City Water? Start with what sanitizer your water supplier uses. Chlorine and chloramine call for different carbon choices.
If your city water has a disinfectant taste or smell, a carbon filter may be the right next step. It can also reduce the disinfectant reaching a downstream softener. The first decision is the media. The second is the filter design and size.
Chlorine or chloramine?
Check your water supplier’s current water-quality report or treatment information. Do not try to identify the disinfectant by smell alone. If the utility changes disinfectants seasonally, account for both in your selection.
Standard activated carbon
For routine chlorine reduction, start with a granular activated carbon system sized for the flow you need.
Catalytic carbon
Choose the catalytic carbon option for chloramine treatment. Check the system’s performance and contact-time requirements for that application.
Both our standard granular activated carbon (GAC) and our catalytic carbon are made from coconut shell. Coconut-shell carbon is a good all-around choice for general water treatment. Choose standard GAC for routine chlorine treatment and catalytic carbon for chloramine. Select the correct option when ordering. A tank labeled “carbon filter” does not establish that it contains catalytic carbon.
Carbon treatment does not soften water or reduce total dissolved solids (TDS). If you also have hardness, use a softener sized for the water reaching it. For drinking-water TDS reduction, see the RO guide.
Upflow or backwashing?
Upflow carbon: clean city water only
An upflow design passes water upward through the media without an automatic backwashing valve. It does not need a routine backwash drain cycle, and does not use electricity. We recommend this design only for clean city water. Do not use our upflow carbon filters on well water.
Do not choose it as a substitute for a sediment filter or install it where collected material requires regular backwashing.
View our Upflow Carbon Filters
Backwashing carbon: the gold standard
We consider backwashing carbon the gold standard for whole-house carbon filtration. During normal service, water travels down through the carbon bed. In addition to chemical treatment, the carbon granules provide some sediment filtration as water passes through them. The control valve periodically backwashes the bed to remove collected material and redistribute the carbon. It requires a suitable drain connection and enough supply flow to complete that cycle.
That sediment capture is an added benefit, not a substitute for a dedicated sediment filter when the water carries a significant sediment load. Backwashing removes collected material; it does not restore exhausted carbon’s chemical treatment capacity.
View our Backwashing Carbon FiltersCarbon needs enough contact with the water to do its job. A valve may physically pass more water than the media can treat effectively. Compare the system’s treatment service flow with the fixtures you expect to run together, particularly for chloramine applications.
Empty bed contact time: give the carbon time to work!
Empty bed contact time, or EBCT, compares the size of the carbon bed with the flow through it. Simply put, more carbon or less flow gives the carbon more opportunity to do its job. A small tank may pass plenty of water while providing too little contact time for the treatment you want.
EBCT in minutes = carbon bed volume in cubic feet × 7.481 ÷ flow in gallons per minute.
Use the volume of the carbon media bed, not the tank’s total volume. Do not count support gravel or the empty space above the carbon. EBCT is a standard sizing calculation based on the bed volume as if it were empty; it is not a measurement of the exact time each drop spends between the granules.
About 3 minutes EBCT
2 × 7.481 ÷ 5 = 2.99 minutes
About 1.5 minutes EBCT
2 × 7.481 ÷ 10 = 1.50 minutes
How much contact time does carbon need?
Use these common design targets as a starting point for sizing. They are not universal removal guarantees or a replacement for the selected system’s tested performance. Incoming concentration, water chemistry, carbon type, and the reduction you need all affect the final requirements.
| Application | EBCT target | Selection guidance |
|---|---|---|
| Free chlorine | 2 minutes | Standard GAC is the usual starting point. |
| Routine city-water taste and odor | 2 minutes | Standard GAC; identify the cause if the problem persists. |
| Chloramine with catalytic carbon | 4 minutes | Our recommended carbon choice for chloramine. |
| Chloramine with standard carbon | 10 minutes | Included for comparison. Choose catalytic carbon to avoid the much larger bed needed for this target. |
| Hydrogen sulfide with catalytic carbon | 4 minutes (theoretical) | We do not recommend carbon of any type for hydrogen sulfide removal. This figure is included for technical context, not as a system recommendation. |
| Volatile organic compounds (VOCs) | 7 minutes | A starting point only. The specific compound and concentration determine media suitability and whether longer contact time is needed. |
| PFAS, including PFOA/PFOS | 10 minutes or more | Requires carbon and a system verified for the specific PFAS application, plus testing and a replacement plan. |
We do not recommend carbon of any type for hydrogen sulfide removal. Although removal is theoretically possible with catalytic carbon at a 4-minute EBCT under suitable conditions, that does not make it our recommended treatment. For rotten-egg odors, use our sulfur treatment guide to choose an appropriate system.
