THE POTENTIAL OF ROCKWOOL BIOFILTER MEDIA FOR ODOROUS GAS TREATMENT

The biological treatment of air emissions is a sustainable and cost-effective control technology for industrial and municipal facilities, where biofilters are being widely used to treat odorous waste gases from different sectors. Since the large areas normally required for biofiltration may be a drawback, recent efforts have been made to achieve an effective treatment at even shorter residence times, thereby allowing for smaller reactors. When trying to develop a compact biofilter for these types of applications, extra care must be taken in the choice of biofilter media. The characteristics and properties of the support medium largely govern the overall effectiveness of a biofilter, since the medium serves as habitat for the microorganisms, humidity reservoir and mechanical support for maintenance of the filter bed's internal structure. Rockwool material has a well-defined composition, and its characteristics can be specifically designed, i.e. density, fiber length, amount of binder and hydrophobic/hydrophilic properties, making it a very versatile filter medium.The objective of this study was to analyze seven different rockwool media for their suitability to be used in a multi-stage biofilter for odorous waste gas treatment. The examined parameters included flow distribution and pressure drop, media characteristics, and aptness for immobilization of microorganisms. Three rockwool media were tested in a pilot scale biofilter in three different applications with odor problems: at a restaurant (fryer), pulp mill, and wastewater pumping station. A comparison to an organic material (a compost/peat mixture) is also included. Rockwool fiber mats with pre-set structures were preferable to loose rockwool due to their easier handling, improved gas flow distribution, and developed lower pressure drops (also in comparison to organic media). However, some of the hydrophobic mats had low mechanical and chemical stability. A linear relationship between pressure drop and surface loading was established, even at very high gas velocities.Sampling of the rockwool media revealed varying moisture content between samples, necessitating an effective humidification system to preserve adequate moisture levels in the filter. Enumeration of the biomass showed a wide range of bacteria (heterotrophic, autotrophic and methylotrophic) able to immobilise and grow in the media with cell numbers ranging from 104 to 1010 per gram dry media. Media pH was relatively stable, indicating a certain buffer capacity in the material. The odorous waste gases were complex mixtures with chemically diverse contaminants whose concentrations varied substantially with time and no aldehydes or sulphur compound reductions could be shown in the filter. Conditions in the field differ greatly from those controlled in the laboratory, and a number of operational and process related problems were identified.