Spent caustic

Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful (or spent). Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants. The contaminants have consumed the majority of the sodium (or potassium) hydroxide and thus the caustic liquor is spent, for example, in one common application H2S ("gas") is scrubbed by the NaOH ("aqueous") to form NaHS ("aq") and H2O ("l"), thus consuming the caustic.
Treatment technologies.
Spent caustics are malodorous wastewaters that are difficult to treat in conventional wastewater processes. Typically the material is disposed of by high dilution with biotreatment, deep well injection, incineration, wet air oxidation, Humid Peroxide Oxidation or other speciality processes. Most ethylene spent caustics are disposed of through wet air oxidation.