Grab Samples

A grab sample is an individual sample collected over a period of time not to exceed 15 minutes. Grab samples are usually taken manually, and the sample volume depends on the number of analyses to be performed. The sampler must make sure that sufficient volume of sample is taken to conduct all necessary analytical procedures, including QA/QC. Grab samples represent the conditions that exist at the moment the sample is taken and do not necessarily represent conditions at any other time. Grab sampling is the appropriate method of sampling under the following conditions:

• 40 CFR 403.12(g)(3) requires grab samples for pH, temperature, cyanide, total phenols, oil and grease, sulfides, volatile organic compounds, microbiological tests, and residual chlorine.
• When the effluent is not discharged on a continuous basis (i.e. batch discharges of an hour or less), and only when the batch is continuously stirred or well‐mixed and the pollutant can be safely assumed to be uniformly dispersed.
• When sampling a facility where a statistical relationship can be established between previous grab samples and composite data.
• When the effluent is being screened to see if a parameter is present (NOTE: This is only true when the sample is well‐mixed and representative of the discharge).
• When the waste conditions are relatively constant (i.e. are well‐mixed and homogeneous) over the period of the discharge. In lieu of complex sampling activities, a grab sample provides a simple and accurate method of establishing waste characteristics.
• When an industrial facility or POTW or state has adopted an instantaneous local limit which is based on grab samples.
• Grab samples are useful in characterizing an industrial user’s fluctuations or extremes in wastewater flow and quality (i.e. changes in pollutant concentrations or loadings) and, therefore, are useful in identifying slug loads.

For additional guidance on when it is appropriate to use grab samples for determining compliance, see the EPA memorandum “The Use of Grab Samples to Detect Violations of Pretreatment Standards” at http://www.epa.gov/npdes/pubs/pretreatment_memo_grabsamplesdetectviolations.pdf.

Composite Samples

Composite samples are samples collected over time, either continuous sampling or by mixing discrete samples. Composite samples represent the average characteristics of the wastestream during the compositing period. Composite samples are collected when

• average pollutant concentration during the compositing period is desired
• mass per unit time loadings are calculated
• wastewater characteristics are highly variable

The primary methods of composite sample collection are time compositing and flow proportion compositing. The permit may specify which type of composite sample to use. Composite samples are collected either manually by combining multiple grab samples of by using automatic sampling equipment. Inspectors should consider variability in wastestream flow rate, parameter concentrations, and approved EPA methods when choosing compositing methods, sampling equipment, and QA procedures (Figure 4.1). The compositing methods, all of which depend on either continuous or periodic sampling, are described in the following.

Time‐proportional composite sampling: It is composed of constant volume discrete sample aliquots collected at constant time intervals. This method provides representative samples when the flow of the sampled stream is relatively constant (i.e. when the flow does not vary by more than 10% of the average flow rate over time).

Flow‐proportional composite sampling: There are two methods used to collect a flow‐proportional composite sample.

In the first method, the time between samples is constant, and the volume of each sample is proportional to the flow at that given moment in time (i.e. the volume of the sample varies over time as the flow changes). This is the preferred method of sampling when taking a manually composited sample. This method requires that discrete samples be collected over the operating day and then be manually composited. It is crucial, when using this method, to have accurate flow data continuously recorded during the sampling period (ISCO 2001).

Table 4.5 Composite sampling methods: advantages and disadvantages.

Method	Advantages	Disadvantages	Comments
Time‐proportional composite
Constant sample volume, constant time interval between samples.	Minimal instrumentation and manual effort. Requires no flow measurement.	May lack representativeness especially for highly variable flows.	Widely used in both automatic samplers and manual handling.
Flow‐proportional composite
Constant sample volume, time interval between samples proportional to stream flow.	Minimal manual effort.	Requires accurate flow measurement reading equipment.	Widely used in automatic as well as manual sampling.
Constant time interval between samples, sample volume proportional to total stream flow at time of sampling.	Minimal instrumentation.	Samples must be manually composited. Discrete samples must be taken. Chance of collecting samples that are too small or too large for a given composite volume.	Used in automatic samplers and widely used as manual method.

The second flow‐proportional sampling method involves collecting a constant sample volume for each volume of wastestream flow (e.g. 200 ml sample collected for every 5000 gal of flow) at time intervals inversely proportional to the stream flow. This is the preferred method when taking composite samples using an automatic sampler. This method is based on taking a sample after a set amount of wastewater has been discharged. This method provides representative samples of all wastestreams when the flow is measured accurately. A composite sample should be collected over a workday. If a facility operates and discharges 24 h/day, then the composite sample should be taken as a 24‐hour composite (either flow proportional or time composite). If a facility operates 24 h/day but only discharges wastewater for 6 hours, a 6‐hour composite sample should be collected. In general, composite samples should be collected to assess compliance with categorical standards and local limits, as long as the limits are daily, weekly, or monthly averages, except for those parameters listed above that must be taken by grab sample.

There are numerous cases where composites are inappropriate. Samples for some parameters such as pH, residual chlorine, temperature, cyanides, volatile organics, microbiological tests, oil and grease, and total phenols should not be composited. They are also not recommended for sampling batch or intermittent processes. Grab samples are needed in these cases to determine fluctuations in effluent quality. For bioassays, composite samples are used unless it is known that the effluent is most toxic at a particular time. Some toxic chemicals are short‐lived, degrading rapidly, and will not be present in the most toxic form after lengthy compositing even with refrigeration or other forms of preservation. Require grab samples for bioassays to be taken under those circumstances. In the absence of an expressed sampling protocol in regulation, the duration of the compositing time period and frequency of aliquot collection is established by the state permit writer. Whether collected by hand or by an automatic device, the time frame within which the sample is collected should be specified in the NPDES permit. The number of individual aliquots which compose the composite should also be specified.

Table 4.5 lists the advantages and disadvantages of each composite sampling method. Either manual or automatic sampling techniques can be used. If a sample is composited manually, sample manipulation should be minimized to reduce the possibility of contamination. The inspector must use the sampling method specified in the permit or other control mechanism, which in turn must reflect the requirements at 40 CFR 403.12(g)(3) (CWA 1972).