Short definition
An activated carbon filter uses granular carbon (GAC) or carbon block to adsorb chlorine, chloramine, organic compounds, and bad tastes from water. Standard GAC removes chlorine well but only about 30% of chloramine; catalytic activated carbon removes about 95% of chloramine. Seattle Public Utilities uses chloramine — SPU homeowners should specify catalytic carbon.
What it is
Activated carbon is a highly porous form of carbon (often coconut shell, coal, or wood-based) with enormous internal surface area. Water passes through the carbon and contaminants stick to the pore walls — “adsorption.” The carbon eventually saturates and has to be replaced.
What activated carbon removes well:
- Free chlorine (taste, smell).
- Chloramine (with catalytic activated carbon — standard GAC is inadequate).
- Volatile organic compounds (VOCs).
- Tastes and odors from organic matter.
- Some pesticides and herbicides.
What it doesn’t remove:
- Hardness (calcium, magnesium) — that’s a softener’s job.
- Lead, dissolved metals — RO does this.
- Pathogens — UV or RO.
- Nitrates, fluoride — RO.
- Sediment — sediment filter goes upstream.
Forms:
- Granular activated carbon (GAC) — loose carbon granules; cheaper, faster flow, less effective per pass.
- Carbon block — compressed carbon; more contact, finer filtration, slower flow.
- Catalytic activated carbon — modified carbon with enhanced chemistry for chloramine removal.
Cartridge life: typically 6 months for under-sink point-of-use, 6–12 months for whole-house, varying with usage and water quality.
Why it matters to a homeowner
Activated carbon is the most common water-treatment cartridge in residential use. Refrigerator filters, pitcher filters, and most under-sink drinking-water systems are some form of activated carbon.
The chloramine question is the critical WA-specific decision. Seattle Public Utilities switched from chlorine to chloramine years ago for distribution-system disinfection. Chloramine doesn’t off-gas the way chlorine does — it persists through the plumbing system, into your shower steam, and your coffee. Standard GAC only removes about 30% of it. Catalytic activated carbon removes about 95%. If you can taste a swimming-pool flavor in your Seattle tap water, you have chloramine, and you need catalytic carbon.
When a contractor recommends a “whole-house carbon filter,” ask whether it’s standard GAC or catalytic. The price difference is small ($30–$100 per cartridge); the chloramine-removal difference is large.
The other practical concern: replace cartridges on schedule. An overdue carbon cartridge can release trapped contaminants back into the water — “filter-saturation breakthrough.” Set a calendar reminder.
Common failure modes
- Cartridge overdue — saturation breakthrough; release of trapped contaminants.
- Standard GAC used for chloramine — inadequate; switch to catalytic carbon.
- Reduced flow as cartridge clogs — change schedule slipped.
- Bypass valve mode forgotten after install — water bypasses filter unintentionally.
- Sediment overload (without pre-filter) clogs carbon prematurely.
Common variants
- Granular activated carbon (GAC) vs. carbon block (more contact, finer filtration).
- Standard activated carbon vs. catalytic activated carbon (chloramine removal).
- POE whole-house vs. POU under-sink vs. pitcher / refrigerator filter.
- Carbon vs. RO: carbon for taste/chlorine; RO for dissolved solids, lead, heavy metals.
- Carbon vs. KDF: alternative copper-zinc media; sometimes paired with carbon.
Washington note
The chloramine question is the load-bearing WA detail. Seattle Public Utilities (Cedar/Tolt water) uses chloramine. Tacoma Water, Bellevue, Spokane, and other utilities may use chlorine, chloramine, or both — verify with your specific utility’s water-quality report before choosing carbon type.
For SPU-served homes, specify catalytic activated carbon for any meaningful chloramine removal. This applies to point-of-entry whole-house carbon as well as point-of-use under-sink filters.
For pre-1986 homes regardless of utility, activated carbon alone won’t address lead leaching from old solder — pair POU carbon with reverse osmosis at the kitchen sink for lead removal.
A few specific WA scenarios:
Seattle homeowner can taste chloramine — install catalytic carbon POU at the kitchen sink.
New construction: developer specifies standard whole-house GAC — push back and ask about chloramine spec if you’re on chloraminated water.
Eastside Bellevue (Cascade Water Alliance) — verify per-utility whether chloramine or free chlorine is used; standard GAC may be adequate for free-chlorine systems.
Pre-1986 Seattle home with concern about lead — GAC alone won’t help; add RO at the sink.