The root zone approach to wastewater treatment uses wetland plants and naturally occurring microorganisms associated with their roots to remove contaminants from wastewater. The technique is to develop a special kind of "constructed wetland" through which the wastewater flows. Waste materials are filtered out, taken up and broken down within the wet root zone of the system. Sewage water filtered through a series of reed beds can be purified to recreational, even potable, water standards.
Constructed wetlands remove nutrients, organic compounds and metallic ions from wastewater while increasing oxygen and pH levels. As dirty water enters the wetland, bacteria already present in the system remove the waste by transforming it into food and CO2 for the plants, which in turn let off oxygen, some of which remains in the water and helps nurture the bacteria. An important component of the system are plants which are killed by the pollution. They sink to the bottom and form the basic building blocks for new waste-eating bacteria. As the system is gravity-propelled, untreated wastewater enters the marsh in a variety of places, so no one part of the system receives the brunt of the waste and there is no unpleasant smell.
Surface irrigation provides a viable way to dispose of wastewater effluent, but it suffers from several limitations. Surface irrigation systems – including overland flow, surface drip, flood irrigation, and surface spray-irrigation of forests – often are weather-dependent and limited in disposal capacity. Subsurface irrigation options exist, but such systems also have limitations. For example, leach fields are highly site-specific and, like surface irrigation, their water uptake rates depend on soil type. In addition, standard leach field pipes are often clogged by roots. Another method, aquifer injection, is environmentally undesirable because it can cause water pollution.
Interest has steadily increased in the use of natural physical, biological, and chemical aquatic processes for the treatment of polluted waters. In these, wastewater is treated principally by bacterial metabolism and physical sedimentation. The plants take up nutrients through their roots but perform little actual treatment themselves, serving instead as an excellent substrate for microbial biomass which provides significant treatment.
The water hyacinth Eichornia crassipes has been studied extensively for use in these systems. The major advantages are their extensive root systems and rapid growth rate. Their major limiting feature is cold temperature sensitivity, confining its use to the southern states. Other species, such as pennywort Hydrocotyle umbellata and duckweed (Lemna spp., Spirodela spp., Wolffia spp.), have greater cold tolerances than hyacinths and have also been used in these systems. These systems can provide effective secondary wastewater treatment or nutrient removal, depending on organic loading rate. They have been used most often for either removing algae from oxidation pond effluents or for nutrient removal following secondary treatment.
The predominant mechanism for nitrogen removal is nitrification-denitrification, while phosphorus is removed through plant uptake, microbial immobilization into detritus plant tissue, and retention by sediments. Nitrogen and phosphorus removal by the plants is achieved only with frequent harvesting. Periodic removal of accumulated sludge is required. Where anaerobically generated hydrogen sulphide odour and mosquito breeding are problematic, design modifications such as step-feeding of inflows, recycling of effluent, supplemental aeration, and frequent harvesting of plants are effective.
Constructed wetlands vary in their pollutant removal capabilities, but can effectively remove a number of contaminants. Among the most important removal processes are the purely physical processes of sedimentation via reduced velocities and filtration by hydrophytic vegetation. These processes account for the strong removal rates for suspended solids, the particulate fraction of organic matter (particulate BOD), and sediment-attached nutrients and metals. Oils and greases are effectively removed through impoundment, photodegradation, and microbial action. Similarly, pathogens show good removal rates in constructed wetlands via sedimentation and filtration, natural die-off, and UV degradation. Dissolved constituents such as soluble organic matter, ammonia and ortho-phosphorus tend to have lower removal rates. Soluble organic matter is largely degraded aerobically by bacteria in the water column, plant-attached algal and bacterial associations, and microbes at the sediment surface. Ammonia is removed largely through microbial nitrification(aerobic)-denitrification(anaerobic), plant uptake, and volatilization, while nitrate is removed largely through denitrification and plant uptake. In both cases, denitrification is typically the primary removal mechanism. The microbial degradation processes are relatively slow, particularly the anaerobic steps, and require longer residence times, a factor which contributes to the more variable performance of constructed wetlands systems for these dissolved constituents.
Phosphorus is removed mainly through soil sorption processes which are slow and vary based on soil composition, and through plant assimilation and subsequent burial in the litter compartment. Consequently, phosphorus removal rates are variable and typically trail behind those of nitrogen. Metals are removed largely through adsorption and complexation with organic matter. Removal rates for metals are variable, but are consistently high for lead, which is often associated with particulate matter.
Conventional waste water treatment plants are energy intensive, expensive and require skilled personnel to operate and maintain. Constructed wetlands are cheap and simple to build, require little maintenance, and even offer environmental protection that is pleasing to the eye. They purify waste water as it flows through a substrate planted with wetland plants, where processes such as filtration, adsorption, oxido-reduction and nitri/denitrification predominate.
In the early 1980s, some German researchers had been constructing experimental wetlands, inspired by ancient and traditional methods of using wetlands for water purification. In Denmark, the method was introduced in the mid 1980s as a response to a growing demand in the countryside for technological alternatives to conventional treatment systems which could meet new stringent emission standards and still be affordable. A grassroots movement developed from housing owner associations and similar groups arguing the need for decentralized solutions and lobbying authorities to licence such facilities.
One of the first comercial entrepreneurs was a Danish agronomist. The market boomed for a few years and over 250 constructed wetlands were developed in Denmark. Established in 1984, Danish Rootzone Technique (DRT) is a small advisory body which plans and supervises construction of systems in operation. Such rootzone systems, reasonable in cost and needing only the occasional inspection, are established in ponds 60 cm deep, which are lined with a plastic membrane to prevent sewage seeping out. The ponds are partly filled with carefully blended soil, which is planted with reeds or other water plants. Once the plants are grown, sewage is passed through the pool. The organic material is broken down by soil micro-organisms. The plants absorb nitrogen. Phosphates and heavy metals are bound to the soil. According to tests, the system is 75% to 95% effective in cleaning sewage. Such a system could go on working for 100 to 200 years before it needs a soil change. Hundreds of rootzone systems, looking perfectly natural in the landscape, are already operating effectively in Denmark since 1984.
Whilst markets are now declining in the early innovator countries (Germany and Denmark), the technology is developing and markets are expanding in other countries, particularly in the USA, southern and eastern Europe, and more recently Africa and Asia. Newer applications are to treat total runoff for petrol stations and other producers of oily waste water.
In 1995, a half a square kilometre marsh on the Karon Beach on Phuket Island in Thailand was reported to be used as a water treatment plant. With a capacity to treat 1,000 m3 of wastewater per day, it serviced the homes of the resort town's 1,200 families at almost no cost. The government of Phuket was able to treat wastewater at one-sixth of the construction costs of a conventional mechanical system. Maintenance was cheap and simple; no electricity, no chemicals, no operational staff, no tariff collection. Just one municipal employee to check the water twice daily during its five-day flow through the system and the quality of the effluent into the ocean. The concept is promising in developing countries such as Thailand, which have an abundance of publicly owned land but have moved slowly to build mechanical wastewater treatment facilities because of high costs.
The systems are very popular in rural areas of the southern USA where, depending on the amount of land available, they clean everything from simple municipal waste to drainage from strip mines, hog farms and railway car washing facilities.
Wetlands need time to mature. In the first year their cleaning performance is only around 50%, but does reach 80-90% in subsequent years.
The opportunity to build these kinds of treatment facilities is fading fast. Rapid development is pushing up land prices, while publicly owned land is being encroached on at an alarming rate.