REPRESENTATIVE SAMPLING

A sample should be taken in a way that will represent the wastewater being treated. No matter how good the lab analysis is, if the sample was not correctly collected, the lab data will not be correct. With the large changes in composition and flow rate, getting a representative sample can be very hard. Careful thought, planning, and training must be used to develop and carry out a good sampling program.

Samples may be taken by hand or automatically. Taking samples by hand may be as simple as tying an open bottle to a pole that can be lowered into the wastewater. Table 10-7 explains some of the things that should be done when taking samples by hand. The automatic samplers may be made by the operator or bought.

GRAB SAMPLING

A grab sample is a single sample of wastewater taken over a short span of time, usually less than 15 minutes. This type of sample yields data about the wastewater at one time and place. The grab sample should be used where the wastewater does not change suddenly or change a great deal. For example, grab samples may be used to determine pH and temperature. Grab

Table 10-7.-Procedures for Manual Wastewater Sample Collection

samples are also used when a batch dump or sludge discharge is seen.

COMPOSITE SAMPLES

A composite sample yields data about the wastewater over a longer span of time. A series of grab samples may be taken over a certain amount of time and combined to form a composite sample. These samples should show the time and frequency of the sample; for example, an 8-hour composite of 30-minute grab samples. The composite sample is used to find BOD, COD, suspended solids, and nutrients.

FLOW-PROPORTIONAL SAMPLES

The composite may be flow proportional. For this type of sample, the volume of the sample changes in proportion to the flow. The flowproportional composite sample is most often run for 24 hours with a 2-hour interval between each collection. To collect this kind of sample, the volume needed for the tests and the average daily flow for the plant must be known. Table 10-8 shows the volumes required for some tests. The following formula may be used to find the volume of sample to be taken at each interval.

For example, to collect an 8-hour composite sample with a 2-hour interval, five samples would be needed. If a total sample of 2 liters was needed, the average daily flow was 60,000 gallons (227 cubic meters), and the flow at the first sample time was 45,000 gallons per day (170 cubic meters),

Table 10-8.-Recommendation for Sample Volume and Preservation of Sample

Table 10-8.-Recommendation for Sample Volume and Preservation of Sample-Continued

Table 10-8.-Recommendation for Sample Volume and Preservation of Sample-Continued

then the milliliters required for the first sample could be figured like this:

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