Biofiltration Fundamentals

How Does Industrial Biofilter Work?

In the simplest terms, a biofilter is a device that utilizes natural biological oxidation for the destruction and / or removal of hydrocarbons (or CS2, H2S, NH3), that is to say biofiltration is the degradation of organic and inorganic substances by microorganisms. These microorganisms live in a biofilm coating that resides on the surface of a media composed of organic or inorganic (or a combination thereof) matter. The micro-organisms are stationary in regards to the system as a whole though they are mobile in their localized biofilm area. The process gases containing the contaminants to be treated flow through the media and as these gases flow by, molecules of contaminants pass very near to or directly contact the biofilm where they are absorbed into the biofilm. Noting that the biofilm is primarily composed of water one can see clearly that a compound's solubility in water will greatly impact ease of degradation because if the compound does not enter the biofilm then it cannot be decomposed by the micro-organisms in that biofilm. The biofilm also creates a fixed (more or less) inhabitable space that can only be utilized by a finite maximum number of organisms. The organisms will grow and expand until the available space (biofilm) is filled resulting in situation where no more effective growth can occur. This means that the effective amount of the summation of biomass (dead and alive) in the unit is relatively constant. An example of this is shown in the graph below.

What can be degraded in a Biofilter?

Biofilters can be used to degrade many different kinds of compounds within a wide range of industries. Some of the compounds that can be degraded include:

  • Acetone
  • Aliphatic Hydrocarbons
  • Ammonia
  • Anthranilates
  • Aromatic Hydrocarbons
  • Butadiene
  • Carbon Disulfide
  • Esters
  • Ethanol
  • Ethers
  • Formaldehyde
  • Heptane
  • Hexane
  • Hydrogen Sulfide
  • Isopropanol
  • Isopropyl acetate
  • Ketones
  • Methyl Ethel Ketone (MEK)
  • Methanol
  • n-Propanol
  • N-propyl acetate
  • Pesticides
  • Phenol
  • Pinenes
  • Styrene
  • Terpenes
  • VM&P naphtha

In general, more soluble compounds such as lower molecular weight alcohols, aldehydes and ketones are more easily treated in a biofilter than are aliphatic or aromatic hydrocarbons.

How are compounds degraded?

The bacteria produce enzymes that act upon compounds to break them down into the elementary constitutes of water and carbon dioxide. Each enzyme has the capability of altering a reaction and in fact several enzymes will often work upon the same compound at the same time. The presence of many different species of bacteria means there is wide variety of enzymes available at any single given space and time. These enzymes may remove a water molecule, break a C-H bond or C-C bond or add a water molecule. The fewer the variety of compounds supplied the greater the specialization of the population as a whole. For example if a biofilter is fed nothing but methanol then the reproducing populations will be geared toward the degradation of methanol, that is to say they will produce enzymes that are especially successful at degrading methanol as well as more of those enzymes. A basic understanding of the degradation process can be obtained by the examination of the Michaelis-Menten kinetics model. where:

  • E is the enzyme being analyzed
  • L is the concentration of the compound to be degraded
  • EL is the developed complex (a complex is a group of two or more associated polypeptide chains)
  • P is the product that regenerates the original enzyme

On what processes are Biofilters used?

Biofilters have been employed on various processes and pieces of process equipment such as:

  • Composting
  • Door & Window Manufacturing
  • Flavor & Fragrance Production
  • Food Processing
  • Frying Operations
  • Medium Density Fiberboard (MDF)
  • Paint Spray Booths
  • Particleboard Manufacturing
  • Pet Food Production
  • Petro-Chemical Plants
  • Pharmaceutical Production
  • Photo and Film Production
  • Printing
  • Pulp & Paper Manufacturing
  • Sewage Processing Plants
  • Tanneries
  • Textile Fabrication
  • Wastewater Treatment Plants (WWTP)

How is the size of a Biofilter determined?

Each type of compound or gas has a different potential of being utilized by the bacteria as food and as such a semi-empirical model is generated from test reaction and diffusion data gathered in lab scale, pilot scale, and full scale applications. Once the model is constructed a biofilter's media volume can be determined by entering the concentration of each known compound in the gas stream and not just the concentrations of the compounds to be degraded because bacteria will eat the most easily digested molecules first. As a general rule the more insoluble a compound is the less biodegradable that compound is, or to put it another way the less soluble a compound is more media, and thus bacteria, it will take to degrade that compound.

  • Solubility of Various Compounds in Water
  • Compound mg/l @ 20°C
  • Acetone Miscible
  • Benzene 1,780
  • Butadiene 735
  • Ethanol Miscible
  • Formaldehyde Miscible
  • Heptane 3
  • Hexane 9.5
  • Isopropanol Miscible
  • Methyl Ethyl Ketone (MEK) 27,500
  • Phenol 8,300
  • Styrene 300

What factors can adversely affect Biofilters?

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.

How does increased pressure drop across the media affect plant operations?

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.

How can the physical or mechanical design adversely affect the biofilter?

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).