How POU Activated Carbon Filters Work
By Henry Nowicki, PhD, Richard Capp, George Nowicki, Wayne Schuliger, Christopher Brunning and Barbara Sherman
All activated carbons are not the same and a basic understanding of how activated carbon filters work can help users get the best performance from domestic and commercial activated carbon filters. They do the work of removing/reducing contaminants from water and air. Manufacturers and assemblers must be diligent, however, to avoid unintended consequences. Carbon-filter consumers seeking water purification need to understand and analyze the physical ability of different carbons (and other technologies) to maximize contaminants removal from water and hold them tightly.
User focus and perception
Municipal drinking water plants are mandated by the Safe Drinking Water Act to test the water and provide this information annually (in July) to their customers. This free source of information may be useful if consumers believe there is a problem with the drinking water. They can have the water tested by certified drinking-water analytical laboratories to define possible problems. The municipal supplier should be able to suggest laboratories that provide the appropriate testing services. Transient contaminants and compounds below method detection limits (DL), however, may be missed, not detected and therefore, not reported. This US EPA-mandated annual report is based on a few snap-shot tests only and not continuous monitoring for long periods of time. In addition, it’s possible that contaminants may not be detected even with constant monitoring. This makes the diligent use of POU systems even more important.
After hearing almost nightly about lead contamination, many carbon-filter consumers may have developed a mistrust of municipal suppliers.1 Consumers like the idea of continuous extra protection these filters offer, if there is doubt about the safety of the water. (Many contaminants cannot be smelled, tasted or seen but can be detected by lab testing.) In that case, a POU filter should be used to polish water the family will consume for drinking and food preparations. (To be toxic, contaminants need to be consumed.) It is very important to note that lead is not easily removed by activated carbons; every filter has its limitations. Independent certification groups, such as NSF International, evaluate POU claims by suppliers and consumers should check for these evaluation certificates when purchasing POU devices.
Carbon filter suppliers need to provide user instructions to help ensure they have read, understand and will follow supplier instructions, at a minimum. Consumers need to be informed that filters do not last forever and must be replaced with new media. Buying carbon filters on sale or in bulk reduces unit cost and they have a long shelf life if protected from air contaminants. They will, however, adsorb contaminants in air and use up to 40 percent of the adsorption spaces volume. Extra filters should be kept in their air-tight packages until being presoaked for as long as 72 hours before use.
Users should also be advised not to use filters longer than recommended. Filters have a recommended number of gallons treated before replacement and early replacement provides an extra margin of safety to prevent contaminant breakthrough. Carbons are very good at removing chlorine taste and odor because carbon is a reducing agent and converts hypochlorous acid to tasteless chloride ions. Do not use chlorine taste as the end-point for replacement, however, because physical adsorption of contaminants fill adsorption spaces well before chlorine taste breakthrough. Consumers should consider changing POU filters once half of the recommended number of gallons of water have been treated.
Properly wetting activated carbon
The major reason a filter does not perform is that it is not properly wetted.2 If filters are used with air in the pores, contaminants do not have access to these available binding sites. Contaminants in bulk water diffuse to active binding sites via a continuous stream of water. Diffusion is the slowest step in contaminant removal from water. A carbon filter in a supplier-protective wrapping has seven to nine times the atmospheric air compressed in one to five nm-wide micropores. The volume distributions in the carbon filter is 40-percent bulk volume between particles to facilitate water flow; 40-percent pore volume or adsorption space (inside particles volume) to adsorb contaminants and 20 percent is solid carbon. This is referred to as the 40:40:20 rule.2
There are two ways to replace the nano-spaced concentrated air: 72 hours submerged soaking in tap water or using hot water to remove trapped air. Water forms larger conglomerates by hydrogen bonding of water molecules. Conglomerates of hot water are smaller and can better penetrate adsorption spaces than larger, cold-water conglomerates. Replacing filter soaking water with fresh water and turning the filter vertically upside down is also beneficial. Draining helps remove air bubbles. When air in nanospaces is replaced by water, bubbles go into bulk volume between media particles. Simple draining removes these bulk water bubbles. Water inside particles, however, is not removed by draining. Soaking for 72 clock hours is necessary and extra time is acceptable.
POU for continuous water monitoring
A recent article in this magazine discussed the idea of using POU as sentinels for long-term tap-water quality virus monitoring.3-9 Municipal drinking water plants use grab sampling in treatment and distribution systems to monitor contaminants. They cannot, however, detect and report transient contaminants and contaminants below test method detection limits (DL). Continuous monitoring is needed to provide comprehensive detection of transients and contaminants below DLs. POU devices are useful for improving water quality monitoring for continuous, large-volume, long-term assessments of water quality. On a global basis, South America, parts of Europe and Asia are using POU filters to solve drinking-water quality problems. POU is a logical and readily available way to protect the health of the family; POU filtration may be the best (and possibly only) option to protect everyone.
