By Henry Nowicki, H. George Nowicki, Wayne Schuliger and Barbara Sherman
Granular activated carbon (GAC) does not last forever in a water treatment plant (WTP), does need to be periodically monitored for performance, and should be replaced with fresh unused or reactivated GAC. Used GAC, which is removed, can be custom-reactivated at plants dedicated to drinking water carbons for return to the plant. Re-activators use similar chemical processing as original GAC manufacturers. The Gravimetric Adsorption Energy Distribution (GAED) method was used to evaluate the unused-used-reactivated-recycled GAC life cycle. The overall goal of this case study was to compare GAED and classical test methods used to evaluate which used GAC filters need to be replaced with unused or reactivated GAC to decrease customer taste and odor complaints.
Description of WTP-GAC adsorber configuration
Figure 1 illustrates the WTP-GAC adsorber configuration in the plant used in this study. Six GAC gravity filters operated in a parallel down-flow mode. Water and, in some designs, air were introduced through the under-drain system to periodically backwash the carbon to remove filtered dirt.
This configuration and a GAC change-out program helped to maximize both GAC usage, gallons treated per pound of GAC and finished water quality. These multiple parallel GAC adsorbers provided a blended water supply by having adsorber pairs on different change-out schedules. Ideally, the most used pair of GAC filters became those most beneficial to be changed out.
Another way to increase gallons treated per pound was to have two or more adsorbers linked sequentially. Influent went through the lead, then the lag adsorber(s). By monitoring the water between them, it was possible to know when GAC in the lead adsorber was exhausted. By replacing used with unused or reactivated GAC and reversing the flow (making the lag adsorber the lead adsorber), final adsorber effluent is kept at high quality. The lead-lag type system is applicable only to small drinking water plants.
Testing objectives and future recommendations
Requested composite samples were obtained by collecting six samples at different locations from each filter. The testing objective was to define the most-used filter pair based on representative sampling.
Other objectives included:
- Characterize current physical characteristics and adsorptive capacities of existing GAC filter media and unused GAC.
- Compare filter data to new filter media and determine which, if any, of the filters should have their media replaced.
- Establish a baseline for future GAC incoming media,monitoring testing for evaluation for replacement cycles.
All testing results pointed to replacing filters five and six to best improve the supplied drinking water.
Objective one results
Laboratory results for GAC physical characteristics
Tables 1 and 2 summarize the standard American Society Test Methods (ASTM) results for six filters and two unused GAC samples in storage. Typically, carbons in use for a long time lose some minerals and water-soluble components. Most often, the minerals are non-toxic, but arsenic is an exception.
The iodine numbers in Table 1 are statistically the same for all six filters. Iodine numbers provide the milligrams of iodine adsorbed per gram of carbon under ASTM standard operating conditions. Iodine numbers do not differentiate these carbons, in order for the client to know which GAC to change. We have noted this failure of iodine numbers many times. The carbon industry uses iodine numbers more than any other test for aqueous applications. Iodine fills all of the available GAC adsorption spaces, but drinking water GAC applications only use the high adsorption energy sites (2-3 mL per 100 g GAC) to remove trace contaminants (detailed GAED analysis reports can be viewed at www.pacslabs.com).
Particle sizing is critical to maximize gravity flow applications. All study samples passed the WTP gross particle-size distribution requirement; i.e., maximum on number 8 screen of 15 percent, and minimum passing through the 16 screen of five percent on w/w basis (see columns 6 and 8 in Table 2).
Objective two results
Comparing GAC adsorptive capacities
Three different methods were used to compare adsorptive capacities in six filters and two unused GACs: geosmin adsorption, dissolved organic carbon (DOC) removal and GAED full characterizations. The first two test methods have been used for a longer time than GAED, which is relatively new.
Geosmin adsorption on eight GAC samples
Geosmin has a musty, earthy odor and taste when it contains 10 to 20 nanograms per liter (ppt). DOC is one to two milligrams per liter, thus the relative amounts of geosmin to DOC in nanograms is 10 to 20 to 1,000,000 to 2,000,000 per liter.
Even in typical samples spiked with 250 ng geosmin for geosmin removal performance by GAC, it only represents 0.12 to 0.25 percent of DOC, which is a negligible amount compared to DOC concentration. Table 3 summarizes the removal percentages for geosmin using provided standard methods.
