Effluent limitation guidelines: Expect the unexpected By Brad Buecker and Michael McMenus, Kiewit Engineering and Design Co.
Recently there has been much focus on air emissions regulations, as the EPA continues to clamp down on industries that burn fossil fuels, and in particular coal-fired power plants. But effluent limitation guidelines (ELG) are becoming more stringent for wastewater discharge at other facilities, including combined-cycle power plants. In many cases, the tightening regulations are being promulgated by states rather than the federal government, and so much uncertainty exists from state to state. This article examines these issues and techniques for wastewater treatment. The answers can at times be quite complex.
First, a look at coal plants Rather than deal with the expense and effort of complying with new air emissions regulations, owners of many older coal plants have elected to shut down the units. However, a significant number of large plants around the country are still in operation. Many were designed or retrofitted with wet scrubbers for sulfur dioxide (SO2) removal. Liquid purge streams
from wet flue gas desulfurization (WFGD) systems contain a complex mixture of chemical species, including impurities introduced from the coal. The EPA has focused on several, and Table 1 outlines these impurities and the projected discharge limit for each. Mercury, as would be expected, has the tightest limit. Given that the concentration in the purge stream is very slight to begin with, how can it be reduced even further? The answer lies in chemistry. Mercury very strongly and almost completely reacts with sulfide ions (S2-) to form an insoluble precipitate. Mercury can be â€œdropped outâ€? of solution in a clarifier if a sulfide chemical is added to the treatment process. Original chemistry was based on inorganic sulfides, but this chemistry offered two major difficulties. First, some inorganic sulfides are quite hazardous, and thus safety issues with regard to handling are of concern. Secondly, mercury reacts so quickly with sulfide that the precipitates from inorganic sulfide treatment might be so fine that they carry over from a clarifier and cannot be cap-
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