Abstract:
Systems and methods of treating flue gas and wastewater generated by treating the flue gas are disclosed and include introducing the flue gas, a flue gas treatment fluid that removes sulfur dioxide from the flue gas; and an organic acid conditioning agent into a wet-oxidation scrubber/absorber; introducing FGD scrubber wastewater generated by the wet-oxidation scrubber/absorber into an anoxic biological reactor to substantially denitrify the FGD scrubber wastewater; and introducing resulting substantially denitrified FGD scrubber wastewater into an anaerobic biological reactor to substantially reduce the amount of sulfate and/or selected heavy metals in the FGD scrubber wastewater.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from U.S. Provisional Application Ser. No. 60/840,084 filed Aug. 25, 2006, the entire contents of which are incorporated by reference herein. 
     
    
     TECHNICAL FIELD  
       [0002]     This disclosure relates to wastewater treatment methods and systems that remove organic and inorganic pollutants captured in the purge stream from air pollution control equipment.  
       BACKGROUND  
       [0003]     Coal-fired power plants continue to produce a significant proportion of the electricity requirements for the United States. The combustion and gasification of coal is widely recognized as a significant environmental issue due to the potential release of hazardous pollutants. As a consequence, air quality standards continue to tighten. This results in the implementation of scrubbers for emissions control, most notably sulfur dioxide (SO 2 ), from coal-fired power plants.  
         [0004]     Wet scrubber technology with lime slurry/limestone is a proven and commercially established process for flue gas emissions control, particularly SO 2  removal, from coal-fired power plants.  
         [0005]     Wet scrubbers are usually designed with 80 to 95% efficiency of SO 2  removal. However, facilities often use additives such as magnesium-enhanced lime or organic acids to improve process efficiency by 5 to 10%. This is particularly true in light of the market value of so-called SO 2  “removal credits” and the potential for significant economic gain. However, the use of additives at the absorber may cause difficulties with the implementation and performance of downstream biological treatment systems.  
         [0006]     For example, Flue Gas Desulfurization (FGD) process wastewater contains elevated levels of chlorides; significant concentrations of heavy metal contaminants such as chromium, mercury, and selenium; often high levels of nitrates; and a very high solids content that consists primarily of calcium sulfate, calcium carbonate, magnesium hydroxide, and fly ash.  
         [0007]     Treatment of FGD wastewater is a significant need for utility operations. Physical/chemical treatment processes are typically used for neutralization and calcium sulfate desaturation, removal of some heavy metals, clarification and sludge thickening. However, conventional chemical precipitation techniques do not reliably eliminate heavy metal contaminants such as selenium and hexavalent chromium below outfall discharge limits established by newer, more stringent regulatory requirements. Nor do these current practices remove nitrogenous pollution.  
         [0008]     FGD process wastewater is the focus of increasingly stringent effluent requirements, with outfall discharge standards (monthly average and daily maximum) typically established for: 
        pH     Total Suspended Solids (TSS)     Total Nitrogen (TN)     Heavy Metals including but not limited to Arsenic, Chromium, Copper, Mercury &amp; Selenium     Sulfides.        
 
         [0014]     Selenium is an essential micronutrient for animals and bacteria. However, it becomes highly toxic when present above minute concentrations. The oxidized species of selenium, selenate (Se VI) and selenite (Se IV), are highly soluble and bioavailable, whereas reduced forms are insoluble and much less bioavailable. Regulatory limits for soluble selenium remain variable with targets ranging from 800 ug/L down to the U.S. national drinking water standard of 50 ug/L, frequently depending upon the discharge receiving water body.  
         [0015]     Selenium exists in multiple valence states in the natural environment and the impact of selenium speciation on treatment efficiency is known. Notably, Selenium in the form of Selenite (Se IV; SeO 3 ) can be removed with 65 to 85% efficiency using physical-chemical treatment approaches while Selenate (Se VI; SeO 4 ) removal efficiency is limited to &lt;10% with physical-chemical treatment.  
