Now that the final rule has been published in the Federal Register, the stage is set for official implementation of the U.S. EPA’s Steam Electric Power Generating Effluent Guidelines on Jan. 4, 2016. From a wastewater perspective, the updated guidelines (revising regulations from 1982) seek to curb pollution resulting from processes/elements including flue gas desulfurization (FGD), fly ash, bottom ash, flue gas mercury control, and gasification of fuels such as coal and petroleum coke.
Legal analysts at Morgan Lewis & Bockius provided the following summation of the rule and its impacts:
“The rule creates more stringent effluent limitations on arsenic, mercury, selenium, and nitrogen for flue gas desulfurization waste streams and ash transport water. The rule also puts a limit on total dissolved solids in flue gas desulfurization wastewater and creates a voluntary program incentivizing plants to meet the more stringent voluntary limits by 2023. In addition, the rule establishes zero discharge pollutant limits for existing plants’ flue gas mercury control wastewater, and stringent limits on arsenic, mercury, selenium, and total dissolved solids in coal gasification wastewater. The rule is significantly more stringent for new coal or petroleum coke plants, in particular.”
The 2023 deadline referenced is the long end of a compliance schedule determined by a number of factors. The EPA wants compliance “as soon as possible” beginning on November 1, 2018, but no later than December 31, 2023. According to EPA’s technical development document, the flexibility considerations include “time to expeditiously plan (including to raise capital), design, procure, and install equipment to comply with the requirements,” timing of the plant’s National Pollutant Discharge Elimination System (NPDES) permit renewal, and any competing/additional changes being made at the plant to satisfy other standards and regulations.
The EPA has also published its findings on what treatment and control strategies are well-suited to satisfy the new requirements, based on tried-and-true use in the field. The following technologies, then, are not touted as the most innovative; rather, they are the most popular as deemed by an EPA industry survey. Culled from a supporting document to the Final Rule, the most trusted treatment solutions for seven power plant wastewater streams are described below.*
FGD Wastewater Treatment
Surface impoundments (e.g., settling ponds), designed to remove particulates from wastewater by means of gravity, may be configured as one impoundment or a series of impoundments. Impoundments are typically sized to allow for a certain residence time to enable the suspended solids to settle to the bottom. The impoundments are also designed to have sufficient capacity to allow for temporary storage or permanent disposal of the settled solids. Surface impoundments are not designed to remove dissolved metals. Plants may add treatment chemicals to the impoundment, typically to adjust pH before final discharge.
Some plants use chemical precipitation systems instead of or in addition to surface impoundments. Chemical precipitation treatment is a tank-based system in which chemicals are added to enhance the removal of suspended solids and dissolved solids, particularly certain dissolved metals. The dissolved metals amenable to chemical precipitation treatment are removed from aqueous solutions by converting soluble metal ions to insoluble metal hydroxides or sulfides. The precipitated solids are then removed from solution by coagulation/flocculation followed by clarification and/or filtration. Chemical reagents such as lime (calcium hydroxide), sodium hydroxide, and ferric chloride are used to adjust the pH of the water to reduce the solubility of the metal(s) targeted for removal.
Some plants also use sulfide chemicals (e.g., organosulfides or sodium sulfide) to precipitate and remove heavy metals, including mercury. Sulfide precipitation is more effective than hydroxide precipitation in removing mercury because mercury sulfides have lower solubilities than mercury hydroxides. Other metal sulfide compounds also typically have lower solubilities than metal hydroxide compounds. Because sulfide precipitation is more expensive than hydroxide precipitation, plants usually use hydroxide precipitation first to remove most of the metals, and then sulfide precipitation to remove the remaining low solubility metals. This configuration overall requires less sulfide, thereby reducing the expense for the sulfide treatment chemicals.
Some steam electric power plants also treat FGD wastewater using biological treatment systems. An anoxic/anaerobic biological system being used in the industry is effective at removing both metals (total and dissolved) and nutrients. This system is designed to significantly reduce nitrogen compounds and selenium. These fixed-film bioreactors are designed for plug flow operation and have zones of differing oxidation potential that allow for nitrification and denitrification of the wastewater and reduction of metals, such as selenium. The system alters the form of selenium, reducing selenate and selenite to elemental selenium, which is then captured by the biomass and retained in treatment system residuals.
Vapor-Compression Evaporation System
This type of system uses a falling-film evaporator (or brine concentrator) to produce a concentrated wastewater stream and a distillate stream. With pretreatment, such as chemical precipitation and softening, brine concentrators can reduce wastewater volumes by 80 to 90 percent. Plants can further process the concentrated wastewater stream in a crystallizer or spray dryer, which evaporates the remaining water to generate a solid waste product and potentially a condensate stream. The distillate and condensate streams may be reused within the plant or discharged to surface waters.
