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What Are Spent Caustics?
In refining petroleum, people remove sulfur compounds because of their harmful impact on the environment. The desulfurization process uses caustic soda (potassium hydroxide or sodium hydroxide) or another strong alkaline solution. When the process is complete, the contaminants consume most of the hydroxides. We know the resulting noxious effluent as spent caustic.
It is spent because the Environmental Protection Agency defines a spent material as “any material that has been used and as a result of contamination can no longer serve the purpose for which it was produced without processing.”
Spent caustic has a high chemical oxygen demand (COD) concentration, pH level, and total sulfur content. (COD is the amount of dissolved oxygen that must be present in water to oxidize chemical organic materials such as petroleum. There are strict regulations governing the maximum COD allowed in waste waters before one can release them into the environment.) Spent caustics are also a waste product of plants producing paper, soaps and detergents, foods and other products.
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Why Must Spent Caustics be Treated?
The Environmental Protection Agency (EPA), under the Resource Conservation and Recovery Act (RCRA), classifies spent caustics as hazardous waste, with strict handling and disposal requirements because of the high levels of contaminants. Next to radionuclide wastes, spent caustics are the most difficult of all industrial wastes to dispose of. They contain high concentrations of hydrogen sulfide, a deadly gas when exposed to air in quantities as low as 100-500 parts per million by volume.
Hydrogen sulfide has caused the death of workers in water sewers and is also highly explosive when exposed to the atmosphere. Spent caustics from refineries also contain phenols which can prove deadly with skin exposure of only 64 square inches. Improper disposal of spent caustics in Ivory Coast in 2006 resulted in several deaths, as well as illnesses and damage to victims’ skin.
Hazardous wastes do not cease to be dangerous to human health and/or the environment simply because they are being reused, recycled, or reclaimed.Before someone can release spent caustics to water purification plants, they must treat them to neutralize the high pH and drastically reduce the COD concentration.
Are Spent Caustics Always Classified as Solid Waste?
RCRA encourages waste recycling by exempting wastes from classification as solid waste if they are:
- Used directly as an ingredient in a production process without first being reclaimed
- Used directly as an effective substitute for a commercial product without first being reclaimed
- Returned directly to the production process for use as a feedstock or raw material without first being reclaimed
Spent caustic solutions–from petroleum refining liquid treating processes–are excluded from the definition of solid waste when used as a feedstock to produce cresylic or naphthenic acid. This is unless the material is placed on the land, or accumulated speculatively for recycling at some future point. Spent caustics in storage remain classified as solid waste until removed and recycled.