Introduction
Coal-fired power plants generate large amounts of flue gas containing pollutants like sulfur dioxide (SO₂), a major cause of acid rain. To reduce air pollution, facilities use flue gas scrubbers. Flue Gas Desulfurization, also called FGD, systems to remove contaminants before releasing exhaust into the atmosphere. However, this process creates a new challenge: scrubber water becomes heavily contaminated with sulfur, selenium, mercury, and other pollutants. Among these, selenium is especially concerning due to its bio accumulative nature, prompting strict EPA discharge limits under the Clean Water Act. Traditional chemical treatments often struggle to meet these limits, leading to the adoption of advanced technologies like Vibratory Shear Enhanced Processing (VSEP). This membrane-based system removes dissolved solids including selenium with high efficiency, low fouling, and minimal waste.
This article examines how flue gas scrubbers work, the challenges of selenium removal, and how VSEP offers a reliable, sustainable solution for FGD wastewater treatment and environmental compliance.
Flue Gas Scrubber
When coal is burned to make energy, a flue gas is produced that must be exhausted. This flue gas contains Sulfur Dioxide. Previously, the combustion gases that were vented from power plants would cause “acid rain” because of the presence of Sulfur Dioxide. Rules for emissions where changed and power plants installed “scrubbers” that would wash the gas before it is vented to the environment.
This is accomplished by introducing the combustion exhaust into the bottom of a tall column. At the top of the column water soaked in limestone is sprayed as a very fine mist that falls like rain inside the column. A venturi (cyclone) effect is also added for good mixing. As this “scrubber water” cascades down, it collects Sulfur, Selenium, Mercury, Boron, and other elements. The contaminants in the scrubber water will increase as the scrubber water is recirculated over the gas, and so a purge stream is needed to maintain scrubber water clean enough to collect more contaminants from the gas. The purge water becomes the wastewater from a flue gas scrubber that is high in Selenium and other pollutants. This water must be treated prior to surface water discharge to freshwater bodies.
The scrubber is used to clean the air/exhaust before letting it out to the environment, so the flue gas leaving is clean. However, the pollution problem simply moved from the air into the water. While the flue gas is now clean, the water is now very dirty and requires treatment. Flue Gas Scrubber is also known as Flue Gas Desulfurization (FGD) since the primary function is to remove Sulfur Dioxide.
Discharge Compliance
Because the Clean Water Act and the Selenium discharge limit present a challenge to the steam electric power industry, the EPA is writing a special rule for FGD wastewater. This rule will cover Selenium and other contaminants, and the final rule is expected by April 2014.
Power plants have experience with chemical treatment and precipitation for water treatment. But chemical precipitation methods may not be able to achieve the low-level limits required. Biological treatment using special Sulfur reducing bacteria, which also reduces Selenium to elemental Selenium may work. Elemental Selenium or zero-valent Selenium is insoluble and can easily be removed from the effluent by attaching itself to various media. But power plants are not widely experienced in operating biological systems. Operating a biological system requires different skills and more attention than physical/chemical processes. Biological systems are subject to upset conditions if the bacteria doing the work die off, or if changes in loading, pH, or temperature exceed limits. This would add complexity to the power plant operation.
Power plant operation in the past has been an almost completely automated process that is practically unattended. There are very few operators and engineers when compared to other industrial operations. Operating wastewater treatment plants is more of a hands-on operation and will require power plants to recalibrate the way they think about personnel.
Selenium
Selenium is a bio-accumulant. Selenium is taken into the body by some fish and birds where it accumulates and does not leave. As the levels accumulated get to be high, deformities occur in offspring. This was first discovered in the Kesterson reservoir in California. Kesterson is a sink in the central valley where agricultural and hillside run-off collects and then evaporates. The shale deposits near the surface in the coastal mountains have high levels of Selenium. As this runs off and is concentrated at Kesterson, migratory birds are affected. Enzymes the birds use to make eggshells would confuse Selenium for Sulfate and this resulted in weak eggshells and then as a result, lower yields for offspring.
As a result, the EPA included a 5 parts per billion (ppb) discharge limit for Selenium as part of the Clean Water Act. 5 ppb by itself is not harmful, but because Selenium bio-accumulates, it must be minimized in surface waters. This applies to freshwater only. The chronic level for saltwater is 71 ppb as saltwater wildlife have adapted to higher levels.
