Cleaner wastewater makes healthier rivers

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The Grand River in Glen Morris, Ontario, in Canada. The Grand River is Canada’s largest river inflow into Lake Erie. Upgrades to the wastewater treatment plant improved the ecosystem health of this river by reducing the amount of ammonia flowing into the water. Credit: Helen Jarvie

Have you ever thought about where your waste goes? For people who live in cities, it goes to a treatment plant. However, the treated wastewater eventually finds its way into a local waterway. This means it could end up in a nearby stream, river, or lake.

Although wastewater treatment reduces the threat of disease, another problem remains: nutrients. Wastewater contains a large amount of nutrients (nitrogen and phosphorous), including from urine and feces. All plants and animals need nutrients to grow and thrive; however, too much of a good thing is a big problem, particularly for waterways. Rivers get sick when too many nutrients damage the ecosystem. One of the worst offenders is excess ammonia.

“Ammonia is a nitrogen compound produced by the decomposition of organic matter in wastewater. Discharge of ammonia into waterways can have direct toxic effects, but it can also cause significant oxygen depletion that threatens the survival of the aquatic life, including fish,” says Helen Jarvie. Jarvie, a professor of water sciences at the University of Waterloo in Canada, studies how these nutrients affect waterways.

The study was published in the Environmental Quality Magazine.

Jarvie and his team studied what happened when two Canadian cities upgraded their wastewater treatment plants. Waterloo and Kitchener sit along the Grand River. The Grand River is the largest river in Canada that flows into Lake Erie. Over the past decade, the two cities have begun a program called ‘nitrification’ at their wastewater treatment plants. Nitrification converts ammonia into other types of nitrogen.

“Ultimately, this reduces the amount of ammonia in the wastewater that is discharged into waterways,” says Jarvie.

Thanks to these improvements, there was a massive drop in the amount of ammonia entering the river. Before the changes, the two sewage plants were discharging more than 90 metric tons of ammonia per month. In just one year, the Kitchener treatment plant reduced its ammonia emissions by 80%. A decade later, total ammonia production had dropped to less than a metric ton per month, a 99% decrease. Nitrogen was still flowing into the river, but now it was in an amount and form that is less of a problem for dissolved oxygen levels and fish.

Jarvie’s team studied how this drop in ammonia from the sewage affected the river. One of the biggest signs of the health of the waterways was the increase in the amount of oxygen in the water. Too much ammonia depletes oxygen, killing aquatic life. So the Great River Conservation Authority placed sensors in the river to measure how these vital levels of dissolved oxygen changed.

The river’s oxygen levels vary between the hours of the day, when plants produce oxygen, and the night, when oxygen is consumed. The scientists used the oxygen data to assess the river’s overall metabolism, which is the balance between how much organisms produce and how much they consume. When organisms consume too much, they consume too much oxygen.

When ammonia levels were really high, the river’s oxygen levels were depleted overnight. The effects were greatest during the summer when the river was most biologically active. On almost 90% of the summer days before the nitrification treatment, nighttime oxygen dropped below levels needed to support aquatic life. At the end of the study, nighttime oxygen fell below the levels needed to support the most sensitive creatures on only about 6% of summer days.

“This represents a significant improvement in the health of the Grand River ecosystem, as a result of reductions in ammonia loads in the effluent,” says Jarvie.

The river’s metabolism was rebalanced and oxygen levels improved. After improvements to sewage treatment plants, reduced oxygen consumption meant the river was in better overall health.

“This is a great success story,” says Jarvie. “We have shown how investments in wastewater management have produced a significant improvement in the ecological health and water quality of the Great River.”

Improving our waterways will mean addressing all sources of excess nutrients. Jarvie emphasizes that wastewater is only part of the equation. “Agriculture is another very important contributor of nutrients to the Great River, ultimately to Lake Erie and other waterways.”


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More information:
Helen P. Jarvie et al, Fingerprints and Metabolic Regimes of Rivers Reveal Ecosystem Responses to Improved Wastewater Treatment, Environmental Quality Magazine (2022). DOI: 10.1002/jeq2.20401

Provided by the American Society of Agronomy

Citation: Cleaner Wastewater Makes Healthier Rivers (2022, Oct 17) Retrieved Oct 17, 2022 at https://phys.org/news/2022-10-cleaner-wastewater-healthier-rivers.html

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