Work on filter economics, environment impact and health
Compare the personal cost of filters to bottled water in plastic containers. Bottled water sales have surpassed soda or carbonated beverage sales. Research indicates that these plastic containers are bad for our environment, as much of this plastic is converted to small pieces that concentrate in the oceans. Birds and wildlife confuse these small plastic pieces as edible food.10 Floating ocean plastic four- and six-mm diameter beads are fouled by dimethyl-sulfide producing algae, creating a dinner bell for many seabirds.
During a normal human life span, susceptibility to contaminants changes. Prenatal and up to about six years of age, the human body does not have a fully developed immune system and rapid growth can concentrate contaminants. Also, elderly and illness-prone, as well as immunocompromised individuals with cancer and other health problems, are at greater risk from drinking-water contaminants. POU is a good option to polish municipal drinking water and protect these at-risk groups.
One needs a disciplined schedule to change drinking-water filters, hot-water heating and cooling systems, refrigerators, water filters in shower heads and POU drinking-water filters. There should be an app or calendar entry for filter installation and an alert for changing out filters because they do not last forever. Once adsorbates break through, a contaminant can be several orders of concentration higher than influent water levels. This phenomenon is called roll-over, when carbon concentrates a contaminant early in its life cycle and later it was desorbed by a stronger binding adsorbate.
- Ragan, Stevan. Drivers for POU/POE Filtration use. IACC-35 Hall of Fame Plenary Lecture. 2014.
- Schuliger, Wayne. “The Blame Game.” Industrial Wastewater. January/February 2001. Wayne provides and short course titled “Activated Carbon Process Systems”.
- Reynolds, K.A. “Household POU Filters: Sentinels for Long-term Tap Water Quality Monitoring.” WCP International. pp. 62-64. March 2017.
- Reynolds, K.A.; Mena, K.D. and Gerba, C.P. “Risk of waterborne illness via drinking water in the United Sates.” Rev Environ Contam Toxico. 2008; 192:117-158. www.ncbi.nlm.nih.gov/pubmed/18020305.
- Morris, R.D. and Levin, R. Estimating the incidence of waterborn infectious disease related to drinking water in the United States. (233). 1995.
- Colford, J.M.; Roy S; Beach, M.J.; Hightown, A; Shaw, SE and Wade, TJ. “A review of household drinking water intervention trials and an approach to the estimation of endemic waterborne gastroenteritis in the United States.” J Water Health. 2006; 4 (Supply 2); 71.doi:10.2166/wh.206.018.
- Pepper, L.; Rusin, P.; Quintanar, D. l.; Haney, C.; Josephson, K. and Gerba, C. “Tracking the concentration of heterotropic plate count bacteria from the source to the consumer’s tap.” Int J Food Microbiol. 2004; 92 (3): 289-295. doi: 10.1016/j.ijfoodmicro.2003.08.021.
- Capp,Richard and Henn, Earl. Qualitative Detection of Adsorbates from Spent Activated Carbons. Calgon Carbon Company Standard Operating Procedure. pp. 1-12. 1985.
- Nowicki, Henry and Capp, Richard. “Analysis of Spent Activated Carbon: Pores Filled/unfilled by GAED and Identifications of Adsorbates by GC-MS and ICP.” International Filtration News. Submitted for publication in 2017.
- Everts, Sarah. “Plastics Lure Seabirds by Smell.” Chemical & Engineering News. p. 8. Nov 14, 2016.
About the authors
Henry Nowicki, PhD/MBA is President and Chief Scientist for Activated Carbon Services–PACS Inc. He directs PACS’ new carbon-focused product development and has been awarded eight SBIR contracts on carbon. Nowicki consults and teaches the popular two-day course, Activated Carbon Adsorption Principles, Practices and Opportunities and is Chairperson for September 2017 IACC-40 in Pittsburgh, PA. Contact him and team members by email, Henrypacs@aol.com, by phone (412) 334-0459 or on the website, PACSlabs.com. Richard Capp is Manager of PACS commercial software to provide activated carbon performance evaluations and support ASTM carbon test methods. George Nowicki, BA/BS has 26 years of carbon experience. At age 14, he developed an AC tester to determine remaining service life. Presently he directs the PACS day-to-day laboratory services for AC industry clients. Wayne Schuliger, PE is a PACS Technical Consultant and teaches PACS course, Design, Operation, and Troubleshooting AC Adsorber Systems for Water and Air. Christopher Brunning is the PACS webmaster and the Manager of the 3-D printing and manufacturing of carbon objects containing proprietary additives project. Barbara Sherman is Manager for PACS short courses and carbon conference.
About the company
PACS is a 45-year-old firm focused on carbon services for manufacturers and users by providing routine and advanced carbon testing, consulting, R&D projects, software programs to increase productivity, converting a wide variety of waste materials to activated carbons brands as well as offering public and on-site carbon short courses. PACS hosts the bi-annual International Activated Carbon Conference and Carbon School Courses. The next carbon conference and courses program is September 14-15, with carbon courses Sept 11-17 in Pittsburgh, PA. See www.pacslabs.com for more information.