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Arsenic Removal Tech Comparison.pdf | Clean Water Act | Sewage Treatment
Arsenic Removal Tech Comparison.pdf
Exposure to arsenic, a naturally occurring trace contaminant in drinking water, has recently attracted greater regulatory attention in industrial wastewater discharges as well. The discharge of arsenic to a receiving stream is subject to compliance with the National Pollution Discharge Elimination System (NPDES) program. These discharge limits established in the NPDES permit for specific contaminants are determined by the water quality criteria established for the receiving water, ambient levels of the specific contaminants, the established low-flow condition of the receiving water, and the design flow of the proposed discharge from the arsenic treatment process. Depending on these factors, the discharge limit for arsenic in industrial wastewaters can be quite stringent. For example, some power plants in the United States have recently been limited to arsenic discharges as low as 4 μg/L, which is lower than the 10 μg/L arsenic standard for drinking water. This paper presents best available technologies for the removal of arsenic from industrial wastewaters, and also discusses wastewater quality drivers affecting technology selection and treatment residuals management issues.
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01-19-18 MASTER Water Resources Briefing From the MassDEP Water Programs Division Directors
WOTUS House Version Walz
TPDES Permit Issuance Request Letter
United States v. Cumberland Farms of Connecticut, Inc., 826 F.2d 1151, 1st Cir. (1987)
United States v. Zenon-Encarnacion, 387 F.3d 60, 1st Cir. (2004)
Appendix D3 DEIR
Aria Water Treatment System
Alembic Report
Proposed Rule: Revised National Pollutant Discharge Elimination System Permit Regulations for Concentrated Animal Feeding Operations
Rule: Water pollution control: National Pollutant Discharge Elimination System&#8212; Concentrated animal feeding operations; permitting requirements and effluent limitations guidelines and standards; compliance dates extension
Rule: Water pollution control: National Pollutant Discharge Elimination System&#8212; Concentrated animal feeding operations
ARSENIC REMOVAL FROM INDUSTRIAL WASTEWATER DISCHARGES AND RESIDUALS MANAGEMENT ISSUES
Ganesh Ghurye CDM, Texas Jean-Claude Younan SCANA, South Carolina Joseph Chwirka Albuquerque-Bernalillo County Water Utility Authority, New Mexico
Exposure to arsenic, a naturally occurring trace contaminant in drinking water, has recently attracted greater regulatory attention in industrial wastewater discharges as well. The discharge of arsenic to a receiving stream is subject to compliance with the National Pollution Discharge Elimination System (NPDES) program. These discharge limits established in the NPDES permit for specific contaminants are determined by the water quality criteria established for the receiving water, ambient levels of the specific contaminants, the established low-flow condition of the receiving water, and the design flow of the proposed discharge from the arsenic treatment process. Depending on these factors, the discharge limit for arsenic in industrial wastewaters can be quite stringent. For example, some power plants in the United States have recently been limited to arsenic discharges as low as 4 g/L, which is lower than the 10 g/L arsenic standard for drinking water. This paper presents best available technologies for the removal of arsenic from industrial wastewaters, and also discusses wastewater quality drivers affecting technology selection and treatment residuals management issues.
JOURNAL OF EUEC, Volume 1, 2007 2007 Energy and Environment Conference
Journal of EUEC
Prior to discussing arsenic removal technologies, a brief description of issues relating to arsenic speciation and arsenic removal efficiencies is provided in this section. Arsenic Speciation Arsenic can occur in both organic and inorganic forms. In most arsenic-contaminated wastewaters, the inorganic forms predominate. Inorganic arsenic exists as either As III (arsenious acid with an arsenic oxidation state of +3 H3AsO3) or As V (arsenic acid with an oxidation state of +5 H3AsO4). Arsenious acid is a weak monoprotic acid (pKa = 9.22) and exists predominantly as an uncharged molecule below pH 8.2. Arsenic acid, on the other hand, is a triprotic acid (pKas = 2.22, 6.98 and 11.4) and exists predominantly as a divalent anion above pH 7.0. The normally uncharged arsenic species, As III, is poorly removed by most arsenic removal technologies. Therefore, for waters containing As III, pre-oxidation prior to treatment is required for efficient arsenic removal. As III can be easily oxidized using chlorine, potassium permanganate, ozone, or MnO2-based solid oxidizing media (Ghurye and Clifford, 2001, 2004). If pre-oxidation becomes necessary, the discharge limits for oxidizing chemicals such as chlorine will have to be considered during evaluation of arsenic removal processes. Arsenic Removal Efficiency Unlike the arsenic standard for drinking water, which is set at 10 g/L (U.S. Environmental Protection Agency [EPA], 2001), arsenic limits for industrial wastewater discharges can vary significantly, and may even be set below the drinking water standard. Typically, NPDES discharge limits specify a daily and monthly maximum. Therefore, the required arsenic removal efficiency must first be determined, and subsequently the ability of arsenic treatment technologies to meet low or very low arsenic discharge limits must be evaluated. It should be noted that adsorption processes that might otherwise be feasible to meet the drinking water standard of 10 g/L may well fail to consistently meet a much lower discharge limit of say 2 to 5 g/L. Sufficient scientific information is available for most technologies to perform a desktop analysis and select one or two of the most promising technologies for further evaluation. When performing desktop evaluations of treatment technologies, greater importance should be given to technologies that have been studied by third parties. Vendor literature, though informative, is often cited with incomplete information to allow proper scientific comparisons with other media/processes.
FEASIBLE TECHNOLOGIES FOR TREATMENT OF ARSENIC-CONTAMINATED WATERS
A number of technologies are available for arsenic removal from arsenic-contaminated waters, and technologies appropriate for a particular application may be identified based on
2007 EUEC
knowledge of water chemistry, the strengths/drawbacks of these technologies, and residuals management issues. Current state-of-the-art technologies available for arsenic removal are as follows: Ion Exchange (IX) (with or without spent brine reuse) Hybrid Iron-impregnated IX (Fe-IX) Activated Alumina (AAl) & Iron-doped Activated Alumina (Fe-AAl) Ferric Coagulation-Direct Microfiltration (C/MF) Ferric Coagulation-Pressure Filtration (C-PF) Granular Iron Media (GIM) Other Adsorbent Media (Titanium- or Zirconium-oxidebased media).
