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Downstream Mitigation and Pollution Removal
Storm-water and wastewater both have high levels of excess nutrients and other materials that if not extracted from the water stream become environmental pollutants. The natural processes established in Designed Ecosystems are effective at removing these potential pollutants through biological degradation, filtration, sedimentation and adsorption. The contaminants of most concern include organic compounds, suspended solids, excess nutrients (nitrogen and phosphorus), sediment, metals and pathogens.
Biological Oxygen Demand (BOD)
Requires aerobic environments
Organic molecules, such as those found in solid human waste, and other substances that require large amounts of oxygen in order to be broken down constitute BOD. When BOD is released into a natural environment its decomposition can rob fish and wildlife of needed oxygen. A constructed wetland will mechanically filter out most solid BOD as it passes through sand, soil, crushed rock or brick, while dissolved BOD is eaten by microbes. Microbes colonize on the surfaces of plant roots where oxygen is made available as plants photosynthesize and transport oxygen from their leaves to their roots.
Total Suspended Solids
Suspended solids are the non-living organic and inorganic particles remaining in the water column. Suspended solids, like sediment, increase the turbidity of water, reducing light penetration and thus inhibiting plant growth. Suspended solids may clog the gills of fish and other aquatic organisms, and potentially toxic organic compounds and metals bind to the suspended particles.
Wastewater is full of valuable nutrients, which if properly extracted and recycled can create excellent fertilizers. However, when nutrient-rich wastewater is allowed to enter ponds and streams it creates algae blooms, which, like BOD, deplete waters of oxygen needed by other plant and animal species. This process of oxygen depletion due to excessive algal growth is called eutrophication.
Requires aerobic (dry) and anaerobic (wet) environments
Nitrogen treatment requires a two-step nitrification-denitrification process. The transformation of nitrogen is made through the metabolism of microbes as they consume a balanced diet of carbon (which is available in plant tissue and solid human waste), phosphorus (available in wastewater and stored in soil filters), and nitrogen (either as a gas or from organic matter). The first step, nitrification, requires oxygen (aerobic) and occurs almost instantaneously as ammonia (NH4) combines with carbon dioxide in the air (CO2) to form nitrates (NO3) and harmless hydrogen gas (H2). The second step of denitrification is anaerobic and may take up to 7 days. In this step, carbon ( C ) combines with nitrates from step 1 to form the relatively benign gases nitrogen (N2) and carbon dioxide (CO2).
If denitrification is incomplete then harmful nitrates or poisonous nitrous oxide (NO2) may be released. The limiting factor for denitrification in a wastewater treatment system is the availability of carbon for the microbe's balanced diet.
Requires aerobic environment
Dry periods are needed for phosphorous to chemically bond to soil, sand, or brick. The capacity for phosphorus adsorption is limited by the quantity of material to which it can bond. Flooding will release phosphorus. Brick is especially well suited to the task of locking up phosphorus through the formation of iron phosphate.
Particles such as soils, plant and animal matter that is carried in moving water. High levels of sediment will cloud water preventing light and oxygen from reaching aquatic life. Sediment may settle out in a holding tank or be filtered out by coarse substrates and plants.
Metals are readily circulating in our environment and food supply, and hence in our wastewater, even in human waste. However, the major sources of metal contamination are from industrial waste. Metals and other toxic substances found in our water can accumulate in aquatic organisms, mammals, birds, reptiles and humans. Although designed ecosystems often work better to remove such substances from wastewater than conventional wastewater treatment systems, the objective remains to keep such toxic elements out of the wastewater stream. Wetland fill and plants gradually accumulate metals, which attach to soil or organic particles. Some plants are particularly suited to taking up and storing metals.
Substances released by the roots of some plants can kill some pathogenic bacteria. The final step of the water treatment process requires either Ultra Violet sterilization or the addition of low levels of Chlorine. Both options kill pathogens and ensure the cleanliness of the water during storage and transportation in pipes.