General Information on Powdered Activated Carbon
Powdered Activated Carbon (PAC) has been used for a very long time as a treatment option due to its ease of use and economic advantages. PAC can be used either as dry or as a wet slurry, which is a mixture of carbon and water. A main disadvantage of powdered activated carbon is that after use it cannot be reactivated and is also sometimes difficult to dig out of water treatment reservoirs. The main economic advantages, “However, the low capital cost associated with PAC feed system combined with the ability to apply PAC seasonally or for periodic issues can make it an economic alternative based on an annualized cost.”1
PAC can be stored dry or wet. Dry can typically be stored in small 55 lb. bags while wet is transported and stored in a giant slurry. When held in a slurry form, PAC can solidify if sitting for too long a period. Periodic mixing is required to prevent this from occurring. PAC is stored in a wet slurry when it is expected to be used more frequently and stored dry when expected to be used less frequently. Dry storage can be less expensive, and doses can be more accurate. The facility must consider weight increase when transferring dry PAC into silos due to adsorbing moisture from air.
Following are the key design parameters for a PAC feed system:
- Design flow rate – affect dosing requirements, size and number of storage vessels, amount and size of feed equipment
- Influent concentrations of target and background DOC – determines the PAC dose needed
- Treatment objectives – Target contaminants identified, target effluent concentration determined, determine dose and size of equipment
- PAC type – coal, coconut shell, wood
- Design dose and duration – what contaminants, jar tests, seasonally/periodically vs. continuously
- Contact time and mixing – rapid mix (most common as “PAC will remain in suspension through the rapid mixer”), flocculation basins, sedimentation
Factors that affect the powdered activated carbon are the influent concentration, residence time, external mass transfer, adsorbate properties and additional contaminants other than target compound. When determining dose, “Increasing the PAC dose will decrease the reactor effluent concentration as more adsorbate can be removed from solution. However, the percent effluent concentration decrease is not proportional to an equivalent percent increase in PAC dose. For the high TOC water (TOC = 10 mg/L), a 40 percent decrease in the influent geosmin concentration can be achieved with a dose of 5 mg/L, but it takes about 17 mg/L of PAC to yield 90 percent removal.
It has been found that chlorine, other disinfectants and unidentified contaminants may take up adsorption sites which will reduce the PAC adsorption capacity for target compounds. This can leave a high effluent concentration of the target compounds. Increasing the contact time helps with removal efficiency. A minimum of 15 minutes of contact time is recommended but more may be necessary and beneficial to achieve total adsorption capacity of the PAC. Jar Tests are typically done to gain knowledge of how effective the powdered activated carbon will be before installing a new feed system.
Powdered activated carbon is a proven, cost efficient treatment option. Please speak with a General Carbon representative if you should have any questions. Thank you.
1 – Activated Carbon: Solutions for Improving Water Quality by Zaid K. Chowdhury, R. Scott Summers, Garret P. Westerhoff, Brian J. Leto, Kirk O. Nowack and Christopher J. Corwin