Removing Heavy Metals From Power Plant Wastewater to Meet EPA Regulations
Two Environmental Protection Agency (EPA) regulations, intertwined and consistently in flux, are having an impact on waste streams in power plants.
Important power plant wastewater regulations include Effluent Limitation Guidelines (ELGs) for flue gas desulfurization (FGD) and limitations for coal combustion residuals (CCRs) for coal ash ponds. Coal-fired utilities are looking for solutions to remove the heavy metals found in both scenarios.
The EPA 2015 ELG rule regulates discharge of ash transport water and sets limits on the levels of toxic heavy metals that can be discharged from FGD systems at steam electric power generating stations. The proposed regulations were recently updated. (See chart below.)
The need for heavy metals removal in power plant wastewater will be required in almost all FGD systems. In WesTech's interpretation of these ELG regulations, systems that have a high flow rate, low utilization boilers, or systems that will be retired by 2028 may be exempt from meeting proposed EPA regulations for heavy metals removal. All other FGD wastewater systems will need a solution to remove heavy metals to the limits set by the EPA, which are based on a physical-chemical biological treatment system.
Also, the limitations for CCRs will require coal-fired utilities to consider ways to eliminate coal ash ponds and minimize the use of process water in handling CCRs and wet FGD systems.
Utilities that research options for meeting this continually changing regulatory landscape and proactively seek solutions that work for their sites will be one step ahead.
An effective treatment to consider is a physical-chemical biological solution for eliminating heavy metals in both types of coal-fired utility waste streams, for coal ash ponds and FGD systems. For coal ash ponds, these systems may include reaction tanks, clarifiers, and pressure filters. Solutions for FGD systems might include mixing tanks, clarifiers, ultrafiltration, and continuous backwash filters.
Ideal solutions will accommodate varied and unique site conditions. Though the solution may start with a standardized approach, utilities should work with solutions providers who can customize and modify existing treatment methods to meet the unique needs of their site.
The following case studies represent conditions at two coal-fired power plants – one concerned with meeting the ELG rule and one with CCR requirements. In both cases, a physical-chemical solution was customized to eliminate the heavy metals contamination and protect local groundwater.
Case Study 1 – Addressing Heavy Metals Removal for ELG Compliance
WesTech has developed solutions that have proven to be dependable and robust for the removal of heavy metals such as arsenic, mercury, and selenium in FGD wastewater to meet the ELGs proposed by the EPA.
An example of effective FGD wastewater treatment was at a power plant in the Southern U.S. The plant was seeking to remove heavy metals, TSS, and nitrate to ELG levels from their FGD wastewater for discharge. Their issue was solved with a treatment system that included chemical addition, sedimentation, media filtration, and selenium treatment.
Heavy metals, including mercury and arsenic, were reduced to low levels through chemical addition, sedimentation, and filtration. The metals were first precipitated using a metals scavenger. Coagulant and polymer were then added, allowing captured heavy metals removal through sedimentation and filtration.
The sedimentation and filtration technologies provided a way to optimize the utility’s needs for metals removal. Depending on the technology, sedimentation can be used to remove precipitated metals and provide a clear overflow while also thickening solids to produce a dense underflow sludge for improved dewatering. Flocculating clarifiers were selected in this process.
A SuperSand™ continuous backwash filter was used to remove precipitated heavy metals further, as well as to provide a denitrification step. Nitrate removal is not only important for discharge limitations, but can also be a factor in optimizing selenium removal in the additional treatment steps.
The continuous backwashing feature of the filter was also important to allow for steady flow to the biological system used for selenium treatment.
Selenium treatment is often highly dependent on the speciation of selenium and other influent water conditions. The EPA’s proposed best available technology (BAT) is biological selenium removal.
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In this case study, a biological system followed by ultrafiltration was used to effectively remove selenium to the appropriate level. In other cases, such as when a majority of the selenium is of the selenite species, or levels of removal are less stringent, alternate approaches may be considered.
Selenite, like arsenic and mercury, can be precipitated and removed in a physical-chemical system. Ultrafiltration achieves high levels of removal, but other forms of filtration may be adequate in some applications.
Case Study 2 – Removing Arsenic and Heavy Metals to Meet CCR Regulations
WesTech has successfully met customer needs for coal ash pond wastewater treatment by providing mobile, temporary equipment to the site.
Another effective solution was installed at an unlined coal ash pond at a Southern U.S. coal-fired electric generating station.
The pond required closure to protect against heavy metals leakage through groundwater into a nearby waterway. Though the plant was already off-line, there was potential that earthen berms surrounding the pond were at risk of giving way and inundating the waterway with coal ash.
This utility sought a temporary coal ash pond water treatment system that could be used to close the pond. WesTech provided not only equipment, but a turnkey project with constant monitoring. From mobilization to demobilization, WesTech took full responsibility for site operations.
The organization was able to avoid extra staffing and training investment to hire new positions or transfer experienced operators for a temporary project. Looking at the challenges, many organizations like this utility have chosen to outsource their water and wastewater treatment plant operations to professionals.
The utility's technology solution included a physical-chemical treatment system consisting of reaction tanks and sedimentation clarifiers to provide removal of heavy metals, eliminate TSS, and furnish pH adjustment.
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Beginning with the fiberglass-reinforced plastic (FRP) reaction tank, chemicals for coagulation, pH adjustment, or oxidation were added. The tank was mixed with a chemical-resistant hydrofoil mixer.
The wastewater then flowed via gravity from the reaction tank to a 750 gallons per minute (gpm) process treatment train. The first step, WesTech’s RapiSand™ ballasted flocculation system, removed solids and treated mercury to the effluent requirements.
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Treated water from this system was then pumped into a horizontal pressure filter, and solids from the clarifier were sent back to the ash pond. Arsenic removal and further solids removal took place in the horizontal pressure filter, which included multiple cells and allowed for self-generating backwash.
The chem center control container supported and controlled the automation, data logging, and chemical addition for each process stage.
This process also allows for a biological system for selenium removal.
A Strategic Approach
There is some uncertainty in the power industry regarding the impacts of CCR and ELG compliance requirements and timing. Utilities will be making decisions about their future heavy metals waste stream management methods based on the ELG and CCR rules in a landscape of pending revisions where legal challenges will be inescapable.
For instance, the EPA requirements say that utility providers are to take action to meet CCR rules, but it is up to the state to decide what the specific limits will be. Even those states without previously existing formal guidelines now will likely see regulations for discharge in the future.
Though these regulations will drive utilities to make changes, a cost-benefit analysis might reveal treatment technologies that meet the upcoming regulations while also providing future benefits, such as increased boiler efficiency and better water reuse.
Solutions should be considered that not only achieve heavy metals removal but also account for the financial impact on utilities in such areas as operations and cash flow.
With plants varying in size, influent quality, and geography, a way to assure an optimal solution is through customization.
Utilities should be comfortable working with the solutions provider as a trusted partner. Such a partnership will ensure that from design to operation, the most effective methods are used to meet upcoming EPA regulations.