Coal Ash Pond Water Treatment:
29 October 2019
- Coal ash pond water may require treatment as leached pollutants from ash pond water pose human health and ecological risks.
- Each ash pond water has unique water chemistry and requires an integration of multiple treatment solutions. Treatment methods for the most common pollutants of concern in coal ash pond waters are discussed.
- Consider membrane concentrators, minimal liquid discharge (MLD), and zero liquid discharge (ZLD) technologies to reduce brine volumes from treating leachable pollutants.
Treatment Solutions for Coal Ash Pond Waters
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 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 contain 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 the regulators to clean up their coal ash ponds and to treat coal ash pond waters as soon as possible.
Prior to designing a coal ash pond water treatment plant, it is important to know its water chemistry. This will identify the constituents of concern for treatment and the potential scaling risks. A single treatment method unlikely removes 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. There are three options for removing inorganic arsenite and arsenate:
- physical-chemical (phys/chem) precipitation using lime-softening and/or aluminum or iron coagulation;
- physical adsorption using ion exchange (IX) resins, activated alumina, green sand or zero-valent iron-based adsorbents; and
- reverse osmosis (RO).
The phys/chem precipitation method is often preferred as 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:
- ion exchange (IX) using boron-selective resin that can reduce boron concentrations to less than 0.1 mg/L, the spent resins require regeneration with acid and base; and
- reverse osmosis (RO) under a basic condition (pH > 10), a polish step for the treated water may be required to reduce boron to < 0.5 mg/L (a compliance level regulated by some jurisdictions), 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. Option 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 for 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 requiring disposal of. Reverse osmosis will remove both selenite and selenate, management of the reverse osmosis brine needs to be considered. In same cases the brine can be returned to the coal ash pond, however this will increase the pond salt contents over time. Brine management techniques are covered in this article.
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 reacts and binds 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.
Practical Guidance and Considerations
Each coal ash pond water treatment project will have its own unique water chemistry and required treatment goals. Additional constituents of concern not described above may require removal, such as organics, total suspended solids and total dissolved solids. An experienced wastewater treatment team can engineer and price a solution including an initial desktop study to treat 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
(3) Treat for residual elements, boron, and TDS
(5) Remove scaling compounds:
• Solids Management
(7) Minimal liquid discharge (MLD) or
Zero liquid discharge (ZLD)
(2) Treat for selenate, nitrate, and organics
• Anoxic and oxic biological systems
(4) Polish of treated water to meet stringent limits
• Reverse Osmosis or IX
(6) Reduce brine volumes with membrane system
(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, Xtreme Reverse Osmosis) can achieve ultra high brine volume reduction and evaporator crystallizers (AirBreather, SaltMaker) 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.