Selenium affects animals differently. Very little is known about the exact effects given all the various kinds of animals there are. Also, there are many forms of Selenium including Selenate, Selenite, and Selenide. It is not clear if only one or all of these are problematic, so the group has been limited.
Selenium for humans is a vital nutrient, and humans could not survive without it. The limit for Selenium in drinking water is 50 ppb. Ironically, discharging drinking water that is 49 ppb Selenium would meet the requirement for drinking water but would be a violation of the clean water act. Selenium discharge issues exist primarily in certain industries. Coal mine run-off, refinery wastewater, irrigation districts with high Selenium deposits, and coal-fired power plants.
Selenium Chemistry
Selenate has a valence of +6 and is very soluble. Selenate is the most common form of Selenium in wastewater. Selenate has weak sorption capabilities meaning adsorbent materials such as flocculating polymers, carbon, ion exchange, media filters, and other similar technologies have little effect.
Selenate also has very slow reduction kinetics. If Selenate could be reduced to the zero-valent colloidal Selenium form, it becomes insoluble and is more easily removed from the wastewater. But typical oxidation methods such as Ozone, UV, or chlorine treatment are ineffective. Special bacteria can reduce Selenate to Selenite and elemental Selenium, but the process can be slow and reaching very low levels requires large amounts of carbon addition in the form of molasses or ethanol.
Selenite (+4) is also very soluble but is more easily adsorbed because it forms covalent bonds. The reduction kinetics are faster than Selenate. Selenite is not as common in wastewater as Selenate but is more toxic.
Selenium (0) is zero-valent and is a naturally occurring element. In water it is not soluble and is colloidal, meaning that the suspended solids particles are very small, typically <1 micron. Particles this small will not settle but can be filtered or adsorbed.
Selenide (-2) is soluble except in the presence of metals. It can be precipitated using various metals such as copper. It had weak sorption capabilities and is highly toxic, just as Sulfide in the form of Hydrogen Sulfide is dangerous.
Selenium Treatment Options
Zero valent iron adsorption
Digesters with Sulfate reducing bacteria
Vibratory RO (VSEP from New Logic)
Engineered Wetlands (Experimental)
Because the Selenium discharge limit is a relatively new rule, methods for dealing with it in wastewater are new as well. Many of these methods are in early development and the process is ongoing where design engineers measure the advantages and disadvantages of each method. “Best Available Technology” status is still being formulated.
While the special Selenium Bio-digester and the Zero Valent Iron technologies are new and not widely deployed, VSEP with RO membranes has been in production for 25 years and this system has been installed in every continent in the world at more than 400 locations. VSEP technology is mature and established. VSEP is used for many kinds of wastewater including Selenium removal. The benefit of using RO membranes is that they are not selective and will remove all ion species and not just Selenium like the special digester system and the iron filings technology.
Where the specific focus was Selenium removal, New Logic has deployed its vibratory VSEP RO membrane system at four refineries, two coal fired power plants, two coal mines, a California irrigation district, a phosphate mine, and an oil and gas facility in Texas. New Logic installed its first Selenium removal system at full-scale in 2006.
VSEP Technology
When comparing all the possible treatment methods, VSEP stands out with its simplicity, reliability, and economical benefits. Conventional membranes are limited in their abilities. Particles can become lodged in the membrane pores causing fouling. This will cause reduced flow and permeate recovery as well as more frequent cleanings. A laminar boundary layer will form at the surface of the membrane resulting in the formation of a barrier that restricts permeate flow. By applying a shear force to the surface of the membrane to disrupt the boundary, these problems can be decreased or even eliminated.
The VSEP is a crossflow membrane that is able to produce economical flow rates and reliability with fouling resistance due to vibration. The membrane vibrates at a 3/4” displacement at a frequency of 55 per second. The vibration keeps the turbulent flow at the surface of the membrane allowing large molecules to continue moving away from the surface, avoiding fouling and allowing water to pass freely through the membrane. By using a polymeric reverse osmosis membrane, the VSEP can reject all types of dissolved solids, not just Selenium. Boron, Mercury, and many other pollutants are removed. The typical rejection rate for dissolved solids is >95%. With VSEP, the form of Selenium does not matter. VSEP will remove Selenate, Selenite, elemental Selenium, and Selenide.
