Trickling filters 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. Biological treatment 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.
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 function 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 decomposition 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 "nitrification" 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.
Source :
http://en.wikipedia.org/wiki/Biofilter
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