Bio-filters

Bio-filters contact bacteria (in a liquid film) directly with the contaminated air. The contaminants pass from the air to the liquid film where they can be acted upon by the bacteria. The bacteria breakdown the molecules of contaminant or react them with other molecules to provide themselves with energy. They may use enzymes in this process. All bacteria require ammoniacal compounds to form amino acids so that they can build cell material. They also require salts and minerals in suitable levels to grow and function.

Bacterial growth and metabolism can be affected by pH, temperature and the levels of contaminant present. Inhibiting factors can slow the growth and activity down (lack of nutrients, presence of toxins/biocides, competing bacteria etc).

Typically VOC bio-filters produce carbon dioxide and water where as odour control systems produce sulphurous acids, nitrogen as well as CO2.

The more modern systems are much more robust than the earlier open systems. Most are continuously irrigated giving a scrubbing action and water buffer for peaks and troughs. Some of the latest systems are even Counter current (see scrubber section) and hence have a higher removal efficiency than traditional co-current systems.

All counter current systems need to be "passive" media units. This is because active media units would be vulnerable to bed collapse as the base of the bed would deteriorate first.

Passive or permanent media systems can offer significant life cycle cost savings over active media systems as they do not require major media replacements. Nutrient/buffer costs are typically small in comparison to annualised operational costs for media replacement. A key feature of the new permanent media systems is that fact that the system contact time is constant and therefore there is no reason for the system to fail provided that all nutrient and buffering is maintained. Permanent media systems therefore comply with the odour requirements in IPPC permits. Passive systems deterioirate and hence may require constant monitoring to indicate the onset of media replenishment requirements.

Generally speaking the environment for the bacteria needs to stabilised and controlled (this means temperature, pH, nutrient availability and substrate levels). the more control over these parameters the system has, the more robust and reliable the system becomes.

In most cases it is however advisable to include a down stream polisher (particularly for odour control) as this will protect system performance from bio-filter "glitches".