Wastewater Treatment May Cause Imbalanced N / P Ratio in Lakes

Frequent outbreaks of lake eutrophication and harmful phytoplankton blooms have become a major water environment problem at present in the world. The reason can be attributed to the excessive inflow of nitrogen and phosphorus into waterbodies due to intensified human activities, which has become increasingly uncontrollable due to climate warming and other causes. As the most important nutrients essential for life, nitrogen and phosphorus play different roles in the growth and outbreak of lake phytoplankton.

A recent research conducted by Yindong Tong’s team from Tianjin University and some other renowned research institutions at home and abroad indicates that large-scale construction of municipal wastewater treatment facilities is an important contributor to imbalanced ratio of nitrogen to phosphorus in lake phytoplankton. The relevant paper entitledImprovement in municipal wastewater treatment alters lake nitrogen to phosphorus ratios in populated regionswas published in the Proceedings of the National Academy of Sciences of the United States of America, a well-known international academic journal.

It is reported that to master the long-term evolution rules of balance between lake nitrogen and phosphorus is the premise to identify lake nutrient regime changes and to predict harmful phytoplankton blooms. It also lays the basis for formulating lake nutrient control objectives in the future.

After investigating the historical changes of the nutrient regimes in 46 major lakes in China, Yindong Tong’s team has discovered that, in the light of the changing trend in recent ten years, eutrophication has occurred in some of the lakes in eastern China, demonstrated by rapid declining lake phosphorus concentration and basically stable lake nitrogen concentration, as well as escalating stoichiometric ratio of nitrogen to phosphorus. These changes can be significant factors leading to the competitive status of non-N2 fixing cyanobacteria in lakes.

Through the research, it is also discovered that, the total nitrogen (TN) becomes surplus to the total phosphorus (TP) and that serious stoichiometric imbalance of nitrogen to phosphorus occurs in some anthropogenic discharge sources, particularly in municipal wastewater discharge. The ratio of nitrogen to phosphorus in waterbodies increased nearly twice compared with ten years ago, due to the difference of removal efficiency of TN and TP in wastewater treatment.

This research indicates that, to construct municipal wastewater treatment facilities that satisfy the demand of aquatic ecosystem, it is necessary to consider not only nitrogen and phosphorus removals, but also the influence the imbalanced N / P ratio resulted from wastewater treatment has on aquatic ecosystem. It will serve as an important reference for developing discharge standards of municipal wastewater treatment for the sake of the aquatic ecosystem.

By Zhu Jianglin

Editor: Eva Yin