Constructed Wetlands Treat Waste Water with Weeds
Effluent processing all over the world requires large amounts of energy and/or chemicals to treat the waste water, or the waste water is improperly filtered before being returned to the environment. There are a number of solutions to lessen the waste water load while at the same time producing a net benefit. Systems that include the collection of urine to be used as fertilizer and methane digesters that create fuel from feces [see ‘Biogas’ section at bottom of the just-linked article] are a couple of such solutions. Another solution — constructed wetland filtration systems for homes, communities, and industrial sectors — are efficient in both processing ability and energy requirements.
These artificial wetlands provide a near zero energy input way to treat local effluent with no negative side effects. The process is free of both chemicals and odours, provides habitat for wildlife, increases the diversity and aesthetics of any site, and, depending on the toxicity of the inflowing effluent, can potentially create a yield, such as fodder for livestock.
In-ground installations are usually of the horizontal flow variety. This means that effluent enters at one end of a bed and flows, horizontally, under the surface of the bed (gravel, sand, etc). The micro organisms clean the water as it flows through, eventually exiting at the opposite end of the container as treated water.
To install a basic bed, simply dig an appropriate sized hole for your application and fill it with a liner to prevent any waste-water from seeping into the environment. The beds should be deep enough to accommodate at least 30cm of water depth. Fill the bed with gravel, sand, etc. Another few centimeters of substrate above this 30cm area is suggested to accommodate the variance in the water levels that result from rains or heavy use of the waste-water system.
The level of the water can be allowed to rise higher than the substrate, and can even be maintained to create a pond like environment by allowing the water to flow on the surface rather than in the subsurface.
Often a traditional septic tank will still be required as the primary means of treatment. The effluent from the septic tank can then flow to secondary and even tertiary treatments with a combination of reeds, typha, and sphagnum.
It is best to include as many species of filtration plants in the system to counter potential weaknesses present in the individual species. Systems of sphagnum and typha may increase acidity under conditions where calcium carbonate is present in the inflow effluent at rates >100 mg/L. These concentrations of calcium carbonate would not likely occur in any non-mining waste water. Just as in all other parts of permaculture design, diversity yields the best results.
Containers can be constructed with a horizontal flow, as with a traditional ground system, or of vertical flow design. The horizontal works in the same fashion as a ground installed system; effluent enters one end of a sealed bed and exits the other as treated water.
Vertical flow systems instead introduce effluent through the top of the container. Gravity then feeds the effluent down through various layers of filtration-assisting substrates. The container traps oxygen and aids in breaking down ammonium. Vertical flow units can process stronger effluents in a smaller footprint than horizontal flow units. Combination systems can be installed to achieve even higher levels of filtration.
In-ground (above) and container (below) peat beds
In a typha latifolia only filtration bed, there may be an increase in concentration of fine, suspended solids. Reduced iron concentration of effluent being the largest of any measured change in parameters, but sphagnum and reeds show similar levels of iron reduction. Typha dominant beds reduced sulfate levels between 1-4%. Lead is not shown to be absorbed by typha.
Euell Gibbons called cattails the "Supermarket of the Swamps", stating that you could eat their rhizomes, young shoots, and pollen, but, one should avoid eating any vegetation grown in a constructed wetland treatment system with unknown inflow pollutant levels. Avoid feeding the vegetation to livestock to be consumed if the contaminants include heavy metals, but in a family scale system, these metals should not be present in your closed-loop waste water cycle. In this case, feeding the vegetation to livestock, even for consumption, will be safe, but direct consumption of vegetation by humans should still be avoided.
Typha provides a great deal of habitat. Growing 1.5-3m tall, it hosts ducks, geese, red-winged blackbirds, muskrats, turtles, frogs and insects. Typha rhizomes provide a similar niche to reed roots for microbial life. One negative is that it can grow too dense and clog a system, causing overflow. This is remedied with simple cutting back of the vegetation, but it is still bi-annual maintenance that should not be discounted. Around the world it holds status varying from a serious weed in Hungary, to a principal weed in Australia, Germany, Italy, Zimbabwe, Spain, Tunisia, and a common weed in Argentina, Iran, Kenya, Portugal, and the USA.
