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Section No: 6 Page: 4 of 16 Revision No: 2.0 Date: January 15, 1994 =1M_- t1i'MILLiE 6.2.2 Composite Samples In most cases, the to "compositesample" refers to a mixture of grab samples collected at the same sampling point at different times. Sometimes the term "time-composite°A is used to distinguish this type of sample from oth e- composite samples are most useful for observing average concentrations that will be used, for example, in calculating the loading or the efficiency of a wastewater treatment plant. As an alternative to the separate analysis of a IMe number of samples and the computation of average and total results, composite samples represent a substantial saving in laboratory effort and expense. For these purposes, a composite sample r presenting a 24 hour period is considered standard for most determinations. Under certain circumstances, however, a composite sample representing one shift, or a shorter time period,, or a complete cycle of a periodic operation, may be preferable. To evaluate the effects of special, variable, or irregular es and operations, collect compositesamples representing the period during which such discharges occur. For determining components or characteristics subject to significant and unavoidable changes on storage, do not use composite samples. Make such determinations on individual samples as soon as possible after collection and preferably at the sampling point. Analyses for all dissolved gases, residual chlorine, soluble sulfide, temperature, and pH are examples of this type of determination. Changes in such components as dissolved oxygen or carbon dioxide, pH, or temperature may produce secondary changes in certain inorganic constituents such as iron, manganese, alkalinity, or hardness. Use time -composite sples only for determining components that can be demonstrated to remain unchanged under the conditions of sample collection and preservation. Take individual portions in a bottle having a diameter of at least 35 mm at the mouth and a capacity of at least 120 mL. Col!ect these portions every hour, in some cases every half hour or even every five (5) minutes, and mix at the end of the sampling period or combine in a single bottle as collected. If preservatives are used, add them to the sample bottle initially so that all portions of the composite are preserved as soon as collected. Analysis of individual samples sometimes may be necessary. It is desirable, and often essential, to combine individual samples in volumes proportional to flow. A final sample volume of 2 to 3 L is sufficient for sewage, effluents, and wastes. Automatic spling devices are available; however, do not use them unless the sample is preserved described below. Clean sampling devices, including bottles, daily to eliminate biologi and other deposits. 6.2.3 Integrated Samples For certain purposes, the information needed is provided best by analyzing mixtures of grab samples collected from different points simultaneously, or as nearly so as possible. Such mixtures sometimes are called integrated samples. An example of the need for such sampling occurs in a river or stream that varies in composition across its width and depth. To evaluate average composition or total loading, use a mixture of samples representing various points in the cross-section, in proportion to their relative flows. The need for integrated samples also may exist if combined treatment is proposed for several separate wastewater streams, the interation of which may have a significant effect on treatability or even on composition. Mathematical prediction of the interactions may be inaccurate or impossible and testing a suitable integrated sample may provide more useful information.