What is it?
- Open biofilters;
- Enclosed biofilters.
An enclosed biofilter consists of a layer of material supporting a suitable microbial population and placed under a distribution system which supplies the contaminated waste gas stream to the filter uniformly. The gas stream is drawn to the filter by electric fans. Gas flow is from top to bottom or vice versa. The blower, ventilation system and biofilter construction materials should minimise the effects of corrosive waste gas, excess condensate and dust/sludge.
The majority of biofilters in operation are open-bed filters, which are less costly than enclosed biofilters but generally less efficient.
Design, maintenance and efficiency
Parameters such as biofilter media pH, moisture content (relative gas moisture of about 95% and more is required) and inlet gas temperatures affect odour removal capacity.
The moisture balance is regulated by a pre-connected humidifier or gas scrubber, at times in combination with a moistening of the filter material. The relative humidity of the filter material should be below 60% to avoid clogging. The moistening device needs protection against freezing in regions where temperatures are substantially below 0 °C.
For application to warm waste gas streams (> 35 ºC), cooling is necessary, either by mixing with air or introducing a gas scrubber or heat exchanger. Wet scrubbing can be applied as a pre- treatment with the aim of decreasing excessive particulate content, inlet odour concentration and amount of pollutants not suitable for biofiltration.
The residence time to allow an effective abatement depends on the inlet concentration. As a rough guide, a minimum residence time of 30 s to 45 s should be aimed for.
For some odorous compounds (e.g. mercaptans, H2S), abatement efficiencies of 75% are a minimum. Using a scrubber and biofilter in combination can increase performance. For other odorous compounds, the abatement efficiency is somewhat lower. Comparative research for odour abatement efficiencies between scrubbers and biofilters show that biofilters achieve higher efficiencies.
With open biofilters the filter material has to be regularly worked up and disposed of from time to time. Waste gas channelization can occur through the filter bed, thereby decreasing the biofilter efficiency.
The efficiency of biofiltration depends to a great extent on the composition of the waste gas stream. Also, sudden changes in biofilter media conditions (e.g. temperature) can adversely affect the performance of the system. If parameters do change slowly over time then the media become accustomed and efficiencies remain high.
Cooling is necessary for applications with hot airflows (> 38 °C). This can be realised by a mixture of outside air, a (single-pass) water scrubber or a heat exchanger/condenser. The filtering material should be replaced periodically (every 0.5–5 years), depending on the type of packing material and the composition of the gases.
Although biofilters are static in principle and require little mechanical maintenance, experience shows that regular inspection and monitoring are necessary. The abatement efficiency can be excellent during the first few years, but can decrease drastically over a short period of time because of a lack of nutrients, problems with the fluid balance and/or the deterioration of the filter material.
The moisture balance has to be carefully examined because it is critical for the accurate operation of biofilters.
Typical areas for implementation are:
- Wastewater treatment plants
- Composting installations
- Flavourings industry
- Chemicals industry
- Plastic production
- Foodstuffs industry
- Meat and fish-processing industry
Table 1 shows application limits and restrictions associated with biofiltration (adapted from EIPPCB, 2016, Table 3.178).
Table 1. Application limits and restrictions associated with biofiltration.
|Gas flow (Nm3/h)||100-200000
100-400 per m2 of filter surface
|Pressure drop (mbar)||5-20|
|Content of dust, grease and fat||Can cause clogging; pretreatment is necessary|
|Odour concentration (ouE/m3)||20000-200000|
|Climatic conditions||Frost, rain and high ambient temperatures affect the filter material and decrease efficiency|