The Impact of Discharge Criteria on Spent Caustic Treatment Options

Sodium hydroxide, in caustic solutions, is used in the hydrocarbon processing industries to extract and neutralize gases such as hydrogen sulfide and carbon dioxide along with organic acids from hydrocarbon streams. The composition of the produced spent caustic depends on the process generating the waste and on the source of the hydrocarbon stream being washed. Spent caustic is generally classified into three categories: Sulfidic, Naphthenic, and Cresylic. Although all spent caustic streams are environmentally hazardous and require special handling; the sulfidic type spent caustics generated at oil refineries and olefins plants are typically easier to manage. Olefins plants sulfidic spent caustic contains hydrogen sulfide along with some mercaptans, phenols, benzene and emulsified oils. The toxic properties of spent caustic need to be mitigated for additional downstream treatment in a conventional wastewater treatment plant; however, the mitigation must not only lead to an effluent acceptable for biological treatment but also to a level that will enable the downstream treatment scheme to be able to produce an acceptable discharge. Thus Discharge limits for “conventional” pollutants such as biochemical oxygen demand (BOD5), total suspended solids (TSS), fecal coliform, pH, and oil and grease (O&G); for “toxic” pollutants such as the priority pollutants; and for “nonconventional” pollutants such as whole effluent toxicity all need to be considered. Furthermore, sulfidic spent caustics contain high concentrations of sodium sulfide and sodium bisulfide that when biologically converted to thiosulfates are disruptive to the removal of other organics, in addition to generating low pH conditions. Even though pretreatment systems at a minimum convert sulfides to thiosulfates to prepare for biological treatment, the thiosulfates still contribute high organic loads, which may be difficult to treat. Thus a reduction of sulfidic spent caustic COD to 1g/L (Alnaizy), which is considered a higher end value as influent to a downstream biologically based treatment scheme, may not be sufficient to readily allow for compliance with discharge limits. Considering a pretreated sulfidic spent caustic BOD/COD ratio of 0.4, the minimum suggested for biological treatment, the BOD would be reduced to 400 mg/L by pretreatment. If discharge criteria include an average monthly limit of BOD of 10 mg/L, then even assuming a dilution of the pretreated sulfidic spent caustic with other wastewater streams to a mixed BOD level of 100 mg/L would require a 90% minimum average removal. Thus choosing a pretreatment option should consider more than the minimum acceptable influent to a biologically based treatment plant. This paper discusses sulfidic spent caustic pretreatment options with consideration to optimizing the overall treatment scheme for a typical olefins plant. Working back from existing discharge limits faced by several olefins plants, and considering typical biological treatment efficiencies for these plant wastewater streams, the capabilities of different pretreatment technologies will be presented.

Alnaizy, Raafat. Economic Analysis for Wet Oxidation Processes for the Treatment of Mixed Refinery Spent Caustic. Environmental Progress V27 No 3 October 2008.