Frequently Asked Questions
What is the difference between a biofilter and a biotrickling filter?
What determines the applicability of biological treatment technologies?
Various specific question in the order I got them by e-mail
If you use an air pump or a blower, what type is best suitable for a bench-scale biofilter?
What manufacturers are selling the latest thinking in biofilters (would that be 4th generation?)
We were originally thinking of a water scrubber to humidify air to a compost biofilter.
Answer: A biofilter is a biological reactor where polluted air can be biologically treated. The principle of a biofilter is as follows: polluted air is first humidified and then passed through a packed bed of compost, wood chips, activated carbon, etc. or a mixture of these ingredients with mineral nutrients. At the surface of the packing, pollutant degrading microorganisms are present, and degrade absorbed pollutant vapors. The packed bed is usually 1 - 1.5 m. in height, and the residence time of the air between 5 seconds and 3 minutes. Biofilters can be as large as a basketball field, small-scale units are used in my laboratory for research.
In summary, biofilters are low temperature bio-catalytic converters for volatile organic compounds (VOCs), odors or other air toxics.
Answer: A biotrickling filter is a biological reactor where polluted air can be biologically treated. The principle of a biotrickling filter is as follows: waste air is contacted with a scrubbing solution (usually water) in a column packed with random plastic material (Rashig or Pall rings, balls etc.), lava rock, open pore foam or other inorganic support material. On the surface of the packing, a biofilm of pollutant degrading microorganisms forms which aerobically degrade the absorbed pollutants. Since the packing is made of an inert material, a continuous supply of mineral nutrient (N, P, K, +traces) is needed.
In summary, a biotrickling filter is a biological scrubber where simultaneous absorption and biodegradation is achieved.
Answer: A biofilter does not have a free water phase. The air to be treated is simply humidified to near saturation, and passed through a bed of compost, peat, etc. Under optimum condition, no wastewater is produced. Also, in a biofilter, generally no nutrient (N, P, K) additions are needed.
Answer: This is a highly debated question and reports differ on the usefulness of nutrient additions. My personal opinion is that nutrients can temporarily increase removal performance, but over the long run, continuous nutrient additions are detrimental because they stimulate biomass formation and clogging of the reactor will occur in the same manner as clogging occurs in a biotrickling filter (remember that 1 g of N will result in approximately 10 g of dry weight of biomass, i.e., roughly 100 g of wet biomass!). I sometimes observed increases in performance after nutrient additions, however this was not reproducible. On the other hand, biofilter performance often deceases over the long run. Periodic additions of nutrients do help in maintaining steady performance.
Answer: The criteria for application in a biofilter or biotrickling filter are:
The pollutants should be biodegradable
The pollutants should be somewhat water soluble (in fact it's the Henry coefficient which is important), although this is not an absolute rule because compounds such as pinene or hexane have been well degraded and have unfavorable partition coefficients
The biodegradation does not lead to toxic end-products (most important in biofilters, since in biotrickling filters one can flush the end product via the purge)
Last criterion would be "someone has done it before..."
I think that one does not stress enough the fact that a very good problem definition should be made before applying any control technology. This should include air flow, temperature, concentration, composition of contamination, fluctuations in emissions patterns, presence of grease, dust, etc.
Various specific questions
1. What is the most effective way to force the mixture of air and contaminant into a bench-scale biofilter?Answer: The easiest is to have compressed air (lab air), and to use flow meters to regulate the air flow. If compressed air is not available, a diaphragm pump ($150) will work. Always have a spare diaphragm, since they wear off quickly, especially if the pump is used at a lower flow than what it is rated for. Maybe you can find a cheap aquarium air pump with a high enough flow rate.
Answer: See above, blowers are not appropriate for low flow (they don't provide the head). So unless your bench-scale is >16" ID (40 cm), which I do not recommend, use diaphragm pump. Make sure the air is clean (no oil) if the air is dirty, place a small cartridge with activated carbon (Use a piece of pipe, and fill with charcoal + glass wool at both ends) to trap the oil.
Answer: I recommend that you use an internal diameter of 2" to 6" (5 - 15 cm). Smaller is difficult, larger gets expensive. Best to start is about 3-4" (7 - 10 cm). Use a clear material if you have: glass (expensive, breakable), plexiglass (relatively cheap, easy to glue), or clear PVC (my favorite, relatively inexpensive, so easy to glue and find fittings); if budget is a real issue, use schedule 40 PVC, very cheap but you can't see through it, a big disadvantage. A reasonable range of residence time is 10-60 seconds (empty bed basis = bed volume/air flow) depending on the pollutant.
Answer: Have a prehumidification. We use porous stone (aquarium type), and a large carboy (10-20 gallons) filled with water were we sparge the water before we add the pollutant. We heat the carboy to 26-30 C, but if you don't heat, have 2 humidification in series, or make sure the bed does not dry. You may want to add a little bit of water (spray is best) on top of your bed, every 2-5 days.
Answer: 40-70% moisture on a wet weight basis (1 kg of medium has 400-700 g water)
Answer: I suggest that you work at room temperature. I have done various experiments at higher temperature, but it is difficult (heated enclosure, heating coils), and maintaining proper moisture is a headache.
Answer: See above answer, if you work at room temperature and reasonable loads, the temperature won't increase much (a few degrees, if any), if temperature increase is noticed, make sure that you add sufficient water to avoid dry out.
Answer: I am not expert in ammonia removal, but it makes sense to buffer you medium with some crushed oysters, or limestone to make sure that the pH remains neutral. Some biofilters actually work better at low pH (hydrogen sulfide removal) it depends on your objectives and on your pollutants.
Answer: Add buffer (see above) and make sure you monitor the pH of the leachate for any change. You may also measure the pH of the packing, but it is difficult to take a sample in lab scale biofilter without disturbing the experiment.
Answer: Make sure that you have good connections, monitor pressure drop (U tube with water) since you will use soil as packing; always ask yourself questions about what you observe, and the possible reasons for the observed phenomena. Biofilters are simple, but the underlying principles are complex.
Answer: Biofilters are definitely working in cold climates. Pollutant removal may be slower at low temperatures, so the options are 1) to heat the air and insulate you reactor (gets expensive), 2) have a less efficient reactor in winter (thus bigger = more costly). The right balance needs a cost-benefits analysis. In general, biofilters for odor control are less sensitive to low temperature than biofilters for VOC control. You might want to find out vendors who have installed biofilters in cold climate and talk to their customers.
Answer: Electricity, steam, natural gas, whatever is available. If there is some heat that can be recovered somewhere an used to heat the airstream, that's even better.
Answer: I personally do not classify biofilters by their genealogy. The biofilter generation is not an indication on how good it is, I take it as a vendor marketing operation. There are good simple systems, and bad high tech systems (and vice versa). Performance depends who designs the biofilter, and who operates it.
Answer: You are on the right track, it depends on you raw air temperature and relative humidity, the temperature and loading of you biofilter, and finally the size of your scrubber. I understand the scrubber are existing, they may or may not be appropriate. Maybe additional sprinklers on top of the bed might be sufficient. There is some good information on air humidifier choice in our book, and an excellent discussion of moisture control in van Lith, C., Leson, G., and Michelsen, R., Evaluating design options for biofilters, Journal of the Air & Waste Management Association, 47(1), 37-48, 1997.