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Sampling Of Wastewater

The value of laboratory result depends on the integrity of the sample. The object of sampling is to collect a portion of wastewater small enough in volume to be conveniently handled in the laboratory and still representative of the wastewater to be examined. It must be collected in such a manner that nothing is added or lost in the portion taken and no change occurs during the time between collection and laboratory examination. Unless these conditions are met, laboratory results may be misleading and worse than no result.
The location of sampling points and the collection of samples cannot be specified for all wastewater treatment plants. Conditions vary in different plants and the sampling procedure must be adapted to each plant.

– General principals:

1- The sample should be taken where the wastewater is well mixed. This is most easily accomplished if the sampling point is located where the wastewater flow is turbulent, for example, at a tap on the discharge side of a pump, where a free fall from a pipe lone occurs, where the discharge from a pipe is against a baffle as at the inlet of a tank, or just as the flow enters a pipe as at the effluent line from a tank.

2- Large particles should be excluded. Large particles are all those greater than 0.6 mm in diameter. This is reasonable because if one large piece was include in a litter sample, it would mean that wastewater would contain one thousand large pieces per cubic meter of wastewater. Raw wastewater should be sampled after screening where screens are used.

3- No deposits, growth or floating material that have accumulated at the sampling point should be included.

4- Samples should be examined as soon as possible. If held more than one hour, they should be cooled by immersion of the sample bottle in ice water. Bacterial decomposition of wastewater continues in the sample bottle. After one hour, the changes due to such decomposition are appreciable. Cooling the sample greatly retards bacterial action.

5- The collection of proper samples should be made as easy as possible. Sampling points should be readily accessible and proper equipment should be at hand. The easier it is to take proper samples, the more likely it will be done.

6- Sample preservation may be necessary for some chemical constituents. For chemical analyses, samples should be tested directly and if it was difficult, the maximum time for analyzing the samples is 3 hours or samples should be kept in refrigerator.

– Types of Samples Collection:

There are two types of samples that may be collected, depending on the time available, the tests to be made and the object of the tests.

-: Catch Or Grab Samples:

Grab samples are not representative of the average wastewater since they reflect only the condition at the instance of sampling. However, in many plants the time available for sampling is so limited that catch samples must be used. The samples should be colleted at that hour of the day when the treatment plant is operating under maximum load. This usually coincides with the period of maximum flow and occurs at most plants between 9 A.M and Noon. If good operating efficiency is indicated at that time, it is reasonable to assume that plant efficiency will be satisfactory during other periods. Approximately the same sewage is sampled at inlet and outlet.
For some tests, catch samples must be used. Thus, for residual chlorine, PH value, DO test and for bacterial analysis.

-: Integrated Or Composite Samples:

Integrated or composite samples indicate the character of the wastewater over a period of time. The effects of intermittent changes in strengthen and flow are eliminated. The portion used should be collected with sufficient frequency to obtain average results. If the strength and flow do not fluctuate rapidly, hourly portions over a 12 hour period are satisfactory. Generally, integrated samples are used to determine the character of the wastewater to be treated and the efficiency of the treatment units. Integrated samples can be use to measure the BOD and SS tests.