Biofiltration as odor treatment method

A biofilter is one of the cheapest and simplest odor control methods available. However, a good number of conditions must be met for a biofilter to work properly.

How does biofiltration work?

Biofiltration usually involves the use of a filter filling consisting of biological material in a well-defined mixing ratio, or in different layers. Some examples:

  • tree bark
  • compost
  • peat
  • heather
  • coconut fiber
  • root wood
  • expanded clay granules, etc.

This natural “filter bed” or biomass is contained in an open or closed filter enclosure. This filter filling is surrounded by a thin film of water. Some of the contaminants from the gas flow directed through the filter bed already dissolve in the water film; some of the contaminants are retained on the filter material and further broken down by the microorganisms living in the filter bed. These microorganisms must help to further break down the contaminants from the gas flow that remain on the filter filling. The microorganisms therefore feed partly on the remaining pollutants, partly on the filter material itself. Water and oxygen are also essential to the life of the microorganisms, both of which must be available in the biomass during normal operation. The residual products of these decomposition processes are CO2, H2O, sulfate, nitrate and similar.


Biofiltration – biofilter – odor control and emission treatment

Biofiltration : the important parameters for the proper functioning of the biofilter:

  • Constant moisture content: maintaining an optimal moisture content in the biomass is an essential condition for the proper functioning of a biofilter. The biomass should not be too dry, but also not too wet. Too much moisture can cause the biomass to clump together, air resistance increases, oxygen content decreases, the filter in turn becomes a source of odor nuisance instead of combating odor nuisance. It is therefore also of prime importance that the off-gas flow is prehumidified before sending the air through the biofilter. This prehumidifier must therefore be perfectly adjusted in order to create the exact humidity level.
  • Constant air supply (emission flow rate and oxygen): The emission must be offered relatively continuously. If only 7 to 8 hours/day of flue gas is produced, with on top of that weekend, vacation and vacation stops, a biofilter will not be able to offer a solution. The bacteria from the microbiology cannot survive without a constant supply of oxygen. The airflow offered should be dust-free and grease-free.
  • Constant temperature (ambient temperature): for a good functioning of the microorganisms 15 to 25 to max 30 ° C is ideal. Lower temperatures will slow down the action, higher temperatures will put the action into overdrive and throw the entire biomass out of balance.
  • A load that is as constant as possible: A relatively low load is not a problem as such and will only translate into a high removal yield (provided, of course, that the other parameters remain constant). When the load is too high, the biomass will silt up more quickly due to the rapidly increasing microbial population.
  • No toxic and/or acidifying components in too high concentrations: Highly toxic substances kill the microbiology. Excessive concentrations of sulfur, chlorine and nitrogenous organic components and ammonia lead to acidification and reduced efficiency of the biofilter filling. This can be partly counteracted by adding certain additives to the biomass, by replacing the biomass more frequently, or by incorporating a pre-treated chemical scrubber.
  • Adapted biomass: The biomass must be adapted to the offered gas flow. It should not be too fine or too coarse, the air should be nicely evenly distributed over the filter. Achieve an even air distribution across the filter. The filling must be coarse enough to create a sufficiently large exchange surface, but must also contain enough organic material to provide the micro-biology with a habitat in which it can thrive. Additives such as lime and dolomite can prevent premature acidification. Preferably, the biomass is placed on a grid to promote air distribution. For certain specific applications it may be useful to use a biomass inoculated with specific bacteria. In all cases, the biomass should be replaced periodically, in function of the acidification and salinization degree and the pressure drop over the filter (due to compaction of the biomass). The lifespan of a biofilter filling varies greatly and is situated between 6 months and 5 years.

Biofilter advantages

  • Low investment cost (simple construction)
  • Good result for biodegradable components in relatively low concentrations (polishing technique)

Biofilter disadvantages

  • Big footprint, takes a relatively large area
  • Biomass needs to be replaced periodically
  • Exact moisture content and pH need to be monitored accurately, a continuous aeration is vital

Biofiltration : most important application areas

  • Wastewater treatment plants
  • Agro-industry (composting, food industry, meat processing and fish processing industry)