One of the most commonly used FGD technologies for scrubbing pollutants from power plant gas emissions is a limestone forced oxidation (LSFO) scrubber system. In this process, many pollutants end up in the circulating water in the scrubber. To maintain appropriate operating conditions, a constant purge stream is discharged from the scrubber system, and the purge stream contains contaminants from coal, limestone, and make-up water. The purge is acidic and supersaturated with gypsum, with high concentrations of TDS, TSS, heavy metals, chlorides and occasionally, dissolved organic compounds.
State and Federal laws regulate the concentration of pollutants in FGD wastewater prior to discharge to waterways. In some cases (e.g., power plants discharging to large rivers), the wastewater may be suitable for discharge after minor treatment for suspended solids and pH adjustment. However, in many cases, the wastewater requires treatment for the reduction of key contaminants, including suspended solids, COD/BOD, total nitrogen, and selected heavy metals to very low concentrations. The scrubber purge stream is most often treated in a dedicated wastewater facility rather than an existing treatment system, for the following reasons:
- The power plant’s existing wastewater treatment facility may not have adequate capacity.
- The materials of construction of the existing treatment facility most likely are unsuitable for receiving a high chloride stream.
- The treatment facility’s process design may not be appropriate for the very strict wastewater discharge limits likely to be enforced for the FGD wastewater.
FGD wastewater composition can vary significantly from plant to plant. Quantity and quality is affected by the rated capacity of the boiler, scrubber chloride concentrations, efficiency of the fly ash removal, the type and efficiency of the gypsum dewatering system, the type of FGD process used, and the composition of coal, limestone and make-up water.
FGD effluent from a LSFO type scrubber typically comes from the overflow of a secondary hydrocyclone in the gypsum dewatering system and is directed to the equalization tank of the wastewater treatment system.
The FGD wastewater treatment system by Evoqua Water Technologies is a physical/chemical system specifically designed to de-saturate the effluent, precipitate heavy metals, and reduce suspended solids by flocculation, settling and dewatering. After treatment, the contaminants are discharged as a solids cake for final disposal in a landfill. The treated wastewater is discharged to a receiving body of water, or itmay be polished with a continuous backwash gravity filter to further reduce suspended solids prior to discharge.
In some cases, additional treatment may be necessary if lower levels of organics, selenium, or total nitrogen are required. The solution for treating these contaminants is typically some form of biological treatment.
All of the treatment steps in our process are based on proven technology, in operation throughout the world, in similar applications. Evoqua Water Technologies uses our extensive experience to design a customized solution for each specific plant’s characteristics, while maintaining the flexibility to meet changing future conditions resulting from variations in coal burned, limestone used, and process water consumed.