Joe S
Community Member
Biological Filtration: Overview and Comparisons
Joe Szczebak
Biological filtration is primarily responsible for removing nitrogenous waste from aquaria and promoting gas exchange (i.e., O2, NO2, CO2) for pH maintenance. This is usually achieved in two main steps. First, using some type of biofilter, nitrification by chemoautotrophic bacteria oxidizes inorganic ammonia (NH4+) to nitrate (NO3-). Second, the accumulated NO3- is removed from the system via uptake by aquatic plants (e.g., mangrove trees), macroalgal cultures, or regular water changes. In general, a biofilter is any media that effectively cultures units of nitrifying bacteria and exposes those cultures to the system’s nitrogen.
Considerations for efficient biofiltration:
(1) Large surface area media to maximize available nitrifying bacteria biomass. A large amount of small media will usually produce the highest amount of surface area for nitrification.
(2) Thin nitrifying biofilm to ensure sufficient bacterial contact time with nitrogen. If the bacterial layer is too thick (more than several layers), many of the bacteria will not gain access to the nitrogen source and are thus wasted.
(3) Maximal bacterial access to O2 and a carbon source (e.g., bicarbonate), and minimal exposure to light (light inhibits nitrification).
(4) Appropriate water speed that cycles as much water through the filter as possible but still gives the bacteria enough contact time with the water to nitrify.
(5) Some type of shearing capacity to ensure the bacterial biomass is not clogged or smothered (like raking the leaves off your lawn).
(6) Low maintenance and operational cost.
Fortunately, many of the popular biofilters on the market incorporate these considerations into their compact and efficient designs. Some common biofilters include submerged beds, trickle filters, floating bead filters, fluidized beds, and rotating biocontactors (bio-wheels).
Note: Reef tanks do not necessarily require additional biological filtration due to the live rock and other substrates associated with their aquascaping and design. The below biolfiters are discussed so serve as a general source of information for all hobbyists.
Aerated Submerged Bed
An aerated submerged bed (Fig. 1A) is a common filtering technique usually incorporated into the sump of a system. Water enters the filter from an inflow pipe placed several inches away from the water surface. When the water from the inflow splashes into the filter container, the water disrupts the surface boundary layer. This disruption enhances gas exchange (O2 in, CO2 out). Many inflows are equipped with a drip plate, which distributes the incoming water over a larger surface area to maximize contact at the water surface. The water then passes through very small diameter media (e.g., fine gravel or limestone) that serves as a high surface area culture medium for nitrifying bacteria.
Trickle filter
A trickle filter (Fig. 1B) is very similar to an aerated submerged bed filter; however, in a trickle filter, the media type is different and it sits above the water level in the filter container. This creates a moist aerobic environment for nitrification. Bio-balls and other lightweight plastic media are common for trickle filters. Again, Many inflows are equipped with a drip plate, which distributes the incoming water uniformly over all media in the filter container.
Figure 1: Aerated submerged bed (A) and trickle (B) filters.
Floating bead filter
A floating bead filter (Fig 2) is a closed system connected inline to the system’s plumbing. Pressurized water is pumped into the filter and forced through a layer of floating media (plastic beads, usually). Many floating bead filters are equipped with a wash impeller to hose down the beads to remove clogs and shear the media. Because this is a closed system, no gas exchange occurs in the filter.
Figure 2: Floating bead filter.
Fluidized bed
Fluidized beds (Fig 3), like floating bead filters, are closed system filtration units. This filter contains fine-grain silica sand that provides a very high surface area environment for nitrifying bacteria. Water is pumped into the bed from the bottom of the container and forced through the silica sand, causing the sand to suspend into the water column. The water flow into the filter allows you to control how much sand suspension (and thus available nitrifying surface area) is used. The flow must be adjusted so that the sand suspension level does not reach the outflow port of the filter, or the sand will be pulled out of the filter with the exiting water.
Figure 3: Fluidized bed.
Rotating biocontactor
In a rotating biocontactor (Fig 4), water is injected into a hollow cylinder of horizontally-stacked disks of screen or filter media that is constantly rotating and ~40 percent submerged. This creates a moist and aerated environment for the nitrifying bacteria and also serves as a shearing tool. Rotating biocontactors are very efficient but must be constantly moving. The bio-wheels that are incorporated into many hang-on-the-back filtration units operate using these principles.
Figure 4: Rotating biocontactor.
Comparison of Biological Filtration Techniques
After water passes through the biofilter, the resulting NO3- will accumulate in the system until it is removed by water changes, or is taken up by plants or heterotrophic sources.
This article introduced the major concepts and components of common biofilters and offered some pros and cons of each. It is important to note that there is immense flexibility within and among these components, and the exact combination and utilization of these components depends on the particular characteristics of the aquarium, as well as the preferences and opinions of the hobbyist.
