If you are a building or facilities manager, you're probably quite preoccupied with air quality at the moment. (Heck, you don't have to be a building or facilities manager to be preoccupied with indoor air quality right now.) You have a decent idea of most enemies you need to defeat; viruses, bacteria, dust, pollen, particulate matter, and fungi etc. However, there's one villain who can be a bit more enigmatic: VOCs. But what exactly are VOCs, and how are they removed from the air? In this two-part article, we will give you the secrets needed to defeat this menace. This is part two.
How to remove VOCs from the air?
We've summarized air purification technologies in a previous article, so why are we reviewing them again? First, VOCs are tricky little buggers. Unlike airborne particles, you can't nab them with a true HEPA filter. We want to focus on filtration systems and air purifiers for VOCs specifically. Secondly, our previous article explained air purification technologies for commercial, public, and residential use. Since VOCs are so relevant to manufacturing, though, we'll also want to cover industrial methods of air purification in addition to these other domains.
Note: For this guide, we are especially indebted to the paper 'Removal of Volatile Organic Compounds from Polluted Air' from the Birla Institute of Technology and Science. If a specific fact has no other citation, it is from this work.
We spoke about adsorbent media, especially activated carbon filters, before. As a refresher: contaminated air is pushed through a porous media (porous to increase the surface area). The molecules of the contaminants (in this case, VOCs) stick to the surface of the material, and clean air passes out the other side.
If you want to know how many pounds of carbon you need for your purposes, the EPA released some equations you can use in this guide. Activated carbon is also used as a complementary filter paired with other air purification technology.
Advantages: Adsorption is one of the most tried and true methods of VOC removal.
Disadvantages: The adsorbent media, such as carbon, will eventually become "full" preventing it from adsorbing more VOCs. Also, because the VOCs are only mechanically clinging to the surface of the media, changes in moisture, temperature, or air pressure can cause them to separate and re-enter the air.
One variation on this method: Chemisorption uses a chemically treated media which chemically (rather than mechanically) binds VOCs to its surface. This eliminates the chance of VOCs entering the air once they encounter the chemisorbant media.
Facilities used: Public (schools and healthcare facilities), residential, commercial (offices and warehouses), and industrial
ABsorption (not to be confused with aDsorption above) bubbles contaminated air through a chamber of water. Since many VOCs are soluble in water, the VOCs are absorbed, cleaning the air.
Advantages: In certain industrial settings where this is desirable, absorption allows the VOC to be reclaimed and distilled.
Disadvantages: Absorption is a complicated mechanism which requires a great deal of maintenance. Naturally, it also produces wastewater. As a final detriment, absorption doesn't work on those VOCs which are not soluble in water.
Facilities used: Industrial
Condensation is exactly what it sounds like; contaminated air is passed through a chamber where either the temperature is reduced, or the pressure is increased. This causes many VOCs to become liquid and fall out of the air.
Advantages: Like absorption, condensation allows the VOC to be reclaimed and distilled.
Disadvantages: Also like absorption, condensation requires high maintenance. It also doesn't work on VOCs whose boiling points are too high.
Facilities used: Industrial
Biofiltration humidifies contaminated air (if needed) and then draws it through a bioactive medium; in other words, air is passed through a porous substance (such as peat, wood chips, or straw) containing a complete ecology of microorganisms which chow down on contaminants in the humidified air.
Advantages: Biofiltration is an excellent method for treating VOCs, odors, ammonia, and chlorinated hydrocarbons. Biofiltration can be cheaper than other industrial or agricultural methods of air treatment. Plus, it has a green reputation.
Disadvantages: Biofiltration, as a living ecology, requires regular monitoring and maintenance. Rodent control may also be a factor. Sometimes, dust and debris from the bioactive medium itself can clog the fans.
Two variations on this method: Biotrickling and bioscrubbing wash contaminants out of the air using sprayers and fans. In biotrickling, contaminants are washed into a bioreactor (a chamber of water already containing microorganisms). In bioscrubbing, the contaminated water is later treated separately.
