Evaluating Biofiltration for Air Pollution Control

Evaluating Biofiltration

Interest in biofiltration is growing, but literature about this technology often is not specific about the applications or gas streams that it can effectively treat. This overview discusses several possible applications and may lead a company to an alternative way of solving a VOC emissions compliance or odor problem.

As biofiltration captures the interest of U.S. Industry, many companies are trying to determine if they can apply the technology to their air emissions. When they investigate biofiltration, companies often discover a wide range of literature on the subject but few answers on the applicability to their specific gas streams. And, for many large companies, research is initiated. Industry organizations such as the Petroleum Environmental Research Forum, financed by major oil companies and others, are investigating the applicability in the petrochemical industry. The furniture manufacturing industry has financed biofiltration pilot research. In the wood industry, Weyerhaeuser has conducted extensive research.

Without waiting for results from research of others, individual companies often rely on pilot studies to determine the applicability and design for them. Before committing to a pilot study, an upfront evaluation will answer some basic mechanical and financial decisions about biofiltration and whether it will suit your company.

Establish the Objective

Biofiltration accomplishes the same objective as incineration, except the process biologically, rather than thermally, oxidizes the volatile organic compounds (VOCs) to CO2 and H2O. In the case of compounds containing sulfur, nitrogen or chlorine, the oxidation products are mineral salts. The primary interest in biofiltration today is to reduce hazardous air pollutants (HAPs) for regulatory compliance or odor control. The technology is usually applied to gas streams with dilute concentrations, typically 1,500 ppm or less, but sometimes as high as 5,000 ppm. Suitability is highly dependent on the mix of compounds, which is discussed in detail later in this article.

Beyond this general objective, the answer to several questions may further point to the applicability of biofiltration in lieu of conventional approaches:

· Do removals have to be reported to a regulatory authority?
· What are the removal requirements?
· How are the removal requirements measured? Is it EPA method 25, no odors past the property line, or gam chromatograph/mass spectrograph (GC/MS) tests to determine component specification of the gas stream?
· Do compliance reports have to be filed with regulatory authorities?

Odor Control Objectives

Companies in search of odor abatement fall into a wide range of applications.

Public odor complaints from VOC emissions usually drive the search for abatement equipment. The VOCs may not be considered hazardous and, therefore, regulatory compliance is not necessary. However, local government authorities have the power to stop production under nuisance laws. The attempted solutions often take two extremes.

The Home-Built Abatement Remedy. An animal rendering plant, waste water treatment facility of pet food manufacturer is located in a remote area. Their production emits a very unpleasant odor but is located in a sparsely populated area. As a good neighbor, they want to provide odor abatement without experiencing the high cost of incineration or the water treatment headaches of chemical scrubbers. It is in this arena that crude home-built biofilters are often employed. These systems can be inexpensive to construct and operate and can be quite effective, but they still must be properly engineered to meet air treatment objectives.

In its simplest form, the biofilter involves a hole that is excavated to distribute the flue gas, and peat moss, pine bark or gravel to cover the piping. Air moves through the medium with the assistance of a fan. While this system can provide odor abatement, the removal efficiencies vary substantially. Rain, snow, freezing and dehydration all wreak havoc on these systems. Furthermore, leachate is a concern to regulatory authorities.

The Sophisticated System. A flavor or fragrance producer is located in a large metropolitan area surrounded by residential development. Again, there are no requirements to reduce HAPs. However, the municipality or state permitting requires "no odors past the property line". In this situation, consistent removal efficiencies must be maintained or local authorities can shut down production.

Under these circumstances, the biofilters must be sophisticated systems which have: 1) totally enclosed insulated reactors, 2) pretreatment systems for particulate removal, humidification and temperature conditioning, 3) program logic controlled (PLC) operating systems to maintain the proper moisture content of the medium and to log data, 4) filter media to provide the most effective biodegradation at the lowest pressure drop, and 5) test ports to measure the inlet and outlet concentration.

VOC Compliance Objectives

Biofilters used to comply with VOC regulatory limits fall into two primary categories.

Based on a Limit of Emissions of X Lbs./Yr. They wish to increase production which would push them over their permitting requirements. If they can obtain 50% to 75% removal of specific gas streams they can proceed with plant expansion.

In this situation, the biofilter must operate on a consistent reliable basis. Demonstrating compliance may require semiannual or annual method 25 a testing.

Based on a requirement to remove 95% of VOCs and demonstrate this compliance by continuous emission monitors. In either case, realize that most regulatory authorities are not yet well educated in biofiltration technology. This is due mainly to the lack of operating data. This deficiency will change as more commercial applications evolve and as more projects post longer operating records substantiating the technology. However, some of the existing crude biofilter designs and failures have created doubts for the regulators. Therefore, if you are considering a biofilter, expect more stringent requirements from both construction and demonstration of the system's capabilities and compliance.

