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If you are the person in charge of sampling the influent (incoming) and effluent (outgoing) wastewater in a facility, then it is important to know the two different types of sampling techniques: grab sample vs. composite sample and what they are used for.

Grab Samples

A grab sample is defined as an individual aliquot (volume of water) taken over a period of time not to exceed 15 minutes. For example, scooping up water in a cup from a bucket. Of course, the cup-and-bucket method just described is just an example – you wouldn’t actually sample wastewater in that way. A grab sample is typically taken manually but some automated samplers can be programmed to take grab samples for you. Grab samples are taken for extreme conditions and for batch discharge operations. Grab samples are preferred for reviewing pollutants that change quickly with time such as: pH, temperature, and bacteria. Now that we’ve discussed grab samples, let’s look at composite samples.

Composite Samples


Composite samples, unlike grab samples, are a collection of aliquots taken over an extended period of time and are based on either a flow or time basis. The time frame is usually over the course of 24 hours or over the course of one day’s wastewater discharge. Composite samples are appropriate for showing the average discharge over a period of time and can be either time or flow proportional. All of the aliquots are combined in a composite sample and are delivered into a single sample container. Composite samples can be taken manually or automatically using a sampling device.

For more information

The pollutants recorded on your discharge permit will tell you which of the sampling types is required (grab sample vs. composite sample). If you’d like more information on our sampling services, fill out our contact form and we will get right back to you.