Units incinerating sewage sludge emits mercury into the air that eventually deposits into water, where it changes into methylmercury, an environmental toxicant, through contact with some microorganisms.

The proposed rule, which will be finalized in 2011 and become effective in 2015, would apply to both multiple hearth and fluidized bed incinerators that are typically located at wastewater treatment facilities.

Different air pollution standards will be used for units incinerating sewage sludge at other types of facilities such as commercial, industrial and institutional incinerators to cut mercury emissions from these units by more than 75%.

The proposal, which is expected to incur annualized costs of approximately $105m for all currently operating units to comply with the proposal standards, is expected to yield health benefits ranging from $130m to $320m in 2015.