Using continuous nitrate measurement to control aeration in an aerated-anoxic process

1. A method of treating wastewater, the method comprising:

introducing the wastewater into an aerated anoxic treatment unit configured to perform bacterial nitrification and bacterial denitrification processes;

flowing an oxygen containing gas into the wastewater in the aerated anoxic treatment unit at a first flow rate;

measuring a concentration of nitrate in the wastewater in the aerated anoxic treatment unit;

comparing the measured concentration of the nitrate to a target nitrate concentration;

adjusting the flow of the oxygen containing gas into the wastewater in the aerated anoxic treatment unit to a second flow rate in response to a difference between the measured concentration of the nitrate and the target nitrate concentration;

maintaining the second flow rate for a time period sufficient for the wastewater to be treated in the aerated anoxic treatment unit and form an anoxically treated liquid having a reduced concentration of the nitrate;

introducing the anoxically treated liquid into an aerobic biological treatment unit;

providing conditions for the anoxically treated liquid to be treated in the aerobic biological treatment unit to form an effluent;

measuring a concentration of nitrate in the effluent;

determining a correlation between the measured concentration of the nitrate in the effluent and the measured concentration of the nitrate in the wastewater in the aerated anoxic treatment unit; and

setting the target nitrate concentration based on the determined correlation to achieve a concentration of the nitrate in the effluent below an effluent nitrate target concentration.

2. The method of claim 1 , wherein adjusting the flow of the oxygen containing gas to the second flow rate comprises decreasing the flow of the oxygen containing gas from the first flow rate to the second flow rate responsive to the concentration of the nitrate in the wastewater in the aerated anoxic treatment unit exceeding the target nitrate concentration.

3. The method of claim 1 , further comprising measuring a concentration of ammonia in the wastewater in the aerated anoxic treatment unit.

4. The method of claim 3 , wherein adjusting the flow of the oxygen containing gas to the second flow rate comprises increasing the flow of the oxygen containing gas from the first flow rate to the second flow rate responsive to the measured concentration of ammonia.

5. The method of claim 4 , further comprising setting a target ammonia concentration to have a value of about five milligrams per liter.

6. The method of claim 1 , further comprising measuring a concentration of ammonia in the effluent.

7. The method of claim 6 , wherein setting the target nitrate concentration comprises setting the target nitrate concentration to achieve an ammonia concentration in the effluent below about one milligram per liter.

8. A wastewater treatment system comprising:

an aerated anoxic treatment unit configured to perform bacterial nitrification and bacterial denitrification processes;

an aerobic biological treatment unit;

a first nitrogen species concentration sensor included in the aerated anoxic treatment unit and configured to output a signal indicative of a concentration of nitrate in wastewater present in the aerated anoxic treatment unit;

a second nitrogen species sensor configured to output a signal indicative of a concentration of nitrate in effluent of the aerobic biological treatment unit; and

a control system configured to:

introduce wastewater into the aerated anoxic treatment unit;

flow an oxygen containing aeration gas into the wastewater in the aerated anoxic treatment unit at a first flow rate;

measure a concentration of nitrate in the wastewater in the aerated anoxic treatment unit using the first nitrogen species concentration sensor;

compare the measured concentration of the nitrate to a target nitrate concentration;

adjust a flow of the oxygen containing aeration gas into the wastewater in the aerated biological treatment unit to a second flow rate responsive to a difference between the measured concentration of the nitrate and the target nitrate concentration;

maintain the second flow rate for a time period sufficient for the wastewater to be treated in the aerated anoxic treatment unit and form an anoxically treated liquid having a reduced concentration of the nitrate;

introduce the anoxically treated liquid into the aerobic biological treatment unit;

control conditions of the aerobic biological treatment unit such that the anoxically treated liquid is treated in the aerobic biological treatment unit to form the effluent;

measure a concentration of nitrate in the effluent using the second nitrogen species concentration sensor;

determine a correlation between the measured concentration of the nitrate in the effluent and the measured concentration of the nitrate in the wastewater in the aerated anoxic treatment unit; and

set the target nitrate concentration based on the determined correlation to achieve a concentration of the nitrate in the effluent below an effluent nitrate target concentration.

9. The system of claim 8 , further comprising a third nitrogen species concentration sensor configured to output a signal indicative of a concentration of ammonia in the wastewater in the aerated anoxic treatment unit.

10. The system of claim 9 , wherein the control system is further configured to increase a flow of the oxygen containing aeration gas responsive to the signal from the third nitrogen species concentration sensor being indicative of a concentration of ammonia in the wastewater in the aerated anoxic treatment unit exceeding a target ammonia concentration.

11. The system of claim 10 , wherein the target ammonia concentration has a value of about five milligrams per liter.

12. The system of claim 8 , wherein the control system is configured to decrease a flow of the aeration gas responsive to the signal from the first nitrogen species concentration sensor being indicative of the concentration of the nitrate in the wastewater in the aerated biological treatment unit exceeding the target nitrate concentration.