Why are nitrogen and phosphorus removed from some wastewater?

In environmental engineering, nitrogen and phosphorus are removed from wastewater primarily to prevent cultural eutrophication, a process where excess nutrients accelerate the aging of aquatic ecosystems.12

When these nutrients enter lakes and streams, they trigger rapid growth of algae and algal blooms. As this algae dies, microorganisms consume large amounts of dissolved oxygen to decompose the organic matter. This depletion of oxygen, or hypoxia, creates an environment that can no longer support fish and other aquatic life, often leading to massive die-offs and the degradation of water quality.2314

Additionally, specific forms of nitrogen pose direct risks:
* Ammonia: This compound is toxic to fish and exerts an oxygen demand on receiving waters as it converts to nitrates.1
* Nitrates: Beyond their role in eutrophication, high levels of nitrates in water supplies can cause serious illness in humans.5

Because these nutrients are not sufficiently removed by standard secondary treatment, tertiary treatment methods—such as nitrification-denitrification or chemical precipitation—are employed when environmental standards require higher water quality.16