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“Legacy P” Poses Long-Term Challenge To Water Quality

Scott Gordon WisContext

Phosphorus is one of the most important components in the ongoing struggle to balance agricultural prosperity with water quality. When farmers fertilize their fields with this essential nutrient, plants use some, with precipitation carrying excess amounts into nearby bodies of water, fueling algal blooms that can kill fish and and endanger drinking water quality.

But phosphorus that lingers in the soil, sometimes for decades, is troublesome as well.

What scientists call “legacy phosphorus” — or “legacy P,” a common abbreviation — exists in a sort of nutrient limbo.

Agriculture Phosphorus

Phosphorus can easily bind to soil particles, a state in which plants don’t usually access it. The nutrient generally must be in water plants take up in order for them to use it. Moreover, as farmers spread more fertilizer, new quantities of phosphorus are made available to crops. But as land erodes or heavy rains hit fields, some of that older phosphorus will make its way into streams, rivers, lakes and ultimately groundwater. In coming decades, greater quantities of legacy phosphorus could get flushed into water sources due to climate change, which is projected to increase heavy precipitation events in the Midwest.

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Turning Phosphorus in Wastewaster into Fertilizer

Chicago officials boast that the Stickney Water Reclamation Plant is one of the world’s largest sewer treatment plants, handling the waste of 2.3 million people.

The Stickney Water Reclamation is the biggest single source of phosphorus in the entire region that drains into the Mississippi River. Combined with other sewage plant releases a state task force concluded that these plants are responsible for about half of the phosphorus pollution in rivers that drain into the Mississippi.

Stickney Water Reclamation Plant

The need for more aggressive and widespread action is especially acute in Illinois, which by most estimates is the largest contributor of phosphorus and nitrogen pollution to the Gulf of Mexico.

Gulf Pollution

A new $31 million project will help to reduce that pollution by diverting wastewater through three reactors that use catalysts to form tiny, nutrient-rich “pearls” for the fertilizer industry. The district estimates the equipment will produce up to 10,000 tons of slow-release fertilizer a year and reduce the Stickney plant’s phosphorus discharges by about 30 percent.

More still needs to be done, but this project is a good start.

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