By Gary Battenberg
In part 2 of this series, we listed the cross section of organic contaminants that carbon has high affinity for and a brief description of their use. It is especially important to obtain a complete water analysis that identifies the contaminants and quantifies the levels of those contaminants that exceed the maximum contaminant level (MCL). After interpreting the analysis, designing the appropriate activated [backwash] carbon filter or contactor may be specified.
In this installment, we will look at those contaminants that activated carbons have good affinity, which will yield acceptable results.
Good Carbon Affinity
This list of contaminants has shown that activated carbon (AC) yields acceptable results for reduction/removal to meet customer quality requirements and/or specification.
- Acetic Acid: Used as a chemical reagent to produce chemical compounds like vinyl acetate. Also used in the production of vinegar, which is typically 4 percent by volume.
- Amines: Used in the manufacture of dyes, drugs such as antihistamines, and epoxy resin curing agents.
- Heavy Metals: Including Arsenic, Cadmium, Chromium, Lead, and Mercury may be removed from water when they are organically complexed with the use of a sequestering agent such as sodium hexametaphosphate to prevent precipitation.
- Hydrogen Selenide: A gaseous compound used in semiconductor manufacturing.
- Hydrogen Sulfide: Commonly found in ground water wells, may be removed using catalytic activated carbon, and backwashed periodically to remove the precipitated sulfur element.
- Lead: May be removed using submicron carbon block filters.
- Nitric Acid: Used as laboratory reagent and is a highly corrosive mineral acid.
- Plating wastes: Acidic and alkaline solvents
- Propionaldehyde: Used in the manufacture of plastics, rubber chemical synthesis, and disinfectants.
- Soap: Used for household, commercial, and industrial cleaning chemicals.
- Vinegar: See Acetic Acid.
Backwash Filter or Carbon Contactor?
As previously stated, activated carbon has been accepted as the best all-around adsorbent available for improving the taste and aesthetics of water. However, it is vital to know what contaminants are soluble and which may become insoluble.
Where soluble contaminants are predominant, a contactor is your best option because there is no formation of solids or filter caking to reduce flow and create ascending pressure drop. Whereas insoluble precipitates will accumulate in the filter bed, which requires backwashing to flush away trapped sediment and solids. Looking at the contaminants above, we see that heavy metals, hydrogen sulfide, plating wastes, and soap are candidates for backwashing carbon filters because of the presence of solids and/or precipitates that will accumulate in the media bed and will require periodic backwashing to flush away the solids to maintain optimal filter performance.
Let us look at an example of a backwash carbon filter for this type of remediation.
When designing a backwash filter for insoluble suspended matter, it is important to remember that filter size, media type, and volume are in direct proportion to the target contaminant(s) and service flow rate. The typical industry standard activated carbon for suspended matter removal is 12 X 40 mesh (1.650 mm X 0.417 mm) bituminous coal, which requires a backwash rate of 9 gpm per square foot of bed surface area at 55 degrees Fahrenheit water temperature for a 35 percent bed expansion, whereas an 8 X 40 mesh (2.360 mm X 0.589 mm) requires a backwash rate of 16 gpm per square foot of bed surface area at 55 degrees Fahrenheit 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 out the suspended solids to waste and for reclassification of the carbon to expose new adsorption sites to the influent stream. Keep in mind that higher water temperatures will require higher backwash rates. Before choosing the type of carbon to use, make sure there is sufficient water for backwash at a minimum of 30 psi when speaking with your supplier.
For a calculated service flow rate of 5 gpm, a standard 13” X 54” fiberglass tank containing 2.5 cubic foot of 12 X 40 mesh bituminous coal is generally accepted as sufficient to remove suspended matter at 6 gpm per square foot.
Example: a 13” diameter tank has a square foot (sq ft) area of 0.921. Multiply the square feet by 6.0 gpm and you arrive at 5.5 gpm service flow rate. To determine the empty bed contact time for this filter, multiply the media volume 7.48 gallons per cubic foot (cu ft) of media. In this case, 2.5 cu ft x 7.48 = 18.7 gallons / 6 gpm = 3.1 minutes empty bed contact time.
To confirm that this filter selection will function properly, you must calculate the required backwash rate. The 12 X 40 mesh requires a 9.0 gpm backwash rate. Therefore, 0.921 sq ft X 9.0 gpm = 8 gpm backwash @ 30 PSI and 55 degrees Fahrenheit water temperature.
The same empty bed contact time will apply in this case when we look at the remaining contaminants in our list, which includes acetic acid, amines, hydrogen selenide, nitric acid, propionaldehyde, and vinegar. However, since this [contactor] does not require periodic backwashing, we can achieve the same flow characteristics with a 12” x
52” media tank with 2.5 cu ft of activated carbon. The tank volume is 3.32 cu ft capacity. Adding 2.5 cu ft carbon and 0.18 cu ft of gravel underbed, we have a total of 2.68 cu ft, which is within the volume capacity limits of the tank.
Example: a 12” diameter tank has a square foot area of 0.785. Multiply the cubic feet by 6.0 gpm and you arrive at 5.5 gpm service flow rate. To determine the empty bed contact time for this filter, multiply by the media volume 7.48 gallons per cubic foot of media. In this case, 2.5 cu ft x 7.48 = 18.7 gallons / 6 gpm = 3.1 minutes empty bed contact time.
When in doubt as to which type of carbon to use for a specific water remediation application, always consult your carbon media supplier for support and be sure to include flow, pressure, pH, and water temperature when seeking application assistance. In the final part of this series, we will look at fair, limited application and poor, not
recommended applications for activated carbon.
About the author
Gary Battenberg is a senior business development manager for Argonide Corporation. His four decades of experience in the water industry covers mechanical and adsorptive filtration, ion exchange, UV sterilization, reverse osmosis, and ozone technologies. He has worked in the domestic, commercial, industrial, high-purity, and sterile water treatment arenas. A contributing author to WC&P and a member of its Technical Review Committee since 2008, Battenberg was voted one of the magazine’s Top 50 most influential people in the water treatment industry in 2009. He can be reached by email at [email protected] or by phone at (407) 488-7203.
About the company
Founded in 1994, Argonide Corporation is a family-owned business focused on water filter manufacturing. Today, Argonide sells filtration cartridges and systems all over the world, commercially and residentially, across many applications and markets. Products include NanoCeram, DEAL carbon blocks (diatomaceous earth media coated with aluminum oxide-hydroxide), and the COOLBLUE Microbiological Water Purifier System.