John.taylor@libertyhydro.com Page 1 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

The Problem: Coal-burning power plants face an issue with tightening legislation concerning the 

amount of selenium that can be released from Flue Gas Desulfurization (FGD) wastewaters. The

industry has been investigating many other technologies with underwhelming results. The industry 

needs a solution that is effective, economical and reliable. 

The Solution: Liberty Hydro, Inc. has developed a solution for the removal of selenium from 

wastewater. The Liberty Hydro ZVI BlueTM system is a patented technology in which finely-divided 

zero-valent iron (ZVI) is attached to an open-pore polymer foam. The foam media allows easy flow of 

the water while enabling adequate contact and reaction time with the ZVI. The selenium in solution is 

reduced by the ZVI to elemental selenium which is permanently absorbed by the foam media. 

Background: The form of selenium usually found in natural water systems is either selenate ion 

(SeO4

2-

) or selenite ion (SeO3

2-

) but other compounds can be present, such as selenide ion (Se2-), 

selenocyanate (SeCN-) or organic selenium compounds. Selenium in surface water is a particular 

problem because it is bio-accumulated by plants and animals. Selenium is an essential nutrient for 

humans at low concentrations but is toxic to all mammals at higher levels. 
 

John.taylor@libertyhydro.com Page 2 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

The United States Environmental Protection Agency (US EPA) has established a limit of 5 parts-perbillion (ppb) of selenium in natural water systems, which is the limit normally associated with the 

discharge from coal mines. For coal-fired power plants, the proposed technology-based limit is 10 ppb 

average, with an instantaneous maximum of 16 ppb. 

In recent years, elemental iron or zero-valent iron (ZVI) has been used to treat ground water and 

surface water systems to remove a number of pollutants (e.g., arsenic, selenium, chromium (VI), 

chlorinated hydrocarbons, etc.). The methods for bringing the ZVI in contact with the contaminant 

have varied from 1) direct addition of the metal to the water; to 2) the construction of underground 

permeable reactive barriers containing iron particles.

In all applications, ZVI acts as a strong reducing agent and, when it is in the finely-divided form, is 

particularly effective in reacting with low levels of contaminants. ZVI will effectively reduce all watersoluble forms of selenium and immobilize it as elemental selenium or selenide on the surface of the 

iron. Previously, one of the challenges of reacting ZVI with low ppb levels of selenium was achieving 

an adequate contact time between the ZVI and the selenium while maintain a reasonable water flowrate. Liberty Hydro has achieved this balance with its patented system in which finely-divided ZVI is 

attached to an open-pore polymer foam. 

The ZVI Blue TM system can be operated by gravity feed of the influent water or by a pump if power is 

available. As a minimum, it is recommended that the influent water be filtered prior to the first ZVI 

reactor vessel and this is usually accomplished with a back-flushable sand filter but more recent pilot 

testing have shown that primary treatment of the influent water will also be required in many cases

(i.e., lime softening, coagulation, clarifying, etc.). 

In the early stages of the Liberty Hydro technology, when applied at coal mines, the system was 

comprised of three steel totes in series (see picture below) with the water flow configured downwards 

in each tote. The pH of the water entering each tote is controlled by a carbon dioxide purge which 

adjusts the pH of the water to a target value of 6.2. Since soluble iron (II) is generated during the ZVI 

reduction step, it is important to have a means of removing the iron from the stream exiting the ZVI 

system, especially if the outfall is environmentally sensitive to iron (e.g. entering a trout stream). 

Traditional techniques can be used to remove the iron, such as settling pond and/or aeration. 
 

John.taylor@libertyhydro.com Page 3 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

Schematic of Early Pilot System: 

Data from Coal Mine Installation: 

Raw Water

Sand Filter

Treated Water

ZVI Tote 1 ZVI Tote 2 ZVI Tote 3

Iron Treatment

Carbon Dioxide Cylinders
 

John.taylor@libertyhydro.com Page 4 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

The technology has now advanced to the point where it has been installed at various coal-fired power 

plants, with successful results. 

