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Compliance with one of the latest federal environmental regulations, the Wastewater Restriction Guidelines (ELG), is expensive and complicated. The features and functions offered by the newly available options may make it a more attractive alternative to traditional methods.
In September 2015, the U.S. Environmental Protection Agency (EPA) adopted strict federal restrictions on total suspended solids (TSS), arsenic, and mercury, and finally determined new issues affecting flue gas desulfurization (FGD) wastewater discharge from coal-fired steam generators. Rules, nitrate/nitrite and selenium (Table 1).
Table 1. Steam power wastewater limit guidelines for flue gas desulfurization wastewater (existing source). Source: Environmental Protection Agency
As we all know, the final "Effluent Restriction Guidelines" (ELG) established effluent restrictions based on the economically achievable best available technology (BAT), and based on "technical availability, economic achievability and other statutory factors, designed to reflect the highest Performance in this industry." The ELG rules determine the technical basis of the BAT used in FGD, which includes the "chemical precipitation system [Figure 1], which uses hydroxide precipitation, sulfide precipitation (organic sulfide), and iron co-precipitation , And then anoxic/anaerobic fixed membrane biological treatment system designed to remove heavy metals, selenium and nitrate."
1. Comply with EPA technical basis. This picture shows the entire biological treatment process of flue gas desulfurization (FGD) wastewater provided by Frontier Water Technology after physical/chemical precipitation. Source: Frontier Water
In biology-based wastewater treatment methods, specialized microorganisms are cultivated in engineered bioreactors to maintain environmental conditions and promote the conversion of pollutants from soluble and toxic forms into solid products that can be safely treated and disposed of. In the case of selenium, when nitrate (a more favorable electron acceptor for energy) is exhausted, certain bacteria will undergo microbial respiration, and the bacteria breathe selenate as an alternative terminal electron acceptor to produce selenite Salt.
The process continues to reduce the oxidation-reduction potential (ORP) through selenite to form an insoluble precipitate of elemental selenium (Figure 2). Under suitable conditions, this precipitate can be retained in the bioreactor and periodically wasted from the system in concentrated form.
2. Beneficial bacteria are working. Microbial respiration is used to reduce selenate to elemental selenium. Source: Frontier Water
For the past four years, Frontier Water Systems has been focusing on research, development, design and manufacture of prefabricated modular process equipment solutions for the treatment of selenium, mercury and nitrate in wastewater related to power generation and mining.
The result is the trademarked SeHAWK series of products designed to provide guaranteed water quality performance through time and cost-saving prefabricated solutions. The original SeHAWK series of bioreactors served the mining industry (Figure 3), while SeHAWK FGD products are specifically designed to integrate with physical/chemical pretreatment systems to meet ELG compliance.
3. SeHAWK 500 is a prefabricated bioreactor device for mine water treatment. Courtesy: Border Water
The unique feature of the SeHAWK FGD process is that it contains a fluidized first-stage reactor that continuously discharges a large amount of gas formed during the biochemical conversion of FGD wastewater from aerobic to anoxic form. In traditional downflow reactors, the volume retained and the downtime associated with venting the gas and the additional waste volume generated by flushing must be considered when determining the size.
The SeHAWK FGD system also directly combines fixed membrane anoxic biological treatment with ultrafiltration (UF) to maintain a more consistent effluent quality (Figure 4).
4. Integration. The SeHAWK FGD process combines a bioreactor and ultrafiltration. Source: Frontier Water
In the SeHAWK system, ultrafiltration is used as a pressure-driven process, and separation is achieved by sieving (ie, size exclusion) according to its pore size and molecular weight cut-off. Generally, ultrafiltration membranes of 80 kDa to 150 kDa (equivalent to a pore size of 0.02 μm to 0.03 μm) are used for water treatment applications because they have a good balance between rejection and permeability. This pore size can provide a barrier to viruses commonly found in water sources, and usually produces an effluent with a turbidity of less than 0.2 NTU and a TSS of <1 mg/l.
The integration after ultrafiltration can capture residual selenium and mercury particulate materials that are not retained in the bioreactor. The particulate mercury contained in the coal is released by volatilization in the boiler and converted into elemental mercury Hg0. Part of the total mercury leaving the boiler is captured by particulate control equipment, and the remaining mercury is either absorbed during the FGD process or released into the atmosphere.
Through the use of organic sulfide combined with physicochemical treatment of iron co-precipitation, the dissolved mercury in the form of Hg2 is removed from the FGD wastewater. With the formation of mercury sulfide precipitates, colloidal particles with a diameter of <0.2 μm can be produced and passed through the clarification and bioreactor stages. Although mercury is reduced and retained in the bioreactor, the ELG mercury standard alone is not enough. In these cases, ultrafiltration provides a beneficial polishing step that removes colloidal particles and achieves very low ppt mercury content in the effluent (Figure 5).
5. SeHAWK FGD mercury performance example. Source: Frontier Water
Post ultrafiltration also improves the performance of selenium (Figure 6) because the removal of insoluble selenium associated with the effluent TSS is rejected by the membrane. This method contributes to a smaller footprint and shorter bioreactor retention time.
6. SeHAWK FGD selenium performance example. Source: Frontier Water
In order to meet the compliance targets set by the EPA, it is estimated that 60 to 80 coal-fired stations are working to solve the economic problems of installing and processing in the form of bioreactors, deploying zero liquid discharge systems, or decommissioning. The operation is complete. According to the facility, the time frame for inclusion in the National Pollutant Discharge Elimination System (NPDES) permit is from November 2018 to December 2023.
This timetable puts pressure on utilities, consulting engineers, equipment suppliers and contractors, requiring them to provide accurate cost estimates and project timetables within a short period of time in an uncertain economic and regulatory environment. Given the limited number of teams with the background and ability to provide professional customized solutions of this nature, it is timely and valuable to implement a standardized and repeatable platform quickly and cost-effectively.
Frontier Water estimates that more than 80% of the wastewater flow at these sites will be below 900 gpm. To meet this capacity range, the SeHAWK FGD series of prefabricated modular processing solutions (Figure 7) are designed and constructed around a comprehensive range, including prefabricated tanks, skid-mounted equipment, overhaul platforms and railings. It also integrates with various control platforms, including Emerson's Ovation distributed control system. All equipment uses corrosion-resistant materials, tested and selected according to the opinions of partner public utility companies, and verified in demonstration-scale field tests.
7. SeHAWK FGD 50 installation. Courtesy: Border Water
The SeHAWK FGD system can be expanded by adding unit biological chains and supporting equipment skids to provide consistent water quality for a series of influent water chemistry, as shown in Table 2.
Table 2. SeHAWK FGD model and footprint. Source: Frontier Water
By using this transformative method, the capital expenditure for the implementation of BAT technology (including engineering, procurement, and construction) for ELG wastewater compliance is significantly lower than the original estimate, and the solution can be delivered in a shorter time . ■
— James Peterson PE is the co-founder and CEO of Frontier Water Systems.
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