Representative Sampling

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.

Representative Samples What is representative? Sample should represent or be typical of the wastewater it is collected from. If the true value of BOD 5 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 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.

Population Characteristics 68% 95% 99% Mean or Average

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.

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

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.

Composite Samples Frequently used to estimate average values over a 24-hour period. BOD 5 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 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.

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.

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.

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 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? 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? 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 Accuracy Neither precise nor accurate. Precise, but not accurate. Accurate, but not precise. Accurate and Precise. BOTH ARE NEEDED.

Precision versus Accuracy Neither Precise Accurate Both

Checking for Contamination BLANKS

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

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.

Checking for Precision DUPLICATES

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.

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.

Checking for Accuracy STANDARDS

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.