treatment products that remove these contaminants. Don’t compromise work ethic or personal and company reputations by sidestepping laws, rules and ordinances that apply to the work you do. Work with regulatory agencies…not against them!

    About the author

    Gary Battenberg is a Technical Support and Systems Design Specialist with the Fluid System Connectors Division of Parker Hannifin Corporation in Otsego, MI. He has 36 years of experience in the fields of domestic, commercial, industrial, high-purity and sterile water treatment processes. Battenberg has worked in the areas of sales, service, design and manufacturing of water treatment systems and processes utilizing filtration, ion exchange, UV sterilization, reverse osmosis and ozone technologies. He may be reached by phone at (269) 692-6632 or by email, gary.battenberg@parker.com

     and negatively charged hydroxide ions [OH-]. When water has an equal concentration of H+ ions and OH- ions, it is said to be neutral (pH=7). When water has a greater concentration of H+ ions, it is said to be acidic (pH<7). When a solution has a greater concentration of OH-, it is said to be alkaline (pH>7).


    Oxidation reduction potential, or ORP, is an indication of the degree to which a substance is capable of oxidizing or reducing another substance. ORP is measured in millivolts (mv) using an ORP meter. A positive ORP reading indicates that a substance is an oxidizing agent. The higher the reading, the more oxidizing it is. As such, a substance with an ORP reading of +400 mv is four times more oxidizing than a substance with an ORP reading of +100 mv. A negative ORP reading indicates that a substance is a reducing agent. The lower the reading, the more anti-oxidizing it is. As such, a substance with an ORP reading of -400 mv is four times more anti-oxidizing than a substance with an ORP reading of -100 mv.




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