GAED testing results disclose filters for replacement
GAED reports show characteristic curves for the six used and two unused GACs, from which polynomial equations are derived. This is all the information needed to provide isotherms for organic compounds. (A typical GAED lab report is 25-30 pages. A second author will send an electronic copy upon request or the report may be downloaded from www.pacslabs.com under Advanced GAED Testing). Figure 2 reveals comparative high-adsorption energy cubic centimeters (cc) pore volume per 100 cc of each carbon. These areas contain the binding sites needed for WTP trace applications. Lower-adsorption energy sites (less than 25 cal/cc) have little utility. The data clearly depict that the carbon in filters five and six are the most used and beneficial for strategic replacement.
Author note: Uniformity coefficient is the ratio of the particle diameter corresponding to 60-percent finer on the cumulative particle size distribution curve to the particle diameter corresponding to 10-percent finer on the same distribution curve. Effective size is the particle size in millimeters, which corresponds to 10-per- cent finer on the cumulative particle size distribution curve. Mean particle diameter (MPD) is the weighted average particle size in millimeters, of a granular adsorbent computed by multiplying the percent retained in a size fraction by the respective mean sieve openings, summing these values and dividing by 100.
GAC dissolved organic carbon performance
Table 4 shows DOC removals are in agreement with GAED and geosmin removals performance. All three adsorption performance test results indicate that filters five and six have the greater amount of used GAC, and filters one through four have a lesser quantity of used GAC and still have remaining adsorptive capacities. Replacing the most used GAC in filters five and six should provide the best quality improvement in blended water for the customer distribution system.
Adsorption space usage
In drinking water applications, only a small portion of the original adsorption spaces (pore volume) are filled up. Typically, this application only uses two to three mL of the approximate 50 mL of initial adsorption space per 100 gram of GAC, when the GAC needs to be replaced. When adsorber influent and effluent are the same, the GAC is completely used and needs to be replaced.
Industrial applications often fill 20 to 40 mL of adsorption space when the GAC needs to be changed. Thus, there is typically a large difference between industrial used and unused or reactivated GAC applications, and a relatively small difference for drinking water. Since drinking water applications typically use high-adsorption energy sites selectively, GAED is the only test to provide this critical information directly. Users benefit by purchasing GAC with the highest ultimate loading capacity for their application because time between change outs is longer.
Recommendations for future testing
Future incoming GAC should utilize ASTM and AWWA routine test methods and the GAED advanced test method before installation and monitoring; samples need to be run periodically. WTP managers should collect representative samples of installed GAC in filters, prior to going online. This would provide the best way to compare performance against used carbon when troubleshooting. Representative retained samples are the best way to troubleshoot problems with newly installed GAC, though at present, most carbon users do not employ the practice of retaining samples. Monitoring samples of installed working GAC should be taken on a regular schedule for performance testing.
GAC has an accumulated adsorption memory of what went through its porous structure, because molecules adhere to the surface and fill the adsorption spaces. GAED tests are used to provide the remaining working capacity in performance monitoring GAC samples. This is accomplished by comparing a fully used sample with the original unused GAC sample, tested before use in the WTP filters. This difference provides the usable adsorption space for the application. The difference between two GAED run results (fully used pore volume sample minus partially used pore volume from performance monitoring sample) is what the GAC can still do for the user.
Geosmin and DOC removal tests cost two to three times more than GAED. It is the only test that examines the GAC directly. Other tests rely on indirectly determining GAC status. They assume what is removed from solution is adsorbed by the GAC. Typically, it is better to use a direct measurement than an indirect, which typically are more precise and accurate.
Convenience and turnaround time also favor the GAED test method. Geosmin and dissolved organic compounds removal performance evaluations require water shipments from the WTP with the GAC samples to a qualified testing laboratory. At the lab, contact time between the GAC and influent water was 14 days; in this case study, the client provided the test methods. A three-to-four-week turnaround time using WTP water is normal. Testing the GAC samples directly with GAED can be done in two to three days after the samples are received at the laboratory.
Shipping unstable and variable water can result in bacteriological growth and often water quality has seasonal variations. Used and unused GAC have relatively fixed GAED porous characteristic structures, whereas water samples are more variable.
