Sampling of Wastewaters

The value of any 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 results. 

The location of sampling points and the collection of samples cannot be specified for all wastewater plants.  Conditions vary in different plants and the sampling procedure must be adapted to each plant.  Certain general principles can be listed. 

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 line 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 one-quarter inch in diameter.  This is reasonable because if one large piece was included in a one-gallon sample, it would mean that wastewater would contain one million large pieces per million gallons of wastewater.  Raw wastewater should be sampled after screening where screens or comminutors are used.

 
3. No deposits, growths or floating material that have accumulated at the sampling point should be included.  Obviously, such material would not be representative of the wastewater.  This may be difficult if sampling is at a manhole, but it can be done if care is used.

 
4. Samples should be examined as soon as possible.  If held for 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, proper equipment should be at hand, safety precautions established, and protection of personnel from inclement weather provided, for 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.  The accepted preservatives and the holding times for the various constituents are listed in EPA Manual "Methods for Chemical Analysis of Water and Wastes".

 

Catch or grab samples are not representative of the average wastewater since they reflect only the condition at the instant of sampling.  However, in many plants the time available for sampling is so limited that catch samples must be used.  The samples should be collected 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 this time, it is reasonable to assume that plant efficiency will be satisfactory during other periods.  When catch samples are used to determine the efficiency of a treatment process, the effluent sample should be collected after a period of time corresponding to the flowing-through period so that approximately the same sewage is sampled at inlet and outlet. 
  

Photo Credit:  Virginia Department of Health	For some tests, catch samples must be used.
Thus, for residual chlorine and pH value if two portions were mixed, reactions would occur and the result would not be an average but the result of interaction and have no relation to the wastewater sampled.

Sometimes the object is to determine the effect of a substance in wastewater on the treatment process.  It can happen that the substance is present in high concentration for only a short period of time.  If individual portions were integrated, an average concentration would be obtained.  Thus to determine the high concentration, catch samples should be examined.
 
 
 Composite Samples

Composite samples indicate the character of the wastewater over a period of time.  The effects of intermittent changes in strength 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.  If the fluctuations are rapid, half-hourly or quarter-hourly samples may be required.  The period of sampling may be varied covering four, eight or twelve hours, depending on the personnel available, the use to be made of the results, and effluent monitoring requirements.  Generally, integrated samples are used to determine the character of the wastewater to be treated and the efficiency of the treatment units. 

The rate of wastewater flow must be measured when each portion is taken and the volume of the portion adjusted to the flow by the use of a factor. 
 
 

Equipment

An aluminum dipper six inches in diameter and approximately four inches deep with a long handle is convenient for collecting wastewater from tanks and channels.  For sampling from manholes a one-quart pail which can be fastened to a wooden  pole with a harness snap is used.  Graduated cylinders, sample bottles, and some means of refrigeration are also needed.  Special technique is required for collecting samples for dissolved oxygen and for bacteriological examination. 
 
 

Sampling of Sludge

As in the case of wastewater, the value of sludge analyses depends largely upon the accuracy of sampling.  Thus it is necessary to observe strict precautions in the selection of sampling points and methods of sampling to insure the collection of representative samples at all times. 

To collect samples of sludge from different depths in a tank, a sampling apparatus can be used that is made of cast iron or brass weighted with lead.  It can be lowered into the tank by a link chain which carries markings showing the various depths.  The apparatus is fitted with valves operated by a cord.  A pull on the cord at the desired depth opens the valves and  the sludge flows in at the bottom while air escapes at the top.  A wide-mouthed stoppered bottle attached to the end of a pole can also be used.  The bottle is pushed to the desired depth and the stopper removed by means of an attached cord.  Many separate sludge digestion tanks are equipped with sampling taps at various depths.  When using these taps enough sludge must be discarded to free the lines of accumulated sludge so that the sample collected will be representative of the sludge in the tank. 

To collect samples of sludge when sludge is being drawn or pumped, take catch samples of equal size in a dipper at the start, during and at the end of the period of drawing.  Mix these samples and take about 500 mL for the laboratory sample. 

To collect samples of bed dried sludge, take portions of equal size from several scattered points on the bed, taking care not to include sand, mix thoroughly after pulverizing, and use about 500 grams for the laboratory sample.  Samples of filter cake sludge may be collected by cutting portions of the cake as discharged from the filter.  Doing this with a cookie cutter is a very convenient way to obtain equal size portions at equal time intervals.  These portions can then be examined individually or mixed to yield a composite sample. 

Examination of the samples should be made as soon as possible after collection.