In aquariums and fish farms biological filtration is required to prevent the build up of toxic ammonia excreted by the animals. Ammonia can be lethal to some species in concentrations less than .5 mg/L. It's well known that certain bacteria can oxidize ammonia first to nitrite and finally to nitrate under the proper conditions (called nitrification, where each compound gets converted to something less toxic). Setting up these ideal conditions is the main focus of biological filtration design.
Biological Filters: Pros and Cons of Different Types - Tech Talk 128
In aquariums and fish farms biological filtration is required to prevent the build up of toxic ammonia excreted by the animals. Ammonia can be lethal to some species in concentrations less than .5 mg/L. It's well known that certain bacteria can oxidize ammonia first to nitrite and finally to nitrate under the proper conditions (called nitrification, where each compound gets converted to something less toxic). Setting up these ideal conditions is the main focus of biological filtration design. In simplest terms, all that is needed for biological filtration is an appropriate material (substrate) for the nitrifying bacteria to grow on. Creative people have used everything from hair curlers to lava rock to plastic army men for homemade biofilters. The real science of biofilter design is optimizing the process to get the biggest usable surface area for bacterial growth in the smallest possible space (footprint). Other considerations are choice of substrate, oxygen utilization, biofouling and flowrates.
There are two general classes of biofilters: moving beds and static beds. The static bed is the traditional "trickle filter" where water is sprayed over a bed of stationary substrate. Moving bed filters have the media suspended in the water column inside a container, and the media is kept in constant motion by air, water or a combination of both. The pros and cons of both types:
Static Bed Filters
- Cost. Static bed filters can be made out of lengths of pipe, garbage cans, buckets or almost anything to hold the media.
- Gas exchange. Typically, static bed filters use media with a lot of empty space for oxygen and carbon dioxide (CO2) exchange. While the empty space reduces the surface area available for bacterial growth, it does promote gas exchange for CO2 and supersaturated gas stripping.
- Simplicity of design. There's really not much to break or go wrong with a filter that can be as simple as a Rubbermaid® garbage can filled with lava rock.
- Optimization. The empty spaces in static bed filters that are great for gas exchange give them less than ideal usable surface area. Also, spraying water randomly over the media typically leaves some of the surface area dry (the water evaporates) and therefore unused.
- Biofouling. Because the media doesn't move, layers of bacteria (biofilms) will tend to build up on the media. These can result in lost usable surface area. If pockets of anaerobic bacterial activity form in the thicker biofilms where oxygen exchange is low, you'll have to clean the media occasionally, which can stress or even kill the bacteria in the filter.
- Flowrates. Trickle filters are named that for a reason. If water goes into a static bed filter at too high of a speed, the bacteria can actually be blown right off the media.
Moving Bed Filters
- Optimization. Moving bed filters tend to use smaller media (to keep it in motion) with better surface area to volume ratios. Since the media is submerged, all available surface area can be colonized—a much more compact footprint than for static bed filters.
- Resistance to biofouling. Moving media is constantly bumping into itself, a natural "scouring" action that causes the older, thicker biofilms to slough off. This sloughing also removes the older bacteria, which get rid of ammonia and nitrite slower than younger bacteria. So you don't have to clean the media, which could stress or kill the bacteria, and the biofilter runs at higher efficiency.
- Flowrates. Moving bed filters can typically handle higher flowrates than similarly sized static biofilters.
- Capital cost. Being more complex, moving bed filters are usually more expensive, but this cost must be weighed against the added efficiency of the filter.
- Oxygen depletion. Since they are so efficient and contain higher densities of bacteria, moving bed filters tend to use more oxygen and generate more CO2, which can require additional gas balancing in the water after it passes through the biofilter.
- Most static bed filters are variations of the packed column (trickle filter)—simply an open container filled with biomedia, bio balls or barrels, lava rock, crushed shell or corrugated media (Brentwood). Water enters at the top and drips down over the media. Plenty of gas exchange but relatively low available surface area to volume ratio (SA/V) for its size (30–150 ft2/ft3).
- Fluidized bed sand filter. An up-flow filter that fluidizes a bed of silica sand, keeping it suspended. Extremely high SA/V ratio (1,500 ft2/ft3). The largest amount of nitrification in the smallest package. Also the biggest oxygen user. Unlike static bed filters, can be pressurized.
- Bead filter. A hybrid mechanical/biological filter. Bed is not constantly moving but can be backwashed like a sand filter to scour media. Can also be pressurized. Good SA/V ratio (400 ft2/ft3).
- Low Space Bioreactor. Intermediate SA/V ratio (260 ft2/ft3) between the static bed and fluidized sand filters. Air is introduced into the media bed, preventing the low oxygen conditions common in fluidized bed filters. The aeration also strips CO2 from the treated water, which is a real innovation in biological filtration. Unlike in a fluidized bed sand filter, the bioreactor's aeration will keep the filter from crashing if the pump fails.