Lesson 3:
Sampling
 

Objective

In this lesson we will answer the following questions:

• What guidelines should be followed when taking samples?
• What are grab and composite samples?
• How are samples stored and preserved?
  

Reading Assignment

Along with the online lecture, read pages 2-4 in Simplified Procedures for Water Examination.



Lecture

Introduction

In this lesson we will be considering sampling, which is the process used to select a small portion of water or wastewater for the purpose of determining the characteristics of an entire batch of water or wastewater.  

Sampling is used every day at water and wastewater treatment plants to determine the characteristics of the water.  Sampling may be used to test the finished water for regulatory purposes - to ensure that the treatment plant is treating the water in compliance with regulations.  Or sampling may be used to help the operator tweak the chemical feed rates to make the treatment procedure as efficient as possible.  

You will use sampling in every lab exercise completed in this course.  For example, in the pH lab you'll take a small sample of water and analyze the pH of that sample, then assume that the pH of all of the water at a certain stage in the treatment process has the same pH value as the sample tested.  This is just one of many examples of sampling which you will be exposed to as a treatment plant operator. 

This lesson is concerned with the methods used in sampling.  You will learn how to collect a sample which is small enough to be conveniently handled in the laboratory but which is still representative of the total flow going through the plant.  The sample must be collected in such a manner that nothing is added or lost in the portion taken, and so that no change occurs in the sample's characteristics between the time the sample is collected and the time the laboratory test is performed.  Since the value of any laboratory result depends on the quality of the sample taken, bad sampling can lead to laboratory results which are misleading and worse than no results at all.  

Sampling Guidelines

Safety

Sampling safety is similar to general laboratory safety.  When dealing with wastewater of any sort, the operator must wear rubber gloves to prevent contact of the wastewater with the skin.  Gloves should be thoroughly washed before removal and then hands should be washed using a disinfectant soap.  

In many treatment plants, raw and treated water is piped directly into the laboratory, facilitating sampling.  However, in other plants, the operator must take samples from open bodies of water which can be dangerous.  Do not climb over or go beyond guardrails or chains when collecting samples.  Instead, use sample poles, ropes, or other equipment to safely collect samples.  

 

Sampling Locations

The location from which you take samples will, of course, depend on what you want to discover about the water.  Operators take samples of raw water to determine water characteristics which will influence the treatment procedure, then they take samples of finished water to determine how well the treatment worked.  

But even at a given stage of treatment, you will always have choices about the precise location from which your sample will be taken.  Since you want the sample water to be representative of the water at a given location, you should always take samples from well-mixed areas of tanks or pipes.  If you wanted to determine the quality of drinking water reaching the customer, it would be a bad idea to take a water sample from a dead end in the distribution system since water in such a location is unlikely to be representative of water in the distribution system as a whole.  Similarly, if you are sampling water in the flocculation basin, you should not take your sample from the stagnant water at the very bottom of the basin which is not well-mixed.

When taking samples, you should also be sure that the sampling locations are clearly marked and readily accessible.  You will often be called upon to take samples from the same location day after day and will need to be able to reach the same sampling location repeatedly.  

 

Sampling Equipment

In most cases, the equipment used to collect samples is relatively simple and consists of sampling containers such as jars and beakers.  Only a few tests, such as dissolved oxygen testing, require that specialized devices be used to collect samples.  If the water to be sampled is out of reach, a long-handled aluminum dipper attached to a wooden pole can be used to collect a water sample.  

Sampling containers must be made from a material which is corrosion-resistant and leak-proof, will withstand repeated refrigeration and cleaning, and will not interact with the sample water.  In general, plastic or glass should be used for sample containers.  Do not use metal containers such as coffee cans since they will corrode, cannot withstand repeated use, break easily, and cannot be acid cleaned.  Table 1060:I in Standard Methods lists which sampling container materials are most appropriate for a variety of tests.  

Contamination of sample containers can be prevented by having a different collection container for each sampling location and by cleaning sample containers thoroughly after each use.  Between each collection, the sample container should be cleaned with hot water and a good quality detergent, rinsed thoroughly with tap water followed by distilled water, and allowed to dry completely.  At intervals, the sample containers should also be acid cleaned to remove all residues.  Except in the case of samples for bacteriological or oil and grease tests, sample containers should be rinsed at the time of sample collection with a portion of the water to be sampled.

Sample Quality

As mentioned in a previous section, samples should be taken from locations in which the water is well-mixed and is representative of the body of water being sampled.  In addition, the operator should avoid collecting large or unusual particles in routine samples.  Samples which contain deposits, growths, or floating material which have accumulated at the sample point or on the side walls will tend to result in skewed test results, so these materials should be avoided.  

