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REGULATIONS COMPLIANCE These polymers will participate in typical clarifier flocculation reactions, but where they have captured mercury. Selenium presents an entirely different problem. During combustion, selenium reacts to primarily form the selenite ion (SeO32-). If one examines the periodic table, selenite is analogous to the sulfite ion (SO32-). Selenite is actually an ion that can be rather straightforwardly removed from solution, but in wet scrubbers, most of which are forced-oxidized to produce a gypsum byproduct, selenite is converted to selenate (SeO42-) and this ion cannot be readily extracted from solution. Selenium ions can be removed in wetlands via uptake by vegetation, but obviously this process is very land intensive, with a potential host of issues that make it difficult or impossible at most facilities. An alternative that has been successfully demonstrated is GE’s Abmet® process, in which WFGD wastewater passes through media containing microbiological organisms that capture oxidized selenium species, metabolically convert the ions to elemental selenium and retain the selenium. Beds will reach exhaustion and must be replaced. Exhausted material is disposed in a properly certified landfill.Veolia’s SeleniumZero® Figure 1. Schematic of polymer chains with sulfide groups attached. is another technology that is gaining interest. The idea stems from the chemistry of metals co-precipitation with iron. It tured by conventional filtration media. This problem has been has been well-known for years that a number of metals can addressed by development of flocculating polymers with active be precipitated from solutions by reaction with iron oxides. In sulfide groups. SeleniumZero, the iron oxide is attached to a substrate and the selenium adsorbs to the oxide. Iron oxide co-precipitation also is a technique to remove arsenic, Table 1 – Proposed ELG for FGD Wastewater Best Technology Available [1] although another method is selecAverage of daily values for a tive ion exchange. Many readers Pollutant Maximum for any 1 day 30 consecutive days shall not exceed are no doubt familiar with ion Arsenic, total (µg/L) 8 6 exchange as part of the process to produce high-purity makeup Mercury, total (ng/L) 242 119 water for steam generators. For Selenium, total (µg/L) 16 10 wastewater and other industrial Nitrite/nitrate (mg/L) 0.17 0.13 water treatment purposes, resin beads can be designed with active sites to remove specific ions. Such resins have been developed for Table 2 – Proposed ELG for Nonchemical Metal Cleaning Wastes Best arsenic. Technology Available [1] Even though these technologies Average of daily values for a have been successfully demonstratPollutant Maximum for any 1 day 30 consecutive days shall not exceed ed, treatment of wet FGD purge Copper, total (mg/L) 1.0 1.0 streams is still a very complicated process. The discharge also contains Iron, total (mg/L) 1.0 1.0 significant quantities of calcium, magnesium, chloride and sulfate. Zero liquid discharge (ZLD) treatTable 3 – Proposed ELG for Chemical Metal Cleaning Wastes NSPS [1] ment has not proven to be very successful for these streams. This Average of daily values for a Pollutant Maximum for any 1 day issue is a major reason why dry 30 consecutive days shall not exceed scrubbers have become most popTSS (mg/L) 100.0 30.0 ular for new installations. Oil and grease (mg/L)

20.0

15.0

Copper, total (mg/L)

1.0

1.0

Iron (mg/L)

1.0

1.0

16 ENERGY-TECH.com

APRIL 2015

Profile for Energy-Tech Magazine

April 2015  

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

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