Q. We have an electrocoat system and recently found that we cannot send our anolyte overflow to the drain because we discharge to a small stream that, in dry periods, is 100 percent flow from us. Our NPDES permit has a BOD limit. Our e-coat wastewater pretreatment system is a typical metals precipitation process to meet Metal Finishing Categorical Standards under 40CFR433, which does not remove organics. Very little of the literature out there discusses this organic loading source. It seems that most operate under the assumption that all metal finishing facilities must be served by a sewer system. What is the most effective treatment for reducing the BOD anolyte? Is a small carbon absorption unit on the overflow line the solution? T.S.
A. There is very little data I, too, can find regarding treatment of organic pollutants in the discharge of electrocoat operations. E-coat liquids, as applied, can contain 0–7 percent by weight of solvents, typically polar solvents such as alcohols and ketones. It is these solvents, as well as surfactants and oils/greases from the pretreatment stages, that contribute to BOD5. While your wastewater pretreatment system will remove most oils/grease, it will not remove solvents and surfactants.
BOD5 stands for “biochemical oxygen demand,” and the analysis measures the amount of oxygen that bacteria consume over five days under standard conditions, as they use the organics as food. The more food or organics, the higher the BOD5.
Yes, activated carbon will likely remove the organics, but it may quickly become loaded. If you want to try activated carbon, have a reputable supplier perform bench scale tests to the best type of activated carbon and verify that it can remove enough organics to meet the BOD limit as well as carbon usage; you may discover that its usage is so high that it will be extremely expensive to operate.
Another technology to investigate is evaporation of the waste to concentrate for off-site disposal, although there possibly will be an air pollution issue due to the likely emission of some or all of the solvents.
Lastly, you can investigate biological treatment to reduce BOD. Depending upon the anolyte overflow BOD concentration, flow rate and target BOD to meet your BOD limit, a very-small-package wastewater system, even one designed for residential use, may provide the level of treatment.