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Powdered Activated Carbon

Overview
 
Figure 1: Typical PAC feed locations.
Figure 1: Typical PAC feed locations.

Activated carbon is commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment. Adsorption is both the physical and chemical process of accumulating a substance at the interface between liquid and solids phases. Activated carbon is an effective adsorbent because it is a highly porous material and provides a large surface area to which contaminants may adsorb. Activated carbon is available as powdered activated carbon (PAC) and granular activated carbon (GAC).

PAC is made from organic materials with high carbon contents such as wood, lignite and coal. PAC typically has a diameter less than 0.1 mm and an apparent density ranging between 23 and 46 lb/ft3, depending on the material used and manufacturing process. Iodine and molasses numbers are typically used to characterize PAC. These numbers describe the quantity of small and large pore volumes in a sample of PAC. A minimum iodine number of 500 is specified for PAC by AWWA standards.

PAC is used by water treatment plants on either a full time basis or as needed for taste and odor control or removal of organic chemicals. PAC is can be fed as a powder using dry feed equipment or as a slurry using metering pumps. Dry feed systems are typically used for smaller dosages and where PAC feed is infrequent. Dry feed systems typically include a bag-loading hopper, an extension hopper, a dust collector, a dissolving tank, and an eductor. PAC can also be mixed with water and fed as a slurry. Slurry systems are normally used when PAC is frequently added and the required dosages are high. Slurry systems usually include a storage tank, day tank, and a chemical feeder (either a diaphragm pumps or rotary feeders).

PAC is normally added early in the treatment process and is subsequently removed either by sedimentation or by the filter beds during backwashing. The PAC application point should allow for (1) an adequate contact time between the PAC and organics, and (2) avoid coating PAC particles with other water treatment plant chemicals. A minimum contact time of about 15 minutes is required for most taste and odor compounds; however, significantly longer contact times may be required for methyl-iso-borneol (MIB) and geosmin removal. The PAC should not be coated with coagulants or other water treatment chemicals before the PAC has had sufficient contact time with the source water. Also, PAC should not be added concurrently with chlorine or potassium permanganate as these chemicals will adsorb to the PAC. PAC is usually added at the head of the plant to provide the longest contact time possible before applying other treatment chemicals. Higher PAC dosages may be necessary if PAC is added later in the treatment process to account for reduced contact times and interference with other treatment chemicals.

PAC dosages can range between 1 to 100 mg/L depending on the type and concentrations of organic compounds present. Dosages of 1 to 20 mg/L are typical for nominal taste and odor control. Application of PAC generates additional sludge which is not able to be regenerated as is spent GAC. PAC sludge will contain elevated concentrations of the contaminants removed and must be disposed in accordance with State and federal laws; however, it is not likely to be classified as a hazardous waste.





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