Due to bacteriological growth, water parameter variations, turnaround time and cost, the mini-column technique was developed by Manes and colleagues to replace larger pilot studies.1 These small columns are useful to obtain dynamic performance information to compliment the equilibrium data obtained from GAED tests. These small, dynamic columns have many names in the marketplace, such as rapid small-scale column tests (RSSCT) and accelerated column tests (ACT), etc. These two-to-four-day, small-scale, mini-column simulations have eliminated most pilot scale studies for WTP, because they take so long.
GAED is worth the time to understand and implement into your GAC testing program. GAED has been shown to have many applications and provided the lowest cost test with the highest impact solution to determine strategic replacement of GAC filters at a municipal drinking water plant.
Authors acknowledge Carbon Activated Corporation for providing this project to determine filter replacements and approval to publish. The specific municipal filter plant is a Carbon Activated client and identification cannot be made by the authors.
- M. Manes, Professor Emeritus, Kent State University. The first activated carbon Hall of Fame awardee. Professional conversations withH. Nowicki.
- H. Nowicki, G. Nowicki, and B. Sherman. “GRPD to GAED Sorbent Test Method Name Change.” Water Conditioning & Purification. February 2010, pp. 56-58.
- Water Condition & Purification homepage (www.wcponline.com); search for Nowicki. All articles with Gravimetric Rapid Pore Size Distribution (GRPD) on advanced GRPD testing should now be considered Gravi- metric Adsorption Energy Distribution (GAED).
- H. Nowicki, G. Nowicki, W. Schuliger, B. Sherman. Determining GAC Filter Replacements. Prior conference presentations, Pittsburgh Conference and 25th International Activated Carbon Conference.
- Wayne Schuliger. PACS short course, “Design, Operation and Trouble Shooting Activated Carbon Process Units” and Henry Nowicki’s “Acti- vated Carbon Adsorption: Principles, Applications, Opportunities,” Oct. 9-11, 2010 at IACC-26 Pittsburgh PA, www.pacslabs.com.
- H. Nowicki, G. Nowicki and B. Sherman. “GAED Reveals differences in POU filter media.” Water Conditioning & Purification. March 2010, pp.54-58.
About the authors
Henry Nowicki, Ph.D./M.B.A. is President of PACS and Activated Carbon Services Inc. providing laboratory testing, R&D, won nine SBIR and other government R&D contracts on activated carbons, published over 100 papers and provided numerous PACS Short Courses and talks, fee paid telephone and consulting services, has assisted lawyers as an expert in environmental sciences and activated carbon cases, serves on the WC&P and Filtration News Technical Review Committees and provides a two-day course “Activated Carbon Adsorption Principles and Practices” in public or at client’s time and place.
H. George Nowicki, B.S. is Laboratory Manager and business developer for PACS. George has 12 years of varied activated carbon experiences and advises clients on selecting activated carbon tests based on their specific applications.
Wayne Schuliger, P.E. is Technical Director for PACS, Inc, has 43 years of activated carbon experiences and provides consulting on activated carbon adsorber operation and design. Schuliger provides a short course titled “Design, Operation and Trouble Shooting Activated Carbon Adsorption Systems” in the PACS Activated Carbon School. In addition to courses, Schuliger and Nowicki provide independent testing and provide reviews and recommendations for operating and proposed adsorption process equipment.
Barbara Sherman, M.S. is Manager of operations for PACS for 26 years; she manages the day-to-day PACS business activities and practices and works in PACS Laboratories.
About the company
Professional Analytical and Consulting Services (PACS) and Ac- tivated Carbon Services is in its 26th year of incorporated business. PACS provides routine and advanced testing services, R&D, training consulting services and hosts the International Activated Carbon Conference. To view laboratory testing, consulting, expert witness services, and PACS short courses and conferences please refer to the web site: www.pacslabs.com or call (724) 457-6576
The Gravimetric Adsorption Energy Distribution (GAED) full characterization advanced test method, which directly examines GAC, provides a valuable asset to determine which filters to replace at a water treatment plant (WTP). All activated carbons are not the same; GAED can reveal critical differences for GAC manufacturers and users. This advanced test method is more precise and accurate (and lower cost) than the classical ASTM isotherm, geosmin and dissolved organic compounds (DOC) removal activity numbers test methods. GAC is the best avail- able technology to remove aesthetic taste and odor (T&O) problems and trace toxic organic contaminants from drinking water supplies. More WTP and POU users should know about GAC costs and benefits and the GAED test method utility. A recent article demonstrated use of GAED for differentiating POU activated carbon filters.