Cleaner wastewater makes for healthier rivers

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

Although wastewater treatment reduces the risk of disease, another problem remains: nutrients. Wastewater contains many nutrients (nitrogen and phosphorus), 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, especially for waterways. Rivers become ill when too many nutrients affect the ecosystem. One of the worst culprits is excess ammonia.

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

The study was published in Environmental Quality Journal.

Jarvie and her team investigated what happened when two Canadian cities upgraded their wastewater treatment plants. Waterloo and Kitchener are both on the Grand River. The Grand River is Canada’s largest river, emptying into Lake Erie. Over the past decade, the two cities have started a program called ‘nitrification’ in their sewage treatment plants. Nitrification converts ammonia into other types of nitrogen.

“This ultimately reduces the amount of ammonia in the wastewater that is discharged into water bodies,” says Jarvie.

Thanks to these improvements, the amount of ammonia entering the river went down massively. Before the conversion, the two sewage treatment plants discharged more than 90 tons of ammonia per month. In just one year, the Kitchener wastewater treatment plant reduced its ammonia emissions by 80%. A decade later, total ammonia production had fallen to less than a tonne per month, a 99% drop. Nitrogen still flowed into the river, but now in a quantity and form that are less of a problem for dissolved oxygen levels and for fish.

Jarvie’s team studied how this waste ammonia from the sewage affected the river. One of the biggest indicators of the health of the waterways was the increase in oxygen levels in the water. Too much ammonia depletes oxygen and kills aquatic life. So the Grand River Conservation Authority placed sensors in the river to measure how those vital dissolved oxygen levels were changing.

The river’s oxygen levels vary between daylight hours, when plants produce oxygen, and nighttime, when oxygen is used up. The scientists used the oxygen data to assess the flow’s overall metabolism, i.e. the balance between the amount organisms produce and their uptake. When organisms consume too much, they consume a lot of oxygen.

If the ammonia level was very high, the river would become depleted of oxygen overnight. Impacts were greatest in summer, when the river was most biologically active. For nearly 90% of the summer days prior to nitrification treatment, nocturnal oxygen levels fell below levels required for aquatic life. At the end of the study, nocturnal oxygen fell below the level needed to support the most delicate creatures on only about 6% of summer days.

“This represents an important improvement in the health of the Grand River’s ecosystem as a result of reducing ammonia loads in wastewater,” says Jarvie.

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

“It’s a great success story,” says Jarvie. “We have demonstrated how investments in wastewater management have resulted in important improvements in the ecological health and water quality of the Grand River.”

Improving our waterways means addressing all sources of excess nutrients. Jarvie emphasizes that wastewater is only part of the equation. “Agriculture is another very important source of nutrients for the Grand River, ultimately for Lake Erie and other waterways.”