A biofilter is a layered filter bed of bark or wood that contains bacteria. The bacteria can treat large volumes of odorous air containing low concentrations of pollutants.
A biofilter is a specialised odour treatment method. It should be designed for your business by an expert.
Type of control
- Biological filtration
When to use biofilters
Use when:
- you need to treat volatile organic compounds or gases
- your odorous air has low concentrations of pollutants.
Air with high concentrations of pollutants needs to be pre-treated.
A biofilter is suitable for:
- industries or businesses that produce large volumes of odorous air
- odorous compounds that dissolve in water (soluble) and are biodegradable.
Industries that use biofilters include:
- wastewater treatment plants
- in-vessel composting facilities
- abattoirs
- rendering facilities
- edible oil manufacturers.
Considerations and requirements
Biofilters use living microorganisms to control odour. This means they need more care than other odour controls. Air fed into the biofilter:
- must not be toxic to the bacteria that live within it
- should be as consistent as possible, to maintain the bacterial population.
To make sure the biofilter runs at its most efficient, you need to continuously operate the biofilter and monitor its:
- air temperature
- relative humidity
- back pressure.
How effectively a biofilter treats odour depends on:
- how much odorous air feeds into the biofilter
- how long the air takes to travel through the biofilter (residence time).
Air that is fed into the biofilter must be between 25°C and 45°C and greater than 90% humidity, preferably saturated.
The bed of the biofilter consists of 2 layers that are wood-based, such as shredded logs, wood chips or bark. This is the biofilter media.
You need to replace your biofilter media when:
- there's a significant drop in biofilter back pressure
- the fans pushing air through the biofilter media are using more power
- wood chips in the bed are breaking down where the surface looks muddy
- there's height variation in the biofilter, indicating some areas are compressed or collapsed
- you see uneven distribution of steam in the early morning.
There are waste duties and obligations you must meet when replacing your biofilter media.
Our guidance Biofilter design and maintenance has detailed technical information on biofilter operation.
Biofilter design
Biofilters are complex. Design your biofilter system to:
- deliver odorous air to the biofilter through an open chamber under the bed (plenum)
- water the biofilter bed from above through a drip or sprinkler system – do not overwater the bed.
Locate the biofilter as close as possible to the emission source to:
- minimise the length of ducts
- limit humidity or condensation within ducts
- increase fan efficiency.
Bed layers
The lower bed layer:
- is made of very coarse hardwood material (50 mm to 150 mm)
- should be roughly one-third of the total bed depth
- supports the weight of the bed
- optimises air distribution across the bed
- allows finer particles that break down to wash down through the bed layers.
The upper bed layer:
- is made from coarse material (25 mm to 45 mm)
- should be roughly two-thirds of the total bed depth
- can contain 20% to 40% softer wood and bark
- when replaced or refurbished, should consist of 2 parts sieved used media to 3 parts new media
- allows new media to become populated with the existing bacteria in the biofilter.
The top bed layer (topper):
- is made from fine bark (25 mm)
- should be approximately 20 to 30 cm in depth
- insulates the biofilter in colder months
- retains moisture during dry periods.
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