Using Free Chlorine to Treat Drinking Water: Benefits and Challenges with Stable Residual and Stratification

Using Free Chlorine to Treat Drinking Water: Benefits and Challenges with Stable Residual and Stratification

Disinfection is a crucial step in drinking water treatment, and free chlorine is one of the most widely used disinfectants due to its efficacy in killing pathogens at a relatively low cost. When used correctly, chlorine provides a robust line of defense against waterborne diseases, helping maintain the quality and safety of drinking water. However, using chlorine effectively involves maintaining adequate chlorine residuals throughout the distribution system. Additionally, stratification in storage tanks can lead to uneven chlorine distribution, posing challenges for water utilities.

In this post, we’ll discuss the role of free chlorine in water treatment, its benefits, and the challenges of maintaining residuals and preventing stratification to ensure adequate disinfection and good-tasting drinking water.

The Role of Free Chlorine in Water Treatment

Free chlorine refers to chlorine “available” in the water as hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), both powerful disinfectants. Here’s how it works:

1. Pathogen Inactivation: When chlorine is added to water, it reacts with microorganisms by breaking down their cell walls and disrupting their enzymes, inactivating bacteria, viruses, and other harmful pathogens.

2. Residual Protection: Unlike other disinfectants that may only treat water at the point of contact, chlorine provides a “residual” effect. This means it remains in the water throughout the distribution system, helping control potential contamination. This residual presence is vital for maintaining water quality as it moves from treatment plants to homes and businesses.

However, ensuring that free chlorine remains effective across all points of the distribution system can be challenging due to residual decay and stratification in water storage tanks.

Challenges with Maintaining Chlorine Residuals

Maintaining consistent chlorine levels throughout a water system requires balancing the chlorine dose to be strong enough for disinfection without leading to over-chlorination, which can result in taste and odor issues. Some of the main factors that affect chlorine residuals:

1. Chlorine Demand: Chlorine demand represents the amount of chlorine consumed as it reacts with various organic and inorganic substances in the water. Natural organic matter (such as decaying plant material), ammonia, and even certain minerals can consume chlorine. This reaction reduces the amount of free chlorine, giving a residual effect in the rest of the distribution system. Suppose the demand is higher in certain parts of the distribution system. In that case, it will lead to excessive chlorine consumption, causing the residual to drop below safe levels and increasing the risk of microbial contamination.

2. Effect of Temperature: Temperature plays a significant role in chlorine effectiveness. Warmer water temperatures increase the reaction rate, causing chlorine to dissipate more quickly, leading to uneven residual levels across the system. However, residuals are more stable during colder months, but disinfection can be slower. Adjusting the chlorine dose seasonally is often necessary to account for these fluctuations.

3. Water Age: Water age refers to the time water spends in the distribution system. Longer travel times from the treatment plant to end-users allow more time for chlorine residuals to decay. Dead-end sections of the system, low-flow areas, and storage tanks can all contribute to increased water age, making it harder to maintain consistent chlorine levels.

Stratification in Storage Tanks: A Key Challenge

Stratification, or the formation of layers of water at different temperatures or chlorine concentrations within a storage tank, can lead to inconsistent chlorine distribution. Here’s why it’s problematic:

1. Temperature Gradients: In large storage tanks, temperature differences can create layers within the tank, with warmer water on top and cooler water at the bottom. Since warmer water can cause chlorine to dissipate faster, the top layer may have lower chlorine residuals. This leads to an uneven distribution that can leave parts of the water vulnerable to contamination.

2. Chlorine Decay Variability: When stratification occurs, the chlorine concentration can vary significantly from one part of the tank to another. In worst-case scenarios, some parts of the tank may have little to no chlorine residual, while other areas have higher-than-needed concentrations. This affects water quality and can lead to “pockets” of lower disinfection potential, creating potential health risks.

3. Potential for Nitrification: Stratification can contribute to nitrification, especially in systems using chloramines as a secondary disinfectant. Nitrifying bacteria thrive in low-chlorine, oxygen-rich environments, and stratified tanks can provide ideal conditions for these bacteria to proliferate, especially during warmer weather. Nitrification can further reduce residual levels and produce nitrites and nitrates, which must be minimized in drinking water.

Strategies to Address Residual and Stratification Challenges

To mitigate the challenges associated with chlorine residual decay and stratification, water utilities often implement several strategies, such as:

1. Regular Monitoring and Adjustment: Frequent monitoring of chlorine levels across the distribution system can help operators adjust chlorine doses based on real-time demand. Many facilities use advanced sensors and automation to optimize dosing, ensuring adequate residuals are maintained without overdosing.

2. Tank Mixing Systems: Adding active mixing systems in storage tanks can prevent stratification by constantly circulating water and creating a more uniform chlorine concentration. These mixers help eliminate temperature gradients and ensure chlorine is distributed evenly throughout the tank, reducing the risk of low-residual pockets.

3. Seasonal Adjustments and Booster Stations: In systems with extended distribution networks, booster chlorination stations are sometimes installed to add more chlorine at strategic points. This can be especially useful in systems with high water age or in warmer weather when chlorine decay is faster. Seasonal adjustments to dosing schedules also help accommodate changes in water temperature and chlorine demand.

4. Flushing Programs: Regularly flushing low-flow or dead-end areas in the distribution system helps reduce water age, remove stagnant water, and maintain chlorine residuals. Flushing programs benefit areas with complex or extended distribution systems, where water may sit in pipes for extended periods.

Chlorine Needs Help Along the Way

Free chlorine remains one of the most effective and widely used disinfectants for treating drinking water. It provides a reliable residual to guard against contamination throughout the distribution network. However, maintaining consistent chlorine residuals and preventing stratification in storage tanks can be challenging. Factors such as water quality, temperature, and distribution system design all influence chlorine efficacy, while stratification in storage tanks creates additional obstacles to maintaining even chlorine distribution.

Water utilities can address these challenges and provide safe, high-quality drinking water to their communities by combining monitoring, water tank mixers, and targeted dosing strategies such as a free chlorine-boosting system. Addressing residual and stratification issues is essential to ensuring that free chlorine continues to protect public health efficiently and effectively.

Another topic for consideration when disinfecting drinking water with chlorine is the disinfection byproducts called trihalomethanes. See information about this topic, along with an exciting solution, here: Removing Trihalomethanes From Municipal Ground Reservoirs