Wastewater

Accelerated Remediation Catalysis (ARC) – An Emerging Water Treatment Technology for the Treatment of a Wide Range of Dissolved Phase Organic and Inorganic Contaminants

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The Accelerated Remediation Catalysis (ARC) system is a process that can be applied to reduction or oxidation. For reduction, hydrogen gas and an inexpensive, proprietary catalyst are used to perform a chemical reduction of appropriate contaminants. The application of shear forces that can be achieved by using certain pumps is also a feature that dramatically accelerates reaction times.

On the reduction side, there is data supporting the degradation of 1,4-dioxane (1,4-D), perfluorocarbons (PFCs), chlorinated hydrocarbons, and oxyanions (nitrate and perchlorate). With respect to metals and metalloids such as selenium, these species are precipitated and collected for disposal. ARC is also applicable to oxidative processes for appropriate organics like petroleum hydrocarbons, as well as metals/metalloids that precipitate under high redox conditions. In this application, the oxygen is provided by dilute hydrogen peroxide or peracetic acid with a different catalyst.

To help reduce start-up costs, the ex-situ process uses common tankage, pumps, valves, and process controls that can be obtained from standard vendors. If the process handles low levels of contaminants, it can be constructed of common thermoplastics such as polyvinyl chloride (PVC), polyethylene, and fiberglass.

ARC can operate in either batch or continuous mode. In batch mode, the reaction tank is filled at start-up and the total reaction time is allowed to reach the predetermined level to assure destruction of the constituents of concern (COCs). After this point has been achieved, the process switches to continuous mode, and the reaction tank functions as a single-stage plug flow reactor. The process can be made to be continuous at start-up by simply filling the reactor tank with clean water. The overall retention time for completion of most reactions has been on the order of 10 to 15 minutes. Using reduction, hydrogen used in the catalyst vessel is generated electrochemically at the site, reducing the need to handle compressed gas. Depending on the COC, the reaction will either cause manageable gas evolution, or precipitate out of the water and be recovered by a variety of methods. The insoluble catalyst can be recovered by filtration and recycled back to the reactor vessel.

Case studies where ARC has been used for chemical reduction include:

  • The conversion of 1,4-dioxane to ethanol. Water with 100 μg/l of 1,4-dioxane was reduced to <1 μg/l.
  • The complete destruction of perfluorocarbons to non-detectable concentrations with a fluorine residue of low concentration, as the initial concentrations of perfluorocarbons are generally low.
  • Chlorinated ethenes are easily reduced to ethene and ethane.
  • Trihalomethanes have been reduced from a typical 80 μg/l level to <10 μg/l in 10-15 minutes.
  • Perchlorate levels as high as 100 mg/l are reduced to chloride.
  • Nitrate is reduced to nitrogen gas.
  • Selenium in the form of selenate can be reduced to selenite and removed as a precipitate. Selenate was reduced from 200 mg/l to <1 mg/l.
  • Chlorobenzene at ppm levels is reduced to benzene that is then collected on the low-cost catalyst.

The ARC system can be designed for a wide range of process flow rates. Design of the system is only limited by the required retention time for the reaction. In essence, the system was brought into focus because of the emerging contaminants issue, and it is applied to pump-and-treat systems. This is important because the nature of 1,4-dioxane and PFCs makes in-situ treatment challenging. It is expected that there will be both an increase in the use of pump-and-treat systems and a need for more efficient water treatment technologies, especially since conventional methods of treatment (such as those that use carbon) are limited.

Additionally, because of the low concentrations of reactants in the process, there is typically no detectable heat gain in the reaction vessel. Therefore, cooling of the process is generally not required prior to releasing the treated effluent. Then there are other applications in traditional wastewater treatment, such as removal of selenium from scrub water at coal-fired power plants. The ARC system’s inherent simplicity allows it to be easily scaled so that dealing with the large flow rates encountered in industrial settings is feasible. While the endpoint for ARC treated water is generally to be discharged, a supplementary feature called Advanced Regenerative Process (ARP) can be added as a further polishing step so that beneficial reuse, including human consumption, is an option.

ARC targets those applications where more complicated and expensive systems, such as conventional Advanced Oxidation Processes (AOP), are being used. The chemical usage, energy, and safety features of AOP systems, combined with their operational footprint, suggest they will eventually be replaced by better remedial options like ARC. There are other developing technologies that have similar objectives to displace AOP systems, such as resin-based operations, but ARC presents distinct advantages in cost, efficacy, physical layout, and scalability.

For additional information, please contact Chris Hortert at (800) 365-2324 (chortert@cecinc.com); Steve Koenigsberg at (949) 262-3265 (skoenigsberg@cecinc.com); or Thom Zugates at (602) 644-2163 (tzugates@cecinc.com).

