About Nutrient Removal
Nutrient removal from wastewater consists of treating wastewater to remove nitrogen and phosphorus before it reenters natural waterways. High levels of nitrogen and phosphorus in wastewater cause eutrophication, a process where excess nutrients stimulate excessive plant growth such as algal blooms and cyanobacteria. The decomposition of the algae by bacteria uses up the oxygen in the water causing other organisms to die. This creates more organic matter for the bacteria to decompose. In addition, some algal blooms can produce toxins that contaminate drinking water supplies.
As authorized by the Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program regulates point sources, such as municipal wastewater treatment plants, that discharge pollutants as effluent into the waters of the United States. In recent years, many of the States’ environmental bodies have lowered nutrient limits to arrest eutrophication. Maryland’s effort to protect the Chesapeake Bay and its tidal tributaries is perhaps the most notable example of nutrient removal in the US. Nutrient removal continues to be a growing area of focus for wastewater treatment throughout the world.
The removal of nitrogen and phosphorus require different nutrient removal processes. To remove nitrogen, the nitrogen is oxidized from ammonia to become nitrate through a process called nitrification. This process is then followed by denitrification where the nitrate is reduced to nitrogen gas which is released to the atmosphere and removed from the wastewater.
Nitrification is a two-step aerobic process which typically takes place in aeration tanks. Denitrification requires anoxic conditions to encourage the appropriate biological conditions to form. The activated sludge process is often used to reduce nitrate to nitrogen gas in anoxic or denitrification tanks.
Phosphorus can be removed biologically using polyphosphate accumulating organisms (PAOs) which accumulate large quantities of phosphorus within their cells and separate it from treated water. Phosphorus removal can also be achieved by chemical removal. Once removed as sludge, phosphorus may be stored in a land fill. However, many municipalities and treatment facilities are looking to resell the biosolids for use in fertilizer.