Technology Selection Given the wide array of technologies available, technology selection can often be a challenging aspect of arsenic treatment. A number of technologies cited above, such as the IX process with spent brine reuse or the ferric coagulation-direct microfiltration process, are non-proprietary and thus can be freely used/adapted by the end user. Other technologies require the use of proprietary media, and their advantages/drawbacks may not have been fully explored. The key steps to selecting the best technology include: (1) Wastewater Quality understanding the chemistry of arsenic-contaminated waters; (2) Technology Selection understanding the impact of wastewater chemistry on treatment processes and the ability of selected technology to meet treatment goals; (3) Desktop, Bench and Pilot Studies determining/verifying process efficiency and obtaining scale-up data; (4) Residuals evaluating residuals treatment and handling issues; and (5) Cost performing detailed cost evaluations. The ensuing sections discuss the water quality drivers that affect process selection, the advantages, and limitations of some proven technologies and residuals management issues. Since treatment costs are highly site-specific, they are not discussed in this paper.
Arsenic removal technologies can be broadly classified as purely adsorption-based technologies (for example, ion exchange or granular iron media adsorption), purely membrane-based technologies such as nanofiltration (NF) or reverse osmosis (RO), or technologies that use a combination of adsorption followed by filtration/microfiltration (see Figure 1). Purely membrane-based technologies are generally cost prohibitive for arsenic removal applications, and, moreover, generate large quantities of arsenic-rich brines that require treatment/disposal. Therefore, only adsorption processes, and adsorption followed by filtration (membrane or media filtration), are discussed below.
Arsenic removal processes can be broadly classified into
Adsorption IX GIM AAl
Membrane NF RO
e an r b em M
Figure 1. Arsenic Removal Processes
ADSORPTION-BASED TECHNOLOGIES
Ion Exchange (IX) A schematic of an ion exchange with spent brine reuse process is shown in Figure 2. Developed at the University of Houston (Ghurye, Clifford, and Tripp, 1999; Clifford, Ghurye, and Tripp, 2003), the IX-brine reuse process improved the conventional IX process, which wasted spent brine following regeneration, by reusing or recycling spent brine multiple times. The IX-brine reuse process resulted in substantial salt savings, and also minimized the volume of brine residuals. Arsenic, which is present as an anion, is exchanged for chloride ions on the ion exchange resin. Rapid kinetics is a significant advantage of IX-based processes and empty bed contact times (EBCTs), defined as the ratio of the volume of resin including voids to the flow rate) as short as 1.5 min may be used without compromising arsenic removal performance. Just before arsenic breakthrough (which may be operationally defined as 4 g/L), the column run is stopped and the resin is regenerated (converted back to the chloride form) by simply passing a salt (NaCl) solution through the resin. Arsenic, along with other anions such as nitrate and sulfate are removed from the resin during regeneration, with the waste stream being the spent brine that is typically wasted. If make-up salt is added to this spent brine to bring its chloride content back to a preset concentration, the spent brine can be reused several times before it must be finally treated and disposed. Even with multiple reuses or recycles, arsenic concentration in the treated water can be reduced to less than 1 g/L (Clifford, Ghurye, and Tripp, 2003). For ion exchange, arsenic breakthrough is very sharp (typically occurs in less than 100 bed volumes (BV). Since arsenic breakthroughs are
relatively sharp, virtually all of the arsenic adsorption capacity of IX resins may be utilized prior to terminating a run.
Raw Water As III or As V pH 6.5 9.0 Bicarbonate Nitrate Sulfate Silicate Phosphate Fluoride
NaCl to Regenerate Make-up NaCl Chloride-form SBA Resin 0.3-0.6 mm 1 m deep 1.5 min EBCT Spent Backwash Water FeCl3 to co-precipitate Fe(OH)3.As pH = 5.5 Fe/As=20 (m/m) 99.5% As Rem.
Cl2 to Oxidize As III
Recycle Spent Brine: Arsenic, NaCl, Na2SO4, NaHCO3
Spent Brine Recycle
Fe(OH)3.As Sludge
Arsenic-free (< 2 ppb) Treated Water
Figure 2. Schematic of Ion Exchange with Spent Brine Reuse Process
Water Quality Drivers for the IX Process Of the major anions found in arsenic-contaminated waters, only sulfate has any significant adverse impact on IX capacity for arsenic. This is because sulfate has a greater affinity for a resin than arsenic, and it eventually drives accumulated arsenic off the resin. Unlike granular iron adsorbent media, IX is unaffected by pH (in the range of 5-9), silica, phosphate, or vanadate. Advantages of IX Simple, easily automated process, fast kinetics (EBCT 1.5 min) Requires inexpensive salt for regeneration Cost effective for waters containing high silica/phosphate and low sulfate High ( 99%) arsenic removal efficiency, and the IX process can be tailored to provide effluent arsenic concentration below detection limits (< 2 g/L) at all times Proven track record in full-scale treatment plants In many cases, sufficient data exist to provide accurate predictions of cost and performance without bench testing Disadvantages of IX
Requires prefiltration for iron, manganese, and TOC (total organic carbon), which can foul IX resins Adversely affected by sulfate, arsenic concentration in the effluent can peak above its influent value Requires a large amount of salt (up to 1,660 lbs NaCl/million gallons [MG] treated effluent) Produces a large volume of waste brine (5,000 gallons/MG treated effluent) Waste brine can contain very high arsenic concentrations (up to 20.000 g/L) and will require treatment prior to disposal
Residuals High arsenic spent brines will likely trigger classification of the treatment facility as a large quantity hazardous waste generator (LQHWG) under the Resource Conservation and Recovery Act (RCRA). Fe-Doped Ion Exchange (Fe-IX) Recently developed hybrid iron-resin ion exchangers reportedly have many advantages over conventional strong base anion exchange resins. In addition to functioning as ion exchangers, these hybrid resins also have ferric (hydr)oxide functional groups, which have a high capacity for arsenic adsorption. These new hybrid resins also require regeneration with an alkaline brine solution. Arsenic concentration in waste brine from Fe-IX resins is expected to be several times greater than with conventional IX resins. Therefore, these waste brines will also be classified as hazardous wastes. Based on the chemistry of the Fe-IX process, arsenic breakthroughs will be gradual and not sharp as in the IX process. The Fe-IX process will produce effluent arsenic levels below detection during the initial part of its run, and arsenic concentration will then gradually increase over a period of several hundred bed volumes (one bed volume [BV] is defined as the volume of the media bed including voids). Thus, an Fe-IX run may be terminated to provide an average arsenic effluent concentration of < 2 g/L at all times, but the drawback to this approach is that a bulk of the arsenic adsorption capacity will be left unutilized prior to regeneration, resulting in poor capacity utilization of the Fe-IX resin and concomitantly, resulting in a much greater requirement of salt and base for regeneration per unit throughput of arsenic contaminated water. For example, it may take approximately 20,000 BV for the effluent arsenic concentration to increase from 1 g/L to 5 g/L and an additional 25,000 BV for the effluent arsenic concentration to increase to 10 g/L (Boodoo, 2005). Water Quality Drivers Since Fe-IX resins contain both a resin and ferric (hydr)oxide functionality, they should be affected by the presence of sulfate, silicate, phosphate, and vanadium. However, vendor literature has indicated that Fe-IX resins are unaffected by sulfate, which is the limiting factor in a conventional IX processes. The non-effect of sulfate is plausible, given that an Fe-IX resin may be expected to continue arsenic removal long after the resin sites have reached their arsenic and sulfate capacity. Often contrary to vendor claims, Fe-IX media are affected by the presence of other co-occurring species such as silicate, phosphate, and vanadium all of which reduce arsenic removal capacity. Further, species such as silica may
