A POU workhorse — the carbon block filter

Oct. 13, 2010

You use them all the time, so here’s what they’re all about.

Carbon block filters have been around for many years. Several improvements have been made since they were introduced in the early 1970s, and improvements in performance are ongoing.

The main component in carbon blocks is activated carbon, known as the universal adsorbent because it has the capacity to adsorb many compounds. Carbon block filters are unique because they remove contaminants in water by adsorption, act as a catalyst when chlorine is removed and also remove particulate matter with their mechanical filtration properties.

Their multitude of uses, relatively compact size and low cost explains why carbon blocks have become the workhorse of the point-of use (POU) market.

Carbon block/GAC comparisons
Carbon blocks contain granular activated carbon (GAC) that has been ground to a small particle size. Typically carbon blocks contain "fine mesh" activated carbon. Fine mesh is defined as having the majority of the particles smaller than 80 mesh (0.18 millimeters [mm]) and most particles larger than 325 mesh (0.045 mm).

The smallest GAC used in POU filters is 20×50 mesh, where the particles are smaller than 0.84 mm but larger than 0.30 mm. At times 12×40 mesh GAC, which is even larger, is used in POU filters.

Carbon blocks contain activated carbon particles that are at least 7 to 19 times smaller than GAC. The smaller particle sizes allow carbon block systems to be significantly smaller than a corresponding GAC system treating the same flow rate.

The contact time (the time the water stays in contact with the carbon) for GAC to remove impurities is usually several minutes. For carbon blocks it’s measured in seconds because of the higher removal efficiency produced by smaller particle sizes.

One of the biggest complaints about GAC is that it is dusty. All new GAC filters will release a slug of black water containing carbon fines upon initial start-up. The GAC is usually backwashed or flushed with water before use in an attempt to remove the carbon dust. However, during use GAC filters can still elute carbon dust, which may plug downstream treatment units such as reverse osmosis (RO) membranes.

Carbon blocks eliminate this problem because the activated carbon is bound in a block from which few, if any, carbon fines are released.

How they’re made

The two main methods for producing carbon blocks are compression or molding, and extrusion technology. With either process the activated carbon is blended with a proper-sized binder and sometimes other additives to produce a homogeneous mixture. The mixture is placed into a mold, compressed, and then heated to form a sintered (coherent) block. The block is then cooled and removed from the mold.

The extrusion process is similar, except it’s a continuous process where the mixture is extruded through a die to form one continuous block. After cooling the extruded block is cut to length.

The molding method allows different shapes to be readily made, depending upon the mold shape. The extrusion method is limited to a tubular shape, but with it, production rates are higher than the molding process.

Block types and ratings

The two most prevalent types of activated carbons used for carbon blocks are based on either coconut shell or bituminous coal. The type used may not always be indicated by the block manufacturer. The number of micropores present in the coconut shell-based AC is about 50 percent greater than in bituminous coal-based AC.

The most common method used to distinguish micropore volume is the ASTM (American Society for Testing and Materials) Iodine Number Test. Table 1 compares the iodine number and ash properties of coconut shell and bituminous coal-based AC.

Higher micropore volumes (reflected in higher iodine numbers) in coconut shell AC means they have higher capacity to adsorb small organic molecules, such as volatile organic compounds (VOC). Carbon blocks made with coconut shell activated carbons usually have a higher gallon rating for VOCs like chloroform.

Coconut shell based AC is also purer. (See total ash content in Table 1.) Generally, bituminous coal-based AC will be acid-washed to lower the AC’s residual inorganic ash content.

NSF/ANSI standards

Carbon block’s performance is rated by NSF/ANSI standards. NSF/ANSI Standard 42 addresses reduction of aesthetic contaminants like chlorine, taste and odor, color and particulate matter.

There are six classes used to rank carbon block filters for nominal particulate matter removal. The classes represent particle size ranges which are removed with a minimum 85 percent efficiency (see Table 2). This is referred to as the nominal size rating. Some carbon block manufacturers may rate their carbon blocks with a more severe absolute rating. Absolute rating means 99.9 percent removal of particles greater than the micron size rating.

Carbon blocks with a Class VI rating use coarser fine-mesh AC, have lower pressure drop and are less prone to clogging due to particulate matter in the water. Carbon blocks with a Class I rating use finer fine-mesh AC, have higher pressure drop and are more prone to clogging due to particulate matter in the water.

NSF/ANSI Standard 53 addresses reduction of contaminants that have known health effects. This includes organic chemicals, inorganic chemicals, metals and cysts.

Manufacturers rate their carbon blocks for "X" amount of gallons to remove a specific contaminant. Whatever this rating, during Standard 53 testing the carbon block must last twice (2X) the manufacturer’s rating. While Standard 42 addresses removal of particulate matter, cyst removal is covered under Standard 53. Carbon blocks rated for cyst reduction require 99.95 percent reduction efficiency.

Carbon block additives

Various additives may be added to the carbon block during manufacture to enhance the performance of the carbon block. Additives are used to enhance heavy metal reduction.

Polyphosphate silicate is added to prevent scaling in downstream equipment such as coffee makers. Silver is used as an additive in carbon blocks to give bacteriostatic properties. However, with the high price of silver, its use is becoming cost-prohibitive. New impregnants onto activated carbon are emerging as a safer, more effective and less expensive alternative to silver.

For example, one brand of bacteriostatic AC uses a patented antimicrobial agent impregnated in the activated carbon that protects against a wide range of micro-organisms by rupturing the microbes’ outer cell membranes (whereas silver ions are depleted with continuous use).

Two carbon blocks were recently tested to compare different AC types. Both blocks were made the same way with the same size of AC. One contained normal coconut shell AC, the other contained AC with the antimicrobial agent. The test was run for 22 days at an influent heterotrophic plate count of 240,000 colony-forming units per milliliter (geometric mean). After 22 days, the standard carbon block averaged 80.4 percent reduction in plate count versus 98.5 percent reduction for the carbon block with the antimicrobial agent.

Many different carbon blocks are available with many different performance claims. Matching the right block type to the contaminant(s) you want to remove is critical to achieve the desired contaminant reduction.

Robert Potwora is technical director for Carbon Resources, LLC, a supplier of activated carbon products, equipment and services based in Oceanside, CA. He has 30 years experience in the activated carbon industry. He is currently vice chairman of the ASTM D28 Committee on Activated Carbon. He may be reached at 760-630-5724 by email at [email protected].

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