This document discusses representative sampling and quality assurance/quality control procedures. It covers topics such as types of samples, ideal sampling locations, data quality objectives, and examples of proper and improper sampling techniques. Quality control measures like blanks, duplicates, and standards are described to ensure sample accuracy, precision, and to check for contamination in the sampling and analysis process. Maintaining proper sample handling and preservation techniques as well as adhering to hold times for analysis are also important aspects of quality control.

1. Representative Sampling
and Laboratory QA/QC

2. Agenda
 Language of Sampling
 Types of Samples
 The Unspoken Assumptions
 Ideal Sampling Locations
 Data Quality Objectives
 Examples:
The Good, The Bad, and The Ugly

3. Introduction
 Sampling usually given little thought
relative to analysis, even though critical.
 Largest errors in results from sampling.
 Samples are taken:
 At the wrong place.
 At the wrong time.
 And of the wrong type.
Result: Non-representative results.

4. Samples 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.

5. Representative 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

6. Representative Samples
 Representative
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.

7. Population Characteristics
68%
95%
99%
Mean or Average

8. Standard 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.

9. Types of Samples
 Grab Samples
 Exactly 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

10. Grab 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.

11. Composite 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.

12. Composite Samples
 Consideration 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.

13. How to Composite
 Simple Composite – Add equal volumes
of samples collected from different
times or locations. Mix thoroughly.
 Flow Proportional Composite – Volume
of each subsample based on flow.
 Estimate total volume of sample required.
 Estimate total flow over sampling period.
 Calculate sample volume per flow.

14. Simple vs Flow Proportional
Time Flow (MGD) NH3 -N Simple Flow Prop.
Midnight 10 12 12 120
4 a.m. 15 15 15 225
8 a.m. 18 20 20 360
Noon 28 40 40 1120
4 p.m. 26 37 37 962
8 p.m. 14 14 14 196
NH3-N Conc, (mg/L) in Sample = 23.0 26.9
Avg Daily Flow 18.5 MGD
Total Lbs - Simple 3549
Total Lbs - Proportional 4146
Difference of 15%

15. Manual 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.

16. Examples of Autosampler Misuse
 The BOD5 Producing Equalization
Basin.
 The No-Flow at Low-Flow Problem.
 The 1 Day MCRT Nitrifying Basin.

17. Unstated 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.

18. 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.

19. Guidelines for
Representative Sampling
 Recommended Sample Containers:
 HDPE appropriate for most analyses.
 Trace metals, oil and grease, volatiles
should be sampled in glass containers.
 Pre-cleaned or sterilized containers for
phosphorus testing and fecal coliforms.
When compositing or aliquoting, mix
samples well before pouring.

20. 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.

21. Agenda
 Language of Sampling
 Types of Samples
 The Unspoken Assumptions
 Ideal Sampling Locations
 Data Quality Objectives
 Examples:
The Good, The Bad, and The Ugly

22. DQOs
 There are a lot of choices in sampling.
What type of sample to take.
Where to collect the sample.
What time of day to collect the sample.
 How do we know what we need?
What data quality objectives (DQOs)
are all about.

23. DQOs
 A sampling and analysis plan of attack.
 Plan of attack is determined by
answering these questions:
 Why are we taking the sample?
 What do we want to know?
 How will the data be used?
 What level of QA/QC is needed?
 Who will take the samples?

24. Why Collect the Sample?
 For process control:
 Wasting calculations.
 Calculation of unit process efficiency.
 Estimating plant capacity.
 For permitting:
 Required analyses for DMR.
 Required analyses for biosolids disposal.
 Quantifying receiving water quality.

25. 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.

26. 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.

27. What level of QA/QC
is Needed?
 Field and laboratory?
 Frequency of QA/QC Samples?
 Permit required analysis – every time.
 Process control – weekly perhaps.
 Certified standards needed?
 Outside laboratory involved?

28. Who Will Take the Samples?
 Daytime sampling and analysis not
usually a problem, but….
 Nights and graveyards?
Week-ends?
 May limit types of sampling to be done.
 Autosamplers eliminate this problem,
but still need to be checked.

29. Example Problems

30. Plan of Attack
 Why are we taking
the sample?
 What do we want to
know?
 How will the data be
used?
 What level of
QA/QC is needed?
 Who will take the
samples?
 Sampling location.
 Grab or composite.
 Frequency of
sampling.
 Analytes needed.
 QA/QC required.

31. Calculate MCRT
 Where to collect
samples?
 Type of samples to
collect?
 If multiple basins are
in use?
 If basins are
independent?

32. Estimate TF Performance
 For average
performance.
 For peak
performance.
 Develop DQOs.
 What if there is
recycle?

33. Quality
Assurance/Control

34. Why Do We Do It?
 To Check for Contamination
 To Verify
 Precision
 Accuracy
 To Determine if Interferences are
Present
 ENSURES DATA QUALITY and
GIVES CONFIDENCE!

