Legionella in drinking water is regulated under EPA’s Surface Water Treatment Rule through a treatment technique requirement. There is no min/max Cl2 requirement to treat for Legionella. The efficacy of chlorination is affected by many factors, including chlorine concentration, contact time, pH, temperature, turbidity, buffering capacity of the water, the concentration of organic matter, iron and the number and types of microorganisms in the water system (in biofilms and free-living). For more information on the use and effectiveness of Legionella control strategies in premise plumbing systems, please read this EPA document: https://www.epa.gov/ground-water-and-drinking-water/technologies-legionella-control-premise-plumbing-systems
The Surface Water Treatment Rule requires that public water systems adding chlorine and using a surface water supply or a groundwater supply under the direct influence of surface water monitor for the presence of the residual disinfectant in the distribution system or at the entry point to the distribution system. The disinfectant level must be at least 0.2 mg/L at the entry point to the distribution system and detectable in at least 95 percent of samples collected within the distribution system.
The Stage 1 Disinfectant/Disinfection Byproducts Rule requires that public water systems that use chlorine maintain a residual disinfectant level of less than 4.0 mg/L as a running annual average.