Biofilter

From Wikipedia, the free encyclopedia

Biofiltration is a pollution control technique using living material to filter or chemically process pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in air.

Examples of biofiltration include;

• Bioswales, Biostrips and Biobags
• Constructed wetlands and Natural wetlands
• Slow sand filters
• Green belts
• Living walls
• Riparian zones, Riparian forests, Bosques

Contents 1 Control of air pollution 2 Water treatment 3 Further reading 4 External links

[edit] Control of air pollution

When applied to air filtration and purification, biofilters use microorganisms to remove pollution. The air flows through a packed bed and the pollutant transfers into a thin biofilm on the surface of the packing material. Microorganisms, including bacteria and fungi, immobilised in the biofilm, degrade the pollutant. Related technologies include biotrickling filters and bioscrubbers.

The technology usually finds application in treated malodorous compounds at low concentrations. Industries employing the technology include food and animal products, off-gas from wastewater treatment facilities, pharmaceuticals, manufacturing etc. Compounds treated are typically single or mixed volatile organic compounds and recent success has been obtained with hydrogen sulfide. Very large airflows may be treated although a large area is then required -- a large biofilter may occupy as much or more land than a football field -- this being one of the principal drawbacks of the technology. One of the main challenges is to prevent the packing media from drying out during operation. The air is normally humidified before it enters the bed and some form of watering system installed. Properly controlled, the packing media may last for a decade or more before it needs replacement. A number of companies offer proprietary packing material although many operators opt for other materials such as compost, peat moss or wood chips.

Although widely employed, the research community is still unsure of the physical phenomena underpinning biofilter operation, and information about the microorganisms is only just emerging. Design methods are based on experience or, if the pollutant has not been treated before, pilot-scale testing. A biofilter is a fairly simple device to construct and operate and so often presents a cost-effective solution provided the pollutant is biodegradable, at fairly low concentration and at a reasonable temperature. For high flows of air, a biofilter may be the only cost-effective solution. There is no secondary pollution (unlike the case of incineration) and degradation products (for VOCs) are some additional biomass, carbon dioxide and water. Leached water from the media has very high BOD and must be treated before disposal, although many operators recycle part of it to the watering system to reduce operating costs.

[edit] Water treatment

Biofilters have been used to filter water for various end uses for almost two centuries. Biological treatment has been used in Europe to filter surface water for drinking purposes since the early 1900s and is now receiving more interest worldwide. Biofiltration methods are also common in wastewater treatment, aquaculture and greywater recycling as a way to minimize water replacement while increasing water quality.

For drinking water, biological water treatment involves the use of naturally occurring micro-organisms in the surface water to improve water quality. Under optimum conditions, including relatively low turbidity and high oxygen content, the organisms break down material in the water and thus improve water quality. Slow sand filters or carbon filters are used to provide a place on which these micro-organisms grow. These biological treatment systems effectively reduce water-borne diseases, dissolved organic carbon, turbidity and colour in surface water, improving overall water quality.

Use in Aquaculture -- The use of biofilters are commonly used on closed aquaculture systems, such as recirculating aquaculture systems (RAS). Many designs are used, with different benefits and drawbacks, however the fuction is the same -- reducing water exchanges by converting ammonia to nitrate. Ammonia (NH₄⁺ and NH₃) originates from the brachial excretion from the gills of aquatic animals and from the decomposistion of organic matter. As ammonia-N is highly toxic, this is converted to a less toxic form of nitrite (by Nitrosomonas sp.) and then to an even less toxic form of nitrate (by Nitrobacter sp.). This "nitification" process requires oxygen (aerobic conditions), without which the biofilter can crash. Furthermore, as this nitrification cycle produces H⁺, the pH can decrease which necessitates the use of buffers such as lime.

[edit] Further reading

Biofiltration for Air Pollution Control, Joseph S. Devinny, Marc A. Deshusses and Todd S. Webster, Lewis Publishers, 1998, ISBN 1-56670-289-5.

[edit] External links

• Software for Soakaway and SUDS systems design
• Caltran article on bioswales and strips for storm runoff
• Description of VOC reduction using biofiltration
• What is a Biofilter
• Odor Control - The Green Solution to Air Pollution