Sewage is made up of carbonaceous waste and a small amount of nitrogen waste. Nitrogen is a found in proteins and in ammonia, an important compound in sewage. In domestic sewage the ammonia could make up about a half of the total BOD. Urine is the major source of ammonia in domestic sewage. Ammonia needs a lot of oxygen and certain bacteria to decompose it to nitrate. This process is called nitrification.
There are a large number of bacteria that carry out carbonaceous oxidation but nitrifying bacteria require special attention. This is because the numbers of nitrifying bacteria are low, reproduction is slow, oxygen requirements are high and reaction with ammonia is slow and reduced further in cold weather.
Nitrate is a compound containing nitrogen and oxygen. Complete nitrification occurs when all ammonia has been converted to nitrate. The nitrate is then broken down via a process called denitrification. Denitrifcation occurs under anoxic conditions, when there is no dissolved oxygen in the sewage, and the bacteria utilise the oxygen in the nitrate and release nitrogen gas, as bubbles. If these bubbles attach themselves to sludge it may be lifted and float to the surface.
Denitrification can be a nuisance if it occurs in the wrong place. Anoxic zones or tanks are sometimes created to ensure denitrification occurs where it can be properly controlled. Anoxic conditions can occur in final settling tanks. This can cause rising sludge when the liberated nitrogen gas buoys up the sludge and lifts it to the surface resulting in high SS in the effluent. Nitrate compounds behave as nutrients when discharged to rivers and can encourage the growth of algae. The EA sometimes requires nitrate to be removed from sewage effluent and the consent may place limits on nitrate compounds. When this is the case, nitrification and denitrification will form part of the treatment process.