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How Activated Carbon Filtration Removes VOCs from Air and Water
Activated carbon filtration is commonly used to remove volatile organic compounds, or VOCs, from contaminated air and water during environmental remediation projects.
The same basic treatment principle applies to both vapor-phase and liquid-phase systems: contaminated air or water passes through a vessel containing activated carbon, and organic contaminants are retained on the carbon’s internal surface.
What Are Volatile Organic Compounds?
Volatile organic compounds are carbon-based chemicals that can evaporate into the air under normal environmental conditions. Many VOCs are associated with fuels, solvents, degreasers, industrial chemicals, coatings, and petroleum products.
VOCs may be found in contaminated soil gas, extracted subsurface vapors, groundwater, process water, air-stripper exhaust, and industrial ventilation streams.
Examples of contaminants that may be evaluated for activated carbon treatment include:
- Benzene, toluene, ethylbenzene, and xylenes
- Gasoline and petroleum-related hydrocarbons
- Trichloroethylene and tetrachloroethylene
- Other chlorinated solvents
- Industrial solvents and cleaning compounds
- MTBE and other fuel-related compounds
- Odor-producing organic compounds
Activated carbon performance varies by compound. A carbon product that works well for one contaminant may have less capacity for another, so the treatment media should be selected for the project’s specific contaminant profile.
How Does Activated Carbon Work?
Activated carbon is manufactured to contain an extensive network of pores. These pores create a very large internal surface area within a relatively small amount of carbon.
As contaminated air or water flows through the carbon bed, molecules are attracted to and retained on the carbon surface. This process is called adsorption .
Activated carbon generally captures contaminants rather than chemically destroying them. Once the available adsorption capacity has been used, the carbon must be replaced, regenerated, reactivated, or otherwise managed according to the application and applicable requirements.
How Does Vapor-Phase Carbon Filtration Work?
Vapor-phase carbon systems treat contaminated air. They are often connected to soil vapor extraction equipment, vacuum blowers, air strippers, tank vents, process exhaust systems, and other sources of VOC-containing vapor.
The contaminated vapor stream enters a carbon vessel and passes through a bed of activated carbon. VOC molecules are adsorbed onto the carbon while the treated air continues toward the system outlet.
Common vapor-phase applications
- Soil vapor extraction system exhaust
- Air-sparging and vapor-extraction projects
- Air-stripper off-gas treatment
- Storage-tank and process vent treatment
- Odor and organic-vapor control
- Temporary treatment during pilot studies
- Polishing treatment downstream of other equipment
Vapor-phase systems may use one carbon vessel or multiple vessels arranged in series. A common lead-and-lag arrangement directs the vapor through a primary vessel and then through a secondary vessel.
The secondary vessel provides additional treatment if contaminants begin passing through the primary vessel. It can also provide an opportunity to monitor the primary vessel before VOCs reach the final system outlet.
How Does Liquid-Phase Carbon Filtration Work?
Liquid-phase activated carbon systems treat contaminated water. Groundwater or process water is pumped through a vessel containing granular activated carbon or another selected filtration medium.
As the water moves through the carbon bed, dissolved organic contaminants are adsorbed onto the carbon. The treated water then exits the vessel for discharge, reuse, additional treatment, or another project-specific destination.
Common liquid-phase applications
- Groundwater remediation
- Pump-and-treat systems
- Industrial process-water treatment
- Construction dewatering treatment
- Air-stripper water polishing
- Petroleum-hydrocarbon removal
- Solvent and chlorinated-VOC treatment
- MTBE treatment using specialized carbon media
Liquid treatment systems may include particulate filters, oil-water separation, air stripping, specialty media, or other pretreatment stages before the water enters the activated carbon. Effective pretreatment can help protect the carbon bed from suspended solids, oil, biological growth, and other materials that may reduce performance.
Vapor-Phase vs. Liquid-Phase Carbon Filtration
| Selection Factor | Vapor-Phase Filtration | Liquid-Phase Filtration |
|---|---|---|
| Treatment stream | Air, soil gas, or process vapor | Groundwater, process water, or wastewater |
| Typical measurement | Airflow in CFM | Water flow in GPM |
| Typical equipment | Drums, radial vessels, or tank-based adsorbers | Drums, pressure vessels, tanks, or treatment skids |
| Common pretreatment | Moisture separation and particulate filtration | Solids filtration and oil or sediment removal |
| Important operating factors | Humidity, temperature, pressure drop, and airflow | Contact time, pressure, water chemistry, and flow rate |
| Performance monitoring | VOC sampling at vessel outlets | Water sampling before, between, and after vessels |
What Determines Carbon Filtration Performance?
Activated carbon filtration is not a one-size-fits-all process. Several operating and design conditions affect contaminant removal, carbon usage, and system service life.
1. Contaminant type
Different compounds have different adsorption characteristics. The contaminant list should be reviewed before selecting the type of carbon or specialty media.
2. Contaminant concentration
Higher contaminant loading generally uses the available carbon capacity more quickly. Concentrations may also change during a remediation project, so both normal and maximum expected values should be considered.
3. Airflow or water flow rate
The flow rate affects how long the contaminated stream remains in contact with the carbon. A vessel must be sized to provide sufficient treatment while maintaining an acceptable pressure drop.
4. Contact time
The carbon bed must provide adequate contact between the contaminants and the filtration media. Excessive flow through an undersized vessel may reduce treatment performance.
5. Temperature
Temperature can affect vapor-phase adsorption capacity. Hot vapor streams may need to be cooled before entering the carbon vessels.
