Biofiltration Factors

Virtually any force that affects the biofilm can affect the operation of the biofilter. Factors that can affect the biofilm include: Moisture, temperature, residence time, particulate load, nutrient availability, pH and poisons.

The impact of moisture can be easily visualized, too little and the biofilm dries up and too much and overgrowth of organisms, desired and undesired, can occur. Each biofilter is different in terms of heat generation from the biological oxidation, heat capacity in terms of the mass and type of media, ambient conditions in terms of external heat loss or absorption, biofilter housing insulation coefficients, spacing of water irrigation nozzles and the irrigation nozzles themselves. In order to account for all of these variables industrial biofilters are irrigated with an over bed spray system (OBS) that is controlled by a microprocessor, usually a programmable logic controller (PLC), that allows the user to control water application by zone, by time of day and day of week. Some processes only operate eight hours a day, five days a week, so applying water on a 24/7 schedule would result in uncontrollable biological that would blind off the media resulting in excessive pressure drop. Increased pressure drop usually means either decreased flow or channeling (unbalanced flow through the media) resulting in operational issues with the plant.

  • Temperature has a direct impact biological activity of the bacteria, in other words how fast they can digest any given molecule, how fast they move and how fast they reproduce. Temperatures above 104°F start to encourage the growth of thermophilic microorganisms that do not have the ability to degrade or are less efficient at degrading the compounds. In recent years much money and time has been spent in developing and quantifying organisms that can efficiently degrade the compounds. The below graph is a typical example of the biological activity of mesophilic bacteria. It is important to note that increased biological activity does not necessarily mean increased appetite in terms of the rate of degradation or molecules per hour degraded by a particular micro-organism.
  • In reality temperature effects in a biofilter are very difficult to predict because in addition to affecting biological activity, temperature can also affect the physicochemical properties such as solubility , diffusivity and Henry's law coefficient of the compounds in the gas stream. The chart below shows a biofilter's biological activity of bacteria versus temperature.

Like many other air pollution control technologies the longer any particular pollutant molecule (particulate matter, nitrous oxide, carbon monoxide, etc..) is within the treatment zone of the equipment the more time that equipment has to act upon that molecule. In a biofilter a longer residence time means an increased chance of any particular gaseous molecule will contact a biofilm, be absorbed into the water of that biofilm and subsequently captured and bio-degraded by an organism. Once captured the complexity of that particular compound will dictate the digestion time required to break it down into its simplest elemental constitutes (carbon dioxide, water, etc...).

Particulate matter comes in many different sizes and as such some can affect the biofilter while others do not. The two simplified vast extremes are the very small and the very large. Very small particles such as smoke, viruses and smog can pass through the biofilter without ever actually contacting a single water particle, media particle or biofilm. These particles are so small they follow the path of the flue gas and thus avoid any collisions. The very large particles such as sand, ash, sawdust and pollen can collect on the surface or fallout in the media. Over time these large particles can fill the void spaces that the air uses to travel through the media thus creating excessive pressure drop. A secondary affect can be that these large particles provide food for other organisms present in the media such as slimes and fungi with the result being the same; these organisms fill the voids resulting in excessive pressure drop. The gray area, those particles that are definitively not large and definitively not small, lies in the 1 to 10 micron range and their affect on the system can vary with each individual biofilter.

  • If, gas streams with high concentrations of a particular compound, the primary compound and low concentrations of a secondary compound then the resultant population of bacteria is geared toward the degradation of the primary compound and not the secondary compound, unless of course the secondary compound is part of the degradation path of the first compound. The graph below shows an example of how the inlet concentration has an effect on the elimination capacity of a biofilter.

  • Coupling gas concentrations with degradation rates the below diagram can be used to visualize the relationship between the two. The easier a compound is to degrade the faster it is degraded and thus does not have a chance to move deeper into the biofilm.

One of the keys to successful biological oxidation is the degradation of the compounds. In order for this degradation to take place the compound must be where the bacteria and its enzymes can affect it, which is in the biofilm that is composed mainly of water. If a compound is not very water soluble then the molecules of the compound want to stay in the vapor phase and not dissolve in the water that makes up the biofilm. Coupling solubility, gas concentrations with degradation rates the diagram below can be used to visualize the relationship between the three. Note that from the previous graph the gas concentration is considerably higher than that in the biofilm.

  • To understand how increased pressure drop in the system affects plant operations one must have a basic understanding of how to read a fan curve. In reference to the figure below CFM is the amount of airflow which is usually given in actual cubic feet per minute and SP is static pressure which is usually given in inches of water column. Point of fact there is 27.7 inches of water in 1 PSI. For a given system (fan, biofilter, bioscrubber, duct, etc..) there is a SYSTEM CURVE. This system curve is a plot of airflow (CFM) vs pressure drop (SP). Each fan will have static pressure curve (SP CURVE) developed by the manufacturer of the fan. Dropping a line straight down from the intersection of the SYSTEM CURVE and the SP Curve to the x-axis will yield the amount of airflow that the fan will move. The intersection of the drop line and the BHP CURVE will yield the brake horsepower (BHP) required by the motor to generate the air flow (shown on the second y-axis). Drawing a line left from the intersection of the SYSTEM CURVE and the SP CURVE will yield the static pressure at that intersection point.
  • It is of vital necessity the process engineer or system designer take into account each potential pressure drop source (media, packing, transitions, support grids, direction changes, entrances and exits). Failure to accurately take these pressure drops into account can result in a system that has a desired operating point outside (above or to the right) of the SP Curve or in a situation where there is not enough horsepower available. Of these two the lack of horsepower is the easiest and most economical to fix though it may require pulling new cable and conduit but when you are also considering replacing 300hp motors and variable frequency drives (VFD's) the bill can add up to hundreds of thousands of dollars very quickly.

Mechanical design in a biofilter has ramifications from the inlet nozzle all the way through to the outlet nozzle. Improper inlet design can result in air flow imbalances across the width of the biofilter that can never be corrected short of full reconstruction of the inlet. Improper packing design can result in decreased efficiency removal in the bio-scrubber section as well as increased pressure drop. Improper demister design can allow water carryover onto the media surface that encourage the growth of unwanted micro-organisms. Improper selection of the media itself can result in high pressure drops and encourage the growth unwanted micro-organisms as well such as fungus. Improper outlet design can result in excessive pressure drop (ie in the 3" to 5" range).

Various instrumentation, such as temperature, pressure, pH and humidity, are required on biofilter to monitor and predict the performance of the bacteria. A sample Process and Instrumentation Diagram (PID) is shown