not be readily removed during regeneration leading to a substantial (up to 30%) loss of capacity on subsequent reuse. Advantages of Fe-IX Longer run lengths (throughput) than conventional IX media as arsenic capacity is not affected by sulfate concentration High arsenic removal efficiency; arsenic removal to less than 4 g/L possible, but may lead to inefficient use of media capacity Media more robust than friable granular iron media or activated alumina Some vendors offer off-site regeneration facilities Disadvantages of Fe-IX Requires prefiltration for iron, manganese, and TOC which can foul IX resins Arsenic capacity is reduced by the presence of silicate, phosphate and vanadate Bench and/or pilot testing will be required to predict performance and costs Requires a large amount of salt and sodium hydroxide for regeneration Vendor claims of multiple regeneration with minimal loss of capacity remain unproven If not regenerated off site, the large volume of alkaline waste brine containing very high concentrations of arsenic will require treatment prior to disposal Residuals The extremely high arsenic waste brines will trigger classification as a large quantity hazardous waste generator (LQHWG) under the Resource Conservation and Recovery Act. Activated Alumina (AAl) & Fe-doped Activated Alumina (Fe-AAl) The use of activated alumina for arsenic removal has been known since the early 1980s, and is a proven technology for arsenic removal (Clifford, 1999; Wang, Chen, and Fields, 2000). More recently, activated aluminas have been doped with iron to increase arsenic removal capacity (Rubel, 2003). Conventional and iron-doped aluminas remove arsenic via a surface adsorption/complexation reaction. While conventional aluminas require a low operating pH of around 5.5 to 6.5 (capacities are poor at pH > 6.5), the iron-doped aluminas can be operated at pH values up to 7.5. Note that for both aluminas, adsorption capacities increase with decreasing pH. Due to the incorporation of iron, Fe-AAl media possess greater arsenic capacity than conventional aluminas. Both media suffer from poor kinetics, and require long EBCTs in the range of 5 to 12 min. Comparatively, IX can be operated at an EBCT as low as 1.5 min. The longer EBCT requirement translates to much greater media requirement. Both media are generally used as throwaway media, and are expected to pass the Toxicity Characteristic Leaching Procedure (TCLP) test. A typical process schematic for an adsorbent media process is shown in Figure 3. Typically, raw water is chlorinated/pH-adjusted as necessary and passed through the adsorbent media. The media may be typically backwashed once every month, and is replaced with new media once it reaches a user-defined set-point for effluent arsenic concentration. Usually adsorbent media columns are operated in series (a lead column and a lag column) to maximize their capacity for arsenic and keep effluent arsenic concentrations at a low level.
Chlorine/ Oxidant
Backwash to Waste
Acid or Base (If needed)
Figure 3. Schematic for Arsenic Removal using Adsorbent Media Arsenic breakthrough curves using AAl or Fe-AAl are usually prolonged and may take several thousand bed volumes from the time that arsenic initially makes an appearance in the effluent to complete breakthrough. However, as with Fe-IX, arsenic adsorption capacity utilization will be poor if low arsenic discharge limits are required. Water Quality Drivers Since the mechanism is one of surface complexation (ligand exchange), arsenic removal via conventional or iron-doped media is affected by pH, and adversely affected by the presence of silicate, phosphate and vanadate. Sulfate has no effect on arsenic removal via aluminas, and in fact, sulfuric acid may be used for pH depression prior to treatment via activated aluminas. Advantages of AAl and Fe-AAl Low cost, non-regenerable (not efficiently regenerated), throwaway media. Proven technology; typical arsenic removal efficiencies are in the range of 9095%. Depending on water quality, capacities may be greater than IX media. Disadvantages of AAl and Fe-AAl Will require bench and/or pilot testing to determine performance and cost May not be able to meet low arsenic discharge limits on a consistent basis Requires prefiltration for iron, manganese and TOC, which can foul media Lower capacity than granular iron media, more frequent media replacement
Adsorbent Media Filtrate to Discharge
Slow kinetics compared with IX and Fe-IX media, requires a 5- to 12-min EBCT and hence increased media requirement Adversely affected by silicate, phosphate, and vanadium, which compete with arsenic for adsorption sites Friable media and not as robust as IX or Fe-IX media
Residuals Spent media may pass the TCLP test and be classified as a non-hazardous waste Granular Iron Media (GIM) Adsorption The adsorption of arsenic onto a fixed bed iron-based media has been shown to be effective at removing arsenic from drinking water (Rubel, 2003). These iron-based media are generally referred to as granular iron media (GIM). The two most effective GIM available are granular ferric oxide (GFO) and granular ferric hydroxide (GFH) (no endorsement is implied by the authors) and they have been pilot tested in a number of installations across the United States. The GIM adsorption process is similar to the activated alumina process, and occurs by a process of surface complexation or ligand exchange. Typical arsenic breakthrough curves for some various GIM media may be obtained from Badruzzaman, Westerhoff and Knappe (2004). As with the alumina media, GIM breakthroughs are gradual and may take several thousand bed volumes to complete breakthrough. To obtain low (< 4 g/L) arsenic effluent concentrations, the GIM process will have to be operated in series in a lead-lag configuration. As with the aluminas and Fe-IX, if low effluent arsenic concentrations are required, poor adsorption capacity utilization will result. Water Quality Drivers GIM can generally operate in a higher pH range than activated aluminas due to the higher point of zero charge (pzc) of ferric hydroxide at pH 8.2. GIM adsorption is adversely affected by silicate, phosphate, and vanadate. Sulfate, however, has no effect on arsenic removal via GIM. Advantages of GIM GIM have been shown to have higher capacity than other granular media, which translates to longer run times. Proven technology, simple operation; arsenic removal to less than 4 g/L possible but will lead to poor capacity utilization. Of various metal (hydr)oxides available for arsenic removal, ferric hydroxide has the greatest capacity for arsenic removal. Disadvantages of GIM Pilot testing will be required to predict performance and costs. May not be able to meet a low arsenic discharge limit on a consistent basis. Without prefiltration, iron, manganese, and TOC can foul the granular iron media. Slow kinetics compared with IX media, requires 3 to 6 min EBCT, thus increasing media volume requirement compared with IX. Adversely affected by silicate, phosphate, vanadium, and TOC, which foul GIM.
