Treating Waste Water Containing Hexavalent Chromium
The purpose of this paper is provide information on hazardous compounds found in aircraft washing, stripping and painting operations. These compounds include Hexavalent Chrome, Phenol, Methylene Chloride and Formaldehyde. This paper also provides an introduction to treating the wash waters generated in aircraft washing so that the treated water may be re-used or discharged to the city sewer.
What is Hexavalent Chromium?
Hexavalent Chrome, chemical symbol Cr(VI) also written as Cr(6) ,is a toxic form of chrome that is known to cause cancer and other serious health effects. The number 6, indicates what is called the valence state, or electron charge on the chrome atom. In this case Chromium has a positive 6 charge. For this paper, it is only important to know that chrome can be in two very different forms, Hexavalent Chrome 6 and Trivalent Chrome, Chrome 3. The major difference, and the most important difference is that Hexavalent Chrome is both cancer causing and water soluble where Trivalent Chrome is not cancer causing and it is not soluble in water.Chemical Oxygen Demand, COD in Aircraft Wash Water
In addition to the specific compounds found in the stripping gel, these chemicals add to COD (Chemical Oxygen Demand) of the waste water. COD is used by the EPA and the local regulators as a measure of the waste water strength. For example household waste water generally has a COD of around 350 mg/l, where aircraft wash water will average around 3,000 mg/l and can spike as high as 20,000 mg/l. COD discharge limits are usually around 350 mg/L to 500 mg/l.
The local sanitary sewage authority will often charge a per gallon fee to treat waste water with a high COD. Check your water bill to see if you are being charged.
How do I treat the wash and rinse water?
The wash and rinse water should be treated to remove both the Hexavalent Chromium and the organic compounds; Phenol, Methylene Chloride and Formaldehyde and to lower the COD.
What About Biological Treatment?
Although very common, biological treatment will not work in this application, for several reasons. 1. The chromium cannot be removed in a biological process. The microbes may consume some chrome but they cannot digest it and it simply goes back in to the waste water through excrement or decay of dead microorganisms. 2. Phenol and Formaldehyde (embalming fluid) are toxic to microorganisms. Low concentrations of these compounds will kill or hinder microbial growth needed to treat the waste water biologically. 3. Methylene Chloride, while not toxic, is not easily degraded in a biological process.
What about UV Oxidation?
UV oxidation can oxidize the organic compounds but not the Hexavalent Chrome. UV oxidation can be used in conjunction with other treatment methods. UV Oxidation is also expensive.
What Reverse Osmosis or Micro Filtration?
The issue here is that these methods only treat about 1 gallon for every 5 gallons of waste water. What to do you do with the extra waste water?
What About Ion Exchange?
Ion exchange resins are available for removing Hexavalent Chrome. Resins are expensive and can be exhausted quickly when Hexavalent Chrome concentrations are higher than 100 m/l. Ion exchange resins may be used as a polishing step after removal by precipitation. They will not remove organics or lower the COD.
Aircraft Wash Waters, Generally Require a Two Step Process?
Treating the waste water for both the organic compounds, Phenol Methylene Chloride, Formaldehyde and COD, and removing the Hexavalent Chrome generally requires a two step process.
Step one is the oxidation of the organic chemicals and lowing the COD. While this is not the only way, the simplest and the most cost effective is a process know as the Fenton Process. Oxidation is accomplished by what is know as the Fenton Reagent. The Fenton reagent is a powerful oxidant that is produced by Hydrogen Peroxide in the presence of dissolved iron. The iron acts as a catalysis to produce Hydroxyl Radicals from the hydrogen peroxide. These radicals react with and convert the organic compounds to Carbon Dioxide and Water. This process has been around for a long time, it is well understood and it works. To learn more about the Fenton process see our article Entitled “Oxidation of Organics using the Fenton Reagent”. This process is most easily controlled when done in a batch process.
When done in a batch tank, Hexavalent Chrome can also be removed by simple precipitation. This is accomplished by converting the Cr(IV) to Cr(III) which is the insoluble form, followed by precipitation. This can all be accomplished in the same tank.
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