Washer Discharge Fouled with Oils

Question:

Our plant has a two-stage spray washer to remove oils and dirt from carbon steel stampings. The first stage has an alkaline cleaner while the second stage is a rinse with a rust inhibitor. Although we have had this process for a number of years, the city recently “discovered” this washer during a recent inspection. Because of the amount of oils, the city will not accept the dumps of the first stage cleaning solution even if we neutralize it. We dump the first stage once or twice a week and have found that off-site disposal is very expensive. S.G.

Answer:

Because your washer does not phosphate, it is not regulated under the metal finishing categorical pretreatment standards (40CFR433), but it is subject to local wastewater limits as you have discovered. Apparently, your first stage waste cleaning solution has excessively high oils, but there are cost-effective, even profitable, measures you can take. The key is to remove the contaminants from the cleaning solution and greatly extend its life. Not only does this significantly reduce disposal costs but also reduce chemical costs.

If you are using a non-emulsifying cleaner, I recommend the following concept. First, install an oil/water separator on the first stage’s overflow. This separator will allow the floating oil to be skimmed from the surface and the heavy dirt particles to settle to the bottom. The cleaning solution would then be pumped to a filter that can contain one of several types of media, including cartridge, disc, or expanded plastic, that have an affinity to capture and hold onto oils as well as capturing fine dirt particles. The “clean” filter discharge is then directed back into the first stage.

If you are using an emulsifying cleaner, I recommend you work with your current supplier to see if you can switch to a non-emulsifying cleaner. A number of our clients have successfully made the switch. If you find that you must still continue to use an emulsifying cleaner in order to achieve your desired results, then I recommend the following option.

Redirect the first stage overflow to a small pump tank and then pump through a particulate filter and then to an ultrafiltration membrane filter. The reject stream containing the oils is directed back to the small pump tank while the “clean” cleaning solution flows back into the first stage cleaner tank. Eventually, the small pump tank builds with oil and dirt and needs cleaning out, and the small amount of oily waste would need to be disposed of off-site. While membrane filtration is usually more expensive than an oil/water separator and filtration, it does produce a cleaner solution, thus extending the bath life even more. Furthermore, over the past years, the cost of membrane filtration continues to decline, making it more financially attractive than even several years ago.

To properly size this equipment, you can work with your local suppliers or those listed in the 2005 Products Finishing Directory and Technology Guide (www.pfonline.com/suppliers.html). These concepts do not only improve cleaning quality by continuously removing oils and dirt, we have seen bath life extended up to 10 times. Furthermore, either of these concepts will significantly decrease off-site disposal costs, and, possibly reduce the oil concentration to acceptable levels for sewer discharge.

If after implementing one of these two concepts you still find that the oil concentration in the spent cleaning solution is still too high for sewer discharge, you could implement a fairly inexpensive treatment. I would recommend an elevated batch treatment tank with a mixer. After adjusting pH within acceptable limits, I recommend adding treated clay-based powdered chemical to absorb the residual oils and remove it from the waste stream. After the waste has settled for several days or weeks, the clarified water can then be discharged to the sewer while the settled sludge can be dewatered by gravity through sheet filter media. Chemical suppliers can also be found in the 2005 Directory and Technology Guide (www.pfonline.com/suppliers.html).