Calcium Carbide Waste

Question: While cleaning out a remote building, we found a 5-lb can of calcium carbide in a storage area. How should we dispose of it?

Situation

Caller had found a 5-lb container of calcium carbide at thon's site. Thon knew that it was a hazardous material, but was uncertain about what could be done.

Recommendations

Destroy this small quantity of calcium carbide with water and dispose of the residues safely. (See the Work Plan below for additional detail.)

Analysis

Hazards:

Calcium carbide reacts with water to produce highly flammable acetylene gas. By generating the gas outside in small quantities, it is easy to ensure that you will not reach the lower explosive limit (LEL). Incidentally, acetylene (HCCH), with a molar mass of 26, is lighter than air and will not accumulate in low areas.

The solution that you get after reaction with water will have a pH in the range of 10-11. This is not enough to qualify the liquid as a corrosive hazardous waste, but it is too basic to discharge to a stream or municipal wastewater treatment system.

Regulatory Requirements:

  • Hazardous Wastes: Calcium carbide is a D003 reactive hazardous waste, since addition to water generates a regulated amount of flammable gas. However, this particular site was a conditionally exempt small quantity generator of hazardous wastes (CESQG), so this quantity of calcium carbide would not cause them to be regulated. Thus, there is no restriction on treating the waste.

    If the site had been a small quantity or large quantity generator site, they would have had to include the calcium carbide in their total hazardous waste generated for the month, but they are probably allowed to perform this treatment. The law allows generators to treat wastes in tanks and containers. However, different jurisdictions treat this allowance in different ways. Check with legal counsel or local regulators for further guidance. Note that since the ultimate waste is generated to wastewater, you do not need to generate a land disposal notice.

  • Wastewater: Facilities discharging to a publicly-owned treatment works (POTW) must consult the POTW limits for discharge; here in Indianapolis any pH between 5 and 10 is o.k. Discharges more acid than 5 will corrode the sewer lines, and discharges more basic than 10 may disrupt the aerobic digestion process. If you discharge to a flowing body of water you need an NPDES discharge permit from the state and your pH limits will always be 6-9. (Many Indiana POTWs use the pH 6-9 limits also).

    The reaction will generate fairly high levels of suspended and dissolved solids, but in a limited quantity the dilution from other discharges will often be enough to keep you within the allowed NPDES limits. POTWs only worry about suspended solids at levels that might plug the sewer lines, and they will have plenty of dilution for the small quantity of dissolved solids.

  • Air: Acetylene is not exempt from the VOC rules, so this would be a fugitive emission for sources covered by a Title V permit. However, since 5 lbs of calcium carbide will generate less than 2 lb of acetylene, it would appear to be a less important VOC emission source than aerosol cans at such facilities.

Work Plan

Destroy the carbide: Get a 5-gallon plastic bucket and add about two gallons of water. Place it in a secure area outside away from any combustion sources. Add about a cup of the calcium carbide to the water and allow it to react. Do not make further additions until the solution is cooled. If solid (calcium carbonate/calcium hydroxide) begins accumulates in the bucket, dispose of the wastewater and use new water. Continue the addition until all calcium carbide is consumed; this can be done over a period of days.

Dispose of the wastewater: Use commercial vinegar to treat the wastewater so that the pH is less than 9 and greater than 6. [Let us know if you need a few universal pH indicator strips to check this.] At that point it can be disposed of to a septic system or a wastewater treatment system (local or municipal). If you have the proper NPDES permit, it can even be discharged to the stream if you ensure that your discharge does not exceed the limits for suspended or dissolved solids.

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