Facilities used: Industrial, agricultural
Thermal oxidizers, also known as fume incinerators, pump air into a heated chamber. The heat (around 1500°F) then speeds up the oxidation of VOCs into water and carbon dioxide.
Advantages: Burn, baby, burn. Thermal oxidation is beautiful in its simplicity. It oxidizes 99% of most VOCs. Most thermal oxidizers also make use of a heat exchange to improve energy efficiency
Disadvantages: Thermal oxidation cannot deal with concentrations of VOCs over 25%. It also can produce secondary pollutants such as nitrous oxides. Halogenated VOCs (VOCs bonded to gas molecules in the halogen group) may require additional treatment.
One variation on this method: The reason thermal oxidizers can't deal with higher concentrations of VOCs is that they might explode. Yikes! Luckily, human beings already have a wonderful technology designed to contain an explosion-the internal combustion engine. One method runs VOCs in an internal combustion engine and adds enough fuel to aid oxidation. With this method, high concentrations of VOCs can be safely removed, including halogenated VOCs.
Another variation on this method: A Catalytic Oxidizer is, as you might guess, a thermal oxidizer with a catalyst inside. (A catalyst is a substance that speeds up a chemical reaction but is not used up in said chemical reaction.) You probably already own one type of catalytic oxidizer-the catalytic converter in your car.
Facilities used: Industrial, with some limited use in residential, commercial, and public spaces
Reverse flow reactor
The reverse flow reactor is an adiabatic packed bed reactor that makes deliberate use of variable flow-feed directionals to encourage...hey, come back! This is important! Okay, this one is a bit complicated for us, too. An oversimplified summary would be this: a reverse flow reactor runs a heated, contaminated fluid (air or water) over a catalyst which is fixed in place. The catalyst prompts the VOCs to oxidize. What makes a reverse flow reactor unique though is that the direction of the fluid is changed periodically, transferring heat in such a manner that the system needs less input energy for oxidation to occur.
Advantages: Reverse flow reactors, when maintained and calibrated properly, are extremely energy efficient.
Disadvantages: Reverse flow reactors are complex systems which require an initial investment. In certain other types of packed bed reactors, a catalyst's surfaces may be covered with byproducts from a reaction. This is called catalytic poisoning. We could not verify if reverse flow reactors suffer from this problem or not.
Facilities used: Industrial
Ah, this is the method that goes out to meet those dastardly VOCs in battle, rather than waiting for the VOCs to come to them. Advanced photocatalysis uses a catalyst and oxidation, but there is no heated chamber. Instead, a shielded UV light shines on the catalyst, promoting the generation of reactive oxygen species from water vapor in the air. (Reactive oxygen species-as you might guess from the name-are oxygen-based molecules which love to grab the molecules of VOCs and pull them apart.) These reactive molecules are blown throughout both small and large rooms by a fan, deactivating VOCs proactively.
Advantages: Advanced photocatalysis is proactive. We like advanced photocatalysis so much, it's the method we use ourselves. A laboratory tested our tech on four common VOCs. After the test, formaldehyde was the same as average outdoor levels. Toluene was found to be much lower than outdoor levels. Meanwhile, acetaldehyde and benzaldehyde weren't detected at all. (Source: "Fighting Viral Spread with Air Purification Technology" by Avisha NessAiver, Medstartr Direct.)
Disadvantages: Compared to the other methods, none. UV bulbs do occasionally need to be replaced, but so do carbon filters, packed bed catalysts, etc.
One variation on this method: There is an older version of this method called photocatalytic oxidation (PCO). However, PCO is subject to catalytic poisoning, which results in the incomplete breakdown of VOCs. Older versions of PCO also produced ozone. ActivePure's proprietary cell shape and catalyst have solved these problems.
Facilities used: Public (schools and other facilities), residential, and commercial (offices and warehouses)
A few final words
We hope you found this summary of VOCs and their removal helpful. Of the non-industrial methods, we recommend advanced photocatalysis and adsorbent media. We have units available which combine both methods (and happy customers) in office buildings, warehouses, schools, and homes all over the world. Contact us to start reducing your VOC problem today.
Connect With an NCide Air Quality Expert at Info@NCide.US .