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Sampling Techniques
Wastewater sampling is generally performed by one of two
methods, grab sampling or composite sampling. Grab sampling is just what it sounds like;
all of the test material is collected at one time. As such, a grab sample reflects
performance only at the point in time that the sample was collected, and then only if the
sample was properly collected. Composite sampling consists of a collection of numerous
individual discrete samples taken at regular intervals over a period of time, usually 24
hours. The material being sampled is collected in a common container over the sampling
period. The analysis of this material, collected over a period of time, will therefore
represent the average performance of a wastewater treatment plant during the collection
period.
Numerous industry references list various parameters for wastewater testing and whether
samples should be collected using grab sampling or composite sampling methods. For
example, grab sampling allows the analysis of specific types of unstable parameters such
as pH, dissolved oxygen, chlorine residual, nitrites and temperature. However, the most
widely used indicators of treatment plant performance, including CBOD5 (five day
carbonaceous biochemical oxygen demand), TSS (total suspended solids) and TN (total
nitrogen) require the use of composite sampling techniques. STANDARD
METHODS (20th
Edition, Section 1060 § B, "Collection and Sampling") states "A sample can
represent only the composition of its source at the time and place of collection."
Grab samples may be used to represent "some well-mixed surface waters, but rarely,
wastewater streams" for water quality evaluation. The widely varying flow patterns of
residential treatment plants make it impossible to evaluate performance by analyzing a
single grab sample of effluent. Residential treatment plants receive a frequent number of
short hydraulic surges throughout the day followed by intermittent periods of no flow
whatsoever.
Routine variations in the volume and strength
characteristics of incoming wastewater create fluctuations in the quality of treatment
plant effluent. Therefore, an effluent grab sample taken at one specific time throughout
the daily flow pattern will not be representative of system performance over the entire
day. The CODE OF FEDERAL REGULATIONS (Title 40, Chapter 1, § 133.102) stipulates that
treatment plant performance must be evaluated by tabulating 30-day averages of plant
effluent. By definition, a single grab sample can never be used to evaluate the long-term
performance of any wastewater treatment plant.
While the limitations of determining system performance by
grab sampling are apparent, the use of grab samples for the evaluation of a residential
treatment unit is further compromised if the sample is collected from a location where the
effluent does not have sufficient velocity to keep the effluent solids in suspension.
WASTEWATER SAMPLING FOR PROCESS AND QUALITY CONTROL (Manual of Practice No. OM-1, §
"Representative Sampling") directs that samples be collected "at points
where the sample stream or tank is well mixed." The manual goes on to say "avoid
taking samples at points where solids settling occurs or floating debris is present. These
situations occur normally in quiescent areas, where the velocity of the flow has
decreased." For this reason, plant performance can never be evaluated by a sample of
effluent taken from a pump chamber, distribution box, sump, roadside ditch or any device
that contains effluent below the flow line. The intermittent flow pattern of an individual
residence intensifies this problem by allowing effluent solids to settle out within a sump
during low flow periods. Even the few solids present in a high quality effluent will
settle out when retained within a sump. If only a very few solids settle within a sump
during a low flow period today, they remain and accumulate with additional solids settling
out over successive days. Any sample drawn from a sump will contain effluent suspended
solids combined with days, weeks, months or years worth of accumulated solids. A sample of
liquid from such a sump cannot be analyzed for overall system performance, as it is
scientifically impossible to determine what portion of the solids were suspended in the
effluent and what portion of the solids had accumulated in the sump over a period of time.
Using any sample drawn from a sump to evaluate the performance of a residential wastewater
treatment plant is simply inappropriate. Samples must be taken from a location where the
effluent is free-flowing and has sufficient velocity to keep the effluent solids in
suspension.
Composite samples of effluent collected, stored, analyzed, tabulated and averaged over an
extended period of time provide the only verifiable indication of treatment plant
performance. Collecting and analyzing these composite samples is often an expensive and
time-consuming process. For these reasons, most regulatory organizations recognize
independent third-party certifiers, who use composite sampling methods to conduct
performance evaluation and accurately measure system performance in a standardized,
reproducible setting. Attempting to evaluate a residential treatment system in the field
by analyzing a grab sample taken from a sump or any other containment vessel provides a
compound degree of error and will yield erroneous conclusions about system performance.
Refer to the Norweco Technical Bulletin
EFFLUENT SAMPLING TECHNIQUES FOR RESIDENTIAL TREATMENT SYSTEMS
for complete, detailed instructions on effluent sampling and
analysis of residential treatment units.
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