FGD and DeNOx
NEWSLETTER       

August 2008
No. 364

Pilot Study Shows DBA Use Impacts Biological Selenium Removal in FGD Wastewater

Staged biological treatment concepts were developed by Enos Stover of Stover & Assoc. and Michael Pudvay of Degremont Technologies. Bench-scale continuous flow biological activated sludge treatability studies with FGD wastewater were performed in the laboratory. The bench-scale studies achieved all of the study objectives of heavy metals, organics and nitrogen removal. Upon successful completion of the bench-scale studies, the decision was made to perform on-site larger pilot-scale studies to accomplish the following objectives:

• Develop and confirm design basis for selenium removal

• Develop and confirm design basis for nitrogen removal

• Define effluent concentrations achievable for selenium

• Evaluate metals reduction, precipitation and removal, especially for selenium, chromium and mercury

• Evaluate dibasic acid (DBA) impacts on biological treatment performance

• Optimize and fine-tune organics removal

• Evaluate nitrogen removal and control mechanisms.

The FGD wastewater pilot plant studies demonstrated that biological treatment can effectively remove soluble selenium below 0.1 mg/l. Anaerobic treatment provides selenium removal at ORP values less than -150 mV. Furthermore, the reduced environmental conditions required for both sulfate and selenium reduction were also found to provide the reducing conditions necessary for reduction and precipitation of additional heavy metal species including hexavalent chromium, mercury and vanadium. Aerobic biological treatment as a polishing step was demonstrated to effectively remove organics (BOD) and ammonia-nitrogen.

The pilot studies demonstrated consistent treatment despite highly variable influent selenium concentrations with changing coal supplies and coal washing practices. However, results of this pilot study clearly demonstrate that the addition of scrubber additives to improve SO₂ removal has a significant impact on the biological treatment of FGD wastewaters. Specifically, the use of DBA, containing manufacturing by-products, impacted the speciation of selenium within the wastewater matrix, contributing to the formation of a multitude of unidentified organo-selenium compounds that were surprisingly recalcitrant to microbial removal. Addition of a pure organic acid reagent, such as formic acid, to the absorber was found to enhance the downstream biological treatment process while continuing to provide efficient SO₂ removal.

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