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

Importance of the Topic

Measurement of hexane-extractable material (oil & grease) is a critical indicator of hydrocarbon pollution in wastewater. This operationally defined parameter assesses the presence of aliphatics, aromatics, PAHs and other lipophilic substances that can impair water treatment processes and lead to sewer blockages. Regulatory frameworks worldwide, including US EPA 1664B and ISO 11349, rely on this metric to enforce discharge limits and protect aquatic environments.

Objectives and Study Overview

This work evaluates an automated solid phase extraction (SPE) approach using the SPE-DEX 3100 system for oil & grease analysis in wastewater. The goals are to demonstrate compliance with US EPA Method 1664B performance criteria, compare disk sizes (47 mm vs 100 mm), and assess matrix effects in real wastewater samples.

Instrumentation Used

• SPE-DEX 3100 Oil & Grease Extraction System (Horizon Technology)
• Speed-Vap IV Evaporation System with multi-position rack
• Mettler AE 200 analytical balance
• Pacific Premium SPE disks (47 mm and 100 mm)
• Silica gel sorbent for non-polar isolation
• Hexadecane and stearic acid standards for spiking and calibration

Methodology

An acidified 1 L sample is extracted by SPE disks, dried inline, and evaporated to a constant mass. Two quality checks were performed:

• Initial Precision and Recovery (IPR): four 40 mg/L spikes evaluated for gravimetric recovery and silica gel treated fraction
• Method Detection Limit (MDL): seven replicates at 4 mg/L spike plus blank to determine MDL

Non-polar extracts are isolated via silica gel treatment and re-evaporated for selective analysis.

Main Results and Discussion

The automated SPE method achieved MDLs of 0.68 mg/L (47 mm) and 0.89 mg/L (100 mm), exceeding the 1.4 mg/L requirement. Precision and recovery criteria were met for both disk sizes:

• HEM recovery 96–100% with RSD <2%
• SGT-HEM recovery 88–91% with RSD <3%

Wastewater spikes (40 mg/L) in plant influent demonstrated recoveries of 80–89%, within the 78–114% regulatory window. Automation reduced solvent use, eliminated emulsions, and improved reproducibility.

Benefits and Practical Applications

• Reduced solvent consumption and operator exposure
• Elimination of emulsions and improved sample throughput
• Compliance with stringent regulatory criteria for environmental monitoring
• Adaptability to high-particulate matrices and varying sample volumes

Future Trends and Opportunities

Advances may include integration with online detectors for real-time monitoring, miniaturized SPE formats for field applications, use of greener solvents or sorbents, and coupling with mass spectrometry for compound-specific analysis.

Conclusion

The SPE-DEX 3100 based automated SPE method meets US EPA 1664B requirements, offering robust performance, improved lab safety, and operational efficiency. It provides a reliable platform for routine oil & grease monitoring in wastewater and industrial effluents.

References

1. Ming Yang, Produced Water, Springer, 2011.
2. Stenstrom MK, Fam S, Silverman G, Environmental Technology Letters, 7, 625–636 (1986).
3. EPA Method 1664B: n-Hexane Extractable Material by Extraction and Gravimetry, US EPA, 2010.
4. ISO 11349: Water Quality – Determination of Low-Volatility Lipophilic Substances, ISO, 2010.
5. Standard Methods 5520G: Water and Wastewater Analysis, 21st Edition.