Free available chlorine (FAC) is the active, unbound portion of chlorine in water, ready to disinfect. This form neutralizes contaminants before reacting with other substances. FAC typically exists as hypochlorous acid (HOCl) and hypochlorite ions (OCl-), with HOCl being the more potent disinfectant. Water pH influences the balance between these two forms.
The Role of Free Available Chlorine
FAC plays a fundamental role in maintaining water safety as a powerful disinfectant. When introduced into water, chlorine forms hypochlorous acid and hypochlorite ions, the primary active compounds for disinfection. These compounds work by breaking down the cell walls of microorganisms, disrupting their internal processes, and effectively inactivating bacteria, viruses, and other harmful pathogens. This prevents the replication and survival of disease-causing agents.
This disinfectant action safeguards public health in water systems, including swimming pools and drinking water supplies. Chlorine’s ability to remain in the water, providing a “residual” effect, helps protect against potential recontamination as water moves through distribution networks from treatment plants to homes. Water chlorination has been a standard practice for over a century, significantly reducing the incidence of waterborne diseases such as E. coli, Hepatitis, and Giardia. This ensures water remains safe for consumption and recreational use.
Monitoring and Adjusting Chlorine Levels
Regular monitoring of free available chlorine levels ensures effective disinfection and safe water conditions. Common methods for testing FAC involve test strips or liquid test kits. Test strips are dipped into a water sample and change color, compared to a manufacturer’s chart for concentration. Liquid test kits use reagents like DPD (diethyl-p-phenylene diamine) that react with chlorine to produce a color change, allowing for more precise measurement.
For swimming pools, a free chlorine level between 2.0 and 4.0 parts per million (ppm) is generally recommended. The Centers for Disease Control and Prevention (CDC) suggests a minimum of 1 ppm in pools and 3 ppm in hot tubs. Low FAC levels can be increased by adding chlorine in various forms, such as chlorine gas, liquid bleach (sodium hypochlorite), or solid forms like calcium hypochlorite or dichlor granules. To decrease high FAC levels, methods include stopping further chlorine additions, exposing water to sunlight (which degrades chlorine), or partially draining and refilling. Chemical neutralizers like sodium thiosulfate can also rapidly reduce chlorine levels, though they should be used carefully to avoid over-correction.
Impact of Chlorine Levels
Deviations from appropriate free available chlorine levels can lead to significant consequences. When FAC levels are too low, the water’s disinfection ability is compromised, increasing the risk of pathogen growth and waterborne illnesses. Low chlorine can also result in cloudy water and the proliferation of algae, making the water unsafe and unappealing for use. This insufficient disinfection creates an environment where bacteria and other contaminants can multiply without being effectively neutralized.
Conversely, excessively high free available chlorine levels can cause irritation and health concerns. Elevated concentrations lead to skin and eye irritation, and respiratory discomfort, particularly for sensitive individuals. High levels may also produce a strong, unpleasant odor. Furthermore, excessive chlorine can react with organic matter in the water to form disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs), linked to health issues with prolonged exposure.