Less arsenic adsorption capacity than freshly generated ferric hydroxide, may not be cost effective for larger flow rates or high arsenic (> 20 g/L) waters. Friable media and not as robust as IX or Fe-IX media.
Residuals Spent media may pass the TCLP test and be classified as a non-hazardous waste.
Previous studies have shown that purely membrane based technologies can effectively remove arsenic from potable water supplies (Brandhuber and Amy, 1998). However, due to their comparatively higher costs and the generation of large quantities of contaminant-laded brines, these technologies are not considered feasible for arsenic removal from industrial wastewaters (Chwirka, Stomp, Thomson, 2001). Therefore, purely membrane-based technologies are not further discussed in this paper.
COMBINATION OF ADSORPTION AND FILTRATION TECHNOLOGIES
Ferric Coagulation-Pressure Filtration (CPF) In the ferric coagulation-pressure filtration process, a coagulant such as ferric chloride (or ferric sulfate) is added to the arsenic-containing water with mixing typically provided via a static mixer, followed by a contact tank to provide sufficient time for arsenic adsorption, and finally arsenic-iron precipitate removal in a pressure media sand filter. Sufficient iron dose is provided to meet the target arsenic removal. Typically sand or dual media pressure filters have been used for filtration of the arsenic-precipitated iron solids Fields, Chen, Wang, 2000a, 2000b). More recently, robust light-weight ceramic media have been tested that claim much higher flow fluxes than conventional pressure filters (up to 8 gpm/ft2) and require lower backwash flow rates, thus minimizing the volume of waste generated. During a typical filter run, the initial portion of filtrate is recycled back until the filter ripens (evidenced by reduced effluent turbidity), followed by a production leg where incoming iron-arsenic precipitate is removed efficiently to produce a low arsenic filtrate. The filter run is terminated based either on reaching a user-defined headloss across the filter, effluent turbidity, or filter run time. Efficient arsenic removal will depend significantly on the efficient filtration of precipitated solids (ferric hydroxide). Therefore, even a small breakthrough of solids will render it difficult to achieve low filtrate arsenic concentration using the ferric coagulation pressure-filtration process. Water Quality Drivers Since arsenic removal occurs via ferric hydroxide, water quality impacts are the same as those for GIM. Greater ferric doses may be required to reach a target filtrate arsenic concentration in the presence of competing species such as silicate, phosphate, and vanadate.
Alternatively, a lower ferric dose in conjunction with pH reduction via acid addition may be required to overcome/compensate for the presence of competing ions. Advantages of Coagulation/Pressure Filtration Readily automated, simpler operation than CMF process, proven technology for arsenic removal for small systems. Ferric dose and pH can be varied independently to achieve a target arsenic removal. Arsenic removal to less than 2 g/L possible but may come at the cost of reduced filter run times. Pressure filtration media is not susceptible to fouling by organics in the raw water as is the case with microfiltration membranes. Disadvantages of Coagulation/Pressure Filtration May not meet low arsenic discharge limits on a consistent basis. May not be able to handle surges in influent suspended solids concentrations or may fail to provide sufficient run time when higher coagulant doses are required. Backwash water usually contains less than 0.5% solids, and will require thickening/dewatering prior to disposal. Requires pilot testing to determine the optimum ferric coagulant dose, filter ripening time and backwash interval. Generates a waste sludge that must be handled and dewatered prior to disposal. Precipitation of ferric hydroxides may cause a pH drop (depending on the ferric dose and buffering capacity of the water) requiring neutralization prior to discharge. Residuals Waste sludge may pass the TCLP test and be classified as a non-hazardous waste. Ferric Coagulation-Microfiltration (C/MF) A conceptual C/MF process flow diagram is shown in Figure 4. This process is essentially the same as the coagulation/pressure filtration process except that microfiltration membranes are used for filtration rather than a media filter. Coagulant (ferric chloride or ferric sulfate) is typically added at the optimum dose in-line to a rapid mixer, where the coagulant is instantaneously mixed with the incoming raw water. The coagulated feed stream is then delivered directly to a microfiltration unit for solids removal. In general, lower treatment pHs tend to result in better arsenic removal due to the increased fraction of positively ferric hydroxide surface that results with a decreasing pH. Thus, filtrate arsenic concentrations less than 4 g/L (often less than 2 g/L) are achievable on a consistent basis (Ghurye, Clifford, Tripp, 2004; Chwirka, Colvin, Gomez, and Mueller, 2004). The microfiltration units are backwashed on a periodic basis to remove accumulated ferric hydroxide solids from the membranes. Typically microfiltration systems include compressed air addition to assist with the backwashing. Submerged filtration systems may also be used instead of pressurized MF systems but usually require a larger footprint. Backwashing occurs approximately every 20 to 30 minutes. The flux of the microfiltration unit is a key parameter for effective operation. Flux is defined as the instantaneous flow per unit area of membrane and is typically expressed in
terms of gallons per square foot (of filter surface area) per day or GFD. The microfilter membranes are tubular in shape and the surface area is calculated based on the outside surface of the membrane. Typical fluxes vary from 25 to 75 GFD. Ferric dose, flux, and backwash intervals are typically optimized during pilot testing. The optimum flux is typically dependent on the solids loading to the filters and the potential for membrane fouling from organic carbon in the raw water (Chwirka, Colvin, Gomez, Mueller, 2004).
