What does 'residual chlorine' refer to in water treatment?

'Residual chlorine' refers specifically to the amount of chlorine remaining in the water after the disinfection process has taken place. This residual is important because it indicates that there is still some level of chlorine present, which helps maintain disinfection as water moves through pipes and distribution systems. The presence of residual chlorine is critical for ensuring that any pathogens that may enter the water after treatment are effectively neutralized.

In terms of water treatment efficacy, measuring residual chlorine allows operators to confirm that sufficient chlorine was applied and that the water remains safe for consumption. It also plays a role in the management of chlorinated water systems by preventing regrowth of harmful microorganisms.

The other choices illustrate different aspects of chlorine in water treatment but do not accurately define residual chlorine. The total amount added refers to the initial dose used for disinfection, while the amount that evaporates pertains to chlorine loss due to volatilization. Toxicity to aquatic life also describes chlorinated water but is not specific to the residual quantity of chlorine present after disinfection.