The configuration used for pilot testing involved a series of drums filled with ZVI BlueTM media. The flow 

enters the drum through a pipe in the center of the drum so that the flow is upward through each 

drum, from the bottom of the pipe to the connections in the lid of the drum. It is, again, important to 

have a preceding filtration system because the advanced ZVI BlueTM media, designed for reducing 

metals and metalloids, is also, unfortunately, effective at removing mud, silt, algae and other metals 

that would otherwise properly be managed of by an effective primary treatment system (power plants 

usually call primary treatment a physical chemical system or “phys chem” for short).

The pH adjustment of the influent water at the coal-fired power plant pilots was accomplished by feed 

hydrochloric acid (target pH 6.2). 

The drum system installed at a power plant is shown in the following photograph:
 

John.taylor@libertyhydro.com Page 5 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

Typical schematic configurations are shown in the following diagrams (the lower right schematic shows 

the drum system):

Treatment results for selenium removal are shown below: 
 

John.taylor@libertyhydro.com Page 6 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

Larger pilot units can be installed and Liberty Hydro has designed and prepared preliminary cost 

estimates for full scale systems with a flow of 350 gallons per minute. The initial cost estimate was 

under $20 Million and further development work is likely to reduce that cost. 

Now that Liberty Hydro’s ZVI BlueTM technology has been shown to work at power plants, the logical 

next step is to install larger units and enter joint development agreements with major electric utilities 

to refine the technology for their particular use. This would be a much better arrangement than 

watching the power plants continue to test other technologies that have not worked in initial testing. 

Liberty Hydro offers a cost-effective technology that has been shown to work under the right 

conditions and the goal is to replicate those conditions on larger units so that the most promising 

technology is refined for final installation. 

Chemistry: 

At ppb levels of selenium in the influent water, the primary route of selenium removal is by direct 

reduction by the ZVI or by iron (II) hydroxide. Other mechanisms likely involved adsorption, especially 

in iron oxide/hydroxides or green rust phases that develop in the media.

• 3 Fe0

(s) + SeO4

2-

(aq) + 4 H2O(l) → 3 Fe2+(aq) + Se0(s) + 8 OH-(aq)

• 2 Fe0

(s) + SeO3

2-

(aq) + 3 H2O(aq) → 2 Fe2+(aq) + Se0(s) + 6 OH-(aq)

• SeO4

2-

(aq) + 2Fe(OH)2 (aq) + H2O(l) → SeO3

2-

(aq) + 2Fe(OH)3 (s)
 

John.taylor@libertyhydro.com Page 7 304.941.3020

Technical Explanation for the 

Selenium Remover ZVI Blue TM system 

The reduction reaction is pH dependent with the reaction rate increasing at lower pH. The broad 

target pH range for the ZVI BlueTM system is 5.5 to 6.5 since low influent pH (pH<4) - while increasing 

the rate - will dissolve too much iron. Based on the reduction reaction, the pH increases through the 

bed and will eventually reach basic conditions (pH>8); this is an undesirable condition since it likely

mimics lime softening and will tend to precipitate/coagulate any remaining dissolved solids on the 

media. A desirable design feature is pH control at the inlet to each bed.

It is quite well established in the literature that the kinetics of selenate reduction by ZVI is pseudo-first 

order. Recent data collected during pilot testing is consistent with pseudo-first order kinetics (see 

below).

The first data point in the graph represents the influent concentration to the pilot system (5 drum sets 

in series each containing about 80 gallons of media each). Subsequent data points represent 

increments of 80 gallons of media through the system. At a constant flow rate of 3 

gallons per minute (GPM), the x-axis is equivalent to a time axis. The last data point at 400 gallons bed 

volume is 8 ppb as selenium.

Liberty Hydro welcomes additional questions about its water treatment technology.