Enhancing FGD Wastewater ZLD Design and Insights from Simulated Brine-Solidification

摘要

Treating flue gas desulfurization (FGD) wastewater to comply with water quality-based standards and effluent limitation guidelines (ELGs) has been a challenge to electric utilities. As a result, utilities have been examining alternatives that do not require discharging FGD wastewater. This involves mixing concentrated FGD wastewater with a solid that can be landfilled. CH2M reviews alternatives for achieving this and describes a successful example. The path to zero liquid discharge (ZLD) begins with maximizing the natural evaporative capacity of the FGD system. In our example case study, wastewater disposal was driven by a fines buildup in the system. A fines removal process reduced FGD wastewater flow from 650 to 250 gallons per minute (gpm), by recycling the remaining wastewater back to the FGD scrubber. Next, lime softening was used to remove magnesium, sulfate, fluoride, and boron, which allowed for increased cycles of concentration in a falling film evaporator. Finally, a disposable solid material was produced by mixing the evaporator brine with fly ash. A chemistry and mass balance modeling tool was used to develop the design, which found that our zero discharge design approach was cost-competitive with biological treatment. This paper shares laboratory-scale testing results used to verify the results of our chemistry/mass balance model, to simulate and assess the effectiveness of each treatment step, and to gain additional insights with regards to metals partitioning. Solids were tested for toxicity characteristic leaching procedure (TCLP) and rainwater teachability and evaluated to determine whether salts and metals would cycle up if landfill leachate were returned to the FGD treatment system. We also present results of a third-party evaporator vendor's test results that were used to verify our estimates of lime dosage, magnesium removal, and boiling point rise to brine production.