EPA Finalizes Steam Electric Power Generating Effluent Limitations Guidelines (ELGs)

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The first Federal limits on various metals and other pollutants discharged by steam electric power plants were finalized on September 30, 2015, and published in the Federal Register on November 3, 2015. Limits for arsenic, lead, mercury, selenium, chromium, and cadmium are established in the new rules. EPA notes that steam electric power plant sources make up approximately 30 percent of the toxic and bio-accumulative pollutants discharged into surface waters of the United States by all industrial categories under the Clean Water Act. The Final ELGs set Daily Maximum and 30-Day Average Effluent Limits for discharges from existing and new sources for Flue Gas Desulfurization (FGD) (see 1. below), Gasification (see 2. below), Combustion Residual Leachate (see 3. below), and Chemical Metal Cleaning Wastewaters (see 4. below). Also established are zero discharge requirements for Flue Gas Mercury Control (FGMC), Fly Ash Transport, and Bottom Ash Transport Waters.

The electric power industry has made great strides to reduce air pollutant emissions under Clean Air Act programs, yet many of these pollutants may be transferred to the wastewater as plants employ technologies to reduce air pollution. When metals such as mercury, arsenic, lead, and selenium accumulate in fish or contaminate drinking water, they can potentially cause adverse effects in people who consume the fish or water.

This final rule is the first to ensure that generating stations in the steam electric industry employ technologies designed to reduce discharges of trace metals and other potentially harmful pollutants discharged in the plants’ wastewater. Sources of drinking-water have been identified with increased levels of carcinogenic disinfection by-products (brominated DBPs, in particular trihalomethanes (THMs)) from bromide in the plants’ wastewater. This was tracked from drinking-water utilities’ violations of the THM Maximum Contaminant Level (MCL). Nitrogen discharged by steam electric power plants can also impact drinking-water sources by contributing to algal blooms in reservoirs and lakes that are used as drinking-water sources. Mercury and selenium can bioaccumulate in fish and wildlife, and also accumulate in the sediments of lakes and reservoirs.

The Steam Electric Power Generating Effluent Guidelines and Standards that EPA promulgated and revised in 1974, 1977, and 1982 did not reflect process and technology advances that have occurred in the last 30-plus years (e.g., coal gasification) and the widespread implementation of air pollution controls (e.g., FGD and FGMC). The technological advances have altered waste streams and created new types of wastewater at many steam electric power plants, particularly coal-fired generating stations. Many stations, none-the-less, still treat their wastewater using only surface impoundments, which may be ineffective at controlling discharges of toxic pollutants and nutrients.

1. FGD Wastewater
FGD systems are used to remove sulfur dioxide from the flue gas so that it is not emitted into the air. Dry FGD systems spray sorbent slurry into a reactor vessel so that the droplets dry as they contact the hot flue gas. Although dry FGD scrubbers use water in their operation, the water in most systems evaporates, and the dry FGD scrubbers generally do not discharge wastewater. Wet FGD systems contact the sorbent slurry with flue gas in a reactor vessel, producing a wastewater stream.

Best Available Technology (BAT) required for control of pollutants discharged in FGD wastewater is a chemical precipitation system that employs hydroxide precipitation, sulfide precipitation (organo-sulfide), and iron co-precipitation, followed by an anoxic/anaerobic fixed-film biological treatment system designed to remove heavy metals, selenium, and nitrates. At some stations, this wastewater is managed in surface impoundments, constructed wetlands, or through practices achieving zero discharge. Other technologies have been evaluated or are being developed to treat FGD wastewater, including iron cementation, zero-valent iron (ZVI) cementation, reverse osmosis, absorption or adsorption media, ion exchange, and electrocoagulation.

2. Gasification Wastewater
Integrated Gasification Combined Cycle (IGCC) plants use a carbon-based feedstock (e.g., coal or petroleum coke) and subject it to high temperature and pressure to produce a synthetic gas (syngas), which is used as the fuel for a combined cycle generating unit. After the syngas is produced, it undergoes cleaning prior to combustion. The wastewater generated by these cleaning processes, along with any condensate generated in flash tanks, slag handling water, or wastewater generated from the production of sulfuric acid, is referred to as “grey water” or “sour water,” and is generally treated prior to reuse or discharge.

3. Combustion Residual Leachate from Landfills and Surface Impoundments
Combustion residuals generally collected by or generated from air pollution control technologies comprise a variety of wastes from the combustion process. These combustion residuals can be managed at the station in on-site landfills or surface impoundments. Leachate includes liquid, including suspended or dissolved constituents, that has percolated through or drained from waste or other materials placed in a landfill, or that passes through the containment structure (e.g., bottom, dikes, berms) of a surface impoundment. Most landfills have a system to collect the leachate. In a lined landfill, the combustion residual leachate collected by the liner is typically transported to an impoundment (e.g., collection pond). Some generating stations discharge the effluent from these impoundments containing combustion residual leachate directly to receiving waters, while other stations first send the impoundment effluent to another impoundment handling the ash transport water or other treatment system (e.g., constructed wetlands) prior to discharge.

