A Few Things You Should Know about Chlorine

by Pure Water Annie

Chlorine has saved some lives and it has taken some.


Adapted from articles originally published in the Pure Water Occasional for January, 2012.  

Chlorine is a big part of our lives. It has hundreds of uses in addition to water treatment, but water is where most of us encounter it most frequently and most intimately.

Chlorine (or its near relative chloramine, which we’ll discuss more fully later) is added to most public and many private water supplies to eliminate problems with bacteria, viruses, fungi, and algae. It is also used as an oxidant to aid in the treatment of such well problems as iron, manganese, and hydrogen sulfide.

Chlorine is a powerful germicide. It kills or reduces most disease-causing water contaminants to non-detectable levels. It also eliminates algae and mold that are problems for municipal water systems.

Chlorine, along with improved sanitation, is responsible for the virtual elimination in the US of such serious waterborne diseases as cholera, dysentery and typhoid. Waterborne ailments have not been erased by chlorination by any means, but the problem is minute compared to what it was before chlorination was used.

Lack of clean drinking water and adequate sewage systems is the main health problem in most of the under-developed world.

The effectiveness of chlorine as a disinfectant can depend on a variety of water variables. These include contact time (how long the chlorine remains in the water to do its work), the concentration of chlorine, and the pH, temperature, and turbidity level of the water.

Chlorine remains the disinfectant of choice for municipal suppliers largely because of its price. As compared with other forms of disinfection, such as ultraviolet, ozone, and hydrogen peroxide, it is cost effective. It is also the disinfectant of choice because of its residual effect: Chlorine (and chloramine to an even greater degree) stays in the water and continues to protect against micro-organisms, while UV and Ozone kill on contact but offer little if any “residual” protection. UV works great on wells, and it aids in the treatment program of many cities, but is not often used as the principal disinfectant.

Another problem with chlorine is that when it combines with organic substances in water it creates a group of spin-off chemicals called, variously, THMs (trihalomethanes) or DBPs (disinfection by-products). The full scope of the problems with DBPs is not known, but of the hundreds of chemicals that have been identified, some are known cancer causers and are regulated by the EPA.

Because of the DPB issue (and other issues, like the relatively shorter life-span of chlorine) water suppliers are in greater numbers switching to chloramines, a mixture of chlorine and ammonia, as the disinfectant for public water supplies.

The risks of cancer from contacting and ingesting chlorinated water have been downplayed by public health officials because the alternative of non-treatment is so much more dangerous. The World Health Organization has said that “the risk of death from pathogens is at least 100 to1,000 times greater than the risk of cancer from disinfection by-products (DBPs), and the risk of illness from pathogens is at least 10,000 to one million times greater than the risk of cancer from DBPs.”

The best and most practical method for removing chlorine (and chloramine) from tap water entering the home is carbon filtration. There are many subtleties involved in carbon filtration. Variables like pH, water temperature, flow rate, “mesh” size, arrangement of the carbon, and others can greatly affect carbon’s effectiveness, but the truth is that almost any carbon filter, including the cheap, end-of-faucet units, will do a decent job of chlorine (but not chloramine) reduction from drinking water. (Shower filters, which must handle a much larger volume of water at a higher flow rate, most often are made with KDF rather than carbon.)

Reduction of chloramine is a much more complex process, but the urban legend that says that standard carbon water filters won’t remove chloramine is false. Chloramines can be reduced by carbon, but more residence time is needed—a lot more. Some carbons are more effective with chloramines than others, and the very best, by far, is specially processed “catalytic” carbon.

A Sensible Home Treatment Strategy for Chlorine and Chloramines

The disinfectants in city water are there for a reason. They protect against pathogens. They should be in the water until it reaches your home, but at that point they become a problem rather than an asset. The problems are bad taste, chemical toxicity (which affects some more than others), and serious health issues involving both the disinfectant and its by products.

If your goal is simply to produce better tasting water by removing the taste of chlorine, a simple end-of-faucet filter will do.

If you want improved taste plus protection from chemicals (including DBPs), a serious carbon block filter is needed. The more carbon the better, and the higher the filter quality the better. DBPs are not effectively reduced by small end-of-faucet filters. Treating them requires more contact time and a larger and better carbon-based filtration system.. Multi-stage carbon filters are excellent. Virtually all undersink reverse osmosis units remove chlorine, chloramine, and DBPs easily.

For whole house treatment, a small carbon filter will remove chlorine, but a much larger filter, or a fairly large filter that uses catalytic carbon, is needed for chloramine.


Numerical Wizard B. Bea Sharper ferrets out the watery facts that Harper’s misses

Rank of contaminated drinking water on the World Bank’s 1992 list of preventable environmental hazards — 1

Number of people in the underdeveloped world that lack clean drinking water — 1 billion.

Number of people in the underdeveloped world that lack adequate sewage systems — almost 2 billion.

Parts per million chlorine of common household bleach – 52,500.

Year in which US water utilities began treating water with chlorine — 1908.

Percentage of US water utilities that now use chlorine or its derivatives to disinfect drinking water – 98%.

Year when chloramine was first used to treat water – 1916.

Percentage of US municipalities that now use chloramine as a disinfectant by EPA count – 30%.