Backwash to Thickening/Landfill
Filtrate to Discharge
Coagulant (FeCl3)
Compressed Microfiltration Module Air
Figure 4. Ferric Coagulation Direct-Microfiltration Process Schematic Nominal pore sizes in microfiltration membranes are usually in the range of 0.1 to 0.22 m, leading to a near complete removal of precipitated ferric hydroxide solids. Thus, the C/MF process is capable of very high filtration efficiencies, and is typically able to meet effluent arsenic targets using low ferric doses. For example, raw water arsenic concentrations can be reduced to below detection (< 2 g/L) using a ferric dose 2 to 3 mg/L as Fe (6 to 9 mg/L as FeCl3) in low alkalinity waters (typically containing less than 30 mg/L alkalinity as CaCO3). Water Quality Drivers Since removal is via ferric hydroxide, water quality impacts are the same as those for GIM and ferric coagulation-pressure filtration. However, iron-based processes that use freshly formed or in situ-formed ferric hydroxide typically exhibit greater arsenic removal efficiencies over preformed iron-adsorbent media such as GIM. The reason for the greater efficiencies (more arsenic adsorbed/unit mass of Fe) is the availability of greater surface area available for arsenic adsorption. As ferric chloride is dosed into the arsenic-containing water, and as the ferric hydroxide particles coagulate and agglomerate, a significant amount of internal surface area becomes available for arsenic adsorption. In contrast when using GIM,
the ferric adsorbent particles are already formed, and thus only the external surface area of the adsorbent is available for arsenic adsorption (Ghurye, Clifford, Tripp, 2004). Advantages of Ferric Coagulation-Microfiltration One of the most cost-efficient technologies for removing for arsenic removal from higharsenic waters. Membranes provide much tighter filtration for removal of arsenic-iron solids compared to pressure media filters. Ferric dose and pH can be tailored independently to achieve target arsenic removals; consistent arsenic removal to less than 4 g/L is possible. C/MF process exhibits greater arsenic adsorption capacity (arsenic removal/unit mass of Fe) than GIM. Proven technology. Disadvantages of Ferric Coagulation-Microfiltration More complex than GIM adsorption and pressure filtration. Capital cost of membrane systems is much higher compared to pressure filtration systems. High TOC-containing waters can result in more frequent cleaning requirements for membranes. Requires pilot testing to determine the optimum ferric coagulant dosage, backwashing interval, and flux. Backwash water, usually containing less than 0.5% solids, will require thickening/dewatering prior to disposal. Precipitation of ferric hydroxides may cause a pH drop (depending on the ferric dosage and buffering capacity of the water) requiring neutralization prior to discharge. Residuals Spent media may pass the TCLP test and be classified as a non-hazardous waste.
SITE-SPECIFIC WASTEWATER QUALITY SAMPLING PLANS
Although not discussed in detail in this paper, it is vitally important to obtain data on wastewater characteristics including pH and alkalinity, flow, and important inorganic and organic constituents. This is often achieved through a well-conceived site-specific sampling plan. The sampling plan should ideally be implemented prior to performing the desktop analysis, and should be of sufficient duration to capture seasonal variations in wastewater quality. For example, significant shifts in pH and constituents that interfere with arsenic removal must be sufficiently characterized to ensure that the treatment technologies selected for pilot testing are theoretically able to handle upsets in wastewater quality. Of special importance also are constituents such as total suspended solids (TSS) and total organic carbon (TOC). Although these two constituents do not directly interfere with arsenic removal, they can significantly increase the scope of pretreatment that is required upstream of the treatment processes (Ghurye and Chwirka, 2006).
ARSENIC TREATMENT RESIDUALS MANAGEMENT
Toxicity Characteristics Arsenic-containing wastes are considered hazardous when the arsenic concentration is higher than certain threshold levels. Under the Resource Conservation and Recovery Act (RCRA), a residual from an arsenic treatment facility may be defined as being hazardous waste if it exhibits a toxicity characteristic. For arsenic, the hazardous waste toxicity characteristic criterion is 5.0 mg/L as defined in Title 40 of the Code of Federal Regulations (CFR), Part 261.24. For liquid wastes with less than 0.5% solids, the 5 mg/L criterion is applied to the dissolved concentration of arsenic in the liquid. For liquids, sludges, or solids with a solids concentration greater than 0.5 %, the Toxicity Characteristic Leaching Procedure is used. The TCLP is performed on a sample to determine the leachable concentration of arsenic under mildly acidic conditions. If the concentration of arsenic in the leachate is greater than 5 mg/L, the liquid, sludge, or solid is characterized as a hazardous waste. The TCLP is performed by placing approximately 125 grams of the solid residuals in a glacial acetic acid solution with a resulting pH of 4.8. This solution is tumble-mixed for 18 hours. The solution is then filtered and the filtrate is analyzed for arsenic. The State of California has modified the TCLP procedure by using citric acid in lieu of acetic acid. This modification is called the State of California Waste Extraction Test (WET). The California WET test is applied in addition to the federal regulations in California. Under RCRA, a chemical used in the treatment process is not considered a waste until it is no longer used in the process. At that point, it is considered a waste and if the waste exhibits a toxicity characteristic, the waste will be characterized as a hazardous waste. This is an important concept in that with several of the technologies discussed above there is an intermediate liquid waste that may have a sufficiently high concentration of arsenic to be classified as hazardous. Even though the arsenic can be removed from the hazardous waste at the facility, the facility will either be classified as a hazardous waste generator or as a treatment, storage, and disposal facility, both of which have specific RCRA requirements. If an arsenic-containing waste is discharged to NPDES permitted discharge (such as a municipal wastewater treatment facility), the RCRA permitting requirements do not apply. However, this is not a likely option for arsenic residuals classified as hazardous because most industrial pretreatment programs prohibit the discharge of hazardous wastes to the sanitary sewer system. Large Quantity Hazardous Waste Generators Arsenic treatment systems which have onsite regeneration (ion exchange and activated alumina) will be classified as large quantity hazardous waste generators (LQG) if they generate brine quantities greater than 1,000 kg per month. This regulation is specified in 40CFR Part 262.34. In addition, depending on the jurisdiction of state, the state permitting agency may define the arsenic facility as a hazardous waste treatment, storage, and disposal facility. The requirements of 40CFR Part 262.34 also apply if the treatment facility stores their hazardous waste on site for more than 90 days.
