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.
As a generalization, pore diameters are usually categorized as follows:
micropores 5,000 Angstroms (typically 5000-20000 A)