35. Contamination
 Results in a false positive.
 Caused by dirty glassware and improper
sampling or handling techniques
 Can happen at any stage of sampling or
analysis
 Happens when we add something to the
sample
 Examples: Phosphorus, Fecal Coliforms, BOD5

36. Precision versus Accuracy
 Neither precise nor accurate.
 Precise, but not accurate.
 Accurate, but not precise.
 Accurate and Precise. BOTH
ARE NEEDED.

37. Precision versus Accuracy
Neither
Precise
Accurate
Both

38. Interferences
 Substances in a sample that
cause
 False Positives
 False Negatives
 Look for Interferences at the time
of analysis.

39. How do we know if we are:
Free of contamination?
Accurate and precise?
Lacking Interferences?

40. Checking for
Contamination
BLANKS

41. Filter Blank
Only needed when analyzing for
dissolved substances.
 Total Suspended Solids (TSS)
Ortho-phosphorus

42. Filter Blank
 Checks for contamination during filtering.
 Set up and clean filtration apparatus. Special
cleaning should not be done for blanks.
 Filter a volume of ultra-pure water.
 The filtrate is the filter blank.
 The filter blank should be treated like any
other sample.

43. Reagent 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.

44. Limits for Blanks
 Blank values should be less than the
MDL.
MDL = Method Detection Limit.
 Lowest concentration used for
reporting.
 Calculated value that may be different
for different laboratories and analysts.
 See EPA method for how to calculate.

45. Acceptable or Unacceptable?
MDL for Nitrate test is 0.5 mg/L
 Field blank reads 0.2 mg/L
 Filter blank reads 1.7 mg/L
 Reagent blank reads 0.1 mg/L
 Sample results are higher than normal.
What happened?

46. Checking for Precision
DUPLICATES

47. Field 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.

48. Relative Standard Difference
 RSD = ( A - B) * 100
((A+B)/2)
 Where
A = Original Sample Result
B = Duplicate Result
 Results from a field duplicate should agree
within +/- 20% RPD of original sample.

49. Relative Standard Difference
 Original Sample Result – 300 mg/L BOD5
 Duplicate Sample Result – 350 mg/L BOD5
 Calculate the RPD
100*(300 – 350)
((300 + 350)/2) = (50/325)*100 = 15.4%
 Within Limits?

50. Lab 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.

51. Relative Standard Difference
 Original Sample Result – 300 mg/L TSS
 Duplicate Sample Result – 180 mg/L
TSS
 Calculate the RPD
100*(300 – 180)
((300 + 180)/2) = (120/240)*100 = 50.0%
WHAT HAPPENED?

52. Checking for Accuracy
STANDARDS

53. Standards
 Contain a known concentration of analyte.
 Should be within the same range as the
sample concentrations.
 Standard Methods recommends 5 to 50 times
the MDL.
 May be purchased “certified” from outside
vendors.
 Environmental Resource Associates
 Hach, SPEX, VWR Scientific Products, and others

54. Standards
 Standards should be analyzed
 Each time an instrument is calibrated.
 Once per sample batch.
 Once per lot of reagents.
 Standard percent recoveries should be
within + 10% of the true value.
 Exception: BOD5 standard should be
within + 15% of the true value.

55. Percent Recovery Calculation
 Certified Standard Concentration = 45.0 mg/L
 Measured Standard Concentration = 42.0
mg/L
 Percent Recovery = (Measured
Concentration / Certified Value)*100
 % R = (42.0 / 45.0)*100 = 93.3%

56. Additional Quality Control
 Spike Calculations
 Sample Hold Times and Preservation
 Instrument Calibration
 Instrument Logs and Performance
Checks
 Accurate Record Keeping
 Secondary Review of Calculations

57. Hold Times and Preservation
Parameter Preservative Hold time
Alkalinity 4oC 14 Days
Ammonia Nitrogen H2SO4 to pH<2, 4oC 28 Days
BOD5/CBOD5 4oC 48 Hours
COD H2SO4 to pH<2, 4oC 28 Days
Conductivity 4oC 28 Days
Fecal coliforms 4oC 24 Hours
Hardness HNO3 to pH<2, 4oC 28 Days
Nitrate 4oC 48 Hours
Nitrite 4oC 48 Hours
Total Suspended Solids 4oC 7 Days
Total Dissolved Solids 4oC 7 Days
Total Solids 4oC 7 Days
Trace Metals HNO3 to pH<2, 4oC 180 Days

58. 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.

59. Quality Assurance and Quality
Control: Is it All Really
Necessary?
 Permitted Analyses vs. Analyses for
Process Control
 Remember! Process control decisions
are only as good as the data they are
based on.

60. We Do Analytical Work
GOOD
FAST
and CHEAP
Pick Any Two.

61. QA/QC by Standard Methods
 Reagent Blanks – One per 20 Samples
 Duplicates – One per 20 Samples
 Spikes – One per 20 Samples
 Instrument Calibration – Every Time
Used
 Calibration Blank and Three Standards
 Not all QA/QC applies to every
analysis.

62. Questions?