(Perma)Culture and Sanity
Metals such as lead, mercury, arsenic, aluminum, cadmium and more are circulating in our environment and food supply—even in human waste. Metals accumulate in aquatic organisms, mammals, birds, reptiles and humans—the higher up the food chain we get, the more of these metals accumulate in our bodies.
Before discussing how these metals are collected and removed by constructed wetlands (which typically remove metals much more thoroughly than conventional wastewater treatment systems), it's worth noting for the long term that by far the wisest course would be to keep such toxic elements out of wastewater streams in the first place.
Wetland soil and plants remove metals from a waste stream when plants' roots absorb the metals and incorporate them into their tissues. Some plants, including Typha spp. (cattails) and Juncus spp. (bulrush) are particularly good at removing metals from polluted water, removing 90%+ of most metals from wastewater streams.
Using plants to clean water gives clean water, but polluted plant matter. When plants are used to remove metals from water, the plant matter that results must be treated as toxic waste. This usually means periodic collection and burning of the plants, and then disposal of the resultant ash as if it were toxic waste (which, at this point, it is). This points out, once more, the short-sightedness of releasing metals and other pollutants into our water sources, whether this is done in situ before we use the water, or in our homes and industries.
In Nature, sand dunes and wetland marshes act as natural soil filters for contaminants and nutrients—bacteria in soils and colonizing plants' roots strip minerals as well as organic nutrients from water. Plants in wetlands supply the mud with oxygen and carbon, releasing them as gases and exudates through their roots or as organic matter when roots die. Plants' roots support decomposition microbes which actively remove nutrients and pollutants from water and mud. Organic matter and other pollutants would otherwise cause problems such as creating fish toxins, and removing too much oxygen from the water in the process of decomposing (Biological Oxygen Demand—BOD).
Wetlands support both anaerobic and aerobic processes, and have evolved plants which can thrive in both environments. Since both aerobic and anaerobic processes are important for completely cleaning water of organic pollutants, the man-made, or 'constructed' wetlands used to clean water should include both oxygen-rich and oxygen-deprived treatment areas. In fact, alternating aerobic and anaerobic conditions function best for removing organic pollutants, so some parts of our systems will benefit from periodically flooding and then draining to re-admit oxygen to soil pore spaces.
In addition to sand and plants' roots, gravel, brick, and earth also filter soil and we will use each of these materials to create systems that most-efficiently treat water in the smallest area and to greatest beneficial result. Crushed red brick, for instance, contains iron oxides that are very effective at binding and removing phosphorous from water streams...if plant roots can access the brick, the plants then remove the phosphorous and use it to create yields of foods or plant materials for crafts, construction, mulches, etc.
This is very different from the situation in conventional waste treatment facilities where phosphorous and other valuable nutrients are removed and then simply abandoned—buried in landfills, dumped into rivers or leached into groundwater (although in some cases nowadays phosphorous-containing municipal sludge is turned into compost, possibly questionable because of heavy metals but probably better than abandoning the phosphorous).
Constructed wetlands are combinations of vertical 'soil' filters (again, many materials function as the 'soil' for different cleaning purposes) through which water percolates downward or flows upwards, and horizontal soil filters through which wastewater is directed as in a canal or underground stream. The filters or canals can be intermittently or permanently flooded, creating aerobic, combination aerobic and anaerobic, and purely anaerobic conditions. For best results, aerobic conditions should follow anaerobic conditions, with a combination of aerobic and anaerobic ('faculative') processes in the middle.
By combining different aerobic conditions and a variety of filtering materials with a diverse collection of aquatic and riparian plant species, constructed wetlands mimic the powerful cleansing effects of natural marsh ecosystems. Constructed wetlands can clean wastewater better than conventional systems without using chemicals and can be designed as parks (as Beaumont, Tx has done), as wildlife refuges or as productive agroforestry systems. Similar in initial cost to conventional wastewater treatment plants, maintenance costs are far lower for constructed wetlands since they use little or no chemicals, energy or labor to maintain.
How natural and constructed wetland systems work to clean polluted or waste water; pros and cons of constructed wetlands as compared with conventional systems.
Issues surrounding how to clean polluted water, and how to keep it clean in the first place!
A description of the working parts of a constructed wetland wastewater treatment system, describing their biological functions and variations.
Reprinted from (Perma) Culture and Sanity Website