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Dual-stage biological process for removing nitrogen from wastewater

  • US 5,650,069 A
  • Filed: 09/08/1995
  • Issued: 07/22/1997
  • Est. Priority Date: 09/08/1995
  • Status: Expired due to Term
First Claim
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1. An activated sludge process for biologically removing nitrogen from wastewater, comprising the steps of:

  • a) forming mixed liquor from influent wastewater and activated sludge;

    b) treating the mixed liquor in a first nitrogen removal stage, including the substeps of;

    i) directing the mixed liquor into a first anoxic zone,ii) maintaining the mixed liquor under anoxic conditions within the first anoxic zone for a sufficient time period to permit denitrifying microorganisms to reduce nitrates and nitrites in the mixed liquor to elemental nitrogen,iii) transferring a portion of the mixed liquor from the first anoxic zone to a first aerobic zone,iv) aerating the mixed liquor within the first aerobic zone for a sufficient time period to permit nitrifying microorganisms to convert ammonia and organic nitrogen in the mixed liquor into nitrates and nitrites, andv) circulating the mixed liquor between the first anoxic zone and the first aerobic zone to effectuate denitrification and nitrification;

    c) treating the mixed liquor in a second nitrogen removal stage by passing the mixed liquor through a second anoxic zone and a second aerobic zone to effectuate further denitrification and nitrification;

    d) transferring the mixed liquor treated in the first and second nitrogen removal stages to a final clarifier where settled sludge is separated from purified supernatant;

    e) retaining the sludge in the final clarifier for a period of time sufficient to permit denitrifying microorganisms in the sludge to adapt to respiring nitrates and nitrites instead of oxygen;

    f) recycling a portion of the sludge from the final clarifier to form the mixed liquor; and

    g) recycling another portion of the sludge from the final clarifier into the mixed liquor between the first nitrogen removal stage and the second nitrogen removal stage so as to increase the population of microorganisms in the second nitrogen removal stage that have adapted to respiring nitrates and nitrites instead of oxygen.

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