Passive vs Active DeChlorination

brass faucets,dripping water,fotolia,gardensMunicipal water decision makers face the choice of which dechlorination chemical to employ in their water system maintenance program.  AWWA standards issued in 2010 now require that most field water discharge be dechlorinated before release into the local environment. This issue involves a choice between which chemicals to be used in water dechlorination. The following discussion reviews important points to consider when choosing between chemicals to be used in field dechlorination projects.

Neutralizing Chlorine with Minimum Risk to Staff and Environment

In the realm of tools for dechlorination, chemicals play a vital role in achieving maximum neutralization of chlorine residual in the field.  While there are a number of chemicals to choose from for dechlorination, which will be discussed later, two stand out as the best choice for field use; Ascorbic Acid and Calcium Thiosulfate.

All other chemicals available to neutralize chlorine affect other elements in the water and are better left for use in controlled applications such as Water and Wastewater Treatment plants. Only two chemicals are safe for the environment in field applications, the safest under all chlorine residual levels being Calcium Thiosulfate Solution, with Ascorbic Acid being the next best due to the affect it has on pH at high chlorine residual levels.

In selecting between Ascorbic Acid and Calcium Thiosulfate, a number of points need to be considered. Before we explore these points further we need to clearly establish why they are the only choice for use in field dechlorination

pH, Dissolved Oxygen and Their Effect on the Environment

The first thing to understand about other chlorine neutralizing chemicals is that they can have a big effect on water pH and Dissolved Oxygen.  Release of such waters to the environment can cause severe problems for plants, animals and aquatic life.  In addition many of these chemicals are hazardous to people and animals and require special handling to prevent harm to personnel.

A very important issue to keep front and center in your considerations when choosing chemical, is state and federal regulations regarding discharge into the surface water environments. EPA and state environmental agencies have specific rules regarding how water must be treated before release into the surface environment.  It is important to understand that waters free of chlorine can still cause issues if after chlorine removal water pH or dissolved oxygen levels are outside mandated guidelines.  If waters released to environment harm aquatic life legal issues will arise.  If the process of removing chlorine affects waters in a way that result in fish kills, utilities and their contractors will be subject to legal action and fines resulting from release of those waters.

The following chart displays all potential dechlorination chemicals and their relative characteristics when employed in chlorine neutralization.


pH Impact

Oxygen Scavenger

NFPA Rating


Sulfur Dioxide (SO 2 )





Sodium Thiosulfate (Na2S2O3)





Sodium Sulfite (Na2SO3)





Sodium Bisulfite (NaHSO3)





Sodium Metabisulfite (Na2S2O5)





Ascorbic Acid (Vitamin C)





Sodium Ascorbate




See Note *

Calcium Thiosulfate (CaS2O3)





* Sodium Ascorbate is 30 - 40% higher in cost then ascorbic acid and only comes in granular form. Sodium ascorbate is 30% sodium so it might affect receiving waters with sodium levels.

As the table above illustrates many of the choices for dechlorination are best reserved for use in controlled environments.  Only three are suitable for use in field dechlorination because of their relatively mild effect on plants and aquatic life, plus their fairly safe handling characteristics.  Sodium Ascorbate is safe for the environment like CTS, however it is higher in cost than ascorbic acid, so for this discussion we will only compare CTS to Ascorbic Acid

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Making Field Dechlorination Safe and Simple

When it comes to choosing between Ascorbic Acid and Calcium Thiosulfate, Calcium offers a number of advantages over Ascorbic Acid. Biggest of all the advantages is cost.  Calcium is made in the US while all of the Ascorbic Acid sold in the world currently comes from China.  With a lower cost to manufacture and dramatically smaller shipping distances, the cost advantage of Calcium Thiosulfate as your Dechlorination chemical of choice is difficult to dispute.

Second on the list of advantages is that Calcium Thiosulfate commonly comes ready-to-use in solution.  While Ascorbic Acid commonly comes packaged in tablet or granulate form, Calcium’s availability in liquid form makes it easier to deploy and use.  Calcium is highly stable when in liquid form.  Ascorbic Acid when dissolved into solution has a short usable life, due to oxidation.  Once mixed, Ascorbic Acid must be used or disposed of because its tendency to break down due to oxidization.  Surplus Calcium can be reused on other dechlorination projects because of its relative stability.

The third advantage of Calcium over Ascorbic is its very low toxicity.  Calcium does not scavenge oxygen and is very neutral pH, which results in very pH neutral discharge waters after neutralizing chlorine residuals - posing no adverse effect on plants and aquatic life that are exposed to it.  In fact Calcium Thiosulfate is commonly used as a plant nutrient in agriculture as well as in treatment of some medical conditions and is so safe you can drink it, though it has a very noxious taste.  Calcium Thiosulfate is also NSF 60 listed as safe to use in the processing of drinking water.

Applications and Advantages

Though this discussion weighs heavily for Calcium Thiosulfate Solution as the preferred choice, Ascorbic Acid does have its points to consider.  In situations where only potable water is being dechlorinated Ascorbic is comparable with Calcium in achieving necessary chlorine neutralization.  Though it requires extra handling over CTS, operations that are familiar with its use will suffer no adverse impact by staying with Ascorbic.

Choosing the right chemical for dechlorination also involves consideration for how the chemical will be employed.  Ascorbic is manufactured as a granular/powder from and is remanufactured into a tablet, and Calcium comes in liquid form.  Each form works best in specific applications. 

When employing a chemical injection device to neutralize chlorine coming out of a pipeline system, Calcium is easier to handle and more effective in reducing chlorine residual levels to what is required for release to surface water runoff.  When dealing with chlorine residual levels necessary for pipeline neutralization Calcium also offers a distinct advantage over Ascorbic in lower cost of treatment per pound of chlorine residual, making it more cost effective than Ascorbic Acid.

When choosing which chemical to use in field dechlorination, one must be careful to select a chemical that will adequately neutralize the chlorine residual concentrations in the water being treated while leaving water pH and dissolved oxygen at levels safe for aquatic life and plants.  For field dechlorination ascorbic acid and calcium thiosulfate solution are your best choices, depending on chlorine levels being treated. Field dechlorination can create a number of avoidable issues if not handled in the proper way, and chemical is as big a decision as method and equipment.

To learn more about chemical choices for your dechlorination needs download our free report Making the Right Choice in Field Dechlorination today.  This free report outlines the different equipment and methods available along with chemical choices; and compares the performance of each in the variety of applications.

Making the right choice

While the different chemicals discussed in our free report Making the Right Choice in Field Dechlorination each have strong points, one manufacturer’s chemical offers the lowest cost of use in field dechlorination.   No-Chlor Calcium Thiosulfate Solution offers the most value in dechlorination in field environments. When used in conjunction with an H2O Neutralizer chemical injector No-Chlor completes a system that has the lowest Total Cost of Ownership of any dechlorination system currently on the market.


To review how No-Chlor Calcium Thiosulfate Solution can meet you dechlorination needs get your free copy of Making the Right Choice in Field Dechlorination visit our website at and learn more about how our active dechlorination system can help you meet your needs.

To learn more about the entire H2O Neutralizer product line visit our webiste at:

Measurement Technologies Inc

P.O. Box 2195, Redmond, WA 98073
Toll Free 1-877-889-8482 /  Phone 1-425-836-8683  / Fax 1-425-484-6664