optimizing mercury removal processes for industrial wastewaters

Transcription

1 Water Technologies & Solutions technical paper optimizing mercury removal processes for industrial wastewaters Authors: Gerald Walterick, Jr. and Larry Smith, SUEZ. introduction Mercury (Hg) removal from both air emissions and industrial wastewater, especially coal-fired power plants in the U.S., has been a topic of extensive study for the last decade. Much of the initial focus has been on controlling air emissions of mercury contaminants from coal-burning power plants which were identified as the single largest source of airborne mercury emissions and specifically targeted by the U.S. Environmental Protection Agency s (EPA s) Clean Air Mercury Rule1. This problem was addressed by the installation of wet flue gas desulfurization (FGD) systems at many coal-burning facilities. The FGD process has resulted in a significant reduction in air emission of mercury, but has done this by transferring the mercury contaminants to a wastewater stream. Other industries such as petroleum refining, natural gas recovery and other light and heavy industries also generate mercury contaminated wastewaters. Much of the recent research has been focused on removing mercury from industrial wastewaters. Mercury typically occurs at low parts per billion (ppb) levels in industrial wastewaters. The severe toxicity of some mercury compounds and the tendency of these compounds to bioaccumulate in aquatic ecosystems have led to very stringent wastewater discharge regulations to keep mercury out of the environment. The Ohio River Valley Sanitation Commission (ORSANCO) standard of 12 parts per trillion (ppt) discharge limit on Hg for discharges into the Ohio River and the Great Lakes Water Quality Initiative standard of 1.3 ppt Hg for discharges into bodies of water in the Great Lakes Basin are examples of U.S. guidelines. Meeting these limits presents a significant challenge to many industries. This paper will describe the use of the MerCURxE* technology, which includes a patented chemical product developed by SUEZ to aid in the removal of mercury to these low levels in industrial wastewaters. Known as MetClear* MR2405, this metals precipitant, when used in conjunction with other unique coagulants and flocculants from the SUEZ portfolio, provides significant removal of both soluble and insoluble mercury. mercury occurrence and speciation The primary source of mercury contaminants in coalburning power plants is the coal. Mercury concentrations vary with coal grade, chloride content and origin, but are typically in the range of micrograms per gram (μg/g), equivalent to lb. /ton.2 Although many organic and inorganic mercury compounds exist, for the purpose of this discussion, mercury contaminants will be categorized into three different species that are related to the chemical or physical process that would remove them. These are: elemental mercury (Hg0), ionic mercury (Hg+2) and particulate mercury (Hgp). Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. Nov-12

2 Figure 1: Coal combustion FGD system mercury speciation Figure 1 illustrates the fate of various Hg species through a typical coal combustion process. Prior to combustion, mercury is primarily present in the raw coal as naturally occurring mercuric sulfide, which would be classified as particulate mercury (Hgp). During combustion, most of the mercury associated with the coal is volatilized to Hg0 and Hg2+. Some small fines of Hgp may carryover with the fly ash and a small amount of Hgp may remain with the bottom ash. As the combustion products proceed toward the exhaust stack, they cool down and Hg0 may be oxidized to Hg2+. The flue gases then pass through an Electrostatic Precipitator (ESP) or Fabric filter (FF) to remove "fly ash" particulates, including Hgp. Many systems also include Flue Gas Desulfurization (FGD), which is a process originally intended to remove sulfur dioxide, (SO2), from coal combustion processes, but is also an effective means of removing mercury, particularly Hg2+ which is soluble in the scrubber slurry. Oxidants and catalysts are often incorporated into the process to promote oxidation of Hg0 to the more soluble Hg2+ species. This enhances the removal of mercury by the FGD process. Page 2

3 waste treatment processes A significant portion of the airborne mercury contaminants removed from flue gas is transferred to aqueous waste streams that must be treated prior to discharge. The mercury species of most concern in wastewaters are soluble mercury (Hg2+) and particulate mercury (Hgp). Treatment processes to handle these wastewaters can be as simple as a settling pond as shown in Figure 2, below. Figure 2: settling pond diagram A more complex wastewater treatment system, specifically designed to incorporate chemical precipitation reactions required for adequate treatment of highly contaminated FGD wastewater streams is shown in figure 3. Figure 3: FGD wastewater treatment system This type of system typically includes several separate unit operations, including: desaturation (addition of lime to precipitate sulfate as gypsum), equalization (to stabilize influent ph and water chemistry), metals precipitation, coagulation, clarification and filtration. Biological waste treatment processes may also be included to remove organics, nitrogen compounds and selenium. Page 3

