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Research Shows More Efficient Nitrogen Reduction Necessary in Waste Water Treatment

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Large-scale and rapid improvement in wastewater treatment is a common practice in developing countries. Traditionally, this practice is thought to be beneficial for the mitigation of water eutrophication and restoration of freshwaters, since it is effective in removing the nutrients from wastewaters. However, a recent publication on PNAS (Proceedings of the National Academy of Sciences,USA) has pointed out that, the imbalanced removal of nitrogen and phosphorus elements from municipal wastewater might promote the nutrient imbalance in freshwater lakes located in the populated regions. This study is performed by Dr. Yindong Tong, from the School of Environmental Science and Engineering, and his international collaborators.

Like human beings, a healthy aquatic ecosystem needs a balanced diet, with nothing too much and a right balance between elements (known as “nutrient stoichiometry”). Healthy humans need a “healthy” diet, with sufficient fresh fruits and vegetables, and a moderate amount of meat. Similar balance between nitrogen and phosphorus is also important for aquatic ecosystems.

For instance, nitrogen enrichment relative to the phosphorus could favor plankton species with strong competitive abilities for using phorphosus such as toxin-producing Microcystis spp. and Planktothrix spp. The per-cell production of nitrogen-rich toxins (e.g., microcystins) can also be enhanced by nitrogen enrichment under the oligotrophic conditions.

In Dr. Tong’s study, a decadal water monitoring result from 46 freshwater lakes in China has revealed that lake phosphorus concentrations have decline quickly while little or no declines were observed for lake nitrogen concentrations. These unparallel changes in nitrogen and phosphorus have resulted in widespread increase of nitrogen/phosphorus ratios in majority of the investigated lakes. This might explain why blue-green algae persist in lakes such as Lake Taihu and Lake Chaohu, despite the efforts to stop them.

“In addition to reducing the concentrations of nitrogen and phosphorus in lakes, their ratio is also a critical factor in the growth of toxic algae”, Dr. Tong stated in his paper.

To know the drivers responsible for this trend, the researchers traced the changes of major anthropogenic sources (e.g., crop and livestock farming, municipal wastewater discharges) within the study period. They found that, only the municipal wastewater discharge had a quick increase in nitrogen/phosphorus ratios. In wastewater treatment, ammonium can be microbially converted to N2 by nitrification and denitrification, a process that requires careful “tuning” of facility operations.

However, removing P (phosphorus) is a much simpler process only involving coagulation, flocculation, and sedimentation. “This may cause an imbalance in the natural ratio between nitrogen and phosphorus. Even if the total level of nitrogen and phosphorus is lowered, this imbalance will facilitate the growth of toxic blue-green algae”, Dr. Tong said.

The research team also proposed that that the challenge of imbalanced lake nutrient ratios caused by improved sanitation is not confined to China but also likely occurs in other countries since “clean water and sanitation” are targeted by the 17 sustainable development goals recommended by the United Nations and many developing countries worldwide have built or are constructing numerous wastewater treatment plants.

Dr. Tong suggested that the focus on water quality improvement should lie not just with reducing nutrients, but also with the ratio between nitrogen and phosphorus, which addresses the needs for a healthy aquatic ecosystem.

Full article available at:

https://www.pnas.org/content/117/21/11566/tab-article-info

By Tong Yindong from the School of Environmental Science and Engineering

Editor: Eva Yin