A Guide to Biological Air Scrubbers

Biological air-cleaning systems are an effective alternative to filtration and activated carbon in many industrial applications, especially those with ultrafine particles or highly toxic contaminants

Companies must maintain high air-quality standards, both indoors and outdoors, to ensure continued quality control and employee health and safety. Additionally, companies need to prevent environmental cross-contamination that can impact final product quality.

Air contaminants fall into many categories across different industrial sectors, including volatile organic compounds (VOCs), aerosols, particulate matter and ultra-fine (UF) particles, as well as viruses and bacteria. These contaminant types vary widely in size and properties. These dramatic variations make it difficult for a single air-cleaning technology to comprehensively handle such contaminant mixtures.

There are a number of air-purification methods encountered in industry, including activated carbon and various types of filters and vents, as well as biological air-cleaning units.

Bioreactor-based air-cleaning technology can provide comprehensive destruction by both capturing and destroying all types of airborne contaminants. While other air-cleaning systems, such as traditional filters, may struggle to remove particles as small as 0.3µm, bioreactors can offer complete contaminant destruction all the way down to 0.0001 µm, and can clean air in large facilities, such as industrial, manufacturing and laboratory sites. Bioreactor systems can be used within many industrial sectors, including: pharmaceuticals; chemical and pesticide manufacturing; animal health and agriculture; and food and flavors, among others. In many circumstances, biological air scrubbers outperform conventional air cleaners or scrubbers in both capture and destruction of airborne contaminants. Several use cases for bioreactor-based air-cleaning systems are described in more detail throughout this article.

 

How it works

A typical bioreactor air-cleaner (biological oxidizer) captures charged and uncharged particles and scrubs air with enzyme-infused water. Microorganisms are immobilized on a support structure and generate enzymes as needed. An example is shown in Figure 1, where the support is a spiral-shaped membrane. Immobilization allows the microorganisms to be re-used over and over again, making them efficient and inexpensive.

Dirty air enters the bottom and clean air exits at the top. Contaminants in the water are destroyed by oxidative enzyme action — for example, benzene can be totally converted to CO₂ and water, leaving behind no residual benzene. The water can be cleaned and recycled to the top of the scrubber, thus conserving water. Such bioreactor units are standalone systems and typically will not require a permit. The units do not use a filter and are easily cleaned, typically once every two or three months.

Water is pumped up from the bottom of the reactor, where it flows down the membrane, keeping the media wet. Water is distributed evenly from above the biosupport, exposing the contaminants to the enzymes, which are activated and begin bio-oxidation upon water contact. Non-digestible solids, such as metal dust and fibers from insulation, are washed down to the bottom of the tank for later removal.

As water flows into the tank, it forms an internal water curtain that follows Bernoulli’s Principle of fluid mechanical attraction, as well as electrical-field grounding to pull in aerosols and other contaminants for biological destruction. Enzymes subsequently and rapidly oxidize contaminants in the water. The bio-support membrane is designed to support contaminant contact with the enzymes due to turbulence created by cyclone action.

Bioreactors usually have a fan to capture those contaminants that are not electrically charged by suction into the unit. The unit is operated in a scrubber mode — that is, the air coming in is washed and scrubbed by a water stream created by a pump at the bottom of the scrubber. It is then cleaned and exits at the top of the scrubber. The pump is electrically grounded. Water is pumped from the bottom of the scrubber to a distribution plate at the top. The distribution plate spreads the water evenly over a spiral sheet, which has a large surface area. The air moves via turbulent flow with abundant back-mixing. This creates intimate and extensive air-water contact. Suction action according to Bernoulli’s Principle increases air-water contact by forming small bubbles of air that are sucked into the water. The water falls back down in the scrubber, and is re-circulated to the top.

One of the most important functions of bioreactors in air-cleaning applications is the rapid and continuous destruction of the contaminants by oxidative enzymes in the water, in real time. This allows for the cleaned water to be recirculated inside the scrubber. The need for adding fresh process water is eliminated. There is no wastewater stream.

Example: Charged contaminants. Electrically charged contaminants, or contaminants that behave like aerosols, are captured by the electrically grounded water. Some particles are so strongly charged that they have been observed to slam into the water from all directions, including moving against the exiting airflow at the top of the scrubber. For instance, in a styrene off-loading facility in Dubai, the control-room building air inlet was protected by a carbon filter and the entire building was under high pressure such that all air was discharged to the outside. Styrene was entering the building through doors in spite of the discharging air flow, causing a nuisance and health issues for the workers. Bioreactors can help in such scenarios.

