Florida Wastewater Practice Test

Which sampling method is acceptable for CBOD5 and nutrients at flows less than 100,000 GPD?

1. Continuous samples

2. Composite samples

3. Grab samples

4. Automated samples

The correct answer is: Grab samples

For measuring Carbonaceous Biochemical Oxygen Demand over a 5-day period (CBOD5) and nutrient concentrations in wastewater flows less than 100,000 gallons per day (GPD), grab samples are the most suitable method. Grab sampling involves taking a single sample at a specific point in time. This approach is particularly useful for smaller flows, as it provides a snapshot of the wastewater characteristics at that moment. CBOD5 measurements are sensitive to the timing of sampling because they can fluctuate significantly based on various factors such as inflow variations, time of day, and operational conditions. Under conditions where flows are less than 100,000 GPD, grab samples can effectively capture the necessary data without the complications that may arise from more complex sampling methods. Composite samples (which involve combining multiple samples taken over a period) are often better suited for larger, more variable systems where averaging over time can help represent broader fluctuations. Continuous and automated sampling methods are typically employed for larger treatment plants or more dynamic flows, ensuring a more representative dataset through consistent monitoring. Thus, grab sampling aligns well with both the flow conditions and the practical requirements for accurate assessment of CBOD5 and nutrient levels in lower-flow wastewater scenarios.