Constructed wetlands are engineered systems that use natural biological processes involving wetland vegetation, soils, and microbial activity to reduce the concentrations of metals, nutrients, and TSS [total suspended solids] in wastewater. High temperature, chemical oxygen demand (COD), nitrates, sulfates, boron, and chlorides in wastewater can adversely affect constructed wetlands performance. To overcome this, plants typically dilute FGD wastewater with service water (i.e., supply water used widely throughout the plant for a variety of uses) before it enters a constructed wetland.
EPA identified three plants that treat their FGD wastewater using constructed wetlands. The constructed wetlands used to treat FGD wastewater typically are designed to treat only the FGD wastewater (and the service water used for dilution); however, because these systems are open to the environment, they also receive stormwater from the surrounding areas.
EPA identified several other technologies that have been evaluated for treatment of FGD wastewater, including iron cementation, reverse osmosis, absorption or adsorption media, ion exchange, and electro-coagulation. Other technologies under laboratory-scale study include polymeric chelates, taconite tailings, and nanoscale iron reagents. Most of these technologies have been evaluated only as pilot-scale studies.
Design/Operating Practices Achieving Zero Discharge
EPA identified four design/operating practices available enabling plants to eliminate the discharge of wastewater from wet FGD systems: 1) several variations of complete recycle, 2) evaporation ponds, 3) conditioning dry fly ash, and 4) underground injection.
Of the 145 plants that generate wastewater from FGD processes, 28 plants (19 percent) operate in such a manner that they do not discharge wastewater to surface waters or POTWs [publicly owned treatment works]. Many of the plants in the southwestern U.S. that generate FGD wastewater use evaporation ponds that do not discharge.
Fly Ash Transport Water
Fly ash separated from boiler exhaust by electrostatic precipitators (ESPs) or fabric filters is collected in hoppers located underneath the equipment. From the collection hoppers, the fly ash is either transferred as dry ash to silos for temporary storage or transported (sluiced) with water to a surface impoundment (i.e., ash settling pond). Plants that generate fly ash transport water use surface impoundments to manage the wastewater. EPA has not identified any facilities using more advanced treatment, such as chemical precipitation or biological treatment, to treat fly ash transport water. EPA identified 393 generating units (at 144 plants) that wet sluice at least a portion of fly ash. Wet sluicing systems use water-powered hydraulic vacuums to withdraw fly ash from the hoppers. The ash is pulled to a separator/transfer tank, combined with sluicing water, and pumped to the surface impoundment to remove particulates from the wastewater by means of gravity, before discharge to a receiving stream.
Many coal and oil-fired power plants design their fly ash handling systems to minimize or eliminate the discharge of fly ash handling transport water. Such approaches include:
Wet Vacuum Pneumatic System
These systems use water-powered hydraulic vacuums for the initial withdrawal of fly ash from the hoppers, similar to wet sluicing systems. Instead of sluicing the ash to a surface impoundment, these systems capture the ash in a filter-receiver (bag filter with a receiving tank) and then deposit the dry ash in a silo.
Dry Vacuum Pneumatic System
These systems use a mechanical exhauster to move air, below atmospheric pressure, to pull the fly ash from the hoppers and convey it directly to a silo. The fly ash empties from the hoppers in to the conveying system via a material handling valve.
These systems use air produced by a positive displacement blower to convey ash directly from the hopper to a silo. Each ash collection hopper is equipped with airlock valves that transfer the fly ash from low pressure to high pressure in the conveying line. The airlock valves are installed at the bottom of the hoppers and require a significant amount of space. Retrofit installations of pressure ash handling systems may require raising the bottom of the hopper.
Combined Vacuum/Pressure System
These systems use a dry vacuum system to pull ash from the hoppers to a transfer station, where the ash is transferred from the vacuum (low pressure) to ambient pressure. From the transfer station, the fly ash is transferred via airlock valves to a high pressure conveying line. A positive displacement blower conveys the ash to a silo. Because the airlocks are not located under the hopper, combination vacuum/pressure systems have the space advantages of dry vacuum systems.
Oil-fired units or other units that generate a low volume of fly ash may use manual or systematic approaches to remove fly ash (e.g., scraping the sides of the boilers with sprayers or shovels, then collecting and removing the fly ash to an intermediate storage destination or disposal).
