Which type of sampling is used to collect oil and grease samples?

• A. Grab samples
• B. Composite samples
• C. Field duplicates
• D. Triplicate samples

Answer: (A)

Oil and grease in wastewater is unevenly distributed and tends to separate, often concentrating in a surface layer during short holding times. Because of this, capturing the actual concentration at a specific moment is best done with a grab sample taken at the time and place of interest. A grab sample preserves the instantaneous level of oil and grease as it exists in the discharge, which is how the measurement is designed to reflect the condition of the effluent being released. In contrast, a composite sample mixes contributions over a period, which can smooth out peaks and may not represent the true amount of oil and grease being discharged at any given moment, since oils and greases can separate and move within the water column. Field duplicates and triplicate samples, on the other hand, are used for quality control and precision checks rather than the fundamental sampling approach itself.