Adapted from: Pennsylvania DEP (1998). Working With Nature - New Wastewater Technologies for Pennsylvania - Onlot and Small Flow Systems.
Most biological recycling of organic nutrients occurs in the upper layers of the soil, called the "bio-active zone." Typical wastewater systems, such as septic systems and leach fields, are placed below the bio-active zone allowing little nutrient recycling to take place. A constructed wetland allows the nutrients in wastewater effluent to be beneficially used by aquatic plants and microorganisms.
compared to open surface wetlands and are more commonly used for individual households. By keeping the water below the surface of the gravel medium, there is less chance of odors escaping, less human contact, less chance of mosquito breeding, and faster "treatment" of the water due to more of the water being exposed to the microbially populated gravel surfaces and plant roots. The subsurface water is also less inclined to freeze during cold weather.
Constructed wetlands generally consist of one or more lined beds, or cells. The gravel media in the cells should be as uniform in size as possible and should consist of small to medium size gravel or stone, from one foot to three feet in depth. A layer of sand may be used either at the top or the bottom of a gravel medium, or a layer of mulch and topsoil may be applied over the top of the gravel. In some cases, gravel alone will be used with no sand, mulch or topsoil. The sides of the wetlands are bermed to prevent rainwater from flowing into them, and the bottom may be slightly sloped to aid in the flow of graywater through the system. A constructed wetland for a household, once established, requires some maintenance, mainly the annual harvesting of the plants, which can be composted.
In any case, the roots of aquatic plants will spread through the gravel as the plants grow. The most common species of plants used in the wetlands are the cattails, bulrushes, sedges and reeds. Graywater is filtered through the gravel, thereby keeping the growing environment wet, and bits of organic material from the graywater become trapped in the filtering medium. Typical retention times for graywa-ter in a subsurface flow wetland system range from two to six days. During this time, the organic material is broken down and utilized by microorganisms living in the medium and on the roots of the plants. A wide range of organic materials can also be taken up directly by the plants themselves.
Bacteria, both aerobic and anaerobic, are among the most plentiful microorganisms in wetlands and are thought to provide the majority of the wastewater treatment. Microorganisms and plants seem to work together symbiotically in constructed wetlands as the population of microorganisms is much higher in the root areas of the plants than in the gravel alone. Dissolved organic materials are taken up by the roots of the plants, while oxygen and food are supplied to the underwater microorganisms through the same root system.18
Aquatic microorganisms have been reported to metabolize a wide range of organic contaminants in wastewater, including benzene, napthalene, toluene, chlorinated aromatics, petroleum hydro carbons and pesticides. Aquatic plants can take up and sometimes metabolize water contaminants such as insecticides and benzene. The water hyacinth, for example, can remove phenols, algae, fecal col-iforms, suspended particles and heavy metals including lead, mercury, silver, nickel, cobalt and cadmium from contaminated water. In the absence of heavy metals or toxins, water hyacinths can be harvested as a high-protein livestock feed. They can also be harvested as a feedstock for methane production. Reed-based wetlands can remove a wide range of toxic organic pollutants.19 Duckweeds also remove organic and inorganic contaminants from water, especially nitrogen and phosphorous.20
When the outdoor air temperature drops below a certain point during the winter months in cold climates, wetland plants will die and microbial activity will drop off. Therefore, constructed wetlands will not provide the same level of water treatment year round. Artificial wetlands systems constitute a relatively new approach to water purification, and the effects of variables such as temperature fluctuations are not completely understood. Nevertheless, wetlands are reported to perform many treatment functions efficiently in winter. One source reports that the removal rates of many contaminants are unaffected by water temperature, adding, "The first two years of operation of a system in Norway showed a winter performance almost at the same level as the summer performance." Some techniques have been developed to insulate wetland systems during the colder months. For example, in Canada, water levels in wetlands were raised during freezing periods, then lowered after a layer of ice had formed. The cattails held the ice in place, creating an air space over the water. Snow collected on top of the ice, further insulating the water under-
It is estimated that one cubic foot of artificial wetland is required for every gallon per day of graywater produced. For an average single bedroom house, this amounts to about a 120 square foot system, one foot deep. Some constructed wetland situations may not have enough drainage water from a residence to keep the system wet enough. In this case, extra water may be added from rain water collection or other sources.
Aquatic plants used in constructed wetland systems can be divided into two general groups: microscopic and macroscopic. Most
Figure 9.7: AQUATIC PLANTS
Figure 9.7: AQUATIC PLANTS
normal water line
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