Ch 11 Sampling. The Nature of Sampling Sampling Population Element Population Census Sampling frame.
Representative Sampling. Samples vs. Populations The population is the total or all of the possible...
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Transcript of Representative Sampling. Samples vs. Populations The population is the total or all of the possible...
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.
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.
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.
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.