Description

Biofilters are a type of biological odour treatment system, consisting of a layered filter bed. Biofilters contain bacteria which treat large volumes of odorous air before exhausting the treated air to the environment. Biofilters are highly specialised odour treatment methods that need to be properly designed for your business by a suitably qualified person.

Type of control

Biological filtration.

When you would use this?

A biofilter is generally the equipment chosen to treat large volumes of air containing low concentrations of pollutants. Large volumes of air containing higher concentrations of pollutants can still be treated with a biofilter, however, pre-treating will be required.

Suitable for: industries or businesses that produce large volumes of odorous air. The odour causing compounds need to be soluble, i.e. – able to dissolve in water, and biodegradable.

Industries that would use this: biofilters are most commonly seen in waste water treatment plants, in-vessel composting facilities, abattoirs, rendering facilities, or meat processing plants.

Details and considerations

Biofilters use living microorganisms to control generated odour. To ensure the microorganisms are working optimally to remove the odour, biolfilters can require more maintenance than other odour controls. Therefore, appropriate care needs to be taken for them to continue running effectively:

• Biofilters require continuous operation as well as monitoring of at least air temperature, relative humidity, and biofilter back pressure, as this is way of ensuring the biofilter is running at optimum efficiency.
• How well a biofilter treats odour will depend on two main things
  • How much odourous air is fed into the biofilter.
  • How long the air takes to travel through the biofilter, known as the residence time.
• Air fed into the biofilter must not be toxic to the bacteria that live within the biofilter.
• Air fed into the biofilter should be as consistent as possible to maintain efficiency within the bacterial population.
• Inlet air must be between 25°C and 45°C and at > 90% humidity.

Biofilters are complex and require a good understanding of their design to operate efficiently as an odour control. Features recommended for biofilters designed within Australia include:

• An open chamber (known as a plenum) underneath the bed is the preferred method to deliver odorous air to the biofilter.
• Watering through a drip or sprinkler system to provide moisture to the media from above the bed is recommended. However, care should be taken to not over water the biofilter.
• It is often recommended that a roof above the biofilter is installed to limit direct sun and rain exposure. This will help maintain a consistent state within your biofilter bed.
• The biofilter should be as close as possible to the emission source.  This minimises the length of ducts and thereby limits humidity/condensation within them, increasing fan efficiency.
  

The bed of the biofilter consists of two layers which are most commonly wood based (shredded logs, woodchips, barks, etc). Bed layers need to be periodically replaced. Replacement schedules will vary depending on your odour production and should be discussed with the suitably qualified professional. For a comprehensive guide on how to handle waste bed layering, see the section on waste.

Bottom bed layer:
• Very coarse hardwood material (50-150mm) should be used for this layer.
• Optimises air distribution across the whole bed.
• Supports the weight of the bed.
• Allows finer particles from top layer breakdown to be washed down the bed layers.
• Should be roughly one third of the total bed depth.

Top bed layer:
• Generally, contains a mixture of new media and sieved coarse media (2 parts sieved, 3 parts new media).
• Allows new media to become populated with the existing bacteria in the biofilter.
• Can contain 20-40% softer wood and bark.

There are several ways to tell when your biofilter media needs replacing:
• A significant drop in biofilter back pressure.
• Increased power consumption of fans pushing air through media.
• Breakdown of the wood chips in the bed where the surface looks visibly muddy.
• Variation in height of the biofilter indicating some areas are compressed or collapsed.
• Uneven distribution of steam observed during early morning.

Figure 1 - schematic of a biofilter