Activated Carbon Filters

Water is never pure as found in nature. As the ultimate solvent, water dissolves almost everything to a certain extent. The substances that are dissolved or suspended in water constitute the impurities that cause problems in plant equipment. This includes scale deposition, corrosion, fouling, and microbiological growths.

Water quality is evaluated by the layman based on color, odor and taste. If the water looks clean, doesn’t smell bad, and tastes good, then people assume it’s safe to drink. We know, of course, that this isn’t any guarantee that a water supply is fit to drink. Many disease organisms thrive in water. They may not impart objectionable odors or taste, but can quickly spread sickness and death among the general population. In fact, more people have been killed by waterborne pathogens (disease-causing organisms) than by any other means.

Pollution also adversely affects water quality. Industrial chemicals that seep into the ground or runoff from agricultural operations pollute our water. These chemicals may be present in very low concentration, but are known to cause cancer and other harmful ailments.

Many treatment methods have been developed to remove unwanted impurities from water and thereby improve its overall quality. One of these methods is purification by activated carbon. This paper discusses the role granulated activated carbon (GAC) plays in the treatment of water for industrial use. In this discussion we will focus on the following topics

  • Water impurities that can be removed by activated carbon

  • What is granulated activated carbon (GAC)?

  • How does activated carbon work to purify water

  • Bacteria contamination of activated carbon beds

  • Sterilization requirements

  • Safety issues

  • Glossary of terms

WATER IMPURITIES

Carbon filtration is primarily used to remove organics from water. Organic compounds are made from carbon, oxygen and hydrogen. These organic compounds are either natural substances found in the environment or man-made chemicals. Natural organics include decomposition products of animals and vegetation such as the tannins and humic acids that leach into the water from decaying leaves. Animal feces are another primary source of natural organic contaminants.

Synthetic organic compounds are man-made chemicals used for agricultural and industrial purposes. These include:

  • Herbicides and pesticides (weed and insect chemicals)

    • DDT

    • Aldrin

    • Chlordane

    • Heptachlor

  • Industrial solvents (degreasers and cleaners)

    • Methylene chloride

    • Trichloroethylene

    • Benzene

    • Carbontetrachloride

    • Toluene

    • Nitrobenzene

  • Leaking Underground Storage Facilities

    • Gasoline

    • Diesel fuel

    • Landfill leachate

Other chemicals such as sodium hypochlorite (better known as chlorine or bleach) are added to water supplies to control bacteria growth. One unwanted side effect of chlorination practice is that chlorine may react with organic substances to form chlorinated organic compounds. These by-products of chlorination are thought to promote cancer. Chlorine and chlorinated organics may also cause problems in certain industrial applications such as in reverse osmosis systems (RO) where they can attack the RO membranes. In these cases, it’s best to remove the chlorine before it can cause problems.

Other substances such as lead and radon are known to pose a health risk. These are not organic compounds, but are commonly found in well and surface water supplies. Radon is produced from the natural decay of uranium deposits. Lead often leaches from water pipes or from solder in copper plumbing systems. Carbon filtration has the ability to remove lead and radon from water.

WHAT IS ACTIVATED CARBON?

Activated carbon is made from coal or charcoal. Most carbon comes from bituminous or lignite coal. Other sources of carbon are obtained from charred coconut shells or peanut hulls.

Activated carbon becomes “activated” by processing the coal at 2300 oF with steam in an oxygen-free chamber. This produces a carbon substance that has many, many small pores and cracks. These pores create a very large surface area where the impurities can be adsorbed onto the carbon surface. GAC has a carbon surface area of 800 to 1000 square meters (m2) per gram of carbon.

Two forms of activated carbon are in common use — granulated and powdered. Granulated activated carbon is used in filter beds where the water passes through the media. Powdered activated carbon is added to a tank of water, mixed and then removed by filtration. Granulated activated carbon is more common in industrial applications and is the method we will focus on in this discussion.

Although one batch of activated carbon may look the same as the next, this material is available in a variety of particle sizes and performance characteristics. The various properties of activated carbon can be defined by the following parameters

  • Screen size distribution (particle size)

  • Iodine number

  • Molasses number

  • Apparent density

  • Carbon tetrachloride number

  • Hardness number

  • Abrasion number

  • Ash

  • Moisture

As you can see, all activated carbon isn’t the same.

HOW DOES ACTIVATED CARBON WORK?

