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Why is ammonia in wastewater harmful to fish?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

Ammonia is a significant concern in wastewater treatment because it is directly toxic to fish.1

Beyond its direct toxicity, ammonia is harmful to aquatic environments because it exerts an oxygen demand in receiving waters as it undergoes conversion into nitrates. This process depletes dissolved oxygen levels, which is critical because maintaining adequate oxygen is essential for the survival of aquatic species. Furthermore, the resulting nitrates can promote excessive algae growth, leading to eutrophication, a process that further degrades water quality and disrupts the balance of ecosystems.12

Because of these risks, environmental engineering practices often employ methods such as nitrification-denitrification or ammonia stripping to remove nitrogen compounds before treated water is discharged.1