 

Bioreactor enzymes

Enzymes are the workhorses of the bioreactors. They can catalyze the oxidation of contaminants at room temperature and ambient pressure. They can be generated on an as-needed basis by a consortium of microorganisms, which are typically naturally occurring, non-genetically engineered, non-toxic mixtures of several microscopic entities derived from healthy soils and lakes. The microogranisms can naturally re-arrange their population distribution to meet the needs at hand. For example, they may have one population-distribution profile when oxidizing hydrocarbons, and can change their population distribution to destroy chlorocarbons. The microorganisms are available from vendors as a bottled mixture, and can be added periodically (usually weekly or monthly) to replenish the bioreactor. Such microoganisms have been shown to be exceptionally stable at all atmospheric temperature ranges and pH ranges, just as they are robust in their natural ecosystems.

Bioreactor enzymes act very quickly in air-cleaning tasks. Table 1 demonstrates how rapidly they can destroy gasoline vapors. In the example in Table 1, gasoline vapors from an automotive facility were ducted into a small (around 30 in. tall) bioreactor. Vapors entering and exiting the bioreactor were monitored by a standard VOC analyzer. The bioreactor residence time was less than one second. About 2,000 parts per million (ppm) of hydrocarbon vapors were removed per pass, averaging around a 70% removal rate.

Engineering challenges

Air-cleaning bioreactor systems can provide comprehensive destruction of airborne contaminants by both capturing and destroying all classes of airborne contaminants, from VOCs to aerosols and bacteria and more. Air contaminants may be categorized as follows:

1. Ultrafine (UF) particles, such as oils and metals from air compressors and blowers, or from gasoline and diesel engines
2. Smoke, dust and powders that cross-contaminate other products, reducing product quality
3. VOCs, such as formaldehyde, gasoline or acetone; and semi-volatile organic compounds (SVOCs), such as organophosphates and pesticides
4. Gases like ammonia (NH₃), carbon monoxide (CO) and hydrogen sulfide (H₂S)
5. Viruses, pathogens and mold

One air-cleaning technology typically cannot handle all of these contaminants adequately. However, bioreactor air scrubbers have been shown capable of handling all of these contaminants. The Office of Aerospace Medicine and others [1– 5] note that high-efficiency particulate air (HEPA) filtration technology has the limitation of not capturing particles of less than 0.3µm, fumes and oils of less than 0.05µm, VOCs and SVOCs, gases such as CO and certain viruses, all of which are air-quality contaminants of concern. Bioreactors can overcome all of these limitations, as demonstrated by the use cases described later in this article.

In order to purify air, two engineering issues must be considered: fluid mechanics and electrical fields. The fan within the bioreactor generates an air flow, which satisfies the fluid-mechanics requirements. The pump generates an electrically grounded “waterfall” that captures the charged particles, satisfying the electrical-field issue. Once air capture is complete, these contaminants are destroyed by the enzymes, which generate fresh water that keeps the integrity of the electrical grounding. The water must be pure in order to continue to clean the air.

Air contaminants are classified into two physical forms — those that move with airflow and can be sucked into an air cleaner or pushed outside a building, and those that linger and are not blown away with fans or in a breeze. While some of the contaminants that move with normal airflow may be captured by conventional air cleaners, conventional air cleaners do not destroy them. Furthermore, lingering odors are electrically charged, behave like aerosols and act like static cling. They are not sucked into vents. Conventional technology does not capture these charged particles. Bioreactor air scrubbers can capture and destroy both types of contamination. Also, bioreactors can be installed in both indoor and outdoor applications in a wide range of scales (Figures 2 and 3). An example of an outdoor application is in the removal of VOCs that escape as large tanks are being filled. Tank vent emissions are a major concern — as tanks are being filled, highly concentrated vapors can escape, presenting safety and fire risks.

 

Ultrafine particles

UF contaminants, such as particulate matter and powders, are generated by high pressure, temperature and friction. Oil and metal UF particles are generated from the processing of steel and other metals. UF powders may stem from processing pharmaceuticals, food flavorings, additives, perfumes or paints. Such particles can cause cross-contamination when they are generated in manufacturing plants and in packaging plants that handle different products, such as drugs, paints and flavorings, for which contamination is prohibitive. Airborne UF particles from oil droplets and metals are of special concern, because chronic exposure has been shown to cause health risks to personnel [2].

Use case: Manufacturing facility. A manufacturer handling different types of metals had concerns about UF contamination affecting the indoor air quality (IAQ) at its plant. A system of five larger bioreactor units was placed in the area of particular concern, and two medium-sized units were placed outside the area to capture any fugitive emissions.

