Office of Science & Technology - Background Information: Wastewater Treatment at Blue Plains – removal of Carbon (C), Nitrogen (N), and Phosphorus (P)
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Background Information: Wastewater Treatment at Blue Plains – removal of Carbon (C), Nitrogen (N), and Phosphorus (P) Print E-mail
bridges vol. 21, April 2009 / Feature Articles

The influent is pretreated with screens, including the world's largest fine screen installation. Large inerts are held back, before grit is removed in an air-bubbled tank: the heavy particles settle due to gravity, the air bubbles enhance the sedimentation. After this pretreatment, the primary treatment follows: settleable organic particles are removed as primary sludge in a clarifier with low flow velocity. Fats, oils, and grease are skimmed off the surface as scum. Pretreatment and primary treatment are both physical purification methods, while the following secondary treatment is a biological process. Microbes are suspended in basins, actively degrading contaminants - therefore called "activated sludge." Oxygen required for this aerobic process is provided by air diffusers, which keep the dissolved oxygen in the water above 2 mg/l. Microbes oxidize half of the carbon compounds into CO2, the other half are used for buildup of biomass - producing even more activated sludge, even more microbes. This secondary sludge settles in the secondary clarifier and is mostly recycled into the aerated basin - to continue degrading pollutants. Part of the sludge is withdrawn and, along with primary sludge, is dewatered for further treatment. 

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Now that the carbon removal is accomplished, the nitrogen has to be eliminated. Again, it is an aerobic process, requiring air diffusers to oxidize ammonium to nitrate in a process called nitrification. In a following step without aeration, microbes lacking oxygen use nitrate to degrade organics. This process is called denitrification; elementary nitrogen is released into the atmosphere, which is 80 percent nitrogen anyway. Denitrification requires a carbon source, so methanol is added.
One might ask why Blue Plains applies a two-sludge system - removing the C- and N-compounds in two different aerated basins - instead of the frequently used combined removal;  especially since the two-sludge system requires even further C (methanol) to be added. There are several reasons:
  • generally, the slow-growing N-bacteria have difficulty competing with the bacteria that deal with C
  • the C-load in the fairly diluted wastewater can be too low for complete N-removal
  • the design of the plant makes combined treatment difficult: Blue Plains is an old plant. It started operation in 1938, and has been refurbished often since then. The N-removal was implemented in 2000, making Blue Plains an ADVANCED wastewater treatment plant with a two-sludge system: two separate biological process steps for C and for N elimination
The removal of phosphorus is a chemical process:  Iron chloride is added, triggering precipitation of phosphorus. These flocs settle in clarifiers or, at the latest, in a subsequent sand filter. Disinfection completes the treatment. Chlorination is applied to kill pathogens. This then requires bisulfite treatment to remove surplus chlorine before the water - almost tap water quality - is released into the Potomac River. Austrian wastewater treatment plants don't have these final polishing steps: filtration and disinfection. Concerning chlorination, it's a matter of attitude as to what's better or what's worse: having pathogens or chlorination byproducts in the water.



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