Nutrients and other materials which are not extracted from wastewater become pollutants when released unchecked into the environment. These potential pollutants can be removed by natural processes which are established in “designed ecosystems” such as soil filters and constructed wetlands. The resulting purified water can then be reused.
- Biochemical Oxygen Demand (BOD) – requires aerobic (dry) environment. 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.
- Nutrients – Wastewater is chock 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.
Nitrogen – 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 harmless 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.
Phosphorus – requires aerobic environment.
- Dry periods are needed for phosphorus 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.
Metals are readily circulating in our environment and food supply, and hence in our wastewater, even in human waste. The major sources of metal contamination, however, 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.
Particles 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 wetland fill and plants.