Patent Publication Number: US-2013248443-A1

Title: Selenium recovery from bioreactor sludge

Description:
FIELD 
     This specification relates to wastewater treatment to remove selenium and to the recovery of selenium from wastewater. 
     BACKGROUND 
     The following paragraphs are not an admission that any of the information below is common general knowledge or citable as prior art. 
     Selenium is a trace element essential for human health. Selenium is also a precious non-metal with several useful properties. For example, selenium has photovoltaic and conductive properties making it useful in photovoltaic and electronic products. Selenium is also used as a pigment in glass and in vitamin supplements and fertilizer. 
     However, selenium also becomes toxic at very low concentrations. Selenium accumulates in the bodies of plants and fish that live in selenium-contaminated water and in the bodies of wildlife and people that eat those plants and fish. In people, elevated selenium concentrations may cause neurological damage and hair and nail loss. 
     Selenium may be present in soluble forms (selenate and selenite) in wastewater produced in various industrial or agricultural operations. For example, selenium is often present in flue gas desulphurization blowdown water produced in coal fired power plants. Selenium can also be present in some oil refining and mining wastes. Discharge limits for selenium may be set at between 10 parts per billion (ppb) and 50 ppb. 
     International Publication Number WO 2007/012181 describes a biological reactor for removing selenium from wastewater. Selenium removing reactors are sold by the General Electric Company, GE Water and Process Technologies, under the ABMet trade mark. In these reactors, a fixed media bed supports a biofilm of selenium reducing organisms. The organisms reduce selenate and selenite in the wasterwater to elemental selenium, which precipitates from the wastewater. The selenium is retained in the reactor until it is removed in a waste sludge by a periodic flushing operation. 
     SUMMARY 
     The following summary is intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention. 
     The sludge removed from a selenium bioreactor contains elemental selenium and may be classified as a toxic waste. The sludge must therefore be stored or disposed of to prevent selenium leaching into the environment. The cost of storing or disposing of the sludge is significant. On the other hand, the selenium in the sludge is a valuable commodity. Accordingly, recovering the selenium from the sludge produces a useable product and reduces a waste handling and environmental problem. 
     In a process described herein, a sludge containing elemental selenium, microorganism and other solids, for example minerals, is treated to recover the selenium. The sludge is mixed with an acid to dissolve some of the solids. The sludge is then thickened, or de-watered, to remove dissolved solids. The thickened sludge is burned at a temperature below 350 C to remove microorganisms. Elemental selenium is recovered in the form of an ash remaining after the sludge is burned. 
     When combined with a bioremediation process, the process provides for recycling or recovery of selenium from waste. The recovered selenium can be used as a resource for industrial applications. 
    
    
     
