Biological filters are used in a wide range of applications to filter toxins and other waste material from liquids. The biological filters operate by cultivating bacteria on the filter surfaces that "filter" the liquid. Thus, when the liquid to be filtered is brought into contact with the filter surfaces, the bacteria converts the toxins or waste material in the liquid into less harmful compounds. For example, when used in aquariums, the filter may foster the growth of aerobic nitrosomonas bacteria and aerobic nitrobacter bacteria. When the aquarium water is flowed over the filter, the aerobic nitrosomonas bacteria converts waste products, such as ammonia compounds, into nitrites. The aerobic nitrobacter bacteria then converts the nitrites to nitrates. The end result of the conversion (nitrates) are only mildly toxic to fish in high concentrations, and can be controlled by regular water changes, or by other means. The above-described biofilter system may also be used, in a similar fashion, in waste treatment, pollution control, fermentation processes, aquaculture and other settings.
Because oxygen is consumed in the conversion processes carried out by the bacteria, a regular supply of oxygen must be provided to the filter surfaces. The oxygen may be supplied either by dissolved air in the water, or by the adsorption of oxygen from the air directly by the bacteria layer. "Wet/dry" biofilters regularly expose the filters surfaces to air to supply the necessary oxygen to the bacteria. There are basically three types of wet/dry biofilters employed in the state of the art. The first is a "soak/drain filter" wherein a container is partially filled with a high surface area media upon which the bacteria is grown. The container is filled with liquid to bring liquid into contact with the bacteria. The liquid is then drained from the container to allow the bacteria to adsorb the necessary oxygen. Replacement liquid is then introduced into the container, and the fill/drain process is regularly repeated to filter the liquid. The soak/drain biological filter requires either mechanical means to empty the container, or the use of a siphon tube or valve to drain the water. These mechanical means increase construction and maintenance costs of the soak/drain filter system.
A second type of wet/dry biological filter is a "trickle filter" wherein a column, tank or large enclosure is partially filled with a high surface area media upon which the bacteria is cultured. The water to be filtered is trickled through the column, tank or enclosure such that it contacts the bacteria growing on the media. Trickle filters are usually bulky, and require large amounts of space.
A third type of biological filter is a "rotating biological contacter." The rotating biological contacter is typically a finned, generally cylindrical filter body that is rotated to alternately expose portions of the filter to the liquid and to air. The bottom of the rotating filter may be submerged in the liquid to be filtered, or the liquid may be sprayed directly onto the cylindrical body. The cylindrical body may be rotated by the force of liquid, or a separate motor may be used to rotate the cylindrical body. The rotating biological contacter typically has a lower surface area compared to other biological filters which may reduce its effectiveness. Furthermore, the cylindrical body may become unbalanced due to an uneven accumulation of algae, biomass, or other foreign matter, which can inhibit the rotation of the cylindrical body. Furthermore, the rotational speed of the body must also be regulated to avoid throwing water off of the surface of the body by centrifugal forces.
The efficiency of the filtration process is maximized when the greatest population of bacteria contacts a given volume of water. To this end, high surface area porous substrates having large open pores help improve the efficiency of biological filters. Besides providing a high surface area, large pores also reduce the chances of pores being plugged with bacteria or other foreign substances. Ideally, the morphology of the substrate ensures that water can easily move into contact with the bacteria on the surface of the filter, reside in contact with the bacteria for sufficient time, and then be drained to allow air to replace the water to be absorbed by the bacteria.
Accordingly, there is a need for a biological filter which fosters the growth of bacteria, can filter large volumes of liquid relatively quickly, is of simple construction, is compact, and avoids adverse effects due to the buildup of algae or other biomass.