The data from this project, which ran for about 130 days, have been published in Ref. 3 and are presented briefly in Tables 2 and 3. The baseline air-quality measurements, while within regulator limits, were mainly comprising fine and ultra-fine oil particles, along with the metallic dust. Synthetic oils containing various additives were exposed to high temperatures and pressures in the process, creating particulate emissions of oil, metals and trace VOCs. Fine and UF metals and VOCs were also measured.

Within the bioreactors, oils from oil droplets, additives and oil from oil-coated metals were destroyed and rendered non-toxic by biological oxidation, while metal particles suspended in the air (both fine and ultrafine) were isolated.

Metallic fine particles generated over 130 days of continuous, real-time operation were collected from the bioreactor units, analyzed and weighed. Clean metal particles dropped to the bottom of the bioreactors and were collected for sampling. Organic contaminants were oxidized to carbon dioxide and water, leaving only the non-digestible metals in the bottom residue. The units did not require cleaning during the 130-day operation. Data from four workstations are reported in Table 2. Workers reported greatly improved air quality following installation of the bioreactor air-cleaning systems.

Baseline VOCs generated with the oil and metals ranged from 0.5 to 0.9 ppm and were reduced to non-detectable levels (<0.1 ppm). Several observations are made from the results shown in Table 3. First, about 884 g of iron were captured from the air, along with 2.7 g of chromium and 1.6 g of nickel. These are very large numbers of fine and ultrafine particles, and scrubbing them out of the air offers a great deal of relief to workers. Secondly, the approximate locations of the major sources of contamination can be easily identified amongst the site’s seven total bioreactors — for instance, Units 3 and 5 seem to be near “hot spots.” Further, it may be possible to identify the type of metal being ground by friction by studying the unique ratio of chromium, nickel and iron.

 

Smoke, dust and powders

Particulate matter from smoke, dust and powders are readily removable in the same way other UF particles are removed. These airborne particles can cause cross-contamination of products, leading to major quality-control issues.

Use case: Food and flavor production. A food and flavor manufacturer was set to begin processing a batch of mint. The mint odors were everywhere in the plant, contaminating other products and rendering other flavors, such as chocolate, cherry and vanilla, off-specification, even though the mint-processing unit was moved 300 feet away from the other flavor-processing units. Placement of a free-standing bioreactor in the corner of the area removed the mint powder dust and odor, solving the cross-contamination problem with the other products and mitigating quality-control issues.

 

VOCs and SVOCs

Volatile and toxic compounds, such as formaldehyde, methyl methacrylate and vinyl acetate, have been shown to be readily captured and destroyed, even at low concentrations, by bioreactors [3]. Other VOCs that have been shown to be captured and destroyed in bioreactors include: methyl tert-butyl ether (MTBE), which is traditionally thought to be resistant to biodegradation; petroleum naphtha; benzene, toluene and xylenes (BTX); hydrocarbons in fuels; alcohols; aldehydes; ketones in solvents; dipropylene glycol methyl ether; methylene chloride; and paint and chemical solvents.

SVOCs include compounds used in pesticides that can enter the air. These compounds are largely classified as neurotoxins, and it is thus costly and unsafe to have them contaminate air space in manufacturing facilities. In laboratory tests, bioreactors have been used to destroy various SVOCs, including organophosphates like ethion, chlorpyrifos, acephate and diazinon [3].

Use case: Aerospace manufacturer. Acrylates are commonly used in paints, coatings, latex materials, plastics and cosmetic products. An aerospace manufacturing company needed to remove methyl methacrylate from the air in its manufacturing location. A bioreactor was placed inline with the process, and a second bioreactor was placed in a free-standing location nearby to capture fugitive emissions. Following installation of the bioreactors, the site’s manager reported total compliance with respect to strict methyl methacrylate regulations.

Use case: Tank farm. Bioreactors were installed at a tank farm at a major port-terminal facility to capture and destroy vinyl acetate from tank vents. The other air-cleaning system used at the site was activated carbon, which proved to be expensive and ineffective in this application. The bioreactors reduced the emissions by 100% to non-detectable levels.

 

Gaseous air contamination

Gases such as NH₃, CO and H₂S are contaminants that can detrimentally impact manufacturing facilities and worker health. Bioreactors have been employed to mitigate these contaminants in several industrial settings.

Use case: Chemical plant. NH₃ is a toxic gas that affects the health of people and animals, and NH₃ concentrations in air are highly regulated. A major chemical company was processing ink in a 1,500-gal. batch chemical reactor. The process exhausted nearly pure (~1,000,000 ppm) NH₃ gas. The batch reactor was in the middle of a very large plant building, and the plant manager required essentially total NH₃ removal without venting to the outside. A special bioreactor system was set up for this process (Figure 4). The bioreactor captured and destroyed all of the NH₃ offgas.

