A biofilter is one of the cheapest and simplest odour control methods available. However, a good number of conditions must be met for a biofilter to work properly.
How does biofiltration work?
Biofiltration involves the use of biofilter media bed, consisting of organic material in a well-defined mixing ratio, or in different layers. This natural “filter bed” or biomass is contained in an open or closed filter enclosure. This biofilter media bed is surrounded by a thin film of water. Some of the pollutants from the gas stream already dissolve in this water film. The remaining part of the pollutants is retained by filter media bed. In a next step, these retained pollutants are further broken down by the microorganisms in the filter bed. The microorganisms feed partly on the remaining pollutants and partly on the filter material itself. The residual products of these degradation processes are CO2, H2O, sulfate, nitrate, etc.
What are the advantages of a biofilter?
- Simple construction
- Low investment cost
- Good result for biodegradable components in relatively low concentrations (polishing technique)
What are the disadvantages?
- A relatively large surface area needed
- The biomass or media bed needs to be replaced periodically
- Moisture content and pH need to be monitored accurately
- A continuous aeration is vital
Most important application areas of biofiltration
- Wastewater treatment plants
- food industry
- meat and fish processing industry
Important parameters for the proper functioning of a biofilter
Maintaining an optimal humidity in the biomass is an essential condition for the proper functioning of a biofilter. Too much moisture can cause the biomass to clump, air resistance increases and oxygen content decreases. As a result, the filter itself becomes a source of odour nuisance, rather than combating it.
Constant air flow
The gazeous emission flow must be supplied relatively continuously. If only 7 to 8 hours/day of waste gas is produced, a biofilter will not be able to provide a solution. The bacteria in the biofilter cannot survive without a constant oxygen supply. And moreover, the incoming air must be free of dust and grease.
A temperature between 15 and 25 °C is ideal (max 30 °C). Lower temperatures will slow down the action, higher temperatures will unbalance the entire biomass.
A relatively low load is not a problem as such and will only translate into a high separation efficiency. With a too high and/or peak load, the biomass will clog up quickly, resulting in a lower separation efficiency.
No toxic and/or acidifying components
Highly toxic components kill microbiology. Excessive concentrations of sulfur, chlorine, nitrogenous organic components and ammonia lead to acidification and reduced efficiency of the biofilter. This can be partly countered with a number of additives for the biomass. Or by replacing the biomass more frequently or by incorporating a chemical gas scrubber in the emission treatment plant.