Modern Hexane-Extractable (Oil & Grease) Analysis of Wastewater Samples

n-Hexane-extractable material (HEM), often termed oil & grease, is an operationally-defined general measurement used around the world to help assess water pollution due to a variety of hydrocarbons, including dissolved aromatics, benzene, toluene, xylene and dispersed polynuclear aromatic hydrocarbons (PAHs), aliphatics, naphthenic and fatty acids. (1)  The commonly recognised sources include fats, soaps, fatty acids, hydrocarbons, waxes, and oils. (2) It is also used to determine the input into water treatment plants to ensure their continued good operation and to help keep sewer systems from becoming clogged with fats, oil and greases. The measurement of the extracted material is done using a balance in regulatory methods US EPA 1664, ISO 11349 and Standard Methods 5520G, providing a simple and inexpensive detection step. (3,4,5)  A further silica-gel treatment can be used to isolate the nonpolar material in the n-hexane extract.

Hexane extractables can be used to regulate allowable pollution in the US. This is done through a system known as the National Pollutant Discharge Elimination System (NPDES) where allowable pollution is listed by industrial category for regulation. For example, in the US Code of Federal Regulations part 40 section 408.12, Subpart A—Farm-Raised Catfish Processing Subcategory, oil & grease, the federal effluent limitation is based on the amount of seafood processed and cannot exceed 10 kg/kkg of seafood on any one day or an average of 3.4 kg/kkg of seafood over the course of a month.  Similar regulations are seen in Brazil, Malaysia, the Philippines and other countries.

Method 1664 allows use of solid phase extraction (SPE) instead of liquid-liquid extraction with hexane and this has been widely adopted in the US. In addition to using less solvent, there is less chance of an emulsion forming during extraction with SPE, making the process more predictable. The SPE process can be more easily automated, reducing exposure to solvent and improving reproducibility and we will discuss the results from an automated analysis in this work.