         [0016]     It would therefore be helpful to provide an enhanced biological treatment approach to circumvent such problems, optimizing downstream removal of TN and heavy metals from FGD wastewater while maintaining SO 2  removal efficiency at the absorber stage.  
       SUMMARY  
       [0017]     A method of treating flue gas and wastewater generated by treating the flue gas is disclosed and includes: 
        introducing the flue gas and an organic acid conditioning agent into a wet-oxidation scrubber/absorber substantially to remove sulfur dioxide from the flue gas and condition resulting FGD wastewater for downstream biological treatment;     introducing FGD scrubber wastewater generated by the absorber into an anoxic biological reactor to substantially denitrify and/or reduce selected heavy metals in the FGD scrubber wastewater; and     introducing substantially denitrified wastewater into an anaerobic biological reactor to substantially reduce the amount of sulfate and/or selected heavy metals in the FGD scrubber wastewater.        
 
         [0021]     A system for treating flue gas and wastewater generated by treating the flue gas is disclosed and includes: 
        a wet-oxidation scrubber/absorber comprising a flue gas inlet, a flue gas treatment fluid inlet, an organic acid conditioning agent inlet, and a wastewater outlet;     an anoxic reactor located downstream of the wet-oxidation scrubber/absorber to substantially denitrify FGD scrubber wastewater generated by the absorber; and     an anaerobic reactor located downstream of the anoxic reactor to substantially reduce the amount of sulfate and/or selected heavy metals in the FGD scrubber wastewater.        
 
         [0025]     A method of treating FGD scrubber wastewater includes: 
        introducing FGD scrubber wastewater generated by combining flue gas and an organic acid conditioning agent in a wet-oxidation scrubber/absorber into an anoxic reactor to substantially denitrify the FGD scrubber wastewater; and     introducing substantially denitrified wastewater into an anaerobic reactor to reduce the amount of sulfate and/or selected heavy metals in the FGD scrubber wastewater.        
 
         [0028]     A system for treating FGD scrubber wastewater includes: 
        an anoxic biological reactor located downstream of a wet-oxidation scrubber/absorber to substantially denitrify FGD scrubber wastewater generated by the absorber; and     an anaerobic biological reactor located downstream of the anoxic reactor to substantially reduce the amount of sulfate and/or selected heavy metals in the FGD scrubber wastewater.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a schematic diagram of a representative process flow for a wet-oxidation scrubber/absorber system and associated conventional FGD wastewater treatment system.  
         [0032]      FIG. 2  is a schematic flow diagram of a representative biological treatment system for FGD scrubber wastewater.  
     
    
     DETAILED DESCRIPTION  
       [0033]     It will be appreciated that the following description is intended to refer to specific aspects of this disclosure selected for illustration in the drawings and is not intended to define or limit the disclosure, other than in the appended claims.  
         [0034]     This disclosure relates to biological treatment systems for FGD scrubber wastewater, encompassing the feed of a pure organic acid conditioning reagent, such as formic acid, to the wet-oxidation scrubber/absorber and later followed by a combination of anoxic, anaerobic and aerobic staged activated sludge reactors and associated clarification systems for removal of TN, reduction and precipitation of heavy metals and elimination of suspended solids from the FGD purge stream.  
         [0035]     This disclosure also relates to processes for biological treatment of FGD scrubber wastewater, particularly to treatments that improve the removal efficiency of TN and heavy metals including but not limited to selenium.  
         [0036]     Turning now to the drawings in general and  FIG. 1  in particular, it will be appreciated that a selected, representative pollution control system  10  described below removes FGD wastewater contaminants. The treatment system  10  includes a line  12  to add a pure organic acid conditioning reagent, such as formic acid, to absorber  14  as indicated in  FIG. 1 .  
