Tertiary treatment in wastewater treatment plants focuses on further reducing contaminants to a level where the treated water can be safely discharged into the environment or reused. Nutrient removal, particularly of nitrogen and phosphorus, is a critical aspect of tertiary treatment, especially in areas where excessive nutrients can cause eutrophication in receiving water bodies.
One common method for nutrient removal is Biological Nutrient Removal (BNR), which employs bacteria to metabolize nitrogen and phosphorus compounds in the wastewater.
Chemical precipitation is another effective method for phosphorus removal. Chemicals such as aluminum sulfate (alum) and ferric chloride are added to the wastewater, reacting with phosphorus to form insoluble compounds that settle out of the water as a precipitate, which can then be removed through sedimentation or filtration.
Remaining metal ions can be precipitated out of the water through chemical precipitation as well. Calcium oxide (lime) and sodium hydroxide are mixed into the water to raise pH levels which allow the metals to precipitate.
Physical separation techniques, such as filtration and sedimentation, are commonly employed to remove suspended solids, which may contain nutrients like nitrogen and phosphorus. These processes improve water clarity and reduce nutrient levels in the effluent by physically separating particulate matter from the water.
Advanced treatment technologies, including membrane filtration, ozonation, and advanced oxidation processes (AOPs), can further enhance nutrient removal by targeting specific contaminants that may not be effectively removed by conventional methods. While these technologies can be expensive, they are highly effective in achieving stringent nutrient removal requirements.
Constructed wetlands or biofilters can also be utilized in tertiary treatment for nutrient removal. These systems use plants and microorganisms to uptake and assimilate nutrients, thereby reducing nitrogen and phosphorus levels in the water.
Tertiary treatment to remove nutrients typically combines biological, chemical, and physical methods, customized for the wastewater's traits and the desired water quality. Choosing the right treatment depends on factors like nutrient levels in the incoming water, required cleanliness standards, and the available facilities and resources.
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