In addition, regulatory authorities in each state are implementing different requirements. While your project will have to be built to suite requirements of your state and locality, the following list provides some of the potential requirements:

· Design must not allow leachate from the biofilter condensate to contaminate the surrounding soil or groundwater.
· Limiting blowdown or preserving the objective of zero discharges are concerns in some industries, and some biofilter designs may have potential for discharging water occasionally from the humidification system.
· Operating reports must demonstrate the biofilter consistently operating within temperature specifications and the inlet flue gas had 90%+ relative humidity.
· Inlet and outlet test results (EPA method 25a) must prove the removal efficiency. System must be enclosed. Spot testing of open top systems is not satisfactory.
· Filter media must be protected from the elements.

Develop a Gas Stream Specification

After determining your objective, another major step in assessing a biofiltration system involves developing a gas stream specification. The following items are important:

· VOC specification. Use of flame ionization detector (FID) measurements which show the minimum, maximum and average concentration over an extended period of time. GC/MS measurements which show each specific compound and the respective concentration.
· Temperature.
· Relative humidity
· Particulate loading, including condensables.
· Air flow and volume to be treated, in standard cubic feet per minute (scfm) and actual cubic feet per minute (acfm).

Is Biofiltration Applicable?

In most cases, biofiltration can biologically remove the pollutants from the gas exhaust, but the "size" (residence time) - and therefore the capital cost - may not be economically feasible. Using the gas stream specifications, the following guidelines will apply:

Accurate VOC Specifications Must Be Developed. The cost of a biofiltration system will depend upon the size required for the specified removal efficiency. Biofiltration generally is applied to gas streams with less than 1,000 parts per million (ppm) of VOCs, and most commercial applications are found treating streams with VOC concentrations in the 5 to 500 ppm range. This is highly dependent upon the chemical compounds and the mass loading. Different compounds require different residence times; therefore, flow rate and concentration are necessary to make a preliminary assessment.

Alcohols biodegrade very quickly, followed by ketones, then straight chain alkanes and finally aromatics (such as benzene) which take the longest oxidation time. For instance, a 50,000 cfm gas stream containing 50 ppm of ethyl benzene can be abated by biofiltration. However, the four minute - plus resistance time needed to accomplish the task is not economically feasible. The same 50,000 cfm containing 50 ppm of methyl ethyl ketone can be abated by the biofilter in 25 seconds. This presents an economic advantage over competing abatement technologies.

In gas streams with numerous compounds, the bacteria will biodegrade the simple compounds first and then move to the more complex compounds. Here GC/MS data may be required to determine which compounds are removed and which are not. Batch processes which have varying concentration levels (15 to 400 ppm) over a number of days must be properly sized. Continuous monitoring by FID can assist in the preliminary evaluation. The bacteria used to biologically oxidize the VOCs will adapt to the average concentration. If the cycle is short in duration (several times each hour) the biological reactor can usually handle the fluctuating concentration.

Spray booths in the furniture and automotive finishing industry provide a good example. A small furniture manufacturer or local body shop may operate the spray booths for two hours in the morning and two hours in the afternoon. For 20 hours each day the biological reactor does not receive a food source. For four hours each day it receives a spike loading of 800 ppm comprised of toluene, xylene, solvent naphtha. In this situation, the overall removal efficiency of the system is poor. However, if the same two spray booths operate anywhere from eight to 24 hours per day in assembly line production, the concentration level remains more constant, and the biofilter can provide excellent results.

Acidifying components such as H2S methylene chloride or ammonia will generate acids (sulfuric acid, hydrochloric acid, nitric acid) through their oxidation pathway. Over time, the neutralizing agents in the filter medium will be exhausted and the medium will have to be replaced. Again, the mass loading is the critical parameter. An H2S concentration of 200 ppm may present acidification or pH problems in a matter of months. A 5 concentration may allow the filter bed to operate at a neutral pH for five years.

To solve the acidification problems, the biofiltration industry is developing "trickling filters" that control pH and nutrients with chemical dosing pumps. In this variation, the bacteria is grown on inorganic packing which provide a longer filter life.

Temperature Specifications Are Essential to the Design. Most biological reactors today are operating with mesophilic bacteria. Simply stated, they operate between 50°F and 105°F. The higher temperatures (100°F) produce a more active biofilter and less residence time is required. Above 105°F the bacteria and the removal efficiency begin to die off. Lower temperatures reduce the metabolic rate of the bacteria requiring more filter volume to accomplish the same removal efficiency.

There are many commercial biofilters installed to treat exhaust gases outside of this temperature specification. Installations in cold environments use excess steam to heat the flue gas. Biofilters installed on even exhaust (400°F) use both evaporative cooling and forced cooling to bring the flue gas into the effective range for biofiltration. Economics always provides the final answer after deciding the energy cost associated in conditioning the gas temperature.

Relative Humidity Measurements Provide Information for Pretreatment. Removal efficiency is directly related to the biofilter's bacteria count which, in turn, is directly related to the filter medium's moisture content. If a dry flue gas is injected into the biological reactor it will quickly remove the medium's moisture and the bacteria count will drop, as will the VOC removal efficiency. Therefore, prudent design for the biofilter brings the inlet flue gas close to saturation.