When used alone, sphagnum will increase the magnesium content of the out-flowing effluent. It is tied with typha for best iron concentration, but any combination of the three results in similar iron level reductions. Sulfate concentrations are reduced greatest, 12%, when sphagnum is dominant in the bed.
Sphagnum moss has other uses in water filtration. The Oxford Community Center in St. Paul, Minnesota installed a wetlands filtration system, mimicking the natural filtration seen in the pristine lakes of northern Minnesota, for their pool. Within days the pool-goers began commenting that the usually strong chlorine smell of the pool was not present. "Now you don’t smell it anymore," said Lynn Waldorf, aquatics director for the City of Saint Paul. The constructed wetlands resulted in clearer pool water, less irritation by asthma suffering patrons, a savings of $35,000USD in chemicals over one summer, and an additional $100,000USD in revenue from additional visitors.
As with all three plants, sphagnum absorbs iron. Reduced iron levels will correlate to lower microbial content. The bio-film produced by these bacteria corrode the pipes. These bio-films also absorb chlorine in the water, thus increasing the need to always add more. This is where chemical savings come into play; by removing the bio-film the chlorine requirements of a pool system will decrease by 50%.
The company that installed the St. Paul pool wetland has been experimenting with sphagnum filters for home drinking water, exclaiming that in the two years their demo system had been installed many visitors thought they were drinking bottled water.
Lastly, the company is also experimenting with treating spas and home hot tubs with sphagnum moss. When hot tubs are manufactured and tested, the aforementioned bio-film will have already had a chance to form before the hot tub is even sold. By using sphagnum treated water in the test phase, consumers will get a “clean” tub from the factory. Continued use of sphagnum in the hot tub will inhibit further colonization from bacteria as well as reduce the chlorine consumption.
Care in harvesting sphagnum needs to be taken. Much of England’s sphagnum has been harvested non-sustainably, but current practices in New Zealand allow for a carefully monitored 3-year sustainable harvesting cycle. The environment in these bogs is so sensitive, the sphagnum is hand harvested and flown out via helicopter.
The 4m tall common reed, Phragmites australis, is most commonly used in these wetland filter systems, mimicking the natural habitat of flood plains and estuaries. They have the most extensive root system for micro organisms to colonize. This root system increases the porosity of the substrate in which it is grown, resulting in a patchwork of aerobic, anoxic, and anaerobic conditions. There are many other varieties of reed that can thrive in just about any climate. During the growing season the chemical-resistant reeds themselves can account for about 15% of the pollutant removal, taking up and concentrating pollutants. Take proper care to dispose of the tainted reeds; avoid feeding them to livestock and the compost heap if pollutant levels are high.
Reeds can be good for finishing off the treatment of effluent to help reach regulatory levels, or to deal with seasonal use. Off season use, as in a caravan park, can mitigate runoff entering sensitive water courses while guests are using the property whilst still allowing the treated water to eventually reach its water course.
When establishing the reed bed, weeding will likely be required until the reeds have completely colonized the bed. Ideally 70% or more of the bed will be covered with reeds. The other 30% should include some combination of sphagnum or typha for best results. After 3-5 seasons the reeds can be cut back and allowed to regrow from their rhizomes.
A 17-year old demonstration reed bed has been meeting the environmental requirements in England with only basic maintenance. Yearly emptying of the preceding septic tank, which would be necessary in a traditional soak away system, and maintaining the bed by cutting back the growth every few seasons is all that is required. The gravel in the 17-year old system has never been replaced.
The microscopic life in the bed is a main processor of the pollutants. While living in the substrate and oxygen-rich root systems, these micro organisms metabolize the chemicals in the effluent, effectively mineralizing them. With longer processing times, even hard to remove pollutants such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyl (PCB), dyestuffs, amines and glycols can be treated. At the end of the first Gulf War, oil polluted sands in Kuwait were treated with oil metabolizing organisms that live on the roots of the plants in the area.
Gravel allows faster flows, but has lower microbiological activity. For this reason gravel is more often used as a secondary or tertiary treatment substrate. By planting reeds or typha in gravel, you can use it as a primary treatment bed. The plants allow for more micro organisms to colonize the substrate, providing a perfect niche for life.
