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Mechanical Filtration and Biofiltration

Mechanical filtration and biofiltration are very distinct and separate entities, and they must be treated as such. Filtration is the removal of solid waste, whereas biofiltration is the biological process that converts toxic nitrogenous wastes to less toxic nitrate.

Solid waste is typically categorized by its size and specific gravity. Settleable solids are those solids which have a relatively high specific gravity compared to the water in which they exist. They will settle to the bottom. Suspended solids are those in a category that have a specific gravity the same as, or slightly higher than, the water. They tend to stay in suspension. Dissolved solids are those which actually become a part of the water. Suspended solids can be removed by mechanical means such as filter cloth, bag filters and drum filters. Dissolved solids can be removed by reverse osmosis, anion and cation resins, by using activated carbon that will attract the molecules out of suspension or be broken down by biofiltration. Settable solids can be removed by slowing the water down in a sump or swirl separator.

Biofiltration is the aerobic (with oxygen) breakdown of dissolved nitrogenous fish waste. The process is accomplished by two or more strains of autotrophic bacteria. These bacteria are naturally occurring and will ultimately colonize the bio-media in the biofilter as well as the tank and pipe walls. The speed of this process is dependent on temperature, pH, salinity, surface area, flow rate, etc. The process can be accelerated by the addition of aerobic bacteria to the system.

The autotrophic bacteria use oxygen in a two step process to first convert the toxic ammonia (NH3 or NH4+) to nitrite (NO2-). Another strain of bacteria converts nitrite (NO2-) to nitrate (NO3-). Nitrate is much less toxic and typically tolerated by most cultured species until it reaches very high levels. Controlling nitrate is accomplished by diluting with clean water or by using a denitrification chamber that converts nitrate into nitrogen gas (this is an anaerobic process that uses a group of heterotrophic, without oxygen, bacteria). A third method to keep nitrate levels in check is the use of plants. You can have a green water system (using algae), a vegetative filter or even use a hydroponic plant system to remove nitrate.

Regardless of which type of filtering equipment you decide to use, the one thing to keep in mind is to stage the filtration. It is a common mistake to design a system that relies too heavily on a single filtering device to provide all of the filtering requirements of a recirculating system. By staging filtration components, the system will perform at or near its peak.

If you are unsure about the filtration system that is best for you situation, contact us for information and guidance.


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