INDUSTRY Wastewater Treatment
The EPA issued the ELG Proposed Rule in June of 2013
The goal of the EPA Proposed Rule is to strengthen the controls on discharges from certain steam electric power plants by revising technology-based effluent limitations. The reason is that steam electric power plants alone contribute 50 to 60% of all toxic pollutants discharged to surface water by all industrial categories in the United States. Current discharge standards for the steam electric industry were last updated in 1982.
Introduction: Low level metal removal from power plant wastewater
The new EPA Effluent Limitations Guidelines Standards for the Steam Electric Power Generating Point Source Category; Proposed Rule is looking at setting discharge standards as follows for heavy metals with selenium being the number one contaminant:
Proposed Metal Removal Levels
- Selenium: 10-µg/L
- Mercury: 119-ng/L
- Arsenic: 6-µg/L
The point source waste streams to be regulated include:
- FGD (Flue Gas Desulpherization)
- Bottom Ash handling water
- Fly Ash handling water
- Combustion residual leachate
- Non-chemical metal cleaning waste
- Gasification wastewater
- Flue Gas Mercury Control (FGMC) wastewater
Power plant wastewater is generated from multiple point sources. Plant cooling water is sourced from either ground or surface water and cycled up through cooling towers, then discharged, usually off site. For discharge to a surface water, such as a river, the aquatic MCL requires selenium to be 1-µg/L or lower.
Coal fired power plants generate wastewater from coal piles, ash ponds and flue gas desulfurization (FGD), which contains high concentrations of total dissolved solids (TDS), total suspended solids (TSS), nitrates, phosphates and sulfates along with heavy metals selenium, mercury and arsenic.
There are several species of selenium found in the environment (selenide (Se -2), selenite (Se+4), selenate (Se+6) and selenium Se0). Both (selenide (Se -2) and selenium0 are insoluble. Selenate (Se+6, SeO42-) and selenite (Se+4, SeO32-) are the primary oxidation states found ionized in water. Selenate is the most difficult species of selenium to remove as it directly competes with sulfates, which are a predominate contaminant in power plant wastewater.
Crystal Clear Technologies (CCT) has developed and patented functionialized nano-materials branded NMX™, which form solid phase extraction (SPE) materials capable of reaching single digit µg/L to even lower ng/L metal removal in high TDS wastewater. Figure 1 below demonstrates that NMX™ is capable of removing >98% of both forms of ionized selenium; (Se+4/Se+6).
Figure 1 - Selenite (Se+4) and selenate (Se+6) percent (%) removal with CCT NMX™
Figure 2 shows that CCT NMX™ removes close to 80% of selenate (Se+6) in a single pass, which is the most difficult species to remove from wastewater. The selenate levels are 200 µg/L and sulfate levels in sample D are 400 mg/L and sample F is 550 mg/L.
Arsenic removal in FGD wastewater from plant operationswith CCT NMX™ is represented in Figure 3 where 100% of both forms of arsenic (As+3/As+5) in power plant wastewater had elevated levels of dissolved solids.
FGD and Ash Pond Wastewater Metal Removal