What type of pollution controls must be installed if an existing plant emits mercury?

The correct answer is MACT, which stands for Maximum Achievable Control Technology. This standard is specifically designed to limit hazardous air pollutants, including mercury, from existing and new sources in various industries. Under the Clean Air Act, MACT standards require the implementation of the best available technologies that can significantly reduce emissions of toxic pollutants.

The focus on existing plants emitting mercury is crucial since MACT standards are applied to control hazardous air pollutants that pose significant health risks. MACT ensures that plants take stringent measures to limit emissions, thereby protecting public health and the environment.

Regulations related to the other types of controls, such as RACT (Reasonably Available Control Technology), BACT (Best Available Control Technology), and BPCT (Best Practical Control Technology), play roles in different contexts. RACT is more applicable to non-attainment areas and is focused on reasonably available options, while BACT is typically used for new sources or when permitting major modifications to facilities. BPCT applies to water pollution rather than air. Thus, in the context of existing plants emitting mercury, MACT is the appropriate standard to ensure compliance with air quality regulations.