Activated carbon removes organics from water by adsorption. This is a physical phenomenon whereby molecules adhere to a surface with which they come into contact. This is similar but slightly different than absorption, which means to “soak up” such as a sponge soaking up water.

As water passes through the carbon filter bed, the water seeps through the very small pores in the carbon granule. (pore sizes are around 0.2 millionth of an inch) The organic contaminants are removed from the water by physical adsorption onto the surface of the carbon particle.

Granulated carbon filters are similar in design to gravity or pressure filters. Carbon bed depth of up to 10 feet is needed to ensure removal of organic contaminants. Most carbon filters are designed for flow rates of 2 to 10 gallons per minute per square foot.

The water must have adequate retention time in the carbon filter to achieve the desired level of organic removal. The volume of the carbon filter divided by the flow rate determines the Effective Bed Contact Time (EBCT). The EBCT should be 10 minutes or more to insure adequate organic removal. EBCT’s less than 7.5 minutes are not very effective.

Carbon has a limited capacity for organic removal. As the surface of the carbon becomes saturated with organics, the removal capability of the bed is diminished. At this point the carbon bed must be replaced, or alternatively, the carbon can be regenerated by roasting the carbon to remove the organics. Most carbon beds last for years before replacement is necessary.

CHLORINE REMOVAL

Carbon is very effective in removing chlorine and other oxidizing agents such as oxygen, bromine, chlorine dioxide, ozone, and permanganate. These oxidizing agents are removed by direct chemical reaction with the carbon. In this case, the chlorine is not adsorbed onto the carbon particle, but chemically reacts with the carbon to produce carbon dioxide. As a result, carbon filters have an infinite capacity for chlorine removal. The carbon bed does not become saturated with chlorine as is the case for organic removal.

RADON REMOVAL

Radon is thought to promote the development of lung cancer. It is found in water as a by-product of the decay of uranium. Radon migrates through soil and rock into the groundwater, or may seep through cracks in building foundations to contaminate the air.

Radon is removed by granular activated carbon. The radon is held on the carbon particle for many years until it decays into nonradioactive compounds.

THE PROBLEM OF BACTERIA GROWTH

The surface of the carbon particles in the GAC filter bed provides an ideal habitat for the growth of bacteria. Filters that sit idle for 5 days or more are particularly susceptible to bacteria infestation. Research indicates that these organisms are nonpathogenic (do not cause disease). Still, most plants take precautions to control the bacteria populations in GAC filters.

Bacteria growth in carbon filters used ahead of reverse osmosis (RO) systems are of particular concern. Living and dead organisms that leave the filter in the effluent are trapped on the membrane surface. The organic material from the “dead bodies” tends to adhere to the membrane to cause irreversible organic fouling. These foulants are difficult to remove and frequently cause the premature failure of the RO system.

Regular steam sterilization of the carbon bed is the most effective way to control unwanted bacteria populations. The frequency of sterilization is determined by a number of factors including the raw water quality, intended used of the treated water, and overall operating experience with the system.

The typical steps for sterilizing a carbon bed are as follows:

  • Backwash 10 to 15 minutes

  • Drain down 120 minutes

  • Delay to temperature 235 oF

  • Steam sanitize 60 to 180 minutes

  • Slow fill 75 minutes

  • Backwash 10 to 15 minutes

  • Rinse 5 to 15 minutes

SAFETY

Carbon reacts with many oxidizing agents including oxygen. A vessel containing carbon will quickly deplete the oxygen in the air space above the carbon bed. For this reason, never enter a carbon filter vessel without a respirator and proper personal protection.

Activated carbon will burn if exposed to heat and oxygen. Proper fire safety precautions should be observed when storing or handling activated carbon.

SUMMARY

Activated carbon improves water quality by removing the organic purities that adversely affect its color, odor and taste. These include natural organic substances from decaying vegetation and landfill leachate; synthetic organic chemicals such as herbicides and pesticides, organic solvents and cleaners, and gasoline. Other chemicals such as chlorine, lead and radon are also removed by granulated activated carbon.

Operation and maintenance of activated carbon beds is fairly straightforward. Periodically, the bed must be backwashed to remove accumulated solids that have been filtered from the water. The carbon is then steam sterilized to control the growth of bacteria and to keep the effluent sanitary. After several months or years of service, after the carbon has become saturated with organics, the carbon is either replaced or reactivated.

Proper design, operation and maintenance of an activated carbon bed will improved the taste, odor and color of water for potable or process use.

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