Oct 29th 2019
The combustion of coal in thermal power plants generates large volumes of coal combustion residuals (CCR), such as fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) materials. These CCR are collectively referred to as coal ash. For decades, coal ash has been managed by using river or lake water to sluice it into a large surface water impoundment (also called “coal ash pond”) for its final settlement. Coal ash contains substances such as arsenic, boron, selenium, and heavy metals (cadmium, copper, chromium, lead, mercury, among others). As a result, coal ash pond water, if released into the environment through embankment breaches and seepage of the water through the pond bottom as leachate, may contaminate soil, rivers, lakes, and groundwater. Several U.S. power plants are currently required by regulators to clean up their coal ash ponds and to treat coal ash pond waters as quickly as possible.
Prior to designing a coal ash pond water treatment plant, it is important to know the water chemistry. This will identify the constituents of concern for treatment and the potential scaling risks. A single treatment method is unlikely to remove every pollutant to meet all requirements.
Rather, a plant should be designed to meet the treatment objectives and be cost-optimized for the project. This will involve an integration of multiple treatment methods. We hereby provide a “tool box” of treatment methods for the most common pollutants of concern in coal ash pond waters.
Arsenic: Arsenic may exist in coal ash pond waters as inorganic arsenite As(III), arsenate As(V) or organic methylated arsenic compounds. The species of arsenic present in the water will determine the treatment method. Three options for removing inorganic arsenite and arsenate are available:
The phys/chem precipitation method is often preferred beause it can reduce arsenate down to 0.01 mg/L. When arsenite is the predominant species in the water, an oxidation step is required to convert arsenite into arsenate prior to the phys/chem precipitation process.
Reverse osmosis has the benefit of being able to remove both inorganic and organic arsenic. Phys/chem precipitation or physical adsorption is not effective for organic methylated arsenic compounds. Although work is being done on advanced oxidation processes to target only organic arsenic removal (for example, Fenton oxidation), these are not yet commercial and may be expensive.
Boron: Two options for boron-removal are commonly used:
The polish step could be IX or a secondary RO. At an acidic condition (pH < 7), boron exists as boric acid, which is too small in radius to be rejected by an RO; boric acid will pass through RO membranes into the treated water.
Both IX and RO options will generate a waste brine. Options for managing these brines are summarized in this article.
Selenium: Selenium may exist in coal ash pond waters as selenite Se(IV) or selenate Se(VI). Selenite is more reactive and easier to remove than selenate. Phys/chem precipitation using ferrous or ferric reagents is effective in removing selenite but not selenate. Existing selenate removal processes are based on converting selenate into elemental selenium through biological reduction. Although electrochemical reduction processes have been studied to remove selenate, they are not yet commercialized and generate a large volume of solid waste that requires disposal. Reverse osmosis will remove both selenite and selenate, although management of the reverse osmosis brine needs to be considered. In some cases, the brine can be returned to the coal ash pond; however, this will increase the pond salt contents over time.
Heavy metals: Heavy metals (cadmium, copper, chromium, lead, mercury, and zinc) are generally removed by phys/chem precipitation as metal oxides or metal hydroxides. The phys/chem process may not meet stringent discharge requirements, depending on the specific metal and its precipitate solubility. To further reduce heavy metal concentrations after the phys/chem process, a polishing step with reverse osmosis or scavenging agents that react and bind with metals may be required. Most metal scavenging agents are toxic, so care must be taken to ensure that they do not end up in the treated water.
Each project for treating coal ash pond water will have its own unique water chemistry and required treatment goals. Additional constituents of concern that were not described above, such as total suspended solids and total dissolved solids, may require removal. An experienced wastewater treatment team can engineer and price a solution, which includes an initial desktop study of how to treat the coal ash pond water to meet project objectives and identify risks.
An exemplary, end-to-end treatment train is presented below for the removal of pollutants of organics, inorganics and suspended solids. Not all process steps are needed, depending on the specific project requirements, but are included to demonstrate integration of multiple technologies to meet stringent treatment objectives and end-of-life for residuals.
(1) Treat for arsenic, heavy metals and TSS
• Solids Management
(2) Treat for selenate, nitrate, and organics
• Anoxic and oxic biological systems
• Solids Management
(3) Treat for residual elements, boron, and TDS
• Prefiltration
• XtremeRO Reverse Osmosis
(4) Polish treated water to meet stringent limits
• Reverse Osmosis or IX
(5) Remove scaling compounds:
• Solids Management
(6) Further reduce brine volumes with a membrane system
• XtremeROReverse Osmosis
(7) Minimal liquid discharge (MLD) or
Zero liquid discharge (ZLD)
• SaltMaker MultiEffect
(8) Dispose of residuals
• Solids to landfill
• Low volume brine to injection well or offsite disposal company
Saltworks has the experience and product solutions to treat coal ash pond waters. Our modular, advanced desalination technologies (BrineRefine, XtremeRO) can achieve ultra-high brine volume reduction, and our evaporator crystallizers (SaltMaker AirBreather, SaltMaker MultiEffect) for minimal liquid discharge (MLD)/zero liquid discharge (ZLD) can help economically treat coal ash pond waters for discharge and manage residuals for end-of-life disposal.
Please feel free to contact Saltworks for a detailed review of your coal ash pond project, risk and opportunities, as well as options to manage the resulting brine residuals.
Saltworks Technologies is a leader in the development and delivery of solutions for industrial wastewater treatment and lithium refining. By working with customers to understand their unique challenges and focusing on continuous innovation, Saltworks’ solutions provide best-in-class performance and reliability. From its headquarters in Richmond, BC, Canada, Saltworks’ team designs, builds, and operates full-scale plants, and offers comprehensive onsite and offsite testing services with its fleet of mobile pilots.
Saltworks has shipped a novel FlexEDR system to the southern United States that will clean up coal fired power wastewater. Flue gas desulfurization (FGD) wastewater is a byproduct of sulfur scrubbers, installed to prevent acid rain from coal plants.
Heavy industry, manufacturing, and natural resource-based industries often use large volumes of water and generate wastewaters requiring treatment. Saltworks designs, builds, and operates advanced wastewater treatment plants that economically and reliably treat tough industrial wastewaters.
Saltworks is honoured to receive POWER Magazine’s 2020 Water Award for our ground-breaking ‘chloride kidney’ industrial desalination solution.
Reverse osmosis-based brine concentrators are reaching new performance levels not seen before. Saltworks is pleased to report that a substantial pilot plant is currently demonstrating 99% freshwater recovery on cooling tower blowdown (CTB).