Sodium hypochlorite (NaClO) is a strong oxidizing compound widely used in water treatment, wastewater disinfection, and various industrial processes due to its powerful disinfecting and oxidizing capabilities. As a chlorine-based disinfectant, its primary functions include sterilization, deodorization, bleaching, and oxidation of organic matter, with disinfection being the most significant. When dissolved in water, sodium hypochlorite hydrolyzes to form hypochlorous acid (HClO), which can further decompose to release nascent oxygen—a highly reactive species capable of denaturing proteins in bacteria and viruses, effectively killing pathogens. In addition to oxidation, hypochlorous acid can penetrate microbial cell walls and react with intracellular proteins and enzymes, disrupting normal metabolic activity and leading to cell death.
In drinking water treatment, sodium hypochlorite is commonly used for chlorination. The post-chlorination dosage is typically around 2 mg/L of available chlorine, while pre-chlorination dosage should be determined through laboratory testing according to the characteristics of the raw water. When ammonia nitrogen is present, breakpoint chlorination testing is essential to achieve optimal results. Dosing points are usually set at water distribution wells or mixing basins where sufficient contact can be ensured, with a minimum contact time of 30 minutes.
In wastewater treatment, the dosage of sodium hypochlorite is usually three to seven times that used for potable water disinfection, and it should also be verified through pilot tests. Dosing points are typically placed at final discharge points, such as the outlets of sand filters, secondary clarifiers, or designated clear water tanks. Due to its safety and ease of handling, sodium hypochlorite is widely adopted in wastewater treatment plants with advanced treatment processes.
Like chlorine gas, sodium hypochlorite may also produce disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). However, the quantity of these by-products is significantly lower than that produced by chlorine gas, making sodium hypochlorite a safer alternative. Prolonged storage may lead to decomposition and disproportionation reactions, generating chlorate and chlorite—secondary by-products that should be monitored in application.
The main factors affecting the disinfection efficiency of sodium hypochlorite include pH, temperature, contact time, turbidity, and chlorine dosage. Generally, a higher pH reduces disinfection efficiency, so maintaining pH below 7.5 is recommended. While higher temperatures accelerate chlorine decay, they also enhance disinfection efficiency. High turbidity increases chlorine demand, so dosing is typically performed after filtration. Balancing these factors and optimizing the dosing strategy are key to achieving both effective and safe disinfection outcomes.
