July 15, 2008

New strategies for conquering environmental challenges

Angela Neville, JD

Lessons learned in FGD wastewater treatment

Switching gears from earlier presentations that focused on Clean Air Act compliance demands, speaker Thomas Higgins, PhD, PE, with CH2M HILL, addressed the issues related to treating water used in flue gas desulfurization (FGD) systems so it can be discharged and meet the Clean Water Act’s National Pollutant Discharge Elimination System (NPDES) requirements.

“Complicating the matter, one needs to design a treatment system and obtain a discharge permit before the wastewater is generated. It is dangerous to rely on one FGD wastewater composition in designing a treatment system,” said Higgins.

Fortunately, however, according to Higgins, there is a growing body of data on the characteristics of FGD wastewaters taken from different points in the systems. His company contacted the Electric Power Research Institute, FGD wastewater treatment vendors, and scrubber manufacturers for typical FGD wastewater composition. (“Benefits of evaporating FGD purge water” offered an alternative for treating FGD purge water in the March 2008 issue of POWER.)

Because of the highly complex nature of FGD wastewater, several stages are required to treat the blowdown from the FGD scrubber so that wastewater meets NPDES requirements for surface water discharge. Treatment typically consists of the following steps:

1. Calcium sulfate desaturation

2. Primary clarification

3. Equapzation

4. Trace metals precipitation and flocculation

5. Secondary clarification

6. Filtration

7. Solids dewatering