Bottom Ash Transport Water
Bottom ash (at times also referred to as boiler slag) is produced as fuel is burned in a boiler and collected in hoppers or other types of collection equipment directly below the boiler. Generally, boilers are sloped inward, with an opening at the bottom to allow the bottom ash to feed by gravity into collection hoppers. The hoppers contain water to quench the hot ash. Once the hoppers are full, gates at the bottom of the hoppers open, releasing the bottom ash and quench water to a conveying line, where the ash is diluted with water to approximately 20 percent solids (by weight) and pumped to a surface impoundment or a dewatering bin for solids removal. Conveying bottom ash in a water slurry is called wet sluicing. EPA identified 870 units (345 plants) that wet sluice at least a portion of their bottom ash.
Many coal and oil-fired power plants design their bottom ash handling systems to reduce or eliminate the discharge of bottom ash handling transport water. Available technologies include:
Mechanical Drag System
In these systems, the ash collection hopper is replaced with a transition chute that routes the bottom ash to a water-filled trough. In the trough, a drag chain continuously moves the ash to an incline where it is dewatered and then conveyed to a nearby ash collection area. Excess quench water collected in the dewatering system is recycled to the quench water bath.
Although mechanical drag systems require little space under the boiler they may not be suitable for all boiler configurations.
Remote Mechanical Drag System
These systems collect bottom ash in water-filled hoppers and wet sluice the ash to a mechanical drag system located away from the boilers. Sluice water collected from the dewatered bottom ash is collected and reused in the bottom ash handling system. Plants can use remote mechanical drag systems to convert existing bottom ash handling systems with limited space or other configuration limitations. One U.S. plant has installed and is currently operating a remote mechanical drag system to handle bottom ash. At least one additional plant is currently installing a remote mechanical drag system to handle bottom ash. Additionally, a large U.S. power company has been evaluating installing remote mechanical drag systems for several of its plants.
Dry Vacuum or Pressure System
These systems transport bottom ash from the boiler to a dry hopper without using any water. The system percolates air through the ash to cool it and combust unburned carbon. Cooled ash then drops to a crusher and is conveyed via vacuum or pressure to an intermediate storage destination.
Complete Recycle System
Complete recycle systems transport bottom ash using the same processes as wet sluicing systems. Plants can install complete recycle on existing wet sluicing units. Instead of transporting it to an impoundment, the ash is sluiced to dewatering bins, where it is dewatered and moved to storage. The transport (sluice) water is treated to remove solids in a settling tank and is recycled to the bottom ash collection system. Prior to reusing the treated transport water, plants may add treatment chemicals to the water to adjust pH and prevent equipment corrosion.
Vibratory Belt System
Bottom ash deposits on a vibratory conveyor trough, where the plant cools the ash by air and ultimately moves it through the conveyor deck to an intermediate storage destination.
Oil-fired units or other units that generate a low volume of bottom ash, may use manual or systematic approaches to removing ash that accumulates in the boiler (e.g., scraping the sides of the boilers with sprayers or shovels, then collecting and removing the bottom ash to an intermediate storage destination or disposal).
Combustion Residuals Leachate From Landfills And Surface Impoundments
Plants often treat combustion residual landfill leachate with some of the same technologies used to treat FGD wastewater. EPA identified 102 coal-fired power plants that generate and discharge leachate. Based on the responses to the industry survey, 29 of these plants treat the leachate prior to discharge using surface impoundments, constructed wetlands, or biological treatment. In some cases, plants co-treat the leachate with FGD wastewaters and, in some cases, treat the leachate independently.
Based on information from the industry survey and site visits, surface impoundments are the most common type of system used to treat combustion residual leachate from landfills and impoundments. Constructed wetlands are the next most commonly used treatment system. The anoxic/anaerobic biological treatment system used as the basis for FGD wastewater effluent limits in this proposed rule is also being used by one plant to treat leachate, with the leachate mixing with FGD wastewater immediately prior to the bioreactor stage.
Some plants mix the leachate with fly ash prior to disposing the ash in a landfill to control fugitive dust emissions and to improve the handling characteristics of the dry fly ash. Leachate is also used at some plants for dust control around ash loading areas and landfills. Many plants will collect the leachate from a surface impoundment and pump it directly back to the impoundment from which it originated.
Physical/chemical treatment systems are capable of achieving low effluent concentrations of various metals and are effective at removing many of the pollutants of concern present in leachate discharges to surface waters. The pollutants of concern in leachate have also been identified as pollutants of concern for FGD wastewater, fly ash transport wastewater, bottom ash transport water, and other combustion residuals. This is to be expected since the leachate itself comes from landfills and surface impoundments containing the combustion residuals and those wastes are the source for the pollutants entrained in the leachate. Given the similarities present among the different types of wastewaters associated with combustion residuals, combustion residual leachate will be similarly amenable to chemical precipitation treatment.