Joe Szczebak
Biological filtration is primarily responsible for removing nitrogenous waste from aquaria and promoting gas exchange (i.e., O2, NO2, CO2) for pH maintenance. This is usually achieved in two main steps. First, using some type of biofilter, nitrification by chemoautotrophic bacteria oxidizes inorganic ammonia (NH4+) to nitrate (NO3-). Second, the accumulated NO3- is removed from the system via uptake by aquatic plants (e.g., mangrove trees), macroalgal cultures, or regular water changes. In general, a biofilter is any media that effectively cultures units of nitrifying bacteria and exposes those cultures to the system’s nitrogen.
Considerations for efficient biofiltration:
(1) Large surface area media to maximize available nitrifying bacteria biomass. A large amount of small media will usually produce the highest amount of surface area for nitrification.
(2) Thin nitrifying biofilm to ensure sufficient bacterial contact time with nitrogen. If the bacterial layer is too thick (more than several layers), many of the bacteria will not gain access to the nitrogen source and are thus wasted.
(3) Maximal bacterial access to O2 and a carbon source (e.g., bicarbonate), and minimal exposure to light (light inhibits nitrification).
(4) Appropriate water speed that cycles as much water through the filter as possible but still gives the bacteria enough contact time with the water to nitrify.
(5) Some type of shearing capacity to ensure the bacterial biomass is not clogged or smothered (like raking the leaves off your lawn).
(6) Low maintenance and operational cost.
Fortunately, many of the popular biofilters on the market incorporate these considerations into their compact and efficient designs. Some common biofilters include submerged beds, trickle filters, floating bead filters, fluidized beds, and rotating biocontactors (bio-wheels).
Note: Reef tanks do not necessarily require additional biological filtration due to the live rock and other substrates associated with their aquascaping and design. The below biolfiters are discussed so serve as a general source of information for all hobbyists.
Aerated Submerged Bed
An aerated submerged bed (Fig. 1A) is a common filtering technique usually incorporated into the sump of a system. Water enters the filter from an inflow pipe placed several inches away from the water surface. When the water from the inflow splashes into the filter container, the water disrupts the surface boundary layer. This disruption enhances gas exchange (O2 in, CO2 out). Many inflows are equipped with a drip plate, which distributes the incoming water over a larger surface area to maximize contact at the water surface. The water then passes through very small diameter media (e.g., fine gravel or limestone) that serves as a high surface area culture medium for nitrifying bacteria.
Trickle filter
A trickle filter (Fig. 1B) is very similar to an aerated submerged bed filter; however, in a trickle filter, the media type is different and it sits above the water level in the filter container. This creates a moist aerobic environment for nitrification. Bio-balls and other lightweight plastic media are common for trickle filters. Again, Many inflows are equipped with a drip plate, which distributes the incoming water uniformly over all media in the filter container.
Figure 1: Aerated submerged bed (A) and trickle (B) filters.
Floating bead filter
A floating bead filter (Fig 2) is a closed system connected inline to the system’s plumbing. Pressurized water is pumped into the filter and forced through a layer of floating media (plastic beads, usually). Many floating bead filters are equipped with a wash impeller to hose down the beads to remove clogs and shear the media. Because this is a closed system, no gas exchange occurs in the filter.
Figure 2: Floating bead filter.
Fluidized bed
Fluidized beds (Fig 3), like floating bead filters, are closed system filtration units. This filter contains fine-grain silica sand that provides a very high surface area environment for nitrifying bacteria. Water is pumped into the bed from the bottom of the container and forced through the silica sand, causing the sand to suspend into the water column. The water flow into the filter allows you to control how much sand suspension (and thus available nitrifying surface area) is used. The flow must be adjusted so that the sand suspension level does not reach the outflow port of the filter, or the sand will be pulled out of the filter with the exiting water.
Figure 3: Fluidized bed.
Rotating biocontactor
In a rotating biocontactor (Fig 4), water is injected into a hollow cylinder of horizontally-stacked disks of screen or filter media that is constantly rotating and ~40 percent submerged. This creates a moist and aerated environment for the nitrifying bacteria and also serves as a shearing tool. Rotating biocontactors are very efficient but must be constantly moving. The bio-wheels that are incorporated into many hang-on-the-back filtration units operate using these principles.
Figure 4: Rotating biocontactor.
Comparison of Biological Filtration Techniques
After water passes through the biofilter, the resulting NO3- will accumulate in the system until it is removed by water changes, or is taken up by plants or heterotrophic sources.
This article introduced the major concepts and components of common biofilters and offered some pros and cons of each. It is important to note that there is immense flexibility within and among these components, and the exact combination and utilization of these components depends on the particular characteristics of the aquarium, as well as the preferences and opinions of the hobbyist.
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