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Effluent Sampling Port Design
Two types of samples are collected and analyzed to
determine the characteristics of treatment system effluent: grab
samples and composite samples. "A grab sample is one that is
taken to represent one moment in time and is not mixed with any
other samples. A grab sample is sometimes called an
individual or discrete sample and will only represent
sample conditions at the exact moment it is collected." 3
Grab samples are typically used for unstable parameters such as
temperature, dissolved oxygen or pH. "A composite sample is
prepared by combining a series of grab [individual discrete] samples
over known time or flow intervals. A composite sample shows
the average composition of a flow stream over a set time or flow
period if the sample is collected proportional to flow."3
For both sampling methods, a properly designed, located and
maintained sampling port is needed to insure the sample is
representative of the effluent flow stream. Large municipal
treatment plants frequently sample directly from an effluent channel
with sufficient flow velocity to keep solids in suspension.
With this configuration, a well mixed sample is likely. However,
most onsite systems experience intermittent, gravity flow. As
such, the velocities of the liquids and solids vary from turbulent
and well mixed, to slow and consolidated, all the way to quiescent
and well-settled. A properly designed sampling port will allow
a representative sample to be taken by preventing the variables of
flow velocity and solids settling from affecting the sample collected.
INFLUENT vs. EFFLUENT SAMPLING
A proper influent sampling port for onsite
wastewater treatment systems is very impractical, if not impossible
to design. Small commercial or large flow municipal treatment
systems frequently sample the influent before it reaches the
treatment plant. However, by that time, most raw sewage has
been mixed with several types of wastewater, macerated by
comminuting or pumping, or at least has begun to biologically break
down in the collection system. Any of these processes tend to
homogenize the waste enough that a small volume sample (aliquot) is
more likely to represent the characteristics of the total waste
stream.
Due to the short duration of flows from individual water using
appliances, raw waste in a home sewer line is not likely to be
homogenized with waste flowing at any other time period until after
it enters the treatment plant. Whether the waste was macerated
vegetable peelings from a garbage disposal or gray water from the
clothes washer, the sewer line will likely only contain one or the
other at any single point in time. Any sampler trying to
collect influent may experience problems picking up food waste,
paper products or other material that has not begun to biologically
degrade. If it does pick up these concentrated solids in a
small volume sample, the aliquot will not be representative of the
total incoming flow. Consequently, it is usually assumed that
the influent to an onsite treatment system is of typical domestic
character and influent sampling is not performed, but effluent
sampling is possible.
SAMPLING FOR PERFORMANCE
EVALUATION
The primary use of an effluent sampling port is to
evaluate treatment system performance in order to verify compliance
with specific discharge limits, or to compare the performance of
individual systems. However, in actual installations, even
with a properly designed effluent sampling port, performance
evaluation of onsite treatment systems is extremely difficult to do.
When universities or other third party certifiers establish a
standardized format for performance testing, the logistics of
influent and effluent flow patterns become predictable and useful.
By comparison, field evaluations of an onsite treatment system must
take into consideration that influent waste characteristics can only
be assumed, and representative effluent sampling must be collected
in conjunction with a non-standardized flow pattern. In a
typical gravity flow system, the effluent flow pattern parallels the
influent flow rate and time frame. Using the very low effluent
rates typical of an onsite system to actuate a flow-proportional
composite sampler can be very challenging. Coordinating a
time-based composite sampler to a non-standardized flow pattern is
equally challenging, as flow patterns vary from household to
household. Flow patterns also vary within a given household
from day to day and throughout the day. Considering the
difficulties of using a composite sampler on a treatment system
serving an individual home, it is easy to understand why some people
attempt to use grab samples for performance evaluation.
Even the best designed grab sampling port will still
not allow representative sampling for performance evaluation of an
onsite wastewater treatment system. By definition, a proper
grab sample represents the flow stream only at the point in time the
sample was taken. This makes grab samples invalid for use in a
performance evaluation of an onsite treatment system. Effluent
grab samples are inadequate for performance evaluations even in
municipal treatment plants where there is less variation in the
characteristics of both influent and effluent flow streams. To
evaluate this premise, a Field Investigations Section of the U.S.
Environmental Protection Agency conducted a study of sampling
methods and data variability using a 1.5 million gallon per day
treatment plant serving an Air Force base. In this study, grab
samples of the flow stream were collected. Concurrently, flow
proportional composite samples were collected with automatic
sampling equipment at the same sampling location. When a table
was constructed comparing the removal efficiencies calculated by
using proper composite samples to data from the collection of grab
samples "the table clearly indicates the fallacy of relying upon
single grab samples and demonstrates that varying collection time
will change apparent plant efficiency over a broad range.
Looking at the efficiencies resulting from collecting one sample per
day for three days it can be seen that the removals ranged from -103
to +70 percent." 4
In another study conducted by the same organization, where
grab samples were compared with composite samples, their conclusion
was even more forthright. "The data clearly indicates the
inadequacy of relying upon a limited number of grab samples for
determining wastewater characteristics or plant performance."4
Performance evaluations of onsite treatment systems
should only be conducted under controlled conditions. This
includes using a known, representative flow pattern, a properly
designed sampling port and a flow-proportional composite sampler.
It also must be conