A new study is warning that while wastewater treatment has become an increasingly common practice in developing countries to remove excess nutrients and make the water cleaner, the surge in treatment may be harming ecosystem biodiversity by creating an imbalance between phosphorus and nitrogen.
The PNAS study (Proceedings of the National Academy of Sciences of the United States of America), authored by a consortium of scientists, including experts from the Norwegian Institute of Water Research, found that in 2005, the percentage of municipal wastewater being treated on a national scale in China was only about 40%. In 2017, it reached more than 90%. From another perspective, 1,535 facilities existed in 2008, and by 2017 there were nearly 5,000.
Researchers monitored the influence of this wastewater treatment surge on nutrient regimes in 46 major receiving lakes in China and found a stark increase in nitrogen and phosphorus mass ratios with phosphorus decreasing. In other words, there is an imbalance in removal efficiency, with phosphorus much more easily removed during wastewater treatment.
“This growing imbalance has important implications for aquatic ecology that remain poorly considered and understood,” the study states. It argues that it may be time for wastewater treatment plants to develop more efficient nitrogen removal technologies, or for current regulations to be updated.
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One of the potential risks of an imbalance in nitrogen and phosphorus ratios is phytoplankton blooms and toxin production in downstream waterbodies.
In 2015, the United Nations Development Program set a target to halve the proportion of untreated wastewater in the world by 2030. The study’s authors point out that “the challenge of imbalanced lake ratios caused by improved sanitation is not confined to China, but also likely occurs in other countries.”
The study’s authors recommend that short-term strategies for addressing nutrient imbalance could include refining the operations of existing facilities, developing more efficient nitrogen-removal technologies, and introducing new standards that set nutrient ratio targets for effluent discharge. In the longer term, the study’s authors suggest that increasing nutrient recovery from municipal wastewater along with source separation of human excrement may also be promising.
“More effort should be placed on (nitrogen) removal from municipal wastewater in the future to achieve desired water quality outcomes as human society seeks to achieve (sustainable development goals) for sanitation, water, and aquatic ecosystem health,” the study states.