Nitrification is required when a wastewater effluent ammonia-nitrogen limit is specified. Nitrification is the biological oxidation of ammonia to nitrite (NO2) and then to nitrate (NO3). The microorganism responsible for this conversion are the autotrophic species Nitrosomonas and Nitrobacter
Denitrification is required when the wastewater effluent nitrate nitrogen or total nitrogen limit is specified. Nitrates are converted to nitrogen gas through denitrification which is achieved under anoxic (absence of free dissolved oxygen) conditions. Nitrate reducing bacteria are facultative heterotrophs.
Phosphorus can be removed biologically to levels of 1 - 3 mg/l without the addition of chemicals. If the discharge requirement is less than 1 mg/l, chemicals such as alum or ferric chloride can be added for co-precipitation.