The treatment technologies in use at steam electric power plants for gasification wastewater include:
Vapor-Compression Evaporation System
This type of system is identical to the vapor-compression evaporation system described for FGD wastewater. It uses a falling-film evaporator (or brine concentrator) to produce a concentrated wastewater stream and a distillate stream. The concentrated wastewater stream may be further processed in a crystallizer or spray dryer, which evaporates the remaining water to generate a solid waste product and potentially a condensate stream. Facilities may reuse the distillate and condensate streams within the plant or discharge them to surface waters.
Cyanide Destruction System
This system adds sodium hypochlorite (i.e., bleach) to the wastewater in mixing tanks to destroy the cyanide. The cyanide system treats the condensate and distillate streams from both the brine concentrator and crystallizer just prior to discharge.
Flue Gas Mercury Control (FGMC) Wastewater
FGMC wastewater originates from activated carbon injection systems. The system can be configured either upstream or downstream of the primary particulate collection system. EPA identified 73 plants with current or planned activated carbon injection systems. Of these, 58 plants operate upstream injection systems while the remaining 15 plants inject the carbon downstream.
In cases where the injection occurs upstream of the primary particulate collection system, plants collect and handle the mercury-containing carbon with the fly ash. In cases where the injection occurs downstream of the primary particulate collection system, plants collect the mercury-containing carbon in a secondary particulate control system (e.g., a fabric filter). As with fly ash systems, plants collect the mercury-containing carbon in hoppers located underneath the equipment. From the collection hoppers, plants either transfer the mercury-containing carbon as dry ash to silos for temporary storage (67 plants; 92 percent) or transport (sluice) it with water to an ash impoundment (6 plants; 8 percent). Water transport can result in a wastewater discharge, typically an overflow from the impoundment. However, five of the six plants that use water to transport the FGMC waste to a surface impoundment do not discharge any FGMC wastewater and the remaining plant has the capability to handle the FGMC waste using a dry system but sometimes uses a wet system instead.
Coal-fired power plants can minimize or eliminate the discharge of FGMC particulate handling transport water by using the same solids handling technologies that are available for fly ash. These technologies include:
Wet Vacuum Pneumatic System
These systems use water-powered hydraulic vacuums to withdraw dry FGMC waste from the hoppers, similar to wet sluicing systems. Instead of sluicing the FGMC waste to a surface impoundment, these systems capture the FGMC waste in a filter — receiver (bag filter with a receiving tank) and then deposit it in a silo.
Dry Vacuum Pneumatic System
These systems use a mechanical exhauster to move air, below atmospheric pressure, to pull the FGMC waste from the hoppers and convey it directly to a silo. The collected FGMC waste empties from the hoppers into the conveying system via a material handling valve.
These systems use air produced by a positive displacement blower to convey FGMC waste directly from the hopper to a silo.
Combined Vacuum/Pressure System
These systems first utilize a dry vacuum system to pull FGMC waste from the hoppers to a transfer station, and then use a positive displacement blower to convey it to a silo.
Metal Cleaning Wastes
Metal cleaning wastes are generated from cleaning any metal process equipment. Because there are many different processes at plants that use metal equipment, there are a variety of metal cleaning wastes that are generated. The treatment methods used for each of the different types of metal cleaning wastes vary to some degree depending on the specific cleaning operations.
Based on information from the industry survey, surface impoundments and chemical precipitation systems are two of the most common types of systems used to treat metal cleaning wastes. Other types of treatment systems include constructed wetlands, filtration, reverse osmosis, clarification, oil/water separation, and brine concentrators.
In addition to the treatment systems used to control the discharges of metal cleaning wastes, some plants also employ other handling approaches to control or eliminate the discharge of metal cleaning wastes. For example, some plants immediately recycle the metal cleaning wastes back to other plant operations, while other plants evaporate the metal cleaning wastes in the boiler to evaporate the wastewater and eliminate the discharge. Other handling operations reported in the industry survey include offsite treatment, hazardous waste disposal, third-party disposal, mixing with fly ash and landfilling, and deep well injection.
Physical/chemical treatment systems are capable of reducing the concentration of pollutants, including metals, in the wastewater.**
For more treatment information related to power generation — including process water/cooling tower treatment, desalination, cooling water discharge, and additional wastewater and regulatory insight — visit Water Online’s Power Generation solution center.
* Edited from Regulations.gov transcription
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