Surface impoundments are the most widely used systems to treat combustion residual leachate. Some generating stations collect the combustion residual leachate from impoundments and recycle it back to the impoundment from which it was collected. Some generating stations use collected leachate as water for moisture conditioning of dry fly ash prior to disposal, or for dust control around dry unloading areas and landfills.

4. Chemical Metal Cleaning Wastewaters
Chemical metal cleaning wastewaters are generated from cleaning metal process equipment and are most typically treated in surface impoundments and chemical precipitation systems. Other types of treatment and disposal include constructed wetlands, filtration, reverse osmosis, clarification, oil/water separation, brine concentration, recycling, evaporation, off-site treatment, hazardous waste disposal, third party disposal, landfilling after mixing with fly ash, and deep well injection.

Closing Comments
Many power generating stations that are currently using impoundments or basic treatment may find that additional measures are required to achieve the new ELG limits. Table 1 provides a summary of effluent limits for discharges from existing sources, while Table 2 provides a summary of effluent limits for discharges from new sources. Table 3 provides a summary of additional effluent limits that will apply for discharges from new sources that produce greater than 25 megawatts (MW). Power generating stations will likely have issues associated with the treatment of selenium and boron in their FGD blowdown. These compounds can be difficult to treat and are not always readily removed using conventional treatment techniques that are currently employed by power generators. As such, additional treatment processes may be required to satisfactorily remove these compounds. CEC has experience in the treatment and removal of these compounds and can assist with evaluation of power station water balances, wastewater sampling and testing, and wastewater treatment plant design.

If you have any questions about the November 2015 Steam Electric Power ELGs and their potential impacts on your station, please contact Ron Ruocco, P.E., at rruocco@cecinc.com or 855-859-9932.

 

Table 1: Summary of Effluent Limits for Discharges from Existing Sources
(Daily Maximum/30-Day Average)

Steam Electric Plant Process Arsenic
(ug/L)
Mercury
(ng/L)
Selenium
(ug/L)
Nitrate/Nitrite
as N (mg/L)
TSS
(mg/L)
TDS
(mg/L)
O&G
(mg/L)
FGD Wastewater 11 / 8 788 / 356 23 / 12 17.0 / 4.4 100 / 30 20 / 15
Gasification Wastewater 4 / – 1.8 / 1.3 453 / 227 100 / 30 38 / 22 20 / 15
Combustion Residual Leachate 100 / 30 20 / 15

Existing Sources: The final rule establishes Best Available Technology (BAT)-based effluent limits in existing FGD wastewater, existing gasification wastewater, and existing combustion residual leachate discharges. These limits are equivalent to Best Practicable Technology (BPT).

 

Table 2: Summary of Effluent Limits for Discharges from New Sources
(Daily Maximum/30-Day Average)

Steam Electric Plant Process Arsenic
(ug/L)
Mercury
(ng/L)
Selenium
(ug/L)
Copper
(mg/L)
Iron
(mg/L)
TSS
(mg/L)
TDS
(mg/L)
O&G
(mg/L)
FGD Wastewater 4 / – 39 / 24 5 / – 100 / 30 50 / 24 20 / 15
Gasification Wastewater 4 / – 1.8 / 1.3 453 / 227 100 / 30 38 / 22 20 / 15
Combustion Residual Leachate 11 / 8 788 / 356 100 / 30 20 / 15
Low Volume Waste Sources 100 / 30 20 / 15
Chemical Metal Cleaning Wastes 1 / 1 1 / 1 100 / 30 20 / 15

New Sources: For new FGD wastewater, new gasification wastewater, new combustion residual leachate discharges, new low-volume waste sources, and new chemical metal cleaning waste sources, the final rule imposes effluent limitations based on New Source Performance Standards (NSPS).

 

Table 3: Summary of Additional Effluent Limits for Discharges from New Sources
(Generating Stations Producing Greater Than 25 MW)
(Daily Maximum/30-Day Average)

Pollutant or Pollutant Property Once Through Cooling Cooling Tower Blowdown Coal Pile Runoff
Free available chlorine mg/L 0.20 / 0.20 0.50 / 0.20
Total Suspended Solids mg/L 50 / 50
The 126 priority pollutants (Appendix A) contained in chemicals maintenance, except: mg/L (1)
   –   Chromium, total mg/L 0.2 / 0.2
   –   Zinc, total mg/L 1.0 / 1.0

(1) Denotes No Detectable Amount