Depending on the plant configuration, relative humidity can change with ambient conditions. Humidity measurements should be logging in different seasons, times of the day and during process changes. The moisture content of the gas will play a large role in the inlet temperature to the biofilter.

The flue gas should be brought to saturation, below 105°F and above 60°F. If the wet bulb temperature is within this range, simple adiabatic cooling can be designed. Usually, a simple packed tower or quench duct will be sufficient. If the wet bulb temperature is above 105°F, forced cooling must be designed (plate and frame heat exchanger with cooling tower) and condensate water will have to be disposed of. If the wet bulb temperature is below 50°F, designs to heat the flue gas (steam, natural gas heaters) will have to be incorporated.

Particulate Loading Can Restrict the air Flow. If the particulate loading is water soluble and biodegradable, 0.015 grains per dry standard cubic foot (gr/dscf) may be allowed. If the particulate loading is not water soluable and not biodegradable, a minimum of 0.004 gr/dscf is allowed.

Both the filterable particulate (front half) and the condensable particulate (back half) - determined using EPA method 5 - should be reviewed. Biofiltration medias are relatively dense. Obviously, dust loading can create a flow restriction; however, depending upon the composition, the condensable particulates can create just as many problems.

In the paint industry, dried over spray is a focal point. In the animal byproducts industry, condensing fats and oils can create flow restrictions. In the wood industry, pine tars can form. The flavor and fragrance industry uses spray dryers which can promote dust carryover on the filter bed. Polymerizing compounds in the petrochemical industry can also form agglomerations on filter media.

To address these issues, create a biofiltration system vendors have incorporated pretreatment venturi scrubbers for both humidification and dust removal. Wet electrostatic precipitators have been employed, as well as temperature control to maintain the flue gas above the condensable polymerization range.

Gas Flow Measurements Complete the Preliminary Analysis. The gas flow rate is the final biofilter sizing parameter. With the concentration specifications, the mass loading can be determined. There are two primary sizing parameters - reaction and diffusion. Diffusion refers to the ability of the chemical compound to diffuse into the biofilm allowing the bacteria to begin the oxidation process. High air flows with dilute concentrations can require a longer residence time than a low volume gas flow, which is more concentrated.

This is prevalent in boat-building, where styrene is used as a solvent in fiberglass production. To meet OSHA standards, manufacturers have maintained internal air below 50 to 100 ppm by using numerous ceiling fans. Now that styrene has been identified as a HAP, boat manufacturers must find abatement equipment. A biofilter installed to handle 100,000 cfm with 50 ppm styrene can be reduced significantly to size and cost by redesigning the air collection system. Reducing the flow to 50,000 cfm while concentrating the styrene to 100 ppm provides the same mass loading. However, the capital cost is reduced substantially because the diffusion limitation is not as great. The large air flow may reduce the mass transfer driving force on the bed requiring more residence time to accomplish the same objective.

Compare Technologies

Companies searching for cost-effective air scrubbing alternatives should look at various technologies when exploring biofiltration. Operating cost is the key factor that may support feasibility of biofiltration vis-à-vis other technologies, even when capital costs may favor other technologies or appear to show no clear choice. The evaluation should include obtaining quotes for chemical scrubbers, incineration and carbon absorption. It should also include a five-year projection that assesses power consumption, regenerating cost for carbon, fuel costs for incineration and filter replacement for biofilters.

An analysis recently completed for the printing industry provides an example. On a gas stream of 4,000 cfm, the installation cost for a biofilter was $246,000, regenerative thermal oxidizer technology would be $200,000, and a recuperative catalytic oxidizer would cost about $300,000. However, bringing in annual costs, such as utilities and fuel and filter replacement costs, biofilter operation would cost only $4,300 annually, compared to $28,900 for RTO (regenerative thermal oxidation) and $42,300 for catalytic oxidation. The "all-in" costs for five years of operation end up favoring biofiltration. In addition, the cost gap can increase significantly for systems treating higher volume gas flows.

Consider pilot studies, especially for compounds or gas streams which have not been previously tested. They provide essential design criteria. Several biofiltration vendors encourage the approach by supplying pilot units for rent. However, the filtration medias can vary substantially in residence time, pressure drop and replacement life.

A period of two to six months is a reasonable time to accumulate the data needed for a prudent design. A proper blower design requires knowledge of the pressure drop over the media. The design also has a account for compaction rates of the medium from biodegradation. Expenditures will come from gas stream testing for odor (dilution olfactometer/odor panels), continuous FID monitoring or potentially GC/MS analysis for VOC applications.

If your gas stream still appears to be a candidate after reviewing the capital and operating costs and evaluating a possible system based on issues discussed above, an investigation of different designs available on the market is warranted.

For more information, contact Scot Standefer, president of PPC Biofilter, 3000 E. Marshall Avenue, Longview, TX 75601; phone (903) 758-3395; fax (903) 758-6487.

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