The following were extracted from the Internet :-
Also known as trickle filters, wet/dry filters work on the principle that the beneficial colonies of ammonia neutralizing bacteria grow best in the presence of well oxygenated water. By ``trickling'' water over unsubmerged plastic gizmos or other media, wet/dry filters provide a very large air/water surface area. They come in many shapes and sizes. The boom in successful saltwater aquariums in the 1980's can be attributed to the use of this filter type.
Many things can used for the media, with the best providing great amounts of surface area, while at the same time having large openings to reduce the tendency to clog and ensure efficient gas exchange. The problem of clogging of the media can also be reduced by prefiltering the water with an efficient mechanical filter, and (when used) with a protein skimmer.
Another specialized type of filter is designed to help in the control of the accumulation of nitrates, the end product of the neutralization of ammonia by the biological activity of bacteria. These fall into two categories, the anoxic bacterial, and the plant/algal scrubbers (discussed in the next section). It has been discovered that colonies of bacteria which grow in oxygen poor environments can be harnessed to biologically consume nitrate, and release harmless nitrogen gas. This method is achieved in one of two ways. The process was first developed in the 1980's through the use of a box system, coil, or porous foam block which allowed very slow transmission of nitrate-laden water. Inside the box/coil/foam, sugar was placed, and the slow passage of water quickly became anoxic. In these anoxic conditions, bacteria would grow and consume excess nitrate. Many aquarists have reported failure in their attempts at this type of filtration.
More recently, hobbyists have developed similar anoxic conditions below plates at the bottom of their tanks buried in fine sand. In the saltwater systems, these sand beds are referred to as "live sand". In freshwater planted systems, fine grain substrates are allowed to develop anoxic zones which probably also have a denitrification capability.
The Berlin Method of reef aquariums involves the use of large quantities of live rock harvested from tropical reefs. Aquarists report good nitrate control in live rock systems, which, though not well understood, probably involves the denitrification of the nitrates within the interior of the rocks. Another school of thought is that the heavy growths of calcareous algae on the live rocks in Berlin Method reef aquariums consume nitrate.
A new anoxic process to remove nitrates from the sewage wastewater has been installed at the Sewage Treatment Plant (STP). The equipment for denitrification uses two parallel towers with packing, similar to the existing trickling filters, onto which microorganisms will attach for feeding on the nutrients in the passing water. The system was installed between the primary settling tanks and the trickling filters.
The anoxic process operates under conditions of very low free-oxygen so those microorganisms which feed on organic matter must use the oxygen in nitrates for their metabolism.
Both explained the denitrification process differently. Post your opinion here !