How Biological Filtration can works????

A biological filter is quite simply the heart of the RAS system. It's purpose is to convert the waste matter produced by the fish from harmful ammonia into less toxic waste. It is less important to remove solids particles from water than it is to process nitrogen, so if there is to be a compromise between mechanical and biological, err on the side of biological.

In other words, it is much better to allow particles below a certain size to escape back into the pond,while converting a great deal of ammonia to nitrate, than it is to catch every little thing down to a micron or less which in the process would slow the water down to the point where the bacteria have a hard time living (because they're not getting enough oxygen).

The bacteria that convert ammonia to nitrate for us are among a class of bacteria that you may have heard of before. They are the so-called, “nitrogen fixing” bacteria. This means that they take nitrogen that is unavailable to plants in its ammoniacal form, and make it available to plants in an oxidized form.

There are 2 types of bacterial species that colonise the biological filter media. Nitrosomonas sp.bacteria which oxidize ammonia to nitrite, and Nitrobacter bacteria convert nitrite to nitrate.

NH3 + CO2 + 1.5 O2 + Nitrosomonas → NO2- + H2O + H+NO2- + CO2 + 0.5 O2 + Nitrobacter → NO3-

The conversion of ammonia to nitrates is performed primarily by bacteria and other nitrifying bacteria. The primary stage of nitrification, the oxidation of ammonia (NH3) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-).It is important for the nitrites to be converted to nitrates because accumulated nitrites are toxic to plant life

Denitrification

Denitrification is the reduction of nitrites back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.

Ammonia

Ammonia (NH3) is produced by fish as part of their normal metabolic function and is excreted from the gills. The amount of ammonia produced is directly related to the amount of food they eat. Approximately 3-4% of normal 30-40% protein level food will be excreted as ammonia, i.e. for every 100grams of food 3-4grams (3000-4000mg) of ammonia is produced. Fish exposed to unacceptable levels of ammonia risk damage to gills, eyes, fins and skin which can result in them being susceptible to secondary bacterial infection. Using standard drop type tests kits any ammonia reading is considered unacceptable and remedial action should be taken.NitriteAmmonia is oxidized by the Nitrosomonas sp. bacteria in the filter to produce nitrite (NO2). Whilst it is not considered as dangerous as ammonia it can still do serious damage to your fish. High levels of nitrite are likely to stress your fish leaving them susceptible to secondary infection. As with ammonia, target levelsshould be that nitrite is undetectable. Before the fish pond filter can efficiently remove ammonia and nitrite from the fish pond water, it must first become fully colonized with nitrifying bacteria. This can take some time and is a process known as fish pond filter "maturation". Each time a fish is put in the fish pond it will add to the total amount of ammonia being produced. The ammonia level in the fish pond will therefore increase slightly. Because there is more ammonia for the bacteria to utilize, they start to multiply until there are enough to use all of theammonia being produced inside the fish pond. The ammonia level in your fish pond will then fall back to zero.NitrateAs the ammonia level falls, the amount of nitrite produced by the bacteria in the fish pond filter will start to increase. Therefore, the level of nitrite in the fish pond will rise. The increasing nitrite level means that the bacteria that break it down can start to multiply in the fish pond filter until, as with the ammonia, there are enough to use up all the nitrite that is being produced. The nitrite level within the fish pond can then fall to zero. As this occurs, the nitrate level increases.

Conversion of nitrite to nitrate (NO3) is the final stage of the nitrification process.

There is debate as to the possible problems that elevated levels of nitrate may cause.Nitrate and it causes no problem at all. High nitrate may also attribute to green water(phytoplankton). The green water problem can get worst when you clean the biofilter and make water change outs, due to the reduction in bacteria.The bacteria also produces a certain phytoplankton-killing enzyme. As algae starts to grow in the biofilter,or on the walls of the pond, the bacteria loves to feed on this algae, and as it does so it releases the enzyme into the water. Green water is a pain for many reasons. Ultra Violet Clarifier lights will kill single cell phytoplankton algaethat cause green water, and when dead they clump together and can be removed by the filter. However there is sometimes a concern expressed that passing water through the UVC also kills beneficial bacteria.