Polluted Air Treatment

10 Biofilter Rumours and Facts

There was a Time of Rumours - but now it's Time for Facts

RUMOUR #1: Biofilters need a constant airflow and a constant load of VOCs, and after plant shutdowns, the biofilter needs weeks to get up to performance again.

FACT: It’s just a rumour! Measurements on third generation biofilters prove that the well-adapted micro-organisms will “hibernate” (reduce their intensity of metabolism) after a couple of hours when the VOC concentration diminishes. Within advanced biofilter design, during these periods of non-emission of VOCs the airflow is cut down to a minimal level (stand-by operation). There is no necessity to “feed” the biological systems during these time periods, although some technical papers call for this practice. After several days of hibernation, even after a week or two, the biological system of a third generation biofilter will run at full efficiency again within at the most two hours. The level of 90% of full efficiency value is usually reached within 30 minutes.

RUMOUR #2: One should not pile the substrate of a biofilter higher than 1 m but rather spread it out over more area, and it really does not make much difference whether we use soil, mulch, wood chips, or bark.

FACT: It’s just a rumour! The very early biofilter literature indicated that one can exchange width for height in a biofilter, or vice versa, however, this is not the case because biofilters are non-linear systems. Likewise, the selection of the substrate is quite critical. Soil and mulch do not support a sufficient biological culture. Bark and conifer derived wood chips, as well as some hardwood, contain naturally occurring biocides - it would be quite paradox to use that in a biofilter! OSORNO uses special biosubstrates that are produced from regionally available raw materials.

RUMOUR #3: A biofilter is always in danger of flipping into anaerobic conditions that turn the biomass into a smelly, useless material when wet.

FACT: It’s just a rumour! Any properly designed biofilter is far from an anaerobic condition, however, inexperienced design teams may face difficulties in designing a biofilter in the way that anaerobic conditions cannot occur, not even locally. Within a third generation biofilter design, a worst-case protection is built into the system to safeguard the filter even if everything else fails. Our team has in the past been called to rescue a “dead” (non-aerated) biofilter designed by a well known competitor, and within three days the biofilter was fully active again without 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 competing technologies, however, only through poor design is the area dramatically larger. For a third generation biofilter built in Missouri in 1993 by our team, the regular biofilter design as commonly reported in the literature would have called for an area of 2,200 m² however, the calculation of our team demonstrated that only 600 m² are required - a size reduction of 73%! Upon customer request, the filter was built at 1,050 m² to allow for plant expansion. Needless to say - the system performed exceptionally well (96% to 98% measured VOC removal, design goal < 90% removal).

RUMOUR #5: Biofilters can only be used at low concentrations, well below 1,000 ppm for VOCs and at the most for 10 ppm hydrogen sulfide.

FACT: It’s just a rumour! Unfortunately, “ppm” is a very generic description of a concentration, and comparing ppm values given by different people very often means comparing apples and oranges. In biofilter applications, ppm commonly refers to the volume of the gaseous VOC (standard conditions) in air (standard conditions), with the VOC amount calculated on the basis of carbon atoms. Because detectors are mostly calibrated against propane as a calibration gas, and because propane contains 3 carbon atoms per molecule, the ppm values given by different people may differ by a factor of three.

Ordinary biofilters perform well at low concentrations, i.e. up to some hundred ppm. For higher concentrations, advanced design such as the third generation biofilter technology is necessary. Our team has shown that for most solvents or fugitive emissions, concentrations of up to 6,000 ppm (18,000 ppm in carbon equivalents!) will be destroyed efficiently.

For hydrogen sulfide, the licensee of OSORNO’s advanced biofilter technology has been told by a third party biofilter expert in 1997 that concentrations in excess of 1 ppm hydrogen sulfide “cannot be treated with a biofilter”. Not true, just another rumour! It turned out that the advanced biofilter which was installed at that site is sometimes exposed to hydrogen sulfide concentrations well in excess of 200 ppm, with continuing performance in the 99.5%+ removal range!

RUMOUR #6: The VOC removal of biofilters (“efficiency”) is poor and can hardly reach 90%.

FACT: It’s just a rumour! The track record of third generation biofilter applications shows that regularly destruction efficiencies well above 90% can be reached. Within ‘s team track record is an installations for odour/VOC removal in Germany where constantly over 6 years destruction efficiencies of 99%+ have been demonstrated. Likewise, an installation for hydrogen sulfide and odour removal in the US is currently in the fourth year of continuing removal of 99.5%+.

RUMOUR #7: Hydrocarbons cannot be destroyed efficiently in a biofilter because they are hydrophobic and cannot get into the watery biofilm.

FACT: It’s just a rumour! The rumour stems from experience with the ordinary biofilter design. Under the operating conditions of a third generation biofilter, most hydrocarbons will be destroyed. However, the hydrocarbons with very low boiling point (methane, ethane) do not have enough residence time to get biologically attacked while being adsorbed on the lipophilic membranes that are the skin of the bacteria.

RUMOUR #8: Biofilters are not useful for inorganic gases, or VOCs containing hetero-atoms such as halogen, nitrogen, or sulfur.

FACT: It’s just a rumour! Inorganic gases like ammonia, nitrous oxide, chlorine, or sulfur dioxide will be destroyed in a biofilter under the appropriate conditions. Likewise, most hetero-atom containing VOCs are destroyed in a third generation biofilter. OSORNO will advise the customer whether an advanced biofilter system can destroy a given compound.

RUMOUR #9: Biofiltration is so non-scientific and seems to be more of an art than a science.

FACT: It’s just a rumour! However, we must admit that even the most recently written textbook about biofilters is quite misleading. Our team offers all the information you need to determine if your specific problem can be solved by a biofilter.

RUMOUR #10: Biofilters are inexpensive to operate, highly reliable if properly designed, have a long lifetime, and do not generate secondary or hazardous waste.

FACT: It’s NOT a rumour! All four statements are certainly true for a biofilter with the OSORNO design.

Please let us know whether our Biofilter Rumours and Facts were useful.

Published by Osorno Enterprises, Inc., Winnipeg, Manitoba, Canada, March 2001

© H. Peter Hombach, 1994, 2016. All rights reserved

© H. Peter Hombach; 1994, 1996, 2000

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