Velocity Water Services Blog

As you know, chlorine kills bacteria in areas such as swimming pools and treats water at your local treatment facility. Used in the proper dosage, it helps ensure drinking water is safe by breaking down into hypochlorite ions and hypochlorous acid when it comes in contact with water. This reaction is very efficient at killing the microorganisms that make people sick.  

These reactions also impact how much chlorine in the water is still active and how much is not. When it comes to water distribution system (WDS) management, it's essential to understand the difference between free chlorine vs total chlorine to manage effective chlorine residual levels. The only way to know how effective your water disinfection efforts are is to monitor your chlorine residual levels. 

What is the Difference Between Free Chlorine and Total Chlorine? 

Total chlorine is how much chlorine is initially added to water. Once chlorine goes to work, its job is to oxidize metals and then attack viruses, pathogens, and bacteria which will form chloramines. Chloramines remain part of the total chlorine level. However, the parts of the chlorine still doing their job are called 'free chlorine'. This is achieved once breakpoint chlorination is established by adding sufficient chlorine to bond with all the chloramines in the water. The remaining ‘free chlorine’ is used for disinfection.  

For water to remain safe, the chloramine levels in the total chlorine must be lower than the free chlorine levels. More chlorine is needed to combat microorganisms when the chloramine is higher. When chlorine is added to the water system, we get the following: 

  • Chlorine Demand: The difference between the amount of chlorine added to the water and the amount of chlorine residual remaining after a given contact time. I.e., chlorine Demand = chlorine Applied – Chlorine Residual. Applied – Chlorine Residual.The difference between the amount of chlorine added to the water and the amount of chlorine residual remaining after a given contact time. Chlorine Demand = Chlorine Applied – Chlorine Residual,  
  • Total Chlorine:  The total concentration of chlorine in water, including the combined chlorine (such as inorganic and organic chloramines) and the free available chlorine.  
  • Free Available Chlorine Residual: The amount of chlorine available at the end of a specified contact period. This residual may be in the form of hypochlorous acid or hypochlorite ion.  
  • Combined Chlorine: The sum of the chlorine species composed of free chlorine and ammonia, including monochloramine, dichloramine, and trichloramine. Dichloramine is the strongest of these chlorine species.  

Why are Free Chlorine Levels Important for Water Distribution Systems? 

The local WDS carries treated water to consumers for residential, commercial, and firefighting needs. To protect people from getting sick, the system must be maintained properly to reduce the risk of contamination. High-quality water does not present a risk for microbial degradation in the system, with chlorine playing a pivotal role in safe water preservation. There are many factors at play in the preservation and delivery of safe water, including: 

  • Shelf life: The amount of time the water is in the WDS. 
  • Packaging: The storage area and piping used to deliver the water. 
  • Preservative: The disinfectant residual chlorine used to maintain the quality of the water. 

Water contamination can only be avoided if all components are correctly maintained, including a sufficient chlorine residual. 

How is Water Quality Maintained in a WDS? 

A WDS manager must ensure the following steps are taken to maintain high water quality and preclude water contamination: 

  • Maintaining positive pressures and sufficient fire flows 
  • Managing water shelf life with appropriate ways to measure the age 
  • Maintaining chlorine residual for ongoing disinfection 
  • Keeping the distribution system clean 
  • Providing effective water treatment to avoid water degradation once in the system 

Why is a Chlorine Residual Required? 

A chlorine residual in drinking water ensures the right amount of chlorine is added to combat the microorganisms that make us sick. Chlorine residuals also protect the water from recontamination during its shelf life. Maintaining chlorine residual provides consistent, ongoing disinfectants in the distribution system, reducing bacterial growth as well as the formation of biofilm. As a result, public health is protected against widespread waterborne illness. 

How is Chlorine Residual Measured? 

Through routine monitoring at the municipal level, communities track the condition of the WDS at any given time. If a chlorine measurement shows a chlorine residual is lower than required, it increases the risk of illness. The following conditions are common contributors to poor water: 

  • Excessive shelf life 
  • Microbial regrowth 
  • Contact with corrosion byproducts in the system 
  • Cross-connection or contaminants that consume the chlorine 

Dosage testing measures how much sodium hypochlorite solution is needed to maintain proper levels for safe drinking water and the free chlorine residual in the water based on the average storage time. 

How are Low Chlorine Residuals Addressed? 

When the required level of free chlorine residual is not met, the municipality must take the following actions: 

  • Retesting to verify that the chlorine residual does not meet compliance levels 
  • Increasing the chlorine dosage and flush lines to reach an acceptable level should retesting confirm the initial findings 
  • Testing to confirm the residual is compliant 

If residual chlorine tests remain non-compliant, a boil water order is required. Also, if poor water quality is ongoing, an action plan must be devised, which might include the following steps: 

  • A permanent continuous blow-off station 
  • A residual booster station 
  • An increased frequency of testing 
  • Implementation of a routine flushing program. 

An increase in heterotrophic plate count (HPC) is often the first red flag of water quality issues. The goal is to ensure action is always underway to eliminate possible sources of contamination. Through testing, chlorine residual compliance level issues are addressed quickly to avoid community illness. 

The Importance of Minimum Free Chlorine Residual Levels 

To provide a minimum free chlorine residual, water authorities will set parameters to maintain water quality for the local WDS. With proper operating procedures in place, water is monitored for non-compliant levels. As a result, municipalities remain proactive in meeting the highest standards for local water quality and safety. Through the following steps, contamination can be avoided: 

  • Identifying changes in chlorine residuals 
  • Understanding the cause for changes in the chlorine residual in the WDS 
  • Taking corrective action(s) to increase healthy levels of the chlorine residual 
  • Addressing contamination events and why they occur 
  • Ensuring timely boil water orders reach the public 


At Velocity Water Systems, we play a vital role in assisting utilities with maintaining their WDS. We can also help initialize effective flushing practices to maintain safe community water. Reach out to our experts today