         [0037]     The absorber  14  also connects to a particle scrubber  16  and a recirculation tank  18 . The recirculation tank  18  directly receives flue gas treatment fluid through supply line  20  which is indirectly supplied into absorber  14  by way of line  22 . Flue gas treatment fluid may comprise, among other things, a lime/limestone water slurry. Treated flue gases exit absorber  14  through line  24 , are reheated by reheater  26  and then moved to stack  28  by fan  30 .  
         [0038]     On the other end, FGD scrubber wastewater exits absorber  14  through line  32  and enters recirculation tank  18 . Selected portions of FGD scrubber wastewater exit through recirculation tank  18  and may proceed to clarifier  34 . This may be followed by passage of the clarified wastewater to holding tank  36 . Wastewater contained in holding tank  36  can be recycled to recirculation tank  18  by way of line  38 . The partially dewatered sludge may be channeled from clarifier  34  to vacuum filter  40  by way of line  42 , where most of the remaining water is removed. The waste sludge can then be sent to a settling pond or landfill  44 .  
         [0039]     In accordance with selected aspects of this disclosure, FGD scrubber wastewater may also flow from clarifier  34  to additional treatment systems such as a biological treatment system of  FIG. 2  by way of line  46  and as activated by valve  47 .  
         [0040]     Turning now to  FIG. 2 , a selected, representative biological treatment system  48  for FGD scrubber wastewater is shown in a schematic form. The system  48  includes an inlet  50 , a staged suspended growth biological reactor  52  comprising anoxic  54  and anaerobic  56  zones, an intermediate clarifier  58 , an aerobic suspended growth biological reactor  60 , a final clarifier  62 , a storage tank  64  and a filtration stage  76 .  
         [0041]     The biological treatment system  48  of  FIG. 2  can perform the following functions: 
        Anoxic Stage—Denitrification (Nitrate reduction) and/or reduce selected heavy metals     Anaerobic Stage—Selected heavy metal reduction and precipitation, particularly Selenium reduction     Aerobic Stage—Nitrification (ammonia reduction) and organics reduction.        
 
         [0045]     The biological treatment system  48  may receive influent feed from an upstream physical-chemical treatment system such as from clarifier  34 , for example, of  FIG. 1 , in the form of deoxygenated FGD purge wastewater. The biological reactors of the system  48  may include completely mixed, continuous flow, activated sludge reactors.  
         [0046]     The first cell (or reactor  54 ) in the system  48  is the anoxic stage, where nitrates are reduced to nitrogen gas via denitrification reactions. As FGD wastewater is deficient in macronutrients, including ammonia nitrogen and orthophosphorous, as well as many of the micronutrients required to support biological growth, there is a process requirement for supplemental nutrient addition to yield efficient treatment performance. Reactor  54  is thus fed with a biodegradable nutrient blend, containing macro- and micronutrients to maintain microbial growth.  
         [0047]     Nutrients include but are not limited to supplemental carbon such as waste sugar, corn syrup, molasses or the like, urea or the like to provide ammonia nitrogen, phosphoric acid, micronutrients and yeast extract to provide necessary trace metals and growth factors. Fermentation of sugars dosed into the anoxic reactor  54  results in the conversion of sucrose to volatile fatty acids (VFAs) that sulfate/selenium reducing microorganisms are capable of metabolizing efficiently in the downstream anaerobic reactor stage(s). Additional carbon sources such as lactate, acetate or the like may also be added directly to the anoxic/anaerobic reactors to enhance selenium removal by enriching the selenium reducing microorganisms.  
         [0048]     Further, addition of a pure organic acid stream, such as formic acid, through line  12  of absorber  14  provides a means to introduce a biodegradeable carbon substrate to the wastewater that can provide COD to the system for downstream biological removal of nitrates and selected heavy metals. For example, using the COD factor for formic acid of 0.35, a dosage of 200 mg/L formate equates to a theoretical COD dosage of about 70 mg/L.  
         [0049]     The anoxic/anaerobic biological reactor  52  may be an overflow, under-flow weir design which mimics a plug-flow system without the need to incorporate separate reactor tanks that are physically isolated from one another. Other configurations/structures may be used as appropriate. Operational inputs for successful treatment involve targeting the appropriate oxidation-reduction potential (ORP) in the various reactor stages. Thus, the anoxic reactor  54  may preferably be maintained in the range of about −50 to about −300 mV to yield efficient denitrification.  
