Waste Treatment for Paint Stripping

How can we remove paint sludge and color and neutralize the pH from paint stripping acid before discharging waste to a storm drain?


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Q. We will be using concentrated sulfuric acid to strip e-coatings from reject metal parts. The parts will be immersed into the acid solution, and the stripped coating will migrate into that solution. After a day, the acid will be spent because it is full of paint sludge, and we will need to replace it with new acid. Unfortunately, our only outlet for discharge of this waste is to a nearby storm drain, and we know that we will need to remove the paint sludge and color and to neutralize the pH before discharge. Can you advise? T.T.

A. You present a very challenging problem. I will give you my thoughts, and I invite our readers to submit their suggestions as well.

First, I urge you to look into disposing of this waste material as a hazardous waste, either in 55-gal drums or by collecting and storing in a bulk tank and disposing via bulk tanker truck, depending upon the volume. If you decide to store this corrosive hazardous waste in a bulk tank, the tank system needs to comply with U.S. EPA hazardous waste regulations under 40CFR262, Subpart C, and 40CFR265, Subpart J (with a few exceptions), as well as your state’s regulations. (Go to epa.gov/waste/inforesources/pubs/training/tanks05.pdf for further guidance.) As you will see below, treating this waste material will be extremely challenging and costly, and highly regulatory.

Because you propose to discharge to a storm drain that is connected to surface water, you will be required to apply and obtain a direct discharge permit (aka NPDES permit) either from your state or from your U.S. EPA region if your state does not have an approved NPDES program. 

Assuming that your electro-coating process also includes a phosphate washer pretreatment system, the wastewater discharge is regulated under the Metal Finishing Categorical Standards – New Source Performance Standards – 40CFR433.16. However, for a direct discharge, it is very likely that limits for the heavy metals, total toxic organics, oil and grease, and total suspended solids will be lower due to local water quality stream standards. Also, there will likely be other local water quality standards for biochemical oxygen demand (BOD5), chemical oxygen demand (COD), phosphorus, nitrogen, color (as you mentioned), and aquatic toxicity through bio-assay testing. We strongly recommend that as part of your evaluation process to decide whether to dispose off-site or treat on-site that you discuss your proposed discharge with your direct discharge permit writer to determine exactly what will be your permit limits. Our experience has been that a direct discharge can meet the numerical limits for all of the about pollutants but still fail aquatic toxicity, which will require you to even more aggressively treat this waste stream.

Because of the volumes and the high level of treatment that is likely required, we recommend a batch treatment system consisting of holding tank, treatment tank with mixer, filter press, final holding tank, and air-operated diaphragm transfer pumps constructed of thermoplastic bodies with Teflon diaphragms, seals and checkballs.

In setting up your wastewater treatment scheme, we recommend that you work with an experienced water chemistry sales rep who is able and willing to perform jar testing to determine the most effective treatment in terms of meeting your permit limits and costs. Keeping this in mind, we will provide our thoughts.

After the waste stream is pumped into the treatment tank, caustic (sodium hydroxide) is metered into the tank to raise pH, likely in the range of 8 – 8.5. (The typical upper pH limit for direct discharges is 9.) This could be done automatically with a pH sensor and controller or manually with a portable pH meter after several titrations to determine caustic dosage. We prefer using 12.5 or 25 percent caustic since it reacts quicker and makes it easier to control pH as compared with 50 percent. Also, 50 percent caustic gets slushy when its temperature falls in the mid-60°F range and freezes at about 53°F, while the lower caustic concentrations “freeze” around 5–10°F.

After adjusting for pH, you will likely need a paint de-tackifier since, during the stripping process, some of the resins were solubilized, causing the coating to soften.

This is where a water chemistry sales rep can be quite valuable. At this point, we need to coagulate and flocculate the solids so they can be separated from the water. This may require the addition of a coagulant and/or polymer to make this happen.

Once the solids in the wastewater have been coagulated and flocculated (we like to have solids the size of small curd cottage cheese), it is ready for separation by a plate and frame filter press. One of the questions that cannot be answered without pilot testing is whether or not the filter press needs to be pre-coated with diatomaceous earth (DE) or perlite, as well as the need for continuous body feed into the filter press since the coating solids could plug the filter press, resulting in very long filtration runs at low flows, wet and sloppy filter press cake, and high maintenance to keep filter press cloths clean.

Initially, we start up the press at a fairly low feed pressure (around 30 psig) and recirculate the filter press’s discharging water (filtrate) back to the treatment tank until it clears, when it is sent to the final holding tank. As the feed pump slows down, we typically increase air pressure at 20-psig intervals until around 90 psig. When the pump strokes are about 30 sec apart, it is time to shut down the feed pump, blow down the filter press with compressed air, and open the filter press and remove solids.

To remove color and soluble organics (these are measured by BOD5 and COD), we will need to use activated carbon. There are many types of activated carbon, so again, an experienced water quality sales rep can be of great value to determine the best. There are two ways to use activated carbon. First, we can add after coagulation and flocculation in the treatment tank and remove with the filter press. Second, we can recirculate the wastewater in the final holding tank through dual-activated carbon columns (one operating, one standby) until sufficient color and soluble organics are removed. When spent, the activated carbon is removed and disposed of, and then new carbon is placed into the column. Some have found that the life of the activated carbon can be extended by injecting a small amount of hydrogen peroxide (H2O2) solution into the activated column’s feed line; this oxidizer helps break down the color and soluble organics as well as supplies oxygen to bacteria that also break down soluble organics.

In addition to likely very strict limits, direct discharge permits will have numerous other conditions such as sampling frequency, verifiable daily record keeping, instrumentation calibration record keeping and periodic reporting, and some states require a “licensed operator of record.”

As you can see, for a fairly small wastewater direct discharge, costs can become prohibitive, hence, we urge you to consider off-site disposal. 


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