Solving VOC and odor problems

Biofiltration systems take on air pollution challenges around the world

Biological oxidation and biofiltration technology are earning a winning track record quickly when it comes to air pollution control. How? It's simple. Biofilters operate at low ambient temperatures and do not require chemicals or supplemental fuels. This environmentally-friendly technology is cost-effective to operate, easy to maintain, and safe to use in populated areas.

Biological oxidation is proving its success in handling low concentrations of volatile organic compounds and nuisance odors at installations involving industries from food flavoring and fragrance to municipal and industrial wastewater treatment. Further, many large, well known industrial companies around the world chose biofiltration to solve their pollution discharge problems.

The process
Biofiltration see (Figure 1) eliminates unwanted chemical components from offgassing by means of biological oxidation. The biofiltration system ductwork conveys contaminated gases containing volatile organic compounds or odors to a humidification vessel. The system forces the gas to flow upward through plastic packing and falling water. This countercurrent operation saturates the contaminated air with water vapor.

During biological oxidation, organic compounds consisting of carbon, hydrogen, and oxygen degrade fully into carbon dioxide (CO₂) and water. Inorganic components, such as ammonia (NH₃) and hydrogen sulfide yield compounds that form acids. In some cases, these compounds must be neutralized.

The purified gas collects in the bottom of the biological oxidation vessel before exiting. The cleaned air then enters a process blower and discharges to the atmosphere through a stack.

Biological filter material
The performance of any biofilter is largely dependent on high quality filter media. An effective biofilter media has specific characteristics. The filter media should provide nutrients for cell growth and a high specific surface area for attached microorganisms. Also, the media should have a low gas pressure drop, promote uniform gas distribution, compaction, have proper pH, long term operation, and optimal water content.

Microorganisms
Microorganisms are the heart of a biofiltration system. Every aspect of the system should provide optimal conditions for the microorganisms. They need, among other things, carbon and energy sources to synthesize new cell material and growth.

Two groups of aerobic microorganisms--those that require oxygen for life--are important to microbial conversion of contaminants in the filter material. The first is autolithotrophic and the second is heteroorganotrophic. Autolithotrophic microorganisms use carbon--present in the off gas stream as carbon dioxide--to produce new cell material. Autolithotrophic bacteria obtain their energy by converting inorganic compounds.

The majority of microorganisms used for biofiltration are heteroorganotrophic. Heteroorganotrophic bacteria use carbon in the gaseous pollutant as sources for growth and other cell functions. Most organic compounds can be used by heteroorganotrophes. Only the rate at which the bacteria use compounds, differs.

Heteroorganotrophes in biofilters vary somewhat in nature. Some species are not capable of synthesizing the necessary compounds for growth from one, or a few, carbon sources. Additionally, microorganisms require vitamins and certain amino acids for cell production.

Both types are chemotrophic. That means that the required energy for cell function is obtained from chemical reactions. This is in contrast with phototrophic bacteria that use sunlight as an energy source.

There are only a few inorganic compounds in nature suitable for this type of degradation. Of these, only ammonia and hydrogen sulfide are important
to biofiltration.

The gaseous compounds are converted into non-gaseous salts. The salts accumulate in the filter material and, in some cases, must be rinsed from the system periodically.

Nitrosomonas and nitrobacter are the autolithotrophic organisms that convert ammonia and nitrate to nitrate and nitrite, respectively. Thiobacillus is mainly responsible for converting hydrogen sulfide to sulfate.

The most effective biofilter media consists of a mixture of specialized compost, pre-inoculated bacteria, inert spherical material, and limestone.

Specialized compost provides the necessary nutrients for the bacteria. Because of its physical structure, the compost minimizes gas pressure drop. Inert spherical material provides for a low gas pressure drop, uniform gas flow, and resists compaction over time.

The filter material should be preinoculated with specially chosen bacteria during manufacture and mixing. This ensures optimal performance and dramatically reduces acclimation periods compared with non-inoculated media. A calculated amount of limestone in the media provides buffering for each particular application.

Microorganisms in the water film oxidize the contaminants and use the energy released for maintenance of their own cell material and growth.

Moisture content
Complete removal of a target compound depends on the proper operation of the biofiltration media. The compound must be absorbed into a liquid biofilm surrounding solid media particles. For this reason, the biofilter's moisture content is extremely important.

If the biofilm is improperly formed, the absorption step will not be optimized and the biofilter's performance suffers. For the biodegradation step to be most effective, the liquid film must have the proper thickness.

If the biofilm is not thick enough or absent in some part of the media, the ultimate density of microorganisms can not be reached. If this happens, the biofilter performs below expected levels.

On the other hand, if the biofilm is too thick, oxygen does not penetrate the inner portions of the film. If this occurs, anaerobic growth forms in the inner portions of the biofilm and reduces the effectiveness of the system. In addition, excessive water plugs open pores in the media.

Anaerobic growth produces hydrogen sulfide as an extremely odorous byproduct. Anaerobic conditions and density restrictions can be eliminated if the proper moisture content is present in the media.

To ensure proper moisture content, the biofilter media must be weighed continuously. This is possible when the system incorporates support beams resting on load cells to sense the weight of the media. A programmable logic controller continuously calculates the media moisture content based on the filter material's weight. If the weight indicates the bed is below the optimal moisture content, the controller activates spray nozzles above the media bed. These water sprays add water to the media based on a predetermined duration relative to the dryness of the media.

However, this type of a moisture control system only works on a fully enclosed biofilter system. Open top type biofilters are always susceptible to periods of excessive drying and wetness of the media.

Nutrients
Diffusion from the compost particles and from dead biomass supply nutrients to the biofilm. For this reason, compost selection is critical to the success of a biofilter. The most efficient compost is high in nitrogen content and contains acceptable levels of phosphorus. Other types of nutrients are also necessary, but are only required in trace levels.