         [0050]     The role of the anoxic denitrification reactor  54  is important. We found that efficient removal of selected heavy metals such as selenium substantially depends upon sequential substrate removal, specifically the prior elimination of nitrates.  
         [0051]     Additionally, the efficiency of selenium removal is dependent upon the species present in the wastewater matrix. It is known that selenite (Se IV) is somewhat efficiently removed via physical chemical means while selenate (Se VI) requires biological treatment to obtain significant reductions. Surprisingly, we found that efficient biological removal also depends on the nature of complexes, such as organo-selenium compounds, formed within the wastewater matrix and addition of reagent additives to the scrubber/absorber heavily impacted the contaminants formed. We found that many organic complexes of selenium formed as a result of the use of organic acid containing manufacturing waste by-product mixtures at the absorber. These organo-selenium complexes were found to be surprisingly recalcitrant to selenium reduction by the microbial population in downstream biological reactors. We discovered that use of a pure organic acid reagent, such as formic acid, to improve SO 2  removal efficiency at the scrubber further provides downstream advantages by yielding a wastewater matrix that could readily be treated for selenium removal. The staged biological reactors create a reducing environment for the conversion of selenate or selenite to elemental selenium, which precipitates out of solution into the wastewater solids.  
         [0052]     The partially treated FGD wastewater leaves the anoxic reactor  54  substantially devoid of nitrate contamination and flows into the next cell (i.e., the anaerobic reactor  56 ), which in one aspect may be operated at an ORP in the range of about −200 to about −500 mV, where sulfate and heavy metal-reducing organisms begin to remove sulfates and the selected heavy metals from the wastewater. The treated water then flows to an optional third cell (anaerobic reactor stage) to ensure that heavy metals are removed to levels allowing outfall discharge permits to be met.  
         [0053]     The treated effluent from the anoxic/anaerobic biological reactors  54 / 56  may flow into a mix chamber allowing for chemical addition to improve downstream sedimentation within the intermediate clarifier  58 . From the mix chamber of the anoxic/anaerobic reactors  54 / 56 , the treated effluent flows into a settling type intermediate clarifier  58 , where TSS is settled out and the clarifier underflow solids are recycled to the anoxic reactor  54  by lines  66  and  68  as return activated sludge (RAS) or sent to a sludge holding tank (not shown) by line  70  as waste activated sludge (WAS).  
         [0054]     From the intermediate clarifier  58 , the partially treated FGD wastewater flows into the aerobic biological reactor  60  for removal of BOD and ammonia. In one aspect, the aerobic biological reactor  60  includes operation at positive ORP.  
         [0055]     From the aerobic reactor  60 , the FGD wastewater flows into a settling type final clarifier  62 , where TSS is settled out and clarifier underflow solids may be recycled to the head of the aerobic reactor  62  by lines  72  and  74  as return activated sludge (RAS) or sent to a sludge holding tank (not shown) by line  70  as waste activated sludge (WAS).  
         [0056]     Finally, the clarified water flows into a wet effluent well/tank  64  for pumping to pressure filters  76  and ultimately discharge to the environment. The filters may be gravity sand, multimedia or the like type filters.  
         [0057]     Powdered Activated Carbon (PAC) or other adsorbent materials such as charred poultry waste or the like added to the anaerobic and/or aerobic biological reactor will also adsorb any remaining organo-selenium complexes to assist reaching a final effluent selenium concentration that is below about 200 μg/L. PAC can be added at a dosage of about 550 ppm, for example.  
       EXAMPLE  
       [0058]     A biological treatment system comprising of a 2-stage, completely mixed, anaerobic activated sludge reactor having a total reactor volume of approximately 2,000 gallons followed by a 500-gallon clarifier and an aerobic activated sludge reactor and final clarifier was fabricated and installed on a sidestream of FGD scrubber blowdown at an operating power generating station.  
