Final answer:
Phosphorus is removed to prevent excessive algae growth that leads to eutrophication, oxygen depletion, and the formation of dead zones in aquatic environments.
Step-by-step explanation:
Phosphorus is removed prior to discharge to receiving waters since it is a nutrient that can cause excessive algae growth. High concentrations of phosphorus, often from sources such as fertilizer runoff and from sewage, can lead to a process known as eutrophication. During eutrophication, the rapid growth of algae leads to the formation of thick mats that can block sunlight and clog fish gills. Moreover, when the algae die, their decomposition depletes the dissolved oxygen in the water, which is critical for aquatic life, causing death in species such as shellfish and finfish. This often results in a dead zone, an area with very low oxygen levels where most marine life cannot survive.