FOG and Sewer Overflows
It is estimated that 23% to 28% of sanitary sewer overflows (SSOs) are due to fats, oils and grease (FOG) or as much as 19 million gallons from 1998 to 2001 (State of North Carolina, 2004). Our increasingly mobile society has led to a decrease in cooking at home and a significant growth in the commercial food sector. This, in addition to other industries that already deposit oil and grease in the waste stream, contributes to the increase of SSOs.
As part of the Clean Water Act, industries discharging into Publicly Owned Treatment Works (POTWs) are facing new and stricter FOG regulations. Many are faced with limits of 100 ppm of oil and grease. These new limits are forcing industries to monitor their effluent more closely prior to discharge, which means more frequent measurements to ensure compliance.
For POTWs, an important feature of any FOG program is locating and monitoring industries and food service establishments dumping high levels of FOG in the sewer line. Waiting for remote laboratory results can add costs in both time and dollars. A new simplified procedure for FOG analysis based on solvent extraction and infrared absorption can give a regulator or operator an on-site result in less than 10 minutes.
Infrared analysis of oil and grease has been used in the petroleum industry on highly regulated off-shore oil platforms for over 30 years. EPA Methods 413.2 and 418.1 are infrared methods for oil and grease measurement using now-banned Freon to extract the hydrocarbons from the effluent. EPA Method 1664, using hexane as the extraction solvent and gravimetric analysis, is now the standard method replacing Freon methods. This gravimetric procedure requires a skilled laboratory technician and is a time- and equipment-intensive process.
The ASTM passed an alternate method, D 7066-04, using a Freon replacement solvent (S-316) and simplified infrared analysis. There is also an alternate infrared method using hexane extraction and evaporation.
The 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. This can be directly calibrated or converted to the amount of oil in the original sample.
Infrared Absorption and Solvent Extraction
For an infrared measurement, FOG is measured at the C-H absorption band at 2930 cm-1 or 3.4 micrometers (see Fig. 1). Either S-316 solvent, as called for in the ASTM method D 7066-04, or a spectroscopic, hydrocarbon-free grade of perchloroethylene is a good infrared solvent, as both totally lack a C-H absorption band.