Do not select a general-purpose coconut-shell carbon filter for PFAS solely because it meets an EBCT target. PFAS performance depends on the particular media, compounds present, and competing contaminants. Use documented performance for the exact application and test treated water to track breakthrough—the point when contaminants begin passing through the bed. Smell and taste cannot verify PFAS or VOC reduction. These applications may require two tanks in series with sampling points so performance can be monitored before the final outlet.
What does 5 gpm require?
To work backward, use required carbon volume = target EBCT × flow ÷ 7.481.
- Chlorine at 5 gpm: 2 × 5 ÷ 7.481 = about 1.34 cubic feet of carbon.
- Chloramine with catalytic carbon at 5 gpm: 4 × 5 ÷ 7.481 = about 2.67 cubic feet of carbon.
Choose an available system with at least the calculated carbon volume, then check its treatment rating and installation requirements. Size around the fixtures you expect to run together, not the home’s average daily flow. For a backwashing unit, confirm that your water supply can also provide the required backwash flow.
These examples illustrate the design targets; they are not certified reduction claims for a particular model.
Where do I install a carbon filter?
On a straightforward chlorinated city-water installation, carbon filtration installed ahead of the softener can reduce chlorine exposure to the resin. If sediment is present, address it with appropriate pretreatment. Follow each unit’s installation requirements for the final arrangement.
If you use a chlorination system on a well, preserve the required disinfection and contact stage before removing the residual disinfectant. Do not put carbon ahead of a stage that depends on that residual. For well-water odors, follow the sulfur guide rather than assuming a city-water carbon setup is the answer.
How long does carbon last?
For routine free-chlorine treatment, our typical planning estimate is approximately 800,000 gallons at 1 ppm chlorine per cubic foot of carbon before the media needs replacement. That is a chlorine-loading estimate of about 800,000 ppm-gallons per cubic foot—not a concentration of 800,000 ppm.
1 cubic foot of carbon + 1 ppm chlorine ≈ 800,000 gallons
At 2 ppm chlorine, the same rule of thumb gives approximately 400,000 gallons per cubic foot. To estimate your system’s gallon capacity, use 800,000 × cubic feet of carbon ÷ incoming free chlorine in ppm.
This is an approximate planning estimate, not a guaranteed service life. It assumes the carbon is properly sized and operated for the application, and it is not a chloramine, VOC, or PFAS capacity rating.
For optimal performance, we recommend rebedding every five years, even if your estimated gallon capacity has not been reached. Rebed sooner if testing shows the carbon is no longer providing the intended reduction. Rebedding means replacing the carbon inside the tank; backwashing does not renew exhausted media.
For an existing system, choose the correct replacement type and volume from our bulk standard GAC or bulk catalytic carbon. Follow the system’s media-loading and startup-rinsing instructions. If you need a complete system, compare our backwashing carbon filters.
Carbon life depends on the incoming disinfectant load, water use, other contaminants, and operating conditions. A gallon rating tied to a particular chlorine concentration is not a universal replacement interval or a chloramine rating.
Follow startup rinsing instructions and the maintenance schedule. Use a test appropriate to the disinfectant you are treating to compare incoming and treated water. Smell alone is not a complete performance check.
Make the selection in this order.
- Identify chlorine or chloramine and select the corresponding carbon option.
- Choose upflow only for clean city water, never well water. Backwashing carbon is our gold-standard choice when the installation supports its drain and backwash requirements.
- Use the application’s EBCT target and your simultaneous fixture flow to check carbon volume, then verify treatment ratings, dimensions, and installation requirements.
- For a backwashing model, verify the required supply flow and drain arrangement.
Already have a carbon filter? Shop replacement GAC or replacement catalytic carbon.
Have a question this guide and the product specifications don’t answer? Contact us with that question.