Comparing Biological Process and Evaporation for Flue Gas Desulfurization Wastewater Treatment

On November 3, 2015, the Environmental Protection Agency’s (EPA) long-awaited effluent limitations guidelines (ELGs) for the steam electric power generating industry were published in the Federal Register. The new rules, promulgated under the Clean Water Act (CWA), have been in effect since January 4, 2016. The guidelines for the most part target medium and large coalfired power plants (> 50 MW output) that are equipped with wet limestone forced oxidized flue gas desulfurization (LSFO FGD) and/or use water to transport (sluice) bottom ash or fly ash. Dry handling is mandated for fly ash transport and flue gas mercury control (FGMC) waste to achieve zero discharge limitation for all pollutants. For FGD wastewater, EPA established a set of very low effluent concentration limits on mercury, arsenic, selenium, and nitrate/nitrite (as N) along with an existing limitation on total suspended solids (TSS). For bottom ash transport, dry handling or a closed loop system with water recycle to the plant is required to achieve zero discharge of pollutants. Regarding FGD wastewater at existing facilities, the Best Available Technology Economically Achievable (BAT) recommendation is physical-chemical treatment that employs some combination of the following processes: hydroxide precipitation (e.g., lime), sulfide precipitation (e.g., organosulfide), and iron co-precipitation, followed by an anoxic/anaerobic fixed-film biological treatment system.For new sources, evaporation of LSFO FGD wastewater is the recommended BAT, and has as its core a falling-film evaporator (aka a brine concentrator) to produce both a brine and distillate stream. Final evaporation of the brine stream is completed in a thermal crystallizer to generate solid salts for landfill. Existing plants must meet the discharge limitations by the date determined by each facility’s permitting authority; and within a time frame beginning November 1, 2018 but no later than December 31, 2023. The potential five year extra time period serves as an incentive for selection of evaporation as the technology, however, experience has shown that evaporation and crystallization (E/C) can be fraught with difficulties when treating LSFO FGD effluent. E/C is often considered a last resort due to high life cycle costs and maintenance issues. But new brine solidification options are emerging in which no crystallization is needed. Though the chemical precipitation/biological process is less expensive than evaporation/crystallization, E/C reduces the regulatory risk associated with future, more stringent discharge limitations. Many of the problems associated with E/C can be solved with proper design and operation.