A biological filter contains bacteria which convert ammonia to nitrite and nitrobactor converts nitrite into nitrate. Nitrate is plant fertilizer, which is much less toxic than nitrite.
Ammonia occurs naturally in the environment. A small amount of ammonia is generated when lightning strikes and reaches earth in rainfall. But most ammonia is produced by bacteria in water and soil as an end product of plant and animal waste decomposition. It is found in relatively low nontoxic concentrations in soil, air, and water and provides a source of nitrogen for plants.
In soils and water ammonia will go through many complex biochemical transformations. These transformations constitute what is commonly known as the nitrogen cycle.
Ammonia in Water
Water reacts with ammonia to form ammonium and hydroxide ions. Ammonia is often referred to as “unionized ammonia”. Ammonia is toxic to aquatic organisms but ammonium is non-toxic. There exists an equilibrium in water between the toxic ammonia and the non-toxic ammonium. The equation shifts back and forth depending upon existing or introduced environmental changes.
NH3 (aq) + H2O (l) NH3 · H2O (aq) NH4 + (aq) + OH - (aq)
(Ammonia in water) (ammonia+water) (ammonium + hydroxide ions)
The dynamic equilibrium between NH3 and NH4 + is affected by water temperature and pH (acidity). At a pH of six the ratio of ammonia to ammonium is 1 to 3000 but decreases to 1 to 30 when the pH rises to eight (becomes less acidic). Warm water will contain more toxic ammonia then cooler water. When sampling water for ammonia analysis both the temperature and the pH of the surface water body must be measured at the same time the water samples are collected. (See “Ammonia, pH and Temperature Calculator”)
If ammonia is directly spilled into surface water or if water used by a fire department to depress an ammonia vapor cloud is allowed to reach surface water, aquatic life can be harmed. Even at a concentration of 0.02 mg/L (48 hour LC50) unionized ammonia is lethal to some sensitive freshwater fish. That equates to about ½ a cup of unionized ammonia in one million gallons of water. Ammonia is also highly toxic to freshwater invertebrates having a 48-hour LC50 of 0.66 mg/L for Daphnia magna . Again, water contaminated with fertilizer ammonia should not be allowed to enter any storm drains, rivers, drainage ditches, wetlands or lakes.
From Ecological Effects of Ammonia
A mechanical filter stops all solids.
John explains this is both a biological and mechanical filter
From What Is A "Biological Filter"?
By Stan and Debbie Hauter
Your Guide to Saltwater Aquariums.
The biological filter in a saltwater aquarium is nothing more than a place for bacteria to grow. Any surface in an aquarium which comes in contact with tank water and the nitrobacter & nitrosomona bacteria species that you created when you cycled the tank is part of your bilogical filter. The velocity of the water passing over the bacteria also affects the efficiency of the biological. The faster the water moves over the bacteria, the more efficient it is, up to the point where the bacteria is being stripped from the filter surface.
The efficiency (strength) of a biological filter is determined by its surface area and the amount of water passing over it. Some filter medium are more efficient than others. Aquarium glass surface area aside, here are the most popular biological filtration methods:
Canister Style Filters come in many different styles and sizes and can also be multifunctional.
Live Rock / Berlin Systems are the filters of choice for many SW aquarium purists.
Live Sand / Jaubert Systems are also very popular with many reef aquarists.
Undergravel Filters are among the first biological filters created for SW aquarium hobbyists.
Wet / Dry Trickle Filters were the next step after the UGF to dramatically improve efficiency.
Biological Filtration Polls show you which biological filters are popular with other habbyists.
Choosing Your Biological Filter Material shows you which materials are more efficient.
Knowing how each biological filter works and what it requires will make Choosing a Filtration System In 5 Easy Steps much easier.