Chloramine vs. Chlorine in Tap Water: What's the Difference?

Why cities disinfect water

All municipal water systems must disinfect water to kill bacteria, viruses, and parasites. The two most common disinfectants are free chlorine (Cl2) and chloramine (a combination of chlorine and ammonia, NH2Cl). Both are added at the treatment plant, and a residual amount stays in the water as it travels through distribution pipes to your tap. This residual is intentional: it prevents bacterial regrowth during transit.

The shift from chlorine to chloramine

Starting in the 1990s, many large US water systems switched from free chlorine to chloramine. The primary reason: chloramine produces significantly fewer disinfection byproducts (DBPs). When free chlorine reacts with organic matter in water, it forms trihalomethanes (TTHMs) and haloacetic acids (HAA5), which are linked to cancer risk. The EPA tightened DBP limits, and chloramine offered a way to comply while maintaining disinfection.

Today, approximately 30% of US water utilities use chloramine as their primary disinfectant, including most large cities: Washington DC, San Francisco, Denver, Philadelphia, Dallas, and many others.

How they differ

Why it matters for home filtration

This is the most important practical difference for homeowners. Standard granular activated carbon (GAC) removes free chlorine through a fast chemical reaction (dechlorination). It is very effective and requires minimal contact time.

Chloramine does not respond the same way. Standard GAC can only partially reduce chloramine, and the removal rate drops significantly at higher flow rates. To reliably remove chloramine, you need catalytic carbon, which has a modified surface structure that breaks the chloramine bond (between chlorine and ammonia) more effectively.

This distinction is critical when shopping for filters:

• Whole-house carbon filters: If your city uses chloramine, make sure the system specifies catalytic carbon (often marketed as "catalytic GAC" or "chloramine-rated"). Standard coconut shell carbon is not sufficient. Filter guide.
• Shower filters: Most KDF shower filters are designed for free chlorine. Their effectiveness against chloramine is limited. Shower filter guide.
• Pitcher filters: Carbon block pitchers (like Brita) reduce free chlorine effectively but have minimal impact on chloramine at typical pour-through rates.
• Reverse osmosis: RO membranes are damaged by both chlorine and chloramine. The carbon pre-filter in an RO system must be rated for whichever disinfectant your city uses. RO guide.

Special concerns

Aquarium owners

Chloramine is toxic to fish and does not evaporate from water the way free chlorine does. Letting water sit out overnight works for chlorine but NOT for chloramine. Aquarium owners in chloramine cities must use a chemical dechloraminator (like Seachem Prime) or a catalytic carbon filter on their water supply.

Home brewing and baking

Chloramine can kill yeast and affect fermentation. Brewers in chloramine cities need to treat water with campden tablets (potassium metabisulfite) or use an activated carbon filter rated for chloramine.

Dialysis

Chloramine must be completely removed from water used in dialysis machines. Medical facilities in chloramine districts use specialized carbon systems with verified removal.

How to find out which your city uses

Check your city's Consumer Confidence Report (CCR) or look up your city on CheckMyTap. The CCR will list "chlorine" or "chloramine" (sometimes listed as "monochloramine") as the disinfectant. You can also call your utility directly. If you are unsure, assume chloramine and buy catalytic carbon, since it handles both chlorine and chloramine effectively.