Use case: Loading dock. The presence of CO at an enclosed loading dock was causing serious compliance issues. During a 10-day analysis (prior to the installation of a bioreactor) several ultra-large peaks of CO (hundreds of parts per million) and one peak of >800 ppm CO were observed as trucks came into the area to unload. Further, the number of instances where the CO level exceeded 1 ppm was far too high. Finally, the odors created in the loading area were drawn into a public area.

A bioreactor was installed at the loading dock. After the bioreactor installation, there were no ultra-large peaks of CO recorded. In addition, there was an 80% reduction of instances of CO levels above 1 ppm. Employees working in the area noted a substantial improvement in air quality with reduced odors and respiratory problems. Ref. 5 describes a relevant study of aldehyde and CO air contaminants.

Use case: Wastewater treatment plant. Bioreactors were successfully used to treat H₂S in a wastewater treatment plant. Bioreactors destroy H₂S and related sulfur-containing compounds biologically, leaving odor-less and non-toxic elemental sulfur as a residue in the bioreactor water. Below is the mechanism for the biological oxidation of H₂S to S₂:

 

2H₂S + O₂ → S₂ + 2H₂O

 

At the wastewater treatment plant, three bioreactor units were placed indoors, and one bioreactor was placed outdoors, in the layout shown in Figure 5.

Of 19 water samples taken from indoor and outdoor bioreactors, all 19 tested positive for elemental sulfur, indicating that the bioreactors were capturing and destroying fugitive H₂S emissions. Some key findings from this site are outlined below:

3,518,900 parts per billion (ppb) of H₂S were captured destroyed during the test period of 247 days. This is millions of times higher than the H₂S low-odor threshold of 0.5 ppb (developed by the Center for Disease Control’s Agency for Toxic Substances and Disease Registry). This reduction helped improve safety and eliminated odor complaints from neighboring sites

Although the site had other H₂S-control systems in place, the bioreactors still had H₂S to remove, showing that bioreactors are especially effective in sweeping up fugitive odors that other systems may miss

The outdoor bioreactor removed a total of 1,190,000 ppb over a 240-day period spanning winter to spring. Keeping in mind that all the plant outdoor chemical scrubbing systems were in full operation at the time of this test, this very substantial amount of H₂S captured and destroyed by the outdoor bioreactor is an indication of a large H₂S aerosol cloud that conventional chemical scrubbers could not handle ■

Edited by Mary Page Bailey

Acknowledgement

All images provided by authors

References

1. Day, G.A., Aircraft Cabin Bleed Air Contaminants: A Review, Federal Aviation Administration, Office of Aerospace Medicine, Washington, D.C., November 2015.

2. Jones, B.W., others, The Nature of Particulates in Aircraft Bleed Air Resulting from Oil Contamination, ASHRAE Winter Conference, January 2017.

3. Zanni, S., others, Abatement and bio-digestion of airborne contamination in precision mechanics, University of Bologna, May 2015.

4. Space, D.R., others, Experimental Determination of the Characteristics of Lubricating Oil in Bleed Air, ASHRAE Winter Conference, January 2017.

5. Golden, R., Identifying an indoor air exposure limit for formaldehyde considering both irritation and cancer hazards, Critical Reviews in Toxicology, Sept. 2011.

 

Authors

Lisa Routel is the chief strategy officer at BioOx (11 Melanie Ln Unit 23, East Hanover, NJ 07936, Phone: 301-246-0151; Website: www.bioox.us). BioOx is a biotechnology company offering a range of products, including air scrubbers, plant-growth supplements and water supplements, as well as blood-oxygen measurement technology, all of which use a patented biological technology. Routel’s career began as a research scientist in process development at Pfizer, where she optimized reaction conditions for compounds used in clinical trials. She then made a career pivot to strategy, marketing and operations following business school, and she currently leads business strategy at BioOx. She holds a B.S. in chemistry from Gettysburg College in Pennsylvania and an M.B.A. from New York University’s Stern School of Business.

 

Sam Sofer is the chief science officer at BioOx (same address as above). Sofer has more than 30 years of experience in the development and use of enzymes and microorganisms in industrial and wellness applications. He has held prestigious academic positions, as well as leadership positions in industry. His vast experience includes being an engineer at Celanese Chemical Co., professor and department director of chemical engineering and materials science at the University of Oklahoma and the Sponsored Chair of Biotechnology at the New Jersey Institute of Technology. He launched the biotechnology company BioOx, which holds over 30 patents in over 30 countries. Sam has a B.S.Ch.E. from the University of Utah, a M.S. in engineering from Texas A&M University and a Ph.D. in chemical engineering from the University of Texas at Austin.