Composition of Arsenic Treatment Residuals Some typical residuals produced by technologies evaluated in this paper are summarized below. Due to the lack of full-scale data, residuals information on some of the newer technologies such as granular iron media is not available. It should also be noted that most of the information on residual characteristics comes from potable water treatment plants and little to no information is available on residual characteristics from the treatment of arseniccontaminated wastewaters from industrial sources. Nevertheless, the information provided in Table I showing typical residuals generated from the treatment of arsenic-contaminated potable waters should provide some idea of the residuals that may be generated from the treatment of arsenic-contaminated industrial wastewaters.
Table I. Summary of Treatment Residual Characteristics from the Treatment of Potable Arsenic Contaminated Waters
Technology Type of Residual Sludge Sludge Liquid Sludge Liquid Volume Residual (gal/MG) 4,300 9,600 4,000 52,600 664,000 As Concentration (mg/L) 9.3 4.2 10 0.8 0.1 Solids Produced (lb/MG) 180 2,000 23.4 113 NA As in Solids mg/Kg Dry Weight 1,850 165 14,250 3,000 NA
Conventional Coagulation Chemical Softening Ion Exchange Coagulation-Microfiltration Nanofiltration or Reverse Osmosis
General Standards for RCRA Compliance LQGs accumulating hazardous waste on site under 40CFR Part 262.34(a) must comply with the preparedness and prevention procedures of Part 265, Subpart C. Further, LQGs must develop and maintain a contingency plan onsite, as found in Part 265, Subpart D. LQGs must also comply with the personnel training requirements referenced in 40CFR Part 265.16. Before shipping hazardous waste off site to a RCRA facility, a generator must comply with several pretreatment requirements, including: obtaining an EPA ID number, preparing a Uniform Hazardous Waste Manifest (EPA Form 8700-22), and complying with several Department of Transportation requirements. Reporting and Recordkeeping Generators have several reporting and recordkeeping responsibilities under Subpart D of Part 262. These include reporting requirements for shipping hazardous wastes off site, or for those who treat or store hazardous waste on site, submitting a biennial report (EPA form 870013A) to EPA by March 1 of each even-numbered year (40CFR Part 262.41). The biennial report compiles data collected from off-site shipments of waste during the previous calendar year. Under 40CFR Part 262.40, the generator must keep a signed copy of the manifest for at least three years from the date the waste was accepted by the initial transporter. Basins and
tanks that treat hazardous arsenic residuals are subject to the design and installation requirements in 40 CFR 264/265.192 under RCRA. The tank system or component must be designed with an adequate foundation, structural support, and corrosion protection to prevent collapse, rupture, or failure of the unit.
DISPOSAL OPTIONS FOR ARSENIC-CONTAMINATED RESIDUALS
The residuals generated from various treatment technologies discussed above will consist of either liquids, solids, or both. Presented below is a general description of possible disposal methods for liquid and solid residuals generated from arsenic treatment facilities. Residuals Disposal Directly to a Receiving Stream The disposal of liquid residuals containing arsenic directly to a receiving stream will be subject to compliance with the National Pollution Discharge Elimination System (NPDES) program. The limits established in the NPDES permit for specific contaminants are determined by the water quality criteria established for the receiving water, ambient levels of the specific contaminants, the established low-flow condition of the receiving water, and the design flow of the proposed discharge from the arsenic treatment process. Most NPDES permits limit solids discharge to around 30 mg/L. Waste streams with solids concentrations greater than this can not be discharged. The EPA has established regulations or guidance for arsenic under the Clean Water Act (CWA) and the Safe Drinking Water Act (SDWA). Under the Clean Water Act, an ambient water quality criterion for arsenic was established in 1992 for fish consumption to protect human health. This 1992 criterion for fish consumption was set at 0.14 g/L. Further, if that same water was used for drinking and fish consumption, the ambient water quality criterion for arsenic was set at 0.0175 g/L to protect human health. These arsenic water quality criteria represent a one in one million (10-6) cancer risk for arsenic exposure. The Safe Water Drinking Act MCL for arsenic is 10 g/L. To protect the natural environment, the EPA established arsenic criteria for fresh water and marine water chronic and acute conditions. The ambient water quality criteria that may apply to arsenic discharge are summarized in Table II. Table II. Summary of Water Quality Criteria for Arsenic
Arsenic Form Total Arsenic Fresh Water Acute, g/L (Dissolved) Fresh Water Chronic, g/L (Dissolved) Marine Water Acute, g/L (Dissolved) Marine Water Chronic, g/L (Dissolved) Human Health- Fish Consumption, g/L 0.145 Human Health, Water and Fish 0.018 Consumption, g/L SDWA Drinking Water Criteria, g/L (Total) 10 Trivalent Arsenic 360 190 69 36 0.0175
The water quality criterion presented in Table II will be used by state regulators to establish discharge limitations for arsenic depending on the classification of the receiving water. The established arsenic limit will then be written into the NPDES permits. The discharge limitations are calculated by the following mass balance equation: M2 = (Q3*M3-Q1*M1)/Q2 where M1 = the background arsenic concentration in the receiving stream, g/L Q1 = the low flow condition of the receiving stream, MGD M2 = the allowable arsenic concentration of the discharge, g/L Q2 = the design flow rate of the arsenic treatment facility discharge, MGD M3 = the arsenic water quality criterion of the receiving stream, g/L Q3 = Q1 + Q2, MGD The allowable discharge of waters containing arsenic will therefore be impacted by the ability of the stream to assimilate the arsenic without exceeding the arsenic standard of the receiving water. As such, each potential discharge will have specific arsenic discharge limits established by the regulatory agency. In addition, a discharge will likely be required to pass the whole effluent toxicity (WET) test. The WET test will determine toxicity of the effluent regardless of the arsenic concentration and possible synergistic impacts with other contaminants in the water. Given these considerations, direct disposal of arsenic residuals to the receiving stream will only be acceptable for residuals with low levels of suspended solids and arsenic concentrations that comply with surface water quality standards under the NPDES program. Direct disposal will also be limited to regions where discharge of high TDS (total dissolved solids) wastewaters to receiving streams is not a concern. Given these constraints and the requirement that the groundwater treatment facility must be in close proximity to the receiving stream, direct discharge of arsenic residuals to the receiving stream