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Physical-Chemical, biological and ZLD are all used for FGD wastewater

Options for Treating FGD Purge Wastewater • Physical-chemical treatment to reduce total suspended solids, adjust pH, de-supersaturate the purge stream, and reduce heavy metals. Since 2004, about 14 systems have been installed and are operating in the United States; 27 more are under construction. • Biological treatment to reduce selected heavy metals (such as selenium), and/or organics resulting from use of organic acids, and/or to reduce total nitrogen (usually due to ammonia slip from a selective catalytic reactor). This system is usually preceded by a physical-chemical system to protect it from scaling and suspended solids overload. Since 2004, about eight biological treatment systems have been installed or are planned for installation at FGD wastewater facilities. • Zero liquid discharge (ZLD) using thermal units (evaporator, crystallizer, spray dryer). Only one was operated briefly in the United States in the 1990s, but it experienced scaling, plugging, corrosion, and high capital and operations and maintenance costs. Two more have been ordered in recent years, with one under construction and one canceled during construction. • ZLD using deep-well injection. One is in operation in the United States, and one is under construction. This option costs $5 million to $6 million per well, with two to three wells per site. This system also requires a physical-chemical treatment system preceding it to reduce suspended solids and to de-supersaturate the wastewater to prevent scaling in the high temperature, high pressure deep-well environment. • Sludge stabilization by mixing FGD purge with fly ash for land filling. None are currently being built; however, about 15 were constructed in the 1970s-1980s. This approach eliminates the ability to sell fly ash for commercial products and requires extra landfill volume. • Stacking the gypsum for either final disposal or future reclaim. This will absorb some of the purge in the stack. Liquid runoff is collected in holding basins, mixed with ash water, or held in solar evaporation ponds (in favorable climates). A couple of these designs are under construction but could be required to retrofit to treat the gypsum stack runoff. • Constructed wetlands treatment system. Only three have been completed, and performance has been mixed.

Revision Date:  12/5/2011

Tags:  221112 - Fossil Fuel 化石燃料, Xylem, Wastewater, Biological Treatment, FGD, Physical Chemical Treatment, Zero Liquid Discharge