It is estimated that 23–28% of sanitary sewer overflows (SSOs) are due to fats, oils, and grease (FOG). With significant growth in the commercial food sector, in addition to other high-FOG-producing industries such as metal processors, meat packers, cosmetic manufacturers, and industrial laundry establishments, the need for publicly owned treatment plants (POTWs) and industries to regulate and monitor FOG is crucial to public health. Besides clogged sewer lines, high FOG levels also disrupt the effective operation of wastewater treatment plants. An important feature of any FOG program is locating and monitoring high-oil- and grease-producing industries and food service establishments. Waiting a week or more for a remote laboratory result can compound the problem. A simplified procedure for FOG analysis based on solvent extraction and infrared absorption can provide the regulator or operator an on-site result in less than 10 min.
Principle of operation
Measurement by infrared absorption makes use of the fact that hydrocarbons such as fats, oil, and grease can be extracted from water or soil through the use of an appropriate solvent. The extracted hydrocarbons absorb infrared energy at a common infrared wavelength, and the amount of energy absorbed is proportional to the concentration of the oil/grease in the solvent.
Methodology
Figure 1 - InfraCal TOG/TPH analyzer, model HATR-T2.
Figure 2 - InfraCal TOG/TPH analyzer, model CVH.
Three different solvent options are available for on-site wastewater analysis: hexane, S-316, and perchloroethylene (tetrachloroethylene).
- Hexane. U.S. EPA Method 1664 uses hexane as the extractant and a gravimetric procedure for analysis. The gravimetric procedure requires a skilled laboratory technician and is a time- and equipment-intensive process. The InfraCal TOG/TPH (total oil and grease/total petroleum hydrocarbon) analyzer, model HATR-T2 (Wilks Enterprise, S. Norwalk, CT) (Figure 1), uses an attenuated total reflection (ATR) sample plate in which a measured amount of the hexane extract is deposited on the crystal. The hexane evaporates and leaves a film of oil and grease that is then measured by infrared. The minimum detection for the ATR sample plate is 10 ppm, with accuracy within 10%.
- S-316. In July 2004, the American Society for Testing and Materials (ASTM) passed Method D 7066—04 Standard Test Method for Dimer/Trimer of Chlorotrifluoroethylene (S-316) Recoverable Oil and Grease and Nonpolar Material by Infrared Determination. Because S-316 does not contain hydrocarbons, the InfraCal TOG/TPH analyzer, model CVH (Figure 2), has a different sample stage with a cuvette that is filled with the solvent extract. Because more sample is measured, there is a lower detection limit of 2 ppm, with accuracy within 10%. Without the evaporation stage in the analysis, volatile fractions will remain in the solvent and be measured by infrared analysis. This can be a benefit if petroleum fuels such as gasoline are present in a sample and the analyst would like to determine their presence.
- Perchloroethylene. Perchloroethylene (perc), or tetrachloroethylene, is much like S-316. It also requires use of the InfraCal TOG/TPH analyzer, model CVH, and reads down to 2 ppm. It is important to use spectrophotographic-grade perchloroethylene that has not been stabilized with a hydrocarbon compound.
Analysis
The entire analysis, from sample collection to final result, takes less than 10 min. The extraction and measurement procedure involves several simple steps, allowing an operator with minimal training to perform the analysis. The wastewater sample is collected in a container. Hexane, S-316, or perc is added at a ratio of one part solvent to 10 parts sample. After shaking for 2 min, the solvent removes the oil and grease from the sample. A portion of the solvent extract is presented to the InfraCal TOG/TPH analyzer and a measurement result is displayed in under a minute. Only a small amount of extract is needed for analysis; using a 100-mL sample requires only 10 mL of hexane, S-316, or perc. Since hexane is a hydrocarbon, it can be returned to the pretreatment system for removal. S-316 can be reclaimed and reused.
Comparing different methods
Table 1 - Comparison of hexane/infrared method to hexane/ gravimetric method
The definition of FOG is dependent on the procedure and solvent used. Because different testing methods look at different physical properties of FOG, there can be differences in the analysis. The gravimetric method as used in U.S. EPA Method 1664 measures the weight of the oil that was removed from the sample with hexane. Infrared looks at the infrared absorbance due to the hydrocarbons present in the extracted oil. Because most hydrocarbons are various CH groups, the infrared absorbance typically correlates well with weight. Table 1 shows two sets of data comparing the hexane/infrared method to the hexane/gravimetric method (U.S. EPA Method 1664). Taking into account that it is difficult to obtain two identical grab samples from a waste stream, the two methods match very closely.
Ms. Rintoul is Product Manager, Wilks Enterprise, 140 Water St., S. Norwalk, CT 06854, U.S.A.; tel.: 203-855-9136; fax: 203-838-9868; e-mail: [email protected].