Constructed wetlands (CWs) are often mentioned as possible solutions to certain water pollution problems. They certainly are well-studied and reliable large-scale aquatic installations. Where there is plenty of land and no need to use it for other purposes, a CW can be a great ultra-low maintenance water polishing facility. They probably aren’t the right solution if you have a high throughput of water needing polishing, especially if that water has a high concentration of nutrients. CWs don’t really recycle nitrogen (it just gets “mineralized” into the atmosphere), and their effect on phosphorus is to accumulate it as an insoluble precipitate until there is so much you need to dig up and replace the whole shebang, maybe every thirty years. While I’m not opposed to CWs per se, I’m pretty much not a CW person, as I valorize area-efficient, energy-efficient, labor-and-capital-efficient ways to recycle as much nitrogen and phosphorus as possible, and I haven’t seen a way to get many of those requirements out of a CW.
But. What if you think of the CW not as a water polishing facility, but as an ore concentrating factory. One of the worst aspects of a CW, from the nutrient recycling perspective, is its accumulation of insoluble phosphate salts. Granted, you could call it recycling if, every thirty years when you need to grudgingly dig out all that accumulated sediment, you are able to recover that phosphorus, but it’s not like it’s a concentrated source. You probably couldn’t make a profit from it. But what if you could engineer the wetland to precipitate more than just an occasionally recycled fertilizer?
One problem with some wastewaters is their metal ion content. Many of these are toxins like heavy metals, while others are typical industrial metals, like iron. Indeed, iron was once harvested from natural wetlands, where the source water was high in iron and the chemical and biological environment of the wetland precipitated those ions out in the form of various minerals. Possibly you’d be able to design a CW to facilitate this process, although operating costs would likely rise. Many of the unwanted toxic metals, if present, would also tend to precipitate out in these deposits. The idea is to take a known natural process and facilitate it. This is what I do with algae – it already wants to grow, and with a little help it really proliferates. As with this algae process, you would probably have to sacrifice some specificity, as your ore production depends on what wants to happen, not what you want to happen.
Just as there is no landscape that cannot be improved by the presence of a horse (as some claim), there is probably no aquatic bio-process that cannot be enhanced by integration with algal turf scrubbing (ATS, the process I refer to, invented by my colleague and former boss Walter Adey). I’ve pretty much committed to ATS as my humanitarian contribution, so if I look into any environmental situation, I’m mainly interested in it only if it is ATS-integrable. No surprise, then, that if I were to look into improving CW, I would try to figure out what integration of ATS into the wetland process could do. Would I simply withdraw wetland water and discharge it back into the wetland after a single pass? Would I take batches of wetland water out and treat them down to the limit of ATS before discharging back into the wetland? Ultimately, if ATS is so great, maybe the best way to “integrate” ATS into a CW is to replace most of it with ATS, and surround the facility with a decorative screen of wetland.