Understanding the Breakpoint Chlorination Curve

A diagram of the chloramine boosting system with a water tank mixer helps the chlorine analyzer get a better reading of the chemical residual in the entire tank.

Understanding the Breakpoint Chlorination Curve

The challenges in achieving breakpoint chlorination, a process where the addition of ammonia and chlorine to water continues until the chlorine demand is satisfied and a residual is achieved, are as follows:

1. Variability in Water Quality

One of the most significant challenges in achieving breakpoint chlorination is the natural variability of the water itself. Water quality can change depending on a variety of factors, such as:

Seasonal changes: Water may have higher levels of organic matter during certain times of the year, such as after heavy rainfall or during autumn when plant material enters the water. This variability can alter the chlorine demand, meaning more chlorine may need to be added to reach the breakpoint.

Ammonia content: Water supplies containing high levels of ammonia (common in surface water and groundwater influenced by agricultural runoff or wastewater discharge) will require a larger chlorine dose to neutralize the ammonia and form chloramines. Inconsistent ammonia levels can make it difficult to predict how much chlorine will be needed to achieve the breakpoint.

pH fluctuations: Water’s pH can affect chlorine’s impact as a disinfectant. At higher pH levels, chlorine exists more as a hypochlorite ion (OCl⁻), which is less effective for disinfection. Water treatment operators may need to adjust chlorine doses based on pH fluctuations to maintain efficacy and reach the breakpoint.

2. Formation of Chloramines

During the early chlorination stages, ammonia in the water reacts with chlorine to form chloramine (monochloramine, dichloramine, and trichloramine). Chloramines are less effective disinfectants than free chlorine and can cause unwanted issues, such as:

Taste and odor problems: Chloramines, especially dichloramine and trichloramine, can create unpleasant tastes and a chlorine-like odor in the water. If the chlorine dose isn’t sufficient to break down these chloramines and reach the breakpoint, these undesirable compounds can persist in the water.

Reduced disinfection power: Chloramines, mainly when they dominate in the water, are not as potent at killing pathogens as free chlorine. Therefore, an incomplete chlorination process (without reaching the breakpoint) can result in insufficient disinfection and a risk of microbiological contamination.

Achieving breakpoint chlorination requires careful control to ensure that ammonia is wholly oxidized and does not leave behind chloramines that compromise water quality.

3. Chlorine Demand and Organic Matter

Organic matter in water, such as decaying plant material, algae, or industrial pollutants, creates a chlorine demand. This means that a portion of the chlorine added to the water will be consumed by reactions with these organic substances before it can begin disinfecting the water.

High chlorine demand: In waters with significant organic content, much of the chlorine added is “used up” before it can reach the breakpoint. This means more chlorine is needed to neutralize all organics before there is enough free chlorine to disinfect effectively.

Variable demand: The amount of organic material in the water can vary significantly based on location, season, and water source. A treatment facility might need to adjust chlorine doses frequently to account for changes in chlorine demand, which complicates consistently reaching the breakpoint.

4. Potential for Over-Chlorination

While under-chlorination can lead to insufficient disinfection, over-chlorination presents its own set of challenges:

Excess free chlorine: If too much chlorine is added beyond the breakpoint, the water can have excessive free chlorine levels. High free chlorine concentrations can cause the water to have a strong chlorine taste or odor, which is unpleasant for consumers. In addition, high levels of chlorine can react with organic compounds to form disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs). The government regulates these DBPs due to their potential long-term health risks, such as cancer and reproductive issues. Many have benefited from adding a blower to their water storage tanks. This blower reduces the amount of THMs in the headspace of a reservoir or standpipe, allowing more THMs to escape the water itself.

Corrosion risks: Excessive chloramine can accelerate the corrosion of pipes, fittings, and other equipment in the water distribution system. Corrosion can lead to the leaching of harmful metals like lead and copper into the water, further complicating the treatment process and posing health risks to consumers.

5. Nitrification in Distribution Systems: Nitrification, a process where ammonia is converted into nitrites and nitrates by bacteria, is another challenge that arises, particularly after the breakpoint is achieved and chloramines are eliminated:

Nitrification in pipes: When chloramine levels drop or are inconsistent, nitrifying bacteria may proliferate in the distribution system. These bacteria convert ammonia into nitrites and nitrates, depleting disinfectant levels and increasing the risk of microbial contamination in the water. This can lead to taste and odor issues, decreased chlorine residuals, and potential health risks.

Managing nitrification: To prevent nitrification, operators must continuously monitor chlorine and ammonia levels, particularly in extended water distribution networks. This can require frequent adjustments to chlorine dosing or other interventions, such as flushing water mains or increasing monitoring during warmer weather when nitrifying bacteria grow more rapidly.

6. Real-Time Monitoring and Control

Achieving and maintaining breakpoint chlorination requires real-time monitoring of various water quality parameters, including:

Chlorine residuals: Continuous monitoring of chlorine levels throughout the treatment process and distribution system is necessary to ensure that free chlorine levels remain sufficient to maintain disinfection.

Ammonia and organic levels: Regular testing of ammonia concentrations and the overall organic content of the water allows operators to adjust chlorine dosing based on fluctuating water conditions.

Advanced technology needs: Many treatment facilities require advanced control systems, such as SCADA (Supervisory Control and Data Acquisition), and automated dosing mechanisms to adjust chlorine addition in response to real-time data. Installing, maintaining, and upgrading these systems can be complex and costly.

In conclusion, achieving breakpoint chlorination is a significant responsibility for water treatment operators. From dealing with fluctuating water quality to managing chloramine formation, chlorine demand, and potential over-chlorination, operators must continuously play a balancing act to ensure that the water is adequately disinfected without creating excessive disinfection byproducts.

Overcoming these challenges requires constant monitoring, proper dosing, and, often, advanced technology. Reaching breakpoint chlorination is crucial for maintaining safe, high-quality drinking water and ensuring public health. This is why many municipalities choose to have their addition of chloramines automated with a chlorine boosting system.