We are a small generator of hazardous waste and are well within the limits for discharge to the city, and our tanks are about 1,200 gallons. My problem is what to do once chromate conversion coating bath “turns.” If we ship it out in a liquid form, it costs us about $500.00 a barrel at 20 barrels each time and puts us at a large generator status. The solution we have in the tanks right now has been in there for about four years, and my titrations of hexavalent to trivalent chrome indicated that we are getting close to dumping the tank. I know I can drop the chrome chemically but what is next, filter-press or evaporate? R.B.
You have several options, especially considering that it will cost you more than $10,000 for the upcoming disposal. Since you mentioned that your wastewater discharge is in compliance with your limits to the city, I will assume that you have a wastewater pretreatment system with hexavalent chrome reduction. I recommend that you perform a lab scale pretreatment test to determine the amount of chemicals and their costs to determine if it is worthwhile to pretreat it yourself or to continue to dispose off-site. First, estimate pounds of hexavalent chrome by the following equation:
(ppm Cr+6 x volume, gallons × 8.34 × specific gravity) / 1,000,000 = pounds Cr+6.
For conventional chrome reduction, adjust pH to > 2.5 and then add sodium metabisulfite to reduce the chrome; for every one pound of hexavalent chrome to reduce to trivalent chrome, you need about three pounds of sodium metabisulfite and, possibly, about 1.5 pounds of sulfuric acid, but the lab test will give you the more realistic amounts. Next, you can estimate the amount of sludge that you will generate by adjusting the pH of the above sample after chrome reduction, analyze for total suspended solids (TSS), and estimate dry solids generation by the following equation:
(ppm TSS x volume, gallons × 8.34 × specific gravity) / 1,000,000 = pounds dry sludge. With this option, the waste solution could be pumped to a storage tank or totes and very slowly bled into the wastewater pretreatment system’s chrome reduction. If your current wastewater pretreatment system does not have chrome reduction, then I would consider installing a batch chrome reduction treatment tank equipped with a mechanical mixer and ventilation to the outside. Since you have much time to accomplish the chrome reduction, you can maintain the pH to < 5.0 and use less sulfuric acid; reduction may take several days or even weeks to complete. You can also use other reducing materials, such as ferrous sulfate or even scrap iron, but again, reduction may take several days, maybe even longer; also these reducing materials will generate much more sludge due to the iron. Once you confirm that the chrome has been reduced by testing, the treated solution can then be bled into your wastewater pretreatment system at a very slow rate, the trivalent chrome precipitated out of solution, and removed by clarification and/or filtration.
Even if you do not have a wastewater pretreatment system for metals removal, I would still consider installing a batch chrome treatment tank. After chrome reduction, the treatment sequence could be pH adjust to around 8.0 - 8.5, addition of a anionic polymer and settling. You could then test the top clear water or supernatant to make sure it can be discharged below limits. Unless you have a filter press available, the most cost effective means to dewater the remaining sludge, since you have lots of time on your side and quite low volume, is by gravity filtration through a bag filter; the bag will likely need to be changed a number of times to process your waste stream. Before removing the bag during changeout, you could shut off the flow to the bag and allow it to “drip dry” for several days. To further reduce its weight, you could dry it through use of an infrared heater or place it on a tray and into an oven. Fortunately, using heat in this way to reduce the volume and weight of this waste material does NOT require a hazardous waste treatment permit. The expectation is that the amount of sludge that you would need to ship off-site as a hazardous waste would still qualify you as a small quantity generator, that is, you generate between 220 and 2,200 pounds of hazardous waste in a calendar month.
Another option is to investigate the recycle potential of this waste material due to its chrome content. You can contact your state’s waste exchange or waste management agency to inquire if there are any facilities that would accept your waste solution. If a facility can take and use your material “as is,” USEPA and most states would recognize it as a feedstock, not a hazardous waste. The key is the recycling facility MUST be able to use your material WITHOUT any treatment or conditioning in order for it to qualify as a feedstock.
Unfortunately, evaporation is not really an option. Since this waste would still be a hazardous waste due to hexavalent chrome even after pH neutralization, its evaporation would be deemed hazardous waste “treatment” by USEPA and your state EPA, thus requiring a hazardous waste treatment permit. Even chrome reduction before evaporation would be considered hazardous waste treatment. Elementary pH neutralization and treatment that is associated with a wastewater treatment system regulated by the Clean Water Act are exempt from hazardous waste treatment permit requirements.
If you still decide to have this material transported off-site for treatment and disposal, investigate the cost of having this liquid waste disposed by bulk tanker instead of 55 gal drums. We have several clients who have waste process solutions picked up by vacuum tanker because it is less expensive, greatly minimizes labor costs, much safer, and much cleaner.
The regulatory challenge that you have with the disposal of this waste material off-site is that for the month that you remove this waste from the tank and dispose of it, you are classified as a large quantity generator (>2,200 lbs in a calendar month) and are required for that month to meet the much more numerous and stringent requirements for large quantity generators as compared to small quantity generators. We know of a few cases where a small quantity generator was subjected to a hazardous waste inspection, and when the inspector found a hazardous waste manifest showing more than 2,200 lbs shipped at one time, s/he requested a copy of their hazardous waste program in place at the time of the manifest. Fortunately, the facilities were able to show that the waste was generated at a rate of < 2,200 lbs per calendar month and was accumulated before shipment. This being said, I guess you could remove a small amount of your chromate conversion coating each month so as to maintain your bath within its acceptable specification, provided you continue to meet the small quantity generator standards; two requirements of note are that small quantity generators cannot accumulate more than 13,200 lbs of hazardous waste at any one time and cannot store waste for more than 180 days or for more than 270 days if waste is to be transported more than 200 miles. Here again, you are back at very expensive disposal using drums.
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