Soil is often used in a primary or secondary treatment bed. Soil has the added benefit of being able to encourage metal ions, phosphate and sulphate to be deposited. Different soil blends can be used to treat specific site requirements. A small amount of clay in the substrate can trap chemicals while the micro organisms digest them, this can help cope with large, infrequent amounts of effluent being passed through the system. This is often refereed to as a “shock load”.
Peat mediums (PDF) will absorb phosphorous the best, but it accumulates in the peat. Eventually this accumulation reaches equilibrium and the outflow phosphorus levels return to nearly the same as inflow levels. There is still some phosphorous removal, due to suspended solids, but to better control the levels the vegetation, that are absorbing some of the phosphorous, should be regularly harvested. Peat works best as a pre-filter medium, 500mm deep, to remove suspended solids first. Peat filtration also removes any odour and eliminates any insect activity in the constructed wetland.
Septic Tank Effluent v. Peat Effluent over 13 years
Heavy Metals and Acid Mines
By using a cyanide-based extraction technique for heavy metals such as cadmium, nickel, lead, copper,
iron, manganese and aluminum, an acid mine increases pH by 2-3. With such acidity, the heavy metals are easily soluble and can more readily migrate throughout the site. Other effects include destabilizing phosphorous and the now soluble iron pyrite mixing with water and oxygen to create sulfuric acid.
These mines can be cleaned up greatly with the installation of wetland beds. A study in the 1980s in the Appalachian bituminous coal region showed that such a bed populated with typha and sphagnum could reduce acidity of the soil by up to 33%. The iron, manganese and aluminum levels were reduced 20-40%. These results allowed for other vegetation to begin to establish the once far-too-acidic site. The beds were built with only a few hundred millimeters of gravel; the rest was topped with 1 meter of peat. Pipes, 150mm below the surface of the peat — to prevent freezing — introduced the waste water from the mine at timed intervals. The system required fresh peat every 8-15 years, depending on use.
The peat was shown to harbor higher levels of fungal life, protozoa, rotifers, insects, and annelid worms than have been found in other substrates. Organic matter was reduced >90%, total suspended solids were reduced >90%, and fecal Coliform bacteria reduced >99.99%. The typha dominated systems turned over the water in just 19 hours with significantly less subsurface flow than the sphagnum, which took over 150 hours with 60% subsurface flow to turn over its water.
At Eötvös Loránd University (Budapest, Hungary) a study was done on constructed wetlands as effluent treatment. Hungary has many sub-2000 population settlements, of which over 36% do not have any waste water treatment facilities. The studies show that with a reed bed the majority of the processing is done via the micro organisms, but without the plant roots to harbor the micro organisms the treatment is ineffective. The reeds were shown to allow the processing to continue through the winter seasons with very little loss in efficiency.
Four locations were visited during the tests. Felsôcsatár and Egyházasrádóc utilized activated sludge treatment while Kám and Kacorlak each hosted a reed bed. The reed beds performed similar to the activated sludge in most instances, but for less energy and cost. The reed beds outflowed less ammonium and nitrate than the activated sludge processing plants because the reed beds take up and retain more of these chemicals. Phosphates are also found to be more concentrated in the reeds.
The Nimr oil field in Oman is utilizing a series of constructed wetland beds to treat the waste water from drilling activities. The wetland covers 600 hectares and can process a minimum of 45,000 m3/day of contaminated water, while the site produces 800,000m3/day. Additional levels of filtration allow for the recovery of salt from the water after it has been filtered free of contaminants. The project took two years to construct and has an expected productive life of 20 years.
Nimr Treatment System
Systems of all three species showed the largest average change by parameter with a slow flow/long turnover. Typha and sphagnum only systems showed the next highest level of overall efficiency, where Typha systems allow for the fastest flows and highest turnover of effluent. Sphagnum dominant systems, like mixed systems, provided average filtration but with slower turnovers. The residential filtration bed would be fine using any combination of reeds, typha, or sphagnum given there would be a limited amount of heavily contaminated effluent introduced to the system.
The economic and environmental benefits of constructed wetlands filtration systems for small scale use are recognized in chemical- and odor-free treatment, creation of habitat, low setup/maintenance costs of the system, as well as a number of other natural benefits.