         [0059]     The facility initiated pure organic acid addition in the form of formic acid to the absorber to enhance the downstream biological treatment process while continuing to provide SO 2  removal at the wet-oxidation scrubber/absorber. Analysis revealed that the soluble oxyanions of Selenium, SeIV and Se VI, represented a significantly greater fraction of the total Selenium present in the sample matrix upon initiation of the pure organic acid feed to the wet-oxidation scrubber/absorber when compared to results obtained with the feed of an organic acid containing manufacturing by-product mixture. The use of a pure organic acid feed resulted in significant reductions in the levels of complexed selenium present. Subsequently, this resulted in improved treatability of the FGD wastewater with the performance of the biological treatment system greatly enhanced as noted below.  
         [0060]     The staged activated sludge design was found to yield substantially complete denitrification of influent FGD scrubber wastewater under anoxic conditions, consistently achieving effluent NO 3 -N concentrations below 1 mg/L for influent nitrate levels ranging from 200 to 600 mg/L.  
         [0061]     Concurrent removal of sulfate and selenium was demonstrated under anaerobic conditions, with effluent soluble selenium concentrations consistently below 200 μg/L. Furthermore, the reduced environmental conditions required for both sulfate and oxidized selenium reduction was also found to provide the reducing conditions necessary for reduction and precipitation of additional heavy metal species including mercury and hexavalent chromium.  
         [0062]     Surprisingly, the performance of the aerobic reactor, intended to perform polishing treatment for removal of excess CBOD and ammonia nitrogen was found to exhibit significant additional removal of soluble selenium species.  
         [0063]     Furthermore, the aerobic reactor performance was found to be much less sensitive to changes in the influent soluble selenium concentration allowing for quick adaptation to system upsets that result in rapid increase in influent Se levels.  
         [0064]     The benefits brought about by the methods and systems described above may include: 
        The complexity of Selenium speciation within FGD scrubber wastewaters may be reduced or eliminated by feeding a pure organic acid conditioning additive, such as formic acid, to the wet-oxidation scrubber/absorber. Subsequently, this approach improves downstream biological treatment while maintaining SO 2  removal efficiency at the absorber.     Use of a staged biological reactor approach to support the growth of distinct groups of bacteria within the naturally occurring population.     Use of conventional suspended growth activated sludge technology eliminates need to backwash or flush reactors periodically to remove captured waste material.     Reactors are seeded with biomass from natural microbial populations avoiding the need to regularly add “specialized” microbial cultures and thereby reducing annual operational costs.     Treatment approach provides operational flexibility and stable operations/performance under highly variable influent conditions.     Biological removal of selenocyanate forms and other complexed selenium species that are much harder to remove with conventional iron-coprecipitation treatment strategies.     Biological removal of heavy metals, including selenium, in absence of reducing agent feed, such as potassium permanganate, to the biological reactors.        
 
         [0072]     Thus, the use of suspended growth biological treatment methods and systems for the removal of TN and heavy metal contaminants from FGD wastewater offers several potential advantages when compared to conventional chemical precipitation techniques including the use of biodegradable nutrients as opposed to iron-based reagents, elimination of nitrogen-containing pollution, elimination of difficult to remove heavy metal contaminants such as selenate to extremely low levels, production of less sludge and reduced operational expenses.  
         [0073]     Although the above methods and systems have been described generally in accordance with the figures, it should be understood that the above descriptions and figures are merely representative, selected examples. Variations and/or substitutions may be made as appropriate by those skilled in the art. For example, although we have shown selected biological reactors in various shapes and configurations and made from selected materials, it should be understood that such shapes, configurations and materials can be changed as appropriate in accordance with the surrounding environment makes practicable. Also, biological reactors may contain support media to provide a means of attached biological growth in addition to the suspended growth fraction. Of course, other components and steps known in the art may be added to meet various conditions at particular sites.