4 chemical additives Proper selection and application of chemical additives are critical to the success of a wastewater treatment program for mercury removal. Additives used to enhance the removal of contaminants in conjunction with the unit operations described above may include: lime, coagulants, flocculants and heavy metal precipitants. Use of an appropriate precipitant is essential to ensure that soluble mercury contaminants are reduced to low ppt concentrations. Several years of lab, pilot and full-scale testing have determined that some of the most effective mercury precipitants are types like SUEZ s metals precipitant, MetClear MR2405, which has a very strong affinity for mercury and will also precipitate other heavy metals such as silver, cadmium, copper, lead, zinc, cobalt and nickel. experimental Bench scale mercury removal studies (jar tests) were conducted in the laboratory on samples of mercurycontaminated wastewater from several industrial sources. Due to the extremely low concentrations of mercury typically found in these wastewaters, great care was taken to ensure that the test apparatus and sample bottles were meticulously clean. The procedures used for the studies reported here are summarized below: Test apparatus - Mercury removal studies were done in a dedicated clean laboratory using customized apparatus designed and operated to minimize the potential for sample contamination. Samples for low-level mercury analysis were processed in a dedicated clean lab using EPA recommended protocols. Personal Protective Equipment (safety gloves, protective eyewear and protective clothing) were worn by lab personnel at all times. The jar test procedures used for mercury removal studies were customized for each application using a proprietary computer program to design mixing protocols that simulated the mixing conditions and reaction times of the full-scale wastewater treatment process. The use of this program improves the accuracy of bench tests and facilitates scale-up to full-scale processes. results Table 1: composition of test substrates A variety of wastewaters were evaluated, including power plant and refinery wastewaters. The range of chemical compositions of these wastewaters varied widely as shown in table 1. Range of values Parameter Low - High ph Specific Conductance 25C) ,300 P Alkalinity (ppm as CaCO3) M Alkalinity (ppm as CaCO3) Sulfur (ppm as SO4) 97-40,000 Chloride (ppm as Cl) 22-10,100 Hardness (ppm as CaCO3) ,100 Calcium (ppm as CaCO3) ,700 Magnesium (ppm as CaCO3) ,100 Iron (ppm as Fe) < Sodium (ppm as Na) Potassium (ppm as K) Aluminum (ppm as Al) < Manganese (ppm as Mn) Nitrate (ppm as NO3) 4.2-1,800 Phosphate (Total, ppm as PO4) < Silica (ppm as SiO2) Turbidity (ntu) > 4000 Mercury (ppt as Hg) ,000 Page 4

5 Study results with various contaminated wastewaters demonstrated that the low ppt Hg discharge concentrations required for each wastewater could be achieved with proper application of chemical additives. In many cases, the target discharge concentrations were achieved using existing plant unit operations. Figure 5 shows results of tests with an FGD wastewater that contained >14,000 ppt Hg. The target Hg concentration of 1500 ppt was attained by chemical treatment and settling. Increasing the dosage of MetClear significantly improved Hg removal. Figure 4 shows test results for studies done with a pond wastewater. Untreated, this wastewater contained 47 ppt Hg. With chemical treatment and settling, Hg was reduced to 2.5 ppt. The improvement in Hg reduction resulting from the MetClear treatment is clearly evident. Figure 5: FGD wastewater mercury removal Figure 4: pond wastewater mercury removal Page 5

6 Figure 6 is a comparison of treatments with MetClear to treatments with a competitive precipitant. In this FGD wastewater MetClear treatment reduced Hg to below the target concentration of 1500 ppt while Competitor A s product "leveled off" at > 4000 ppt Hg. Figure 7: FGD wastewater mercury removal Figure 6: FGD wastewater mercury removal Another FGD wastewater containing 43,800 ppt Hg was successfully treated to reduce Hg to < 150 ppt using the MetClear product (Figure 7). As previously mentioned, the MetClear product is very effective for removing other heavy metals in addition to mercury. Figure 8 shows the effect of MetClear treatments on the removal of mercury, beryllium, cadmium, copper, vanadium, zinc, cobalt and nickel. Figure 8: MetClear affinity for heavy metals Page 6

