Wastewater Lab · Composite Samples
What Is a Composite Sample?
A grab represents one moment; a composite represents conditions over time.
One Sample Across a Defined Period
A composite sample combines multiple individual samples collected over a defined period. The final sample represents conditions across that period rather than one moment. Many wastewater monitoring programs use 24-hour composite samples, but 24 hours is not universal. The required duration comes from the permit, approved method, or monitoring plan.
Time-Based and Flow-Based
A time-based composite combines sample increments collected at regular time intervals. A flow-based composite links collection to wastewater flow so changing flow conditions are represented. BOD, TSS, and nutrients are common composite-monitoring examples where applicable in wastewater programs. The permit, approved method, and monitoring plan determine the required sample type; none of these examples is a universal rule.
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