will only be feasible for a limited number of facilities. Discharge of Arsenic Residuals to a Sanitary Sewer The discharge of arsenic residuals to a sanitary sewer may be a disposal alternative, providing arsenic concentrations are within the established Technically Based Local Limits (TBLL) of the sewerage authority's Industrial Pretreatment Program. Under NPDES regulations, wastewater treatment plants (WWTPs) must have an Industrial Pretreatment Program in effect to protect the operation of the WWTP, prevent violations of the WWTP NPDES permit, and prevent unacceptable accumulation of contaminants in the WWTP sludge or biosolids. The residuals generated from an arsenic treatment facility will be classified as industrial waste since they contains contaminants that may impact the WWTP (metals, caustic and acidic solutions, and high levels of TDS). As such, the arsenic residuals will need to meet the established TBLLs if they are to be discharged to a sanitary sewer. The TBLLs are computed for each WWTP to take into account the background levels of contaminants in the municipal wastewater. In addition, the estimated flow contributed by the industries
2007 EUEC 17
compared with the municipal flow is used to calculate the allowable contaminant loading to the WWTP. The TBLL is established by the most stringent of the limits that protect the effluent water quality of the WWTP, cause process operational upsets, or cause unacceptable levels of contaminants in the biosolids. The development of TBLLs for each WWTP will result in specific limitations representative of that wastewater system. The discharge limitations for arsenic that the WWTP must meet were discussed above. The computations consider the amount of arsenic removed through the treatment processes at the WWTP that will accumulate in the sludge. Arsenic removal values reported by EPA are 11% to 78% for activated sludge facilities (EPA, 1986). Arsenic may cause inhibition of the biological processes if the concentration is high enough. Reported values of arsenic threshold process inhibition are 0.1 mg/L for activated sludge, 1.5 mg/L for nitrification, and 1.6 mg/L for anaerobic digestion. Arsenic concentrations should be kept below these threshold inhibition values. Note that the arsenic removed in the treatment processes will accumulate in the biosolids and impact the eventual disposal of these biosolids. Based on a number of industrial pretreatment programs established by the authors, the TBLL for arsenic will typically be limited by the contamination of the biosolids as opposed to discharge limitations or process inhibition. However, if the drinking water standard is significantly lowered, the effluent limits for streams classified as a domestic water source will be drastically reduced. This will result in a condition in which the effluent limits will dictate the criteria used to develop local limits, as opposed to the biosolids content. The allowable concentration of arsenic in biosolids will be governed by the 40CFR Part 503 regulations promulgated in 1989 and amended in 1995. The Part 503 regulations specify the following arsenic limits in biosolids as a function of the disposal method: Land Disposal Arsenic Limit....73 milligrams per kilogram (mg/kg) Land Application Arsenic Ceiling Limit 75 mg/kg Land Application Clean Sludge Arsenic Limit.. 41 mg/kg Land disposal refers to dedicated sludge disposal sites that are essentially sludge-only landfills. These facilities are used by municipalities for the disposal of WWTP and water treatment plant (WTP) residuals and typically referred to as Dedicated Land Disposal (DLD) sites. The sludge must be applied to DLD sites in conformance with the 40CFR Part 503 regulations. The land application of biosolids refers to the application of WWTP or WTP residuals to agricultural land for the purpose of nutrient addition and as a soil conditioner. The residuals are applied at a rate in which the nitrogen content of the biosolids meets the agronomic nitrogen requirements of the crop grown and also meets the contaminant limits established in the 40CFR Part 503 regulations. The 41 mg/kg arsenic limit is the value listed in Table III of the Part 503 regulations and has been termed clean sludge. Clean sludge can be land applied with no limitations.
Table III. Arsenic Technically Based Local Limits (TBLL) for Various Cities
City Albuquerque, NM El Paso, TX Phoenix, AZ Tucson, AZ Fresno, CA Newark, NJ Orange County, CA Arsenic TBLL, g/L 51 170 100 400 320 150 2,000
If the arsenic concentration of the residual exceeds the clean sludge limit of 41 mg/kg, the biosolids may still be land applied, but the quantity will be limited to a total cumulative arsenic loading of 41 kg per hectare of land (36.6 lbs/acre). As such, most municipalities will establish TBLLs based on the clean sludge criteria to avoid land application restrictions. To illustrate the impact of the Part 503 regulations on the development of an arsenic TBLL, consider a limit of 41 mg/kg for land application of biosolids. The wastewater removal efficiency for arsenic is typically given as 45% through an activated sludge facility. Assuming that the biosolids production is around 1,200 pounds per million gallons of wastewater treated, the maximum allowable headworks loading will be around 0.109 pounds of arsenic per million gallons of wastewater treated. This equates to a total (background and industrial) influent arsenic concentration of around 13 g/L. The arsenic TBLL will then be calculated based on the background arsenic concentration, the estimated industrial flow, and a factor of safety. For comparison purposes, the arsenic TBLL for several cities is presented in Table III. Table III shows a large variation in the arsenic TBLL caused by the specific characteristics of the individual wastewater systems. The development of an arsenic TBLL will be influenced by the arsenic concentration of the water supply and therefore also by the treatment technology and level of treatment achieved for the water supply. As shown above, when the background arsenic concentration approaches the 13 g/L level, the industrial pretreatment program will result in a lower arsenic TBLL. The ability to discharge arsenic residuals to a sanitary sewer will be determined by the arsenic TBLL and will be different for each community. If the water supply has an arsenic concentration above approximately 13 g/L, it may not be possible to discharge arsenic treatment residuals to the sanitary sewer. In other words, communities that require arsenic treatment will likely have higher background arsenic concentrations in the wastewater, and therefore will limit the possibility of discharge of arsenic residuals to the sanitary sewer. Although a reduction in the arsenic drinking water standard will reduce background arsenic levels in wastewater, disposal of drinking water arsenic residuals to the sewer will increase the influent arsenic concentration at the wastewater plants. This is because in arid areas, wastewater flow is significantly less than the domestic water production rate due to consumptive uses and irrigation. This would also elevate arsenic levels in wastewater effluent and biosolids.
