According to EPA protocol, during a wastewater 24-hour composite sampling event, what must be taken as a grab sample?

According to EPA protocol, during a wastewater 24-hour composite sampling event, what must be taken as a grab sample?

• A. Metals
• B. BOD
• C. Oil and grease
• D. TSS

Answer: (C)

Oil and grease behave differently from the rest of the wastewater: they are hydrophobic and tend to float, forming a surface layer that can be unevenly distributed over time. A 24-hour composite sample blends material from the entire period, which can underrepresent or miss this floating layer. Taking a grab sample captures the actual concentration at a specific moment, often from the surface, ensuring an accurate measurement of the oil and grease fraction. Other common wastewater constituents like metals, BOD, and total suspended solids are more uniformly distributed over the sampling period and can be adequately represented by the composite sample, so they don’t require a separate grab sample.

Oil and grease behave differently from the rest of the wastewater: they are hydrophobic and tend to float, forming a surface layer that can be unevenly distributed over time. A 24-hour composite sample blends material from the entire period, which can underrepresent or miss this floating layer. Taking a grab sample captures the actual concentration at a specific moment, often from the surface, ensuring an accurate measurement of the oil and grease fraction. Other common wastewater constituents like metals, BOD, and total suspended solids are more uniformly distributed over the sampling period and can be adequately represented by the composite sample, so they don’t require a separate grab sample.