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Phosphorus is an essential element for plant life, but when too much of it drains into the water, it causes excessive algae growth.
There are two main types of phosphorus, dissolved and particulate. The particulate type binds itself to sediment particles and erosion control keeps it in check. Dissolved phosphorous isn’t bound to anything and is much more difficult to stop from percolating through soil.
Chad Penn is a research soil scientist at the USDA’s Agriculture Research Service. He’s part of a team developing a filter that removes phosphorus from water.
"You have a filter media that has a high affinity for phosphorus. And so, the water is essentially forced to go through it, flows through the media and the phosphorous absorbs to the media, it accumulates in the media, clean water goes out," says Penn. "Just like all filters, once they’re no longer able to absorb phosphorus, they start to get filled up, you replace the media with new material."
The filters are engineered structures and will last for several years, removing variable amounts of phosphorus depending on feasibility and site constraints. Penn says the design and size of the system will vary, but they all need to be placed where the water naturally converges.
"Water has to flow through them so if it’s overland flow, there are some places in fields where water just naturally goes through it, for example a grass waterway, you can build a filter there. It can’t just skim over the top of it, it has to go through it," he says. "If it’s a pipe, then that makes it a lot easier, the water’s already concentrated in one location and you can force it through your bed of media."