7 key treatment considerations When a mercury-contaminated wastewater stream(s) is identified as a candidate for mercury removal studies, several key steps must be followed to determine the potential for successful treatment. Periodic, routine and historical sampling and analysis of mercury contributing wastewater streams must be implemented utilizing EPA approved protocols for sampling, handling and analysis. The U.S. EPA has published guidelines for proper procedures regarding this. Method 1669, titled Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels is commonly used as a guide for sampling techniques. Analytical techniques to measure low parts per trillion levels were evaluated and in a 2007 memorandum, guidelines for use of these analytical techniques were disclosed11. Analytical method 1631E is currently the method of choice for low level mercury analyses. Common power plant FGD and ash pond systems have a variety of incoming water quality characteristics as shown in Table 1. This variability is one of the reasons why it is so important to routinely analyze and conduct evaluations in the laboratory and on-site to ensure optimum removal is maintained. impact of treatment system equipment design To maintain consistent, optimized mercury removal across any treatment system, all reaction tanks, clarifiers, filtration equipment and in the case of oil refining, both primary and secondary wastewater systems, must be in excellent operating condition. Removal of mercury, other heavy metals and total suspended solids (TSS) are all impacted by the system design and the selection of chemical additives, but that is not the whole answer for achieving effective removal. For pond systems, like bottom ash ponds in coal fired power plants, it is also important to ensure the right chemical feed points are chosen and optimal dosages are maintained. Enough residence time under quiescent conditions is required to facilitate settling of precipitated metals and other suspended solids. Due to the large size of many settling ponds, settling time is typically not a limiting factor. pilot studies On-system application of the treatment programs developed in laboratory jar testing is commonly conducted through plant trials or pilot studies, leading to extended or continuous treatment. Several applications are outlined below to demonstrate the effectiveness of the MerCURxE program for removal of total mercury. Often the requirements for meeting final discharge permit limits are accomplished by treating each candidate stream individually. Treated water may either be combined or discharged separately. In many cases, immediately after the precipitation process, results show excellent removal even though the final discharge limits are not met at that point in the system. Depending on the downstream design, further removal is obtained and lower mercury levels are realized. FGD system # 1 This coal fired power plant FGD system has a traditional design with equalization, reaction tanks and circular clarifier. The treatment program includes the use of a coagulant, MetClear MR2405 and a SUEZ flocculant to remove TSS and mercury. Specific regulatory discharge limits for mercury have not yet been established at this site. Treatment results for several heavy metals in this wastewater are shown in Figure 9. Mercury removal of more than 99% on average has been accomplished, from approximately 30 ppb to less than 0.2 ppb. The inlet loading of most other heavy metals is not significant (less than 1 ppb), compared to iron and mercury. Results indicate that boron, selenium and arsenic are also removed by the combined treatment approach. Page 7

8 Figure 10: FGD total mercury removal Figure 9: FGD wastewater treatment system results FGD system #2 This FGD system includes equalization, desaturation, a primary clarifier, a chemical reaction tank, a secondary clarifier and a continuous backwash sand filter. This filtered effluent is discharged through ash ponds. In this system only the secondary clarifier is treated with chemicals. Additives include lime for ph adjustment, MetClear MR2405, a coagulant and a SUEZ flocculant. Target levels for mercury are less than 200 ppt Hg out of the treated clarifier. Results have shown that mercury can be removed from an inlet range of 230 to 350 ppt down to as low as 65 ppt after the clarifier and as low as 45 ppt after the filters, well below the target goal of 200 ppt. FGD system #3 conclusion The use of MerCURxE chemical technology has been shown to be an effective method for removing mercury from several industrial wastewaters. Incorporating this type of treatment into an overall wastewater treatment program should significantly improve mercury removal. The industry generating the wastewater, the design of the wastewater treatment plant, the operating conditions of the plant and the mercury concentration and speciation in the influent wastewater are all factors that have an impact on the efficacy of a treatment program. Understanding the unique characteristics of each system and the variability of the contaminant loading is vital to the successful removal of mercury from industrial wastewaters. This FGD wastewater treatment system includes equalization, reaction tanks and a clarifier prior to discharge. This system also utilizes a coagulant, MetClear MR2405 and a SUEZ flocculant to remove mercury, other heavy metals and TSS. Results from treatment are shown in Figure 10. Mercury removal in excess of 99.91% was achieved with treatment. Average influent mercury of 84,800 ppt was reduced to 78 ppt across the clarifier then to 18 ppt through the sand filter. This site is also not currently regulated for mercury removal. Page 8

9 pilot studies 1. Basic information (Clean Air Mercury Rule) 2. Schwalb, A.M. and Withum, J.A., "The Evolution of Mercury from Coal Combustion Materials and Byproducts", Mercury Control Technology R&D Program Review Meeting, August 12-12, cfm Mercury Awareness 4. Choi, K.Y., and Dempsey, B., Bench-scale evaluation of critical flux and TMP. JAWWA. July P Factors_Affecting_Mercury_Chemistry_and_Captu re_in_wet_fgd_systems.pdf Blythe, G., Factors Affecting Mercury Chemistry and Capture in Wet FGD Systems 6. USEPA, EPA s Clean Air Rules: An Update, DOE/NETL 2007 Mercury Control Technology Conference, December 11, Pavlish, John H.; Sondreal, Everett A,\.; Mann, Michael D.; Olson, Edwin S.; Galbreath, Kevin C.; Laudal, Dennis L.; Benson, Steven A.; Status Review of mercury control options for coal-fired power plants. Fuel Processing Technology. 82 (2003) Shah, Pushan,; Strezov, Vladmir; Prince, Kathryn; Nelson, Peter; Speciation of As, Cr, Se and Hg under coal fired power station conditions. Fuel. 87 (2008) Diaz-Somoano, Mercedes; Unterberger, Sven; Hein, Klaus; Using Wet-FGD systems for mercury removal. J. Environ. Monit., 7 (2005), Goodarzi, Fariborz; Characteristics and composition of fly ash from Canadian coal-fired power plants. Fuel, 85(2006), /mercury/upload/2007_10_02_pubs_mercurymem o_analyticalmethods.pdf. Analytical Methods for Mercury in National Pollutant Discharge Elimination System (NPDES) Permits memorandum, August 23, Page 9