Recently, many South Island councils, including Nelson City Council, have conducted testing of their biofiltration systems, used to reduce odours from sites such as sewage treatment plants. These tests have shown a wide range of performance by biofilters, with the primary cause of poor performance being a lack of regular maintenance.
Under the Resource Management Act, the primary responsibility for managing air quality lies with regional councils and unitary authorities. They must mitigate any adverse effects of activities on the environment, including the production of offensive odours. Many councils use biofiltration to reduce odours, such as hydrogen sulphide, that are emitted from sewage treatment.
Biofiltration is where organic contaminants in a gas stream pass through a bed of material and adsorb to the surface, where they are broken down by the micro-organisms present. Volatile compounds break down to carbon dioxide, water, mineral salts, and other harmless products. The bed material may be shell, bark, compost or any mixture of these components.
Unfortunately, biofiltration systems are not always operated at suitable conditions to achieve the desired odour control. If biofilters are overloaded, sorption sites are filled faster than they are regenerated by bio-oxidation, resulting in odorous compounds passing through the biofilter into the atmosphere.
Selection of the appropriate biofiltration medium is critical. The medium should have a large active particle surface area to absorb gases, attach microorganisms, and effect good water holding. It must be sufficiently porous to maintain alow pressure drop and provide good drainage. It should have chemical characteristics that help to support and sustain the growth of micro-organisms.
The moisture content, drainage, pH, temperature, nutrient availability of the medium and the pressure drop across the medium must be monitored and controlled within suitable limits to maximise biofilter performance.
Depending upon the medium used, the recommended optimal moisture content normally ranges between 20% and 60% on a weight basis. Options for moisture control include humidification of the biofilter influent gas and irrigation of the biofilter surface.
Biofiltration medium pH is recommended to be in the range of 6 to 8 and should be checked regularly and measured at several depths. Crushed limestone or shell can be added to raise a low pH and extend the life of the biofilter.
Temperature affects not only microbial activity but also the solubility of odorous compounds in water and the sorption capacity of the biofilter medium. Studies have shown that an operating temperature between 20 and 40°C provides optimum results. As temperature affects not only the activity but also the composition of microbial populations, sharp temperature fluctuations in the biofilter should be avoided.
Pictured: A refreshed and serviced biofilter located in the Tasman region.
The microorganisms in biofilters require nutrients for growth. It is generally thought that most biofilter media and influent gases contain enough nutrients so that none need to be added. However some biofilters designed to operate at high loading rates and over long periods of time, may require the addition of nitrogen.
Another key factor in the design and maintenance of biofiltration systems is the pressure drop across the biofilter medium, which is a function of the gas loading rate and is influenced by the type, depth and moisture content of the biofilter medium. Monitoring will detect compaction and indicate when loosening or replacement of the medium is required.
When properly designed, operated and maintained, biofilters have proved to be an effective odour control technology. The biofilter medium chosen must be able to support and sustain a diverse microbial population.
BioFilm, manufactured by Wholesale Landscapes for more than 10 years, is a specialist, branded technical media used in biofiltration systems all over the South Island by local government and private organisations. When working efficiently, BioFilm removes more than 90% of air pollutants, including those that some agencies class as hazardous, such as hydrogen sulphide. BioFilm is a contaminant-free, all-natural product, manufactured from recycled, carbon-neutral materials.
Wholesale Landscapes can provide expert consultants to advise on your specific project. Common services undertaken by Wholesale Landscapes include testing of current conditions, reporting findings and creating recipes and products suitable for your site.