Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, Reducing COD in wastewater
April, 2003

Reducing COD in wastewater

Q. COD level is in the range of 70 - 180 mg/L. (contributed by IPA, hexane, ethyl acetate, acetic acid & others based on GCMS analysis). BOD level is in the range of 10 - 40 mg/L. Treatment system is only neutralization process. Environmental limit is 100 mg/L. We have some amount of microbial growth in the treatment tanks. Will aeration help to reduce COD? Will H2O2 reduce or increase the COD level? What other treatment method can be use to reduce the COD beside biological treatment?

A. I am assuming that your 100 mg/l limit is a COD limit based on the details you listed below in your question. Aeration should help reduce the COD in 2 ways. First it may evaporate some volatiles like hexane. Second it will add oxygen to the water speeding up any biological oxidation of the less volatile organics by the bacteria (microbial growth). The bacteria (if the water is not toxic, and your comments about existing microbial growth suggest that it is not toxic) will help consume the organics, and the bacteria can be filtered out with a sand filter, or settled out with a gravity clarifier.

Biological waste water treatment is based on maximizing the activity of waste water bacteria by adding additional oxygen (air) to the water, oxygen typically being the limiting reagent in supporting an adequate biomass. You may need to control and hold the pH between 7.0 to 8.0 with a pH controller (you did not say if your neutralization was pH, or how tight the pH control already is in you system) in order to consistently reach the 100 mg/l COD limit. You may need to reach a BOD of about 5 to consistently reach the COD limit of 100 mg/l.

I would suggest avoiding H2O2. It will actually increase the COD very quickly. Those that use H2O2 must use a catalyst and later neutralize any unreacted H2O2. I have rarely seen it actually reduce the COD. From what you have told me I would strongly recommend aeration to support the microbial population based on the data you have so far.

If however you had inorganic reducing compounds such as sulfites or reduced iron in the water, a small amount of H2O2 might help, but you would need very tight process control capability using an ORP controller to avoid over treatment with H2O2 and the resulting high COD problem. The problem with H2O2 is that it can react as a reducing agent as well as an oxidizing agent (this is something few people realize, any unreacted H2O2 shows up in COD test) and as such it is difficult to avoid over adding in a treatment step.

Hope this helps. Let me know if you need any advice or design help (consulting) on setting up an aeration system.