       FIGURES 
         FIG. 1  is a schematic process flow diagram of a process for recovering selenium from wastewater. 
         FIG. 2  is a TGA analysis of elemental selenium and sludge microorganisms. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a process  10  for recovering selenium. A feed flow  12  of wastewater containing selenium enters a bioreactor  14 . For example, the feed flow  12  may be flue gas desulphurization blowdown water from a coal fired power plant. In the bioreactor  14 , microorganisms convert soluble forms of selenium into insoluble elemental selenium. The bioreactor  14  may be an ABMet™ reactor available from GE Water and Process Technologies, a business within the General Electric Company. In this form of bioreactor  14 , water to be treated flows through a fixed media bed that supports the microorganisms. The elemental selenium is retained as particles with biomass in the bioreactor  14 . Treated water  16  flows out of the bioreactor  14 , preferably with a selenium concentration reduced to below discharge limits. The bioreactor  14  is periodically flushed producing sludge  18 , which contains biomass, elemental selenium and suspended solids that were present in the feed flow  12 . Other bioremediation processes may also produce an effluent or sludge containing selenium. For example, selenium may be removed from wastewater in a membrane bioreactor containing a suspended growth of selenium reducing organisms. Elemental selenium is discharged in a sludge drawn from the bottom of a process tank or a separate membrane vessel. 
     The sludge  18  is sent to sludge thickening device  20  to produce a thickened sludge  22 . The sludge thickening device  20  may be, for example, a centrifuge, filter press or a belt thickener. Excess water  24  released from the sludge  24  may be sent to a separate wastewater treatment plant or recycled to a point upstream of the bioreactor  14 . The thickened sludge  22  may contain 10-30 wt % solids. The solids comprise cells of microorganisms released from the bioreactor  14 , other suspended solids that were present in the feed water  12  to the bioreactor  14  and are still retained in the thickened sludge  22 , and elemental selenium that has been reduced by the microorganisms. In one sample of a thickened sludge taken from an ABMet reactor treating flue gas desulphurization blowdown water from a coal-fired power plant, the solids in the thickened sludge  22  were composed of about 51% microorganism cells, about 48% other suspended solids, and a small percentage, about 1%, of selenium. A trace amount, less than 0.1%, of nickel was also present. The other suspended solids were primarily minerals such as gypsum particles, fly ash and limestone particles. 
     The thickened sludge  22  cannot be disposed as non-hazardous waste due to its high selenium concentration. In the USA, the thickened sludge  22  would have to be put through the Toxicity Characteristic Leaching Procedure (TCLP) to determine how the thickened sludge  22  must be handled. If the TCLP result is over 1.0 mg/L, which is likely, the thickened sludge  22  must at least be stored in a hazardous waste landfill area. If the TCLP result if over 5.7 mg/L, which is possible, then the thickened sludge  22  must be sent to a waste management company at great expense. In the process  10 , however, the thickened sludge  22  is further treated in a recovery process to remove at least some of the remaining selenium, preferably such that any remaining sludge to be discharged has a TCLP of 1 mg/L or less. 
     In a first part of the recovery process, the thickened sludge  22  is sent to a mixing tank  24 . In the mixing tank  24 , an acid  26  is added to dissolve at least some of the non-cellular or mineral suspended solids. The mixing tank  24  may be maintained at ambient or room temperature, for example a temperature below 40 C or below 30 C. The acid  26  is preferably not an oxide. For example, the acid  26  may be hydrochloric acid (HCl). The mixing tank  24  is preferably stirred to enhance the reaction. 
     A partially dissolved sludge  28  flows from the mixing tank  24  to a solid-liquid physical separation device  30 . The separation device  30  may be, for example, a centrifuge. A liquid portion  30  is removed leaving a further thickened sludge  34  in which the proportion of solids, particularly non-cellular and mineral solids, has been reduced. 
     The further thickened sludge  34  is then treated further to remove the microorganisms. In the process  10 , the further thickened sludge  34  is sent to a furnace  36  and burned, preferably at a temperature low enough to substantially prevent oxidation of the selenium. Referring to the TGA analysis of  FIG. 2 , selenium is oxidized at a temperature of about 350 C while a significant portion of the biomass in the sludge can be burned at about 200 or 250 C. Accordingly, burning the sludge at a temperature of, for example, between 250 C and 325 C removes biomass generally without oxidizing the selenium. 
     In an experiment, a sample of ABMet sludge as described above was treated with hydrochloric acid, further thickened in a centrifuge and then burned at 300 C. The weights of the solids in the sample are given in Table 2 below. As shown in the table, most of the sludge was burned away but most (over 70%) of the selenium was retained. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Experimental results 
               
            
           
           
               
               
               
            
               
                   
                 weight of 
                   
               
               
                   
                 all solids 
                 Weight of selenium 
               
               
                   
                 sample (g) 
                 in sample (ug) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Further 
                 0.6919 
                 1834.9188 
               
               
                   
                 thickened 
               
               
                   
                 sludge 
               
               
                   
                 sludge after 
                 0.3352 
                 1324.04 
               
               
                   
                 burning at 
               
               
                   
                 300 C. 
               
               
                   
                 weight lost % 
                 52 
                 28