Biological Filtration Basics - Tech Talk 125
Biological filtration is one of the most important aspects of filtration design for aquatic animal welfare. There are actually 3 distinct processes that fall under the heading of biological filtration: mineralization, nitrification and denitrification. Nitrification is typically the most commonly discussed and widely known facet of biological filtration and is undoubtedly the most critical since it deals with the breakdown of strongly toxic animal wastes, but all aspects of biological filtration are important and must be considered when designing a functional life support system for aquatic habitats. Biological filtration is one of the most important aspects of filtration design for aquatic animal welfare. There are actually 3 distinct processes that fall under the heading of biological filtration: mineralization, nitrification and denitrification. Nitrification is typically the most commonly discussed and widely known facet of biological filtration and is undoubtedly the most critical since it deals with the breakdown of strongly toxic animal wastes, but all aspects of biological filtration are important and must be considered when designing a functional life support system for aquatic habitats.
Mineralization is a fancy word for decomposition, where complex organic material is degraded by bacteria into its simplest parts. In aquariums and aquaculture, this organic material is typically derived from fecal material and uneaten food from the animals. Heterotrophic bacteria digest this material, preventing it from building up to unsafe levels in displays or farms. The end products of mineralization are mostly inorganic nitrogen and phosphorus. Mineralization is not the only source of nitrogen in aquatic systems. The direct production of ammonia as a waste product from fish (urine) also contributes to the nitrogen load in aquatic systems. Ammonia nitrogen can be toxic to aquatic life (some species will die at levels as low as .5 mg/L). This is why nitrification is so critical. It's important to understand that it is the un-ionized form of ammonia, NH3, that is so toxic. By dropping the pH of a tank, it's possible to convert toxic ammonia, NH3, to NH4+, ammonium, a much less toxic compound. This is easier to do in freshwater systems with pH near or below neutral than it is in marine systems with pH in the 8 to 8.2 range. It's important to realize that pH manipulations are just a "band aid" fix for ammonia management, and a well designed biofilter is still essential.
Several strains of bacteria can use the ammonia nitrogen for food. Nitrosomonas, Nitrosococcus, and Nitrobacter are the best-known organisms responsible for this process. Ammonia is the most toxic end product of mineralization and fish metabolism. Nitrosomonas and Nitrosococcus convert the ammonia (NH3) to a less toxic compound, nitrite (NO2). While less toxic than the ammonia, nitrite can still be lethal to fish in very small amounts (in the 1–5 mg/L range). Finally, Nitrobacter converts the nitrite to nitrate, the least toxic form of nitrogen. Each of these forms of nitrogen—NH3, NO2 and NO3—is progressively less toxic to the animals. The final end product, NO3, is well tolerated by fish but, unless managed, will tend to build up. This brings us to the last biological filtration process, denitrification, where bacteria remove nitrate from the system water.
While nitrate is not strongly toxic, animals don't live with elevated nitrate levels in nature, so keeping nitrate levels low is important in reproducing natural conditions for aquatic life. Historically this was done with periodic water changes in closed systems, but more recently this practice has been frowned upon by local municipalities wanting to limit the discharge of excess nitrogen into natural waterways and sanitary sewer systems.
Instead of water changes, there are two primary methods for denitrification, the carbon fed digester and the sulfur bed digester. Some strains of bacteria known as facultative anaerobes can use the oxygen in the nitrate (NO3) for energy, converting NO3 into atmospheric nitrogen (N2). The carbon digesters require the addition of an organic carbon source to feed the bacteria, typically a short chain alcohol (methanol or ethanol) or a simple sugar. This method has the drawback of having to measure the addition of the carbon compound. Over- or under-dosing can upset the balance of the process or, worse yet, carry unreacted alcohol back to the tank. The sulfur-based method relies on the activity of several bacteria strains that consume sulfur and nitrate without adding other chemicals. Thiobacillis denitrificans and other similar bacteria can use the oxygen in nitrate for energy. The trend in the industry is the sulfur bed digester because of its simplicity.
The NO3 to N2 reaction (called "reduction") consumes alkalinity, so the sulfur is typically mixed with crushed oyster shell or aragonite (a source of carbonate for buffering) to keep the pH stable. Monitoring D.O. is especially important since the environment inside the filter should be oxygen-poor, but not oxygen-free. If too much oxygen is present, the bacteria will respire aerobically (with oxygen) and no nitrate reduction will occur, but if too little oxygen is present, the filter can go anaerobic (without oxygen) and produce toxic hydrogen sulfide.