Treatment of spent caustic waste

1. A method for treating a waste stream comprising:

providing an ethylene production spent caustic comprising at least a first organic compound;

providing a refinery spent caustic comprising at least a second organic compound;

combining at least a portion of the ethylene production spent caustic with at least a portion of the refinery spent caustic in a mixing vessel in a ratio sufficient to produce a combined stream comprising a COD level below that which results in the precipitation of carbonate during wet air oxidation of the combined stream; and

oxidizing by wet air oxidation the combined stream at an elevated temperature and a super-atmospheric pressure sufficient to treat at least a portion of the second organic compound.

2. The method of claim 1 , further comprising:

monitoring a pH of the oxidized combined stream with a pH sensor; and

adding a pH adjuster to the combined stream responsive to a signal from the pH sensor.

3. The method of claim 2 , wherein the pH adjuster is carbon dioxide.

4. The method of claim 2 , wherein the pH adjuster is a base.

5. The method of claim 1 , further comprising biologically treating at least a portion of the oxidized combined stream.

6. The method of claim 5 , further comprising adjusting the pH of the portion of the oxidized combined stream prior to the biological treatment.

7. The method of claim 1 , wherein the step of combining at least a portion of the ethylene production spent caustic with at least a portion of the refinery spent caustic comprises combining at least a portion of the ethylene production spent caustic with at least a portion of the refinery spent caustic in a piping system used to treat the combined stream.

8. The method of claim 1 , further comprising adding at least one of amine wastewater, sour wastewater, wash water, decant slop, and biological sludge to the combined stream.

9. The method of claim 1 , wherein oxidizing the combined stream comprises oxidizing the combined stream at a temperature in a range of between about 200° C. and about 280° C.

10. The method of claim 1 , wherein the second organic compound is a naphthenic compound.

11. The method of claim 1 , wherein the second organic compound is a cresylic compound.

12. The method of claim 1 , wherein the COD level of the combined stream is less than about 125,000 mg/L.

13. A method for treating a waste stream comprising:

providing a sulfidic spent caustic comprising at least one oxidizable compound;

providing a refinery spent caustic comprising at least one organic compound;

combining at least a portion of the sulfidic spent caustic with at least a portion of the refinery spent caustic in a ratio sufficient to produce a combined stream comprising a COD level below that which results in the precipitation of carbonate during wet air oxidation of the combined stream; and

wet air oxidizing the combined stream at an elevated temperature and a super-atmospheric pressure sufficient to treat at least a portion of the organic compound.

14. The method of claim 13 , further comprising:

monitoring a pH of the oxidized combined stream with a pH sensor; and

adding a pH adjuster to the combined stream responsive to a signal from the pH sensor.

15. The method of claim 14 , wherein the pH adjuster is carbon dioxide.

16. The method of claim 15 , wherein oxidizing the combined stream comprises oxidizing the combined stream in a wet air oxidation unit and introducing the carbon dioxide into the combined stream upstream of the wet air oxidation unit.

17. The method of claim 14 , wherein the pH adjuster is a base.

18. The method of claim 13 , further comprising biologically treating at least a portion of the oxidized combined stream.

19. The method of claim 18 , further comprising adjusting the pH of the portion of the oxidized combined stream prior to the biological treatment.