System sizing
The following information addresses the factors listed in Table 2 and explains how each factor affects system size--
the volume of media required for a particular application.

Solubility
Unlike other volatile organic compound and odor abatement technologies, biofilter function and sizing depends on many variables. For example, compounds treated in a biofilter must be water soluble. If a compound is easily transferred from the gas to the liquid phase within a biofilter, the biofilter is sized appropriately small.

However, when trying to remove slowly transferred compounds like aromatics, then the biofilter can become quite large.

As with a packed column, mass transfer into the liquid phase is controlled by a biofilter design. Consequently, water solubility of a compound largely dictates system size.

Biodegradability
Water solubility is not the only variable dictating system size. Once the compound is transferred into the liquid phase within the biofilter, the microorganisms present must oxidize the compound. The speed of this biological step depends on the compound being oxidized or biodegraded.

Molecular weight is usually an indicator of biodegradability. The higher the molecular weight, the more difficult the biodegradation. Also, aromatics are more difficult to break down than straight chain compounds.

Removal efficiency required
In a biofilter, the required removal efficiency is a major factor in determining system size. A biofilter designed to remove 20 ppm of toluene at 95 percent efficiency must be much larger than a biofilter designed for 80 percent removal. In the first case, the amount of contaminant mass that must be removed is greater than the second case. Therefore, the biofilter must have more media for additional mass transfer and biodegradation.

Number of compounds to be removed
The quantity of compounds requiring removal also affects system size. Microorganisms typically degrade more desirable compounds --those easy to break down and high energy yield--before degrading other compounds. The required volume of media does not double or triple with the addition of compounds but can increase system size significantly.

Hydrogen ion
Finally, the impact of hydrogen ion --pH--on biodegradation varies for each compound. The most common occurrences of acidic media formation involve the destruction of hydrogen sulfide that eventually produces sulfuric acid and reduces the pH of the biofilter media, specifically the pH of the biofilm.

Hydrogen sulfide degrading organisms survive at pH levels as low as 2 while volatile organic compound degrading organisms require a near neutral pH. Therefore, if volatile organic compounds must be removed in the same biofilter as hydrogen sulfide, provisions must be made to keep the biofilter neutral.

To prevent acidification, limestone is added to the media in these types of systems. The limestone reacts with the sulfate produced in the biofilter and creates gypsum. Due to the formation of gypsum, only a limited quantity of limestone can be added to a given volume of media. Therefore, the amount of lime, required to maintain a neutral media pH, can dictate the size of some systems.

Regulatory compliance
Many companies use biofiltration, not only for odors, but for volatile organic compound control. Although odor emissions are sometimes regulated in North America, most often volatile organic compound control is regulatory driven. Regulatory agencies typically mandate that 95 percent or more of volated organic compounds be removed.

Biofilters meet these demands economically when the compounds and inlet concentrations are favorable to biofiltration. Namely, gas streams containing highly water soluble compounds with inlet concentrations below 300 milligrams per cubic meter make ideal biofilter applications.

Other technologies
Thermal oxidation or catalytic technology--When is a biofilter more suitable than thermal oxidation or catalytic technology? Some simple screening tools are useful when trying to decide if an application favors thermal oxidation or biofiltration.

Thermal technologies are a better choice most of the time if the inlet concentration of the air stream is in the autothermal range--no supplemental fuel required for continuous operation. Typical autothermal concentrations for catalytic oxidizers are between 500 to 1,000 milligrams per cubic meter of contaminants. For non-catalytic systems, concentrations usually start at around 1,000 milligrams per cubic meter If the required removal efficiency is above 98 percent, thermal oxidation is the correct choice.

With biological systems, microorganisms typically degrade more desirable compounds --those easy to break down and high energy yield--before degrading other compounds.

In addition, biofilters are sized differently than thermal technologies. Biofilters are based on the amount of mass and contaminant that must be removed. The physical size of thermal oxidizers usually are not dependent on loading or contaminant type. Therefore, if the inlet concentration is low, the biofilter required to achieve the desired removal may be quite small. Consequently, a biofilter incurs less capital and operating costs than a thermal oxidizer to perform the same task.

Chemical scrubbers--When is a biofilter more suitable than a chemical scrubber? Both packed bed scrubbers and biofilters require that the compound to be removed be water soluble. The difference in the technologies occurs once the contaminant has been transferred from the gas to the liquid phase. Biofilters use microorganisms to oxidize compounds while chemical scrubbers rely on chemical reactions.

When using chemical oxidation, chemicals must be supplied to the system continuously. The continuous addition of chemical costs money while microbiological reactions do not. However, the capital investment of a scrubber is usually less than a biofilter for a given situation. Again, an exception to this is low inlet concentrations.

Another difference is liquid discharge. A chemical scrubber has a significant liquid effluent that can have a high or low pH. If this is a concern, use a biofilter with minimal liquid discharge.

Activated carbon--When is a biofilter more suitable than activated carbon? The expense associated with activated carbon is not the capital investment but in the carbon replacement and regeneration costs. Carbon systems are designed on the basis of flow and the regeneration cost is set by the inlet load to the carbon. Also, when the carbon can no longer be regenerated, it is disposed of as a chemical waste. Activated carbon achieves high removals but tends to be expensive if the inlet load is high. Biofilters are more expensive to install than carbon system but, if the inlet load is moderate to high, the payback duration of a biofilter can be quite short.

Summary
Although installed in many plants throughout Western Europe for over 15 years, industries in North America have embraced biofiltration technology only recently. Consequently, successful, long-term applications are just now gaining recognition. Potential users are realizing that biofiltration is a low cost, safe and effective solution for volatile organic compound and odor emission problems.