Hach Disinfection Series - Step 3

3. Free vs. Total Chlorine

Free chlorine refers to both hypochlorous acid (HOCl) and the hypochlorite (OCl^–) ion or bleach, and is commonly added to water systems for disinfection. When ammonia or organic nitrogen is also present, chloramines known as monochloramine, dichloramine, and trichloramine will quickly form. Chloramines are also known as combined chlorine.

Total chlorine is the sum of free chlorine and combined chlorine. The level of total chlorine will always be higher than or equal to the level of free chlorine.

Free chlorine is typically measured in drinking water disinfection systems using chlorine gas or sodium hypochlorite to find whether the water system contains enough disinfectant. Typical levels of free chlorine in drinking water are 0.2 - 2.0 mg/L Cl₂, although regulatory limits allow levels as high as 4.0 mg/L.

Total chlorine is measured in drinking water and is also typically measured to determine the total chlorine content of treated wastewater. If you are required to measure and report chlorine levels to a regulatory agency, we advise that you check with your regulator to find whether you are required to measure free chlorine or total chlorine.

Introduction

1. Chlorine Applications in Drinking Water Treatment

2. Chlorine Chemistry

3. Free vs. Total Chlorine

4. Introduction to Chloramination

5. Optimizing Monochloramine Production

6. Dealing with Special Situations when Chloraminating

7. Measuring Free Chlorine in the presence of Manganese and Chloramines

8. Determining an Ammonium Feed Rate for Chloramination

9. UV Methods for DBP Control and UV Disinfection