The VSEP footprint is smaller than either the Zero Valent Ion or the Selenium Digester. Since both of those technologies rely on trapping the Selenium in various media, the sludge volume produced is large. VSEP can achieve high recovery rates and will produce a small volume of liquid reject. This liquid reject is fed to a small evaporator system or dryer to produce dry powdered solids that can be landfilled. The only waste material generated are the solids themselves and does not include adsorptive media.
VSEP Process Description
VSEP’s Reverse Osmosis membrane module is capable of treating FGD Wastewater effluent and providing a filtrate, which is free from suspended solids and very low in Selenium, Mercury, and Boron. The VSEP process does not normally involve any chemical addition and meets the process engineer’s needs for automated PLC controlled process that can run unattended. VSEP membrane modules contain about 1400 Sq Ft (130 m2) of RO filtration membrane. VSEP can be run with modules in parallel in any number needed to meet process flow requirements. Each 84″ tall VSEP module can produce 10-40 gpm (2.2 to 9 m3/hr) of clean water from the effluent. Since the units are modular and can be used in parallel or in series, the number of VSEPs needed can be calculated based on the amount of material to be processed. System throughput is a function of the concentration of dissolved solids. The VSEP module is uniquely capable of high recovery of filtrate due to its scaling resistance. Recoveries of up to 90% of the FGD effluent as clean filtrate are possible. This water can be reused as cooling make up water or discharged if required.
The typical FGD-VSEP wastewater system would consist of a primary stage of VSEP RO filtration with 84” VSEP modules in parallel followed by a second stage of conventional spiral RO filtration to polish the VSEP filtrate. The two-pass RO filtration creates water suitable for surface water discharge from almost any level of Selenium contamination. If the Selenium levels in the wastewater are less than about 300 ppb, then the polishing stage is probably not needed. If Selenium levels are higher than 300 ppb, then polishing with a conventional spiral may be needed to have discharge water less than 5 ppb.
For high recovery, a high pressure VSEP concentrator stage can be used. The reject from VSEP is fed to a multiple effect evaporator for final disposal. The diagram above shows typical VSEP performance.
Chemical pre-treatment or pH adjustment is typically not needed. VSEP can operate at any temperature above freezing and below 60ºC with RO. Variations in temperature or pH will not unbalance the VSEP or cause any upsets, although throughput rates will decrease as temperatures decrease. Increasing temperature reduces the viscosity of water, which helps to improve throughput rates. The expected process temperature is considered during system design to make sure that there is enough membrane area used to process the flow.
Summary
New Logic installed its first full-scale VSEP system that was needed solely for the purpose of Selenium removal in 2006, and the system is still in operation today. While other passive and active treatment methods may have the ability to treat FGD wastewater for discharge, VSEP with RO membranes is the only treatment method that allows the ability to recycle the water and reuse it back on the process. In conjunction with a thermal evaporator for the VSEP reject, the FGD waste stream can be turned into a zero-liquid discharge scenario where there is no discharge at all and therefore compliance with the Clean Water Act and the Selenium limit are of no concern. Other treatment methods may have episodes due to operational sensitivity where discharge limits are not met. This can cause emergency situations for the power plant if not in compliance. VSEP with RO provides consistent separation and Selenium rejection without upsets for those who want to discharge the water but also can provide an option for shutting the valve on the discharge pipe altogether and simply recycle water.
Power plants are often multi-billion dollar investments and so, uptime is critical. Also, power plants are often targeted, especially coal fired power plants. VSEP can facilitate a smooth-running clean coal facility. VSEP is fully automated with a small footprint and lends itself well to the skill sets that already exist at power plant operations.
VSEP with RO followed by a spiral RO polishing stage is the only treatment method capable of making Selenium non-detect in the treated water.
About the Author:
Mr. Greg Johnson is the Chief Executive Officer of New Logic Research and is one of the company’s founders having responsibility for the early development, product design, and commercialization of the VSEP technology.
For more information about how V✧SEP can help you, please contact us at info@vsep.com.
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