 

 

Types of Samples

Introduction

There are two main types of samples which are used in water and wastewater treatment - grab samples and composite samples.  The type of sample taken in a given instance will depend on the type of test to be performed, the frequency of testing, and on permit requirements.  We will explain each test procedure below. 

Grab Samples

A grab sample, also known as a catch sample, consists of a single sample taken at a specific time.  This is the most common type of sample and is the sampling technique you will use for most of your labs.  For example, you took a grab sample when you collected a beaker of raw water and tested it for pH. 

A grab sample has certain limitations.  In essence, a grab sample takes a snapshot of the characteristics of the water at a specific point and time, so it may not be completely representative of the entire flow.  Grab samples are most appropriate to small plants with low flows and limited staffs who cannot perform continual sampling. 

On the other hand, grab samples do provide an immediate sample, and are thus to be preferred for some tests.  Specifically, pH, dissolved oxygen, and total residual chlorine can change very rapidly in water once the sample is removed from the flow, so grab samples are preferred for these tests. 

Grab samples must be collected carefully to make them as representative as possible of the water as a whole.  They should be taken at a time of day when the plant is operating near its average daily flow rate.  If grab samples are used to determine plant efficiency by collecting a raw water sample and a treated water sample, then the collection of the effluent should be delayed long enough after collection of the influent sample to allow for the raw water to pass completely through the treatment process. 

Finally, be aware that mixing two or more grab samples may not result in a result which averages the characteristics of the samples.  Chemical reactions can take place in mixed samples which alter pH and chlorine residual values.  The next section will give you more information on combining samples. 



Composite Samples

A composite sample, also known as an integrated sample, is a sample which consists of a mixture of several individual grab samples collected at regular and specified time periods, each sample taken in proportion to the amount of flow at that time.  Composite samples give a more representative sample of the characteristics of water at the plant over a longer period of time. 

Like grab samples, composite samples have both strengths and weaknesses and are not acceptable for all tests.  The greatest strength of composite samples is their ability to take into account changes in flow and other characteristics of the water over time.  This helps the operator gain an overall picture of the total effects that the influent will have on the treatment process and that the effluent will have on the receiving water.  However, composite samples cannot be used for tests of water characteristics which change during storage (such as dissolved gases) or of water characteristics which change when samples are mixed together (such as pH.)  Table 1060:I in Standard Methods lists whether grab or composite samples are most appropriate for a variety of tests. 

Composite samples are often taken using automatic sampling devices.  These may be set to take a sample every 8, 12, or 24 hours, with the frequency depending on test requirements, on the size of the treatment plant, and on permit requirements. 



Sample Volumes for Composite Samples

One of the most important aspects of a composite sample is that each individual grab sample must be proportional to the amount of flow at the time the sample was collected.  Most automatic equipment used to take composite samples will make these calculations for you and will collect a correctly sized grab sample during each time period.  This section presents a calculation you can use to determine the size of the grab sample at a given time when producing a composite sample manually. 

The volume of sample collected at any given time depends on the volume of flow at that time, the total flow for the day, the total composite sample volume, and the number of individual grab samples to be taken.  The following equation can be used to calculate a grab sample's volume:

For example, the average daily flow at your plant is 11.3 MGD and the total volume of your composite sample is to be 4,000 mL made up of 24 grab samples.  At the time you take your first sample, the plant's flow is 5.2 MGD, so you can calculate the volume of the grab sample to take as follows:

So you should take a 77 mL grab sample during your first sampling of the day. 

 

 

Storage

Introduction

The best test results will be achieved if you analyze all samples as soon after collection as possible.  However, this is not always possible, so you will often be called upon to store, preserve, and treat samples before they are tested.  Once again, I will refer you to Table 1060:1 for specifics on the length of time that may elapse before samples are tested and for the correct preservation technique for each test.  This section merely presents a broad overview of the procedures which may be used. 

Preservation

Except when collecting a grab sample and immediately testing it, storage is a nearly ubiquitous part of the sampling process.  Composite samples must be stored carefully during the time period that compositing is being performed.  Any sample that is being sent off to another lab for analysis will also have to be stored. 

There are a variety of techniques which can be used to preserve samples during storage to prevent their deterioration, including chemical addition and refrigeration.  The type of chemical added to a sample will depend on the nature of the test to be performed, but most samples can benefit from refrigeration during storage.  When refrigerating samples, keep the temperature at or below 4°C but do not allow the samples to freeze.  At this temperature, biological activity is significantly reduced.  However, you should be aware that no preservation technique completely stops biological and chemical action, so samples cannot be preserved for great lengths of time. 

When you are ready to test your sample, you may also need to treat it in some manner before following the test procedure.  Pre-treatment is most necessary when dealing with wastewater samples with heavy solids concentrations or with different sized solids, such as that found in the wastewater influent or in mixed liquor.  These samples should be homogenized in a blender before testing. 

 

 

Review