revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 390
5. POTENTIAL FOR HUMAN EXPOSURE
sludge/soil matrix to the environment by emission to the atmosphere. An additional 5% of mercury
emissions occur via direct discharge of industrial effluent to bodies of water. Mercury emissions from
coal-fired power plants are almost exclusively in the vapor phase (98%) (Germani and Zoller 1988).
Brown et al. (1993) reported that 79–87% of mercury contained in coal was released with the flue gas at
coal-fired power plants. These authors monitored emissions from plants using sub-bituminous C (low
sulfur), lignite (medium sulfur), and bituminous (both low- and high-sulfur) coals. Anthropogenic
emissions, mainly from combustion of fossil fuels, account for about 25% of mercury emissions to the
atmosphere (WHO 1990). These mercury emissions eventually may be deposited on the surrounding soil,
although soil concentrations have not been correlated with distance or direction from such plants (Sato and
Sada 1992). Other potential emission sources include copper and zinc smelting operations, paint
applications, waste oil combustion (EPA 1987f), geothermal energy plants (Baldi 1988), crematories
(Nieschmidt and Kim 1997; WHO 1991), and incineration of agricultural wastes (Mariani et al. 1992).
The incineration of medical waste has been found to release up to 12.3 mg/m 3 of mercury (Glasser et al.
1991). Medical wastes may release approximately 110 mercury mg/kg of uncontrolled emissions from
medical waste incinerators, compared with 25.5 mercury mg/kg general municipal waste, indicating that
medical equipment may be a significant source of atmospheric mercury. The use of scrubbers on the
incinerators may remove up to 51% of the mercury emissions (Walker and Cooper 1992). Other potential
emission sources of mercury emissions to the air include slag from metal production, fires at waste
disposal sites, and diffuse emissions from other anthropogenic sources, such as dentistry and industrial
activities. The anthropogenic mercury contributions are greater in the northern hemisphere than in the
southern hemisphere, and are greatest in heavily industrialized areas.
Balogh and Liang (1995) conducted a 9-week sampling and analysis program to determine the fate of
mercury entering a large municipal wastewater treatment plant. Mercury removal in primary treatment
averaged 79%; and the average removal across the entire plant was 96%. Mercury loading on the
secondary treatment (activated sludge) process was elevated to near plant influent levels due to recycling
of the spent scrubber water from the sewage sludge incinerator control equipment. This internal recycling
of the spent incinerator scrubber water resulted in elevated mercury loadings to the incinerator and reduced
the mercury control efficiency to near zero. Measurements indicated that publicly owned treatment works
(POTWs) can remove mercury from wastewater very effectively; however, approximately 95% of the
mercury entering the plant was ultimately discharged to the atmosphere via sludge incineration emissions.