Constructed wetlands

Blume and I were talking about constructed wetlands the other day and I, in a nod to the Modern Jackass style, told her knowledgeably that they operated by allowing cattails and other semi-aquatic plants to grow in a bed fed by wastewater (true!), and that they were excellent at pulling both Nitrogen and heavy metals out of the soil. “But,” she asked, “why wouldn’t you end up with a bunch of heavy-metal polluted cattails?” Good point, said I. It turns out the wikipedia page I originally skimmed has the answer(s), the first of which is that the plants aren’t really doing the heavy lifting:

Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as they break down organic materials. This community of microorganisms is known as the periphyton. The periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about seven to ten percent of pollutants, and act as a carbon source for the microbes when they decay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment.

And the second of which is that the heavy metals that the plants take up largely end up as part of ecologically useful compounds at the end of the decay process:

Phosphorus is coprecipitated with iron, aluminum, and calcium compounds located in the root-bed medium.

So, as it appears to turn out, if you have Chromium or something in your water supply a constructed wetland might not be your answer. They are, on the other hand, really well suited to removing excess phosphorous and nitrogen, and also for elimination of harmful bacteria and viruses and so on before the excess water drains into groundwater.


One Comment

  1. Posted August 19, 2008 at 5:59 pm | Permalink

    They’re also great with acid mine drainage, which is a huge issue anywhere there’s been mining done (the empty mines fill with groundwater, which is acidified by the waste material and exposed rock, and inevitably leaches out, killing streams).

    The first step is aeration, which means you can do lots of cool fountainy/cascady things as part of the project.

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