Landfill Disposal of Arsenic Residuals The disposal of arsenic residuals at a landfill is a potential method of disposal for arsenic residuals that contain no free water. This means the arsenic residuals must be in a solid form and not contain free water that could drain out of the residual. In addition, the residuals must not have toxic characteristics as defined by the TCLP test or the California WET. If the arsenic-containing residual is determined to be non-hazardous and does not have free water, then it will be accepted at most solid waste landfills. The arsenic residuals must not contain free draining water if they are to be landfilled. The landfill will typically administer a test called the paint filter test to determine if the residual has free water. A sample of the residual is placed in a paint filter and if water drains out, the residual will be rejected. As such, the arsenic residual must be dewatered before it can be landfilled. Depending on the form of the solid arsenic residual, the residual must be dewatered to around 20% to 25% solids before it will meet the paint filter test requirements. Disposal of Arsenic Residuals at a Hazardous Waste Disposal Facility Arsenic residuals that fail to pass the TCLP (or the California WET test in California) must be treated to meet relevant RCRA land disposal restrictions and disposed of at a designated and licensed hazardous waste facility. These facilities are designed to prevent migration of the hazardous contaminants to the environment. As such, these facilities have extensive monitoring and operational requirements that cause the cost of this method of disposal to be much greater than a typical solid waste landfill. In addition, if the arsenic residual is a hazardous waste, its transport to the hazardous waste facility must be manifested and the owner may never be free of the responsibility of that waste. The production of an arsenic residual that is determined to be hazardous should be avoided. Treatment technologies should be evaluated closely to assure that the residuals produced are not classified as hazardous and require disposal at a hazardous waste facility.
Various technologies are available for arsenic removal from industrial wastewaters. These technologies generally use either adsorption or adsorption followed by filtration. Based on the discussion above, the following conclusions are offered: Based on the characteristics of the receiving stream, arsenic discharge limits in industrial wastewaters can be quite low, even lower than the drinking water standard of 10 g/L. The choice of an appropriate treatment technology for arsenic removal depends on treatment goal, i.e., the efficiency required of a treatment process. Technologies that may be feasible for arsenic removal from drinking waters may not always work for industrial wastewaters. Prior to evaluating treatment processes, a site-specific sampling plan should be implemented to obtain important wastewater characteristics. The sampling plan
should be of sufficient duration to capture seasonal changes (if any) in wastewater chemistry. A desktop analysis should be performed to evaluate the feasibility of various available technologies to meet site-specific discharge limits. One or two technologies should be selected for pilot testing. Pilot testing should determine the impact of wastewater chemistry on treatment efficiency and the ability of the treatment process to handle wastewater quality upsets, develop data for establishing design criteria, prepare conceptual facility designs, and estimate capital and operations and maintenance costs. Attention must also be focused on residuals handling and disposal issues. Residuals resulting from pilot plant operation should be collected and analyzed to determine the toxicity characteristic. Simultaneously, a holistic analysis must be performed to ensure that the entire treatment process including waste generation complies with applicable local, state, and federal regulations.
Badruzzaman, M., Westerhoff, P., Knappe, D. 2004. Intraparticle diffusion and adsorption of arsenate onto granular ferric hydroxide (GFH). Water Research, 38 (18):4002-4012. Brandhuber, P., Amy, G. 1998. Alternative methods for membrane filtration of arsenic from drinking water. Desalination 117: 1. Boodoo, F. 2005. Personal communication. Chwirka, J.D., Stomp, J., Thomson, B. 2001. Residuals handing issues for arsenic treatment processes. Presented at the AWWA/WEF Joint Residuals Conference, San Diego, California. Chwirka, K.D., Colvin, C., Gomez, J.D., Mueller, P.A. 2004. Arsenic removal from drinking water using the coagulation/microfiltration process. Journal of American Water Works Association, 96 (3):106-114. Clifford, D.A. 1999. Ion exchange and inorganic adsorption. In Water quality and treatment, 5th ed., chap. 9. New York: McGraw Hill. Clifford, D.A., Ghurye, G.L., Tripp, A.R. 2003. Arsenic removal from drinking water using ion exchange with spent brine recycling. Journal of American Water Works Association, 95 (6):119-130. Environmental Protection Agency. 1986. Quality criteria for water (EPA 440/5-86-001). Washington, DC: Environmental Protection Agency. Environmental Protection Agency. 2001. National primary drinking water regulations; Arsenic and clarifications to compliance and new source contaminants monitoring: Final rule. Federal Register, Vol. 66, No. 14. Fields, K., Chen, A., Wang, L. 2000a. Arsenic removal from drinking water by coagulation/filtration and lime softening plants (E.P.A, Office of Research and Development [ORD] Report, EPA/600/R-00/063). Washington, DC: Environmental Protection Agency. Fields, K., Chen, A., Wang, L. 2000b. Arsenic removal from drinking water by iron removal plants (E.P.A, Office of Research and Development [ORD] Report, EPA/600/R00/086). Washington, DC: Environmental Protection Agency.
Ghurye, G.L., Chwirka, J.D. 2006. Water quality considerations for arsenic removal from groundwaters. Proceedings NGWA Naturally Occurring Contaminants Conference, Albuquerque, NM. Ghurye, G.L., Clifford, D.A. 2001. Laboratory study on the oxidation of As III to As V. US (E.P.A, Office of Research and Development [ORD] Report EPA/600/R-10/021). Washington, DC: Environmental Protection Agency. Ghurye, G.L., Clifford, D.A. 2004. Laboratory study on the oxidation of As III to As V. Journal of American Water Works Association, 96 (1): 84-96. Ghurye, G.L., Clifford, D.A., Tripp, A.R. 1999. Combined nitrate and arsenic removal by ion exchange. Journal of American Water Works Association, 91 (10): 85-96. Ghurye, G.L., Clifford, D.A., Tripp, A.R. 2004. Pilot study of an iron coagulation-direct microfiltration process for arsenic removal from groundwater. Journal of American Water Works Association, 96 (4): 143-152. Rubel, F. 2003. Removal of arsenic from drinking water by adsorptive media (E.P.A, Office of Research and Development [ORD] Report, EPA/600/R-03/019). Washington, DC: Environmental Protection Agency. Wang, L., Chen, A., Fields, K. 2000. Arsenic removal from drinking water by ion exchange and activated alumina plants (E.P.A, Office of Research and Development [ORD] Report, EPA/600/R-00/088). Washington, DC: Environmental Protection Agency.
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