6. Moisture and humidity
Excessive moisture can occupy available pore space and interfere with the adsorption of some VOCs. Vapor systems may require moisture separators, condensate management, or other pretreatment.
7. Suspended solids and oils
Solids, oils, and biological material can foul a liquid-phase carbon bed. Pretreatment may extend the service life of the carbon and reduce pressure-related operating problems.
8. Carbon type and pore structure
Activated carbon can be produced from several raw materials and manufactured with different pore structures. Carbon should be chosen according to the contaminants, treatment stream, required effluent quality, and project conditions.
What Is Carbon Breakthrough?
Carbon breakthrough occurs when contaminants begin appearing at a monitoring point downstream of the carbon bed. This indicates that the carbon is no longer retaining the contaminant as effectively as required at that location.
Breakthrough does not always occur suddenly. Contaminant concentrations at the outlet may rise gradually as the available carbon capacity is used.
Sampling ports between lead and lag vessels can provide an early indication that the primary vessel is approaching the end of its useful service cycle. The vessels can then be changed, rotated, or serviced according to the treatment plan.
Why Are Two Carbon Vessels Often Used?
Two vessels arranged in series provide a lead-and-lag treatment configuration.
- 1. Lead vessel: Receives the untreated air or water and adsorbs most of the contaminant loading.
- 2. Lag vessel: Provides additional treatment and protection if breakthrough begins in the lead vessel.
When the lead vessel is exhausted, it can be removed for service. The lag vessel may be moved into the lead position, and a vessel containing fresh or reactivated carbon can be installed in the lag position.
The exact vessel arrangement and operating procedure should be established according to the project’s treatment requirements and approved system design.
Virgin Carbon vs. Reactivated Carbon
Virgin activated carbon is newly manufactured carbon that has not previously been used in another treatment application.
Reactivated carbon is previously used carbon that has been processed to restore a portion of its adsorption capacity. It may provide a cost-effective option for appropriate environmental and industrial applications.
Selection should consider the contaminant, required water or air quality, regulatory requirements, carbon specifications, project economics, and the intended use of the treated stream.
When Is Specialty Filtration Media Needed?
Standard activated carbon is not necessarily the best treatment medium for every contaminant. Specialty carbon or other filtration media may be selected for weakly adsorbed compounds, metals, petroleum products, emulsified oils, iron, manganese, hydrogen sulfide, and other treatment challenges.
Examples of available treatment media may include:
- Virgin activated carbon
- Reactivated activated carbon
- MTBE-removal carbon
- Polymer-enhanced carbon
- Organically modified clay
- Heavy-metal-removal media
- Manganese greensand
- Anthracite and filter sand
Laboratory testing, pilot studies, carbon usage estimates, and media recommendations can help determine which treatment method is most appropriate.
Activated Carbon vs. Thermal Oxidation
Carbon adsorption and thermal oxidation can both be used to treat VOC-containing vapor streams, but they use different treatment mechanisms.
| Comparison | Activated Carbon | Thermal or Catalytic Oxidation |
|---|---|---|
| Treatment method | Adsorbs contaminants onto filtration media | Oxidizes organic contaminants at elevated temperature |
| Residual material | Spent carbon requires management or reactivation | Combustion byproducts leave with the treated exhaust |
| Energy requirement | Primarily blower or pumping energy | Requires energy to heat the vapor stream |
| Common use | Lower or moderate contaminant loading and polishing | Continuous vapor treatment or higher organic loading |
| Primary maintenance concern | Carbon monitoring and media change-out | Heating system, controls, catalyst, and combustion equipment |
The best choice depends on contaminant concentrations, airflow, project duration, available utilities, disposal requirements, operating costs, permit conditions, and anticipated changes in the remediation process.
Some projects use both technologies. Carbon may be installed as a polishing stage, temporary backup, startup treatment method, or supplemental control device.
Questions to Answer Before Selecting a Carbon System
- 1. Is the treatment stream air, soil gas, or water?
- 2. Which contaminants are present?
- 3. What are the average and maximum concentrations?
- 4. What airflow in CFM or water flow in GPM is required?
- 5. What operating pressure or vacuum must the vessel withstand?
- 6. Are moisture, suspended solids, or oils present?
- 7. What outlet concentration or treatment objective must be met?
- 8. Will one vessel or a lead-and-lag system be required?
- 9. How frequently will sampling be performed?
- 10. Is rental equipment or purchased equipment more appropriate?
- 11. Who will perform the carbon change-out?
- 12. How will the spent media be characterized and managed?
Should You Rent or Purchase Carbon Filtration Equipment?
Rental carbon vessels may be appropriate for pilot studies, temporary treatment, emergency response, construction dewatering, short-term remediation projects, or systems with uncertain initial loading.
Purchasing may be more economical for long-term operation, recurring treatment needs, permanently installed systems, or projects requiring custom controls, manifolds, backwash systems, skid mounting, or specialized engineering.
Evaluating the total project cost should include equipment, filtration media, freight, installation, sampling, pressure drop, carbon change-outs, spent-media handling, labor, and potential system downtime.
How Mako Industries Can Help
Mako Industries supplies carbon-filtration vessels, treatment media, packaged systems, rental equipment, and carbon change-out services for environmental remediation projects.
Available configurations include vapor-phase and liquid-phase systems designed for different flow rates, pressures, vacuum conditions, contaminants, and site requirements.
Mako Industries can also assist with treatment-media selection, vessel sizing, custom skid packages, piping manifolds, controls, startup, training, delivery, pickup, and integration with existing remediation systems.