Representative SamplingRepresentative Sampling

Samples vs. PopulationsSamples vs. Populations

The population is the total or all of the possible answers we might get by sampling.All of the individuals in

this room.Every 100 mL aliquot in 2

MG of influent.We sample because we

can’t count the whole population.

Representative SamplesRepresentative Samples

What is representative?Sample should represent or be typical of

the wastewater it is collected from.If the true value of BOD5 in the wastewater

is 280 mg/L, then the sample should be close to this value.

How do we know the sample is representative?Answer: Statistics

Representative SamplesRepresentative SamplesRepresentative

samples should be very close to the mean value of the population.

How do we know we are close to the mean?

Look at the sample standard deviation.

Population CharacteristicsPopulation Characteristics

68%

95%

99%

Mean or Average

Standard DeviationStandard Deviation

The standard deviation tells us how spread out the data are.

If the mean is 20 and stddev is 2, then 68% of all measurements are between 18 and 22.

Types of SamplesTypes of Samples

Grab SamplesExactly what it sounds like. One sample

collected at a particular point and time.Composite Samples

Multiple samples collected and added together to make one sample.

Time Composite.Space Composite.Flow Proportional Composite.

Manual versus Automatic

Grab SamplesGrab Samples

May be used where population is not changing suddenly or changing a great deal over time.

Must be used for particular analyses:Residual chlorine.Fecal coliform.

Also applicable for estimating performance under a given set of conditions.

Composite SamplesComposite Samples

Frequently used to estimate average values over a 24-hour period.BOD5 loading to aeration tanks.

TSS leaving the WWTP in the effluent.

Gives information over a longer period of time or space.

Permit samples are often flow proportional composites.

Composite SamplesComposite SamplesConsideration must be given to

sample handling and storage during compositing.

We don’t want the sample characteristics to change while we are sampling.

Refrigeration often used to slow biological activity.

Chemicals may also be added as preservatives.

Manual vs. AutomaticManual vs. Automatic

Manual samples are collected by hand.Automatic samples are collected by

machine.

Cautions for automatic samplers:Not necessarily better.Not accurate when collecting <20 mL.Clean frequently; clogging.Variable flows and intake location.

Unstated AssumptionsUnstated Assumptions

For simplicity, we ASSUME that the population we are sampling from is:Normally distributed.Completely mixed.

We also ASSUME that our sample value approximates the population mean.

These assumptions are not always true.

Guidelines for Representative Sampling

Guidelines for Representative Sampling

Samples should be collected:Only where wastewater is well-mixed.In the center of the flow channel.

Horizontally and Vertically.Avoids floating scum and settled solids.

Ensure that samplers and sample containers are clean, uncontaminated, and suitable for the planned analysis.

Guidelines for Representative Sampling

Guidelines for Representative Sampling

Wiers are not good sampling points.Solids settle upstream of weirs.Oils and greases build-up downstream.

Materials tend to collect on the sides and bottoms of channels. Avoid edges.

Before collecting the sample, rinse the sampler and sample container several times.

What Do We Want to Know?What Do We Want to Know?Seems like a simple question……Often neglected in sampling and

analysis plans.Are we interested in:

Average performance?Performance at peak load?Dictates type of sample AND time of day.Dictates sampling location.

How Will the Data be Used?How Will the Data be Used?Internally or externally?Public access?Will results prompt capital

expenditures?Does data need to be legally

defensible?Dictates total number of samples,

analysis method, and QA/QC needed.

Precision versus AccuracyPrecision versus Accuracy

Neither precise nor accurate.Precise, but not accurate.Accurate, but not precise.Accurate and Precise. BOTH

ARE NEEDED.

Precision versus AccuracyPrecision versus Accuracy

Neither

Precise

Accurate

Both

Checking for Contamination

BLANKS

Checking for Contamination

BLANKS

Filter BlankFilter Blank

Only needed when analyzing for dissolved substances.

Total Suspended Solids (TSS)Ortho-phosphorus

Reagent BlankReagent Blank

Ultra-pure water analyzed as a sample.Accounts for differences in reagents

between lot numbers or batches.Often used to “auto-zero” and

instrument.Subtracts out background.Can be a check for contamination.

Checking for Precision

DUPLICATES

Checking for Precision

DUPLICATES

Field DuplicateField Duplicate

A second sample taken at the same time and place as the original sample.

Placed into a separate sample bottle.Checks whether or not the sample is representative.

Tells us how heterogeneous the population is.

Lab DuplicateLab Duplicate

Tests analyst’s ability to take a representative sample from the field sample.

Two aliquots are taken from the same sample bottle and subjected to the same sample preparation and analysis steps.

Don’t confuse a duplicate with a replicate. A replicate is a second reading from the same aliquot.

Checking for Accuracy

STANDARDS

Checking for Accuracy

STANDARDS

Instrument Calibration Instrument Calibration

Minimum of a blank and one standard.Standard Methods recommends a blank

and THREE standards.EVERY time the instrument is used or

once per day.