There was a Time of Rumours – but now it’s Time for Facts


© H. Peter Hombach; 1994, 1996, 2000

Abbreviations used:
ppm  –  part per million
VOC  –  Volatile Organic Compound

10 Rumours and Facts about Biofilters

RUMOUR #1: Biofilters require a constant air flow and load of VOCs. After plant shut-downs, biofilters need weeks to get up to performance again.
FACT: It’s just a rumour! Measurements on BioScrub™ (BioDigestor®) systems demonstrate that well-adapted micro-organisms will “hibernate” (reduce their metabolic intensity) after a few hours when VOC concentrations decline. During these low emission periods, the BioScrub™ (BioDigestor®) air flow diminishes to a minimal level (stand-by operation). There is no requirement to “feed” the biological system, although some technical papers call for this practice. After a week or two of hibernation, the biological system of a BioScrub™ (BioDigestor®) will run at full efficiency again in less than two hours. A 90%  efficiency level is usually reached within 30 minutes.

RUMOUR #2:   The substrate of a biofilter should not be piled higher than 1 metre (3 ft) but rather spread out over a greater area. It really doesn’t matter whether we use soil, mulch, wood chips, or bark in a biofilter.
FACT: It’s just a rumour! Very early biofilter literature indicated that one can exchange width for height in a biofilter, or vice versa. This is not the case since biofilters are non-linear systems. Similarly, the selection of substrate is critical. Soil and mulch do not support a sufficient biological culture. Bark and conifer-derived wood chips, as well as some hardwoods, contain naturally occurring biocides. It would be foolhardy to use substances such as these in a biofilter! Osorno uses special materials called BioSubstrate™ produced from regionally available raw materials.

RUMOUR #3: A biofilter is always in danger of becoming  anaerobic, which turns biomass into a smelly, useless material when wet.
FACT: It’s just a rumour! Any properly designed biofilter operates in far from anaerobic conditions. However, inexperienced design teams may face difficulties in designing a biofilter so that anaerobic conditions never occur. With the BioScrub™ (BioDigestor®) design, worst-case protection is built into the system to safeguard the filter even if everything else fails. Our team has been called on to rescue a “dead” (non-aerated) biofilter designed by a well known competitor.  Within three days the biofilter was fully active again without requiring an exchange of the biological material.

RUMOUR #4: Biofilters need a lot of space, up to several football fields in area for high air volumes.
FACT: It’s just a rumour! It is true that biofilters need more space than other air pollution control technologies. However, with proper design, the area need not be dramatically larger. A BioScrub™ (BioDigestor®) system built in Missouri in 1993 by our team required an area of 600 m2. In comparison, the biofilter design  commonly reported in the literature would have called for an area of 2,200 m².  Our design resulted in a size reduction of 73%! Upon the customer’s request, the filter was built at 1,050 m² to allow for plant expansion. The system performed exceptionally well, exceeding the goal of > 90% removal with a measured VOC removal of 96% to 98%.

RUMOUR #5: Biofilters can only be used at low concentrations, well below 1,000 ppm for VOCs and no higher than 10 ppm for hydrogen sulphide.
FACT: It’s just a rumour! Unfortunately, “ppm” is a generic description of a concentration, and comparing ppm values given by different people often means comparing apples and oranges. In biofilter applications, ppm commonly refers to the volume of  gaseous VOC (standard conditions) in air (standard conditions), with the VOC amount calculated on the basis of carbon atoms. Because detectors are usually calibrated against propane calibration gases, and because propane contains 3 carbon atoms per molecule, the ppm values given by different people may differ by a factor of three. Our team uses ppm values consistently as values based on propane, i.e. in a comparison with carbon or methane based values, our values must be multiplied by three.
Ordinary biofilters perform well at low concentrations, i.e. up to 500 ppm. For higher concentrations, advanced designs such as the BioScrub™ (BioDigestor®) technology are required. With a BioScrub™ (BioDigestor®), most solvents or fugitive emissions, of up to 6,000 ppm (18,000 ppm in carbon equivalents!) will be destroyed efficiently.
In 1997, one of our customers was informed by a third party biofilter expert that  hydrogen sulphide concentrations > 1 ppm “cannot be treated with a biofilter”.  This was just another rumour! The BioScrub™ (BioDigestor®) system that Osorno installed consistently removed > 99.5% of the hydrogen sulphide from emissions with concentrations well in excess of 200 ppm.

RUMOUR #6:  Biofilters are poor at removing VOCs, barely reaching 90%.
FACT: It’s just a rumour! BioScrub™ (BioDigestor®) consistently demonstrate destruction efficiencies well above 90%. One system we installed for odour/VOC removal in Germany demonstarted removal efficiencies of 99%+ for more than 6 years. Similarly, an installation for hydrogen sulphide and odour removal in the U.S. operating for several years has consistently removed 99.5%+ of hydrogen sulphide.

RUMOUR #7: Hydrocarbons cannot be destroyed efficiently in a biofilter because they are hydrophobic and cannot be absorbed into the watery biofilm.
FACT: It’s just a rumour! The rumour stems from experience with ordinary biofilter designs. Under the operating conditions of a BioScrub™ (BioDigestor®) system, most hydrocarbons will be destroyed. However, hydrocarbons with very low boiling points (e.g. methane, ethane) may not have enough residence time for biological degradation through adsorption on the lipophilic bacterial membranes.

RUMOUR #8: Biofilters are not useful for inorganic gases, or VOCs containing hetero-atoms such as halogen, nitrogen, or sulphur.
FACT: It’s just a rumour! Inorganic gases such as ammonia, nitrous oxide, chlorine, and sulphur dioxide will be destroyed in a biofilter under appropriate conditions. Similarly, most hetero-atom containing VOCs are destroyed in a BioScrub™ (BioDigestor®). Osorno can advise customers on whether the BioScrub™ (BioDigestor®) can destroy a given compound.

RUMOUR #9: There is no competent source of information available regarding biofiltration as there is for other air pollution control methods and technologies.
FACT: It’s just a rumour! We admit that even the textbooks on biofilters can be misleading. Osorno offers all the information you need to determine if your specific problem can be solved by a biofilter. Simply complete a site evaluation form and submit it to our office. We will send you a proposal for a BioScrub™ (BioDigestor®) system which addresses your emission control needs, without any obligation to you.

RUMOUR #10: Properly designed biofilters are inexpensive to operate, highly reliable, have a long lifetime, and do not generate hazardous waste.
FACT: All four statements are certainly true for the BioScrub™ (BioDigestor®) design.