Types of Activated Carbon

Powdered Activated Carbon

PAC
Powdered activated carbon, or PAC, is a form of activated carbon with a very small particle size.  Treatment involves adding PAC to water, allowing the PAC to interact with contaminants in the water, then removing the PAC by sedimentation or filtration. 

The feed location of PAC can be at any point prior to filtration.  The most common locations are in the flash mixer or flocculator since these pieces of equipment will mix the PAC into the water very well.  However, some plants feed PAC just before filtration so that the PAC will form a layer on top of the filter and ensure that all water comes in contact with the activated carbon.  Adding PAC just before filtration can cause problems, though, since the small PAC particles can pass through the filters and cause dirty water complaints from customers or can cake filters and result in reduced filter runs. 

Regardless of the feed location, PAC can be added to water using either a dry feeder or as a slurry.  Dry feeders are most often used in small plants when PAC is fed at intervals in response to periodic taste and odor problems.  In contrast, slurries (mixtures of PAC with water) are used in larger plants or when PAC is fed continuously.  Since it is difficult to make the PAC mix with water, the mixer should have an overhead spray system. 

The effectiveness of PAC in adsorbing tastes and odors depends on adequate mixing, contact time, dosage, and on the cause and concentration of the taste/odor problem.  Mixing and contact time are determined by the location at which the PAC is added in the treatment process, so adjustments made by the operator will usually involve only dosage adjustments.  The dosage usually ranges from 1 to 15 mg/L but must be much higher, in the range of 100 mg/L or more, when the PAC is being used to remove trihalomethanes or trihalomethane precursors.  The operator chooses an appropriate dosage using jar tests and the results from odor and taste tests. 

Imhoff cone

If PAC is fed as a slurry, then the actual concentration of PAC in each slurry tank will be different and an accurate dosage can be difficult to determine.  In that case, an imhoff cone, such as the one shown above, can be used to determine the concentration of PAC in the slurry.  Slurry is placed in the cone and is allowed to sit while PAC settles to the bottom of the cone.  The amount of PAC can be measured using the gradations on the side of the imhoff cone. 



Granular Activated Carbon
GAC
Granular activated carbon, also known as GAC, has a larger particle size than PAC with an associated greater surface area.  Like PAC, GAC can remove trihalomethane precursors as well as taste and odor compounds.

GAC contactor

GAC Contactor.

GAC is used as a filter medium, either as a layer in a rapid-sand filter or in a separate filter known as a contactor.  When contactors are used, the contactor is placed downstream of the filter so that turbidity won't clog the contactor. 

Like filters, contactors must be designed to provide adequate contact time of water with the filter medium.  This is done by calculating the empty bed contact time, or EBCT, which is calculated similarly to detention time, as the volume of the filter divided by the flow rate.  The calculation is called "empty bed contact time" because the volume taken up by the GAC in the contactor is not taken into account.  Empty bed contact time should be about ten minutes. 

During operation of a GAC filter or contactor, a variety of factors must be monitored.  If the GAC is part of a filter designed to remove particulate matter as well as to adsorb tastes and odors, then the effluent turbidity should be monitored.  Similarly, the taste and odor contaminants in the effluent should be monitored to determine whether the GAC is operating properly.  The operator should make regular checks for bacteria since microorganisms often grow on GAC filters and result in clogging problems.  Finally, head loss must be monitored as it would be for any other filter to determine when the unit needs to be backwashed.  Washing a GAC filter involves backwashing with a 50% bed expansion and surface washing. 

Although GAC filters can be operated like a rapid sand filter in most ways, backwashing and surface washing are not the only cleaning required for the units.  The entire surface of the GAC will eventually become covered with contaminants, just as a softener's resin will become covered with magnesium and calcium ions.  A GAC filter can typically operate for months or years before reaching this state, depending on the contaminant levels in the influent water.  Once the GAC has reached its adsorption capacity, it must be regenerated using the same heating process used to activate the carbon.  In many plants, GAC is simply replaced rather than investing in the equipment required for regeneration. 


Choosing a Type of Activated Carbon

GAC and PAC each have advantages and disadvantages.  In general, PAC is used more often due to the low initial cost and to the flexibility of dosage which allows the PAC concentration to be adjusted to deal with changing contaminant levels.  However, PAC has a high operating cost if used continuously, cannot be regenerated, produces large quantities of sludge, and can break through filters to cause dirty water complaints by the customers.  In addition, the dust resulting from the small particles of PAC make handling difficult, as does the flammability of the particles. 

GAC becomes a more economical choice in larger systems or where taste and odor must be controlled continuously.  Disadvantages of GAC include a high initial cost to buy the filter or contactor, and the tendency of GAC filters to grow bacteria.