Tipping the scale: Making membranes work for FGD wastewater treatment
By Jon Liberzon, Vice President of BKT Co - 11/12/2018
Jon Liberzon, Vice President of BKT Co, discusses the challenges of removing toxic compounds from flue gas desulfurization (FDG) wastewater, and considers the best available technologies to facilitate compliance with effluent regulations.
To upend the old adage, where there’s fire, there’s smoke. Burning fossil-fuels in power plants produces exhaust gas, which contains elements, including sulfur, that originated in the combusted fuel. Flue gas from coal plants, incinerators and some other facilities must be scrubbed free of sulfur dioxide (SO2) in order to comply with clean air regulations.
Usually, this means installing flue gas desulfurization (FGD) scrubbers, which use sorbents to capture and convert the SO2 released during combustion into bound or dissolved sulfates. In wet scrubbers (the most common type), flue gasses contact a lime slurry which absorbs sulfur and precipitates gypsum, leaving behind a wastewater rich in sulfates. This wastewater must be treated before it is discharged to the environment.
Unfortunately, during combustion, fossil fuels release more than just SO2. Flue gasses contain a veritable cocktail of elements, including heavy metals, that are absorbed in FGD scrubbers and end up in wastewater. Removing these elements represents a major expense for power-plants, which have been known to shut down in response to new discharge regulations. In 2015, the EPA tightened limits on four of these compounds: mercury, arsenic, selenium and nitrate/nitrite. The new, stricter rules were introduced as amendments to the Effluent Limitation Guidelines (ELG), a series of technology-based requirements first established in 1974 and updated five times in four decades. These guidelines establish physical/chemical and biological treatment as the best available technologies to remove these four target compounds from FGD wastewater, but that designation may not last long.
Business as usual: best available technologies
Physical/chemical and biological treatment systems target specific compounds. Biological treatment systems, for example, use bacteria to convert dissolved selenate and nitrates to nitrogen gas and elemental selenium. Physical/chemical systems generally focus on mercury and arsenic.
To manage all four target compounds, multiple treatment processes are required, and the remaining wastewater still contains other constituents which may be regulated at the state or local level, such as chlorides, boron, bromides and other salts. This means that effluent from these systems is not suitable for industrial reuse, and may not even be dischargeable in some cases. For this reason, some power plants have opted for zero-liquid-discharge (ZLD) systems which evaporate the liquid water, leaving the dissolved salts as concentrated solids, which are more easily disposed of. Despite producing pure water that can be reused in cooling towers or other parts of the plant, evaporation remains an incredibly expensive solution for FGD wastewater streams that can run into millions of gallons per day.
Deja-Vu for thermal evaporation?
In the mid-fifties, when scientists began commercializing desalination processes for drinking water from the sea, they used evaporation to separate salt from water. These plants were effective at producing pure distilled water, but they were energy hogs. Evaporative desalination systems have long since been superseded by a new, more energy efficient technology for separation of dissolved solids from water: membranes.
Membrane technology has made huge leaps in the last decade, with prices falling much faster than other treatment technologies. Adoption of reverse osmosis (RO) membranes, which remove all salts from water, has exploded in a range of sectors, from seawater desalination to ultrapure water production, industrial treatment and municipal wastewater reuse.
One sector where this technology has been slow to advance, however, is in FGD treatment. The reason? High concentrations of divalent ions such as sulfates, silicates, calcium, magnesium and barium form hard scales on membrane surfaces during filtration. These sharp or sticky crystals plug up pores and perforate the membrane, greatly reducing its throughput and lifespan. In some cases, scale can rapidly and permanently damage entire membrane trains, forcing budget-busting replacements of entire filtration systems.
Vortex innovation cuts costs and boosts performance
Chemical manufacturers already produce effective, proprietary anti-scalant mixtures which can prevent some scaling, and these are widely used in RO systems. Most membrane systems, however, remain too sensitive to scaling for cost-effective use in FGD applications. Recently, some manufacturers have begun to develop anti-scaling systems based on physical properties as well, which can combine with chemical anti-scalants to shift the balance on membrane adoption in the power sector.
When scaling cations and anions are present in solution above a certain threshold, they will form precipitant scale spontaneously. Scale formation is driven by charge attraction, and chemical anti-scalants work (in an oversimplified sense) by either introducing competing charges, or by disrupting the shape or “stickiness” of scale crystals. These chemicals cannot change the actual concentration of scaling salts in a solution. During membrane filtration, clean water passes through the membrane, leaving charged ions behind at the surface of the membrane. In most systems, the liquid at the membrane surface is relatively static, due to laminar flow dynamics which slow down crossflow as it drags across a surface. This means that concentrations of scaling ions increase rapidly at the membrane surface as clean water passes through the membrane. This up-concentration is what causes FGD wastewater to scale membranes so aggressively.
BKT, a California-based company, has developed a filtration platform called FMX which physically disrupts this process by preventing scaling ions from building up in the first place (Figure 1). Their system uses rotating blades installed between flat-sheet membrane trays to produce tiny vortices of energized fluid that break up the laminar flow layer and mix the wastewater all the way up to the membrane’s surface. In this way, FMX prevents scaling at the root, by eliminating zones of high scalant concentration. The system has so far been tested with nanofiltration membranes, which reject divalent salts while allowing monovalent salts to pass through. Using nanofiltration, BKT maximizes treatment rates and attacks regulated compounds (such as mercury, arsenic and selenium) along with the ions that cause scaling headaches. If a user wants to produce pure, low-salinity water, they can follow the FMX system with an off-the-shelf RO, which will last longer and filter faster when paired with FMX pre-treatment. Concentrated brines from FMX or RO systems can then be disposed of using evaporative technologies at much lower volumes (and lower costs).
This new, physical anti-scalant platform has the potential to dramatically improve the economics of membrane treatment by boosting the rate of flow (flux) through membrane systems, improving contaminant rejection, and dramatically reducing the frequency of chemical cleaning. Less aggressive cleaning helps membranes last longer while reducing chemical costs. A series of demonstration trials at three mega-sized power plants have already treated over 1.5 million gallons of FGD wastewater to-date, and found that FMX can more than double flux rates compared to traditional membrane systems, while reducing both energy draw and cleaning frequency. What’s more, by following the FMX with RO polishing, wastewater can be completely purified and reused as makeup water onsite for a potential water savings of millions of gallons per day, per plant. For all these reasons, anti-scaling membrane systems now present a promising alternative to older processes. In fact, the EPA is currently performing a review of its 2015 rule on FGD effluent, and industry insiders have suggested that membrane technologies are receiving significant attention. The results of that study should be released by next summer. In the meantime, more and more power plants, along with other producers of FGD wastewater, are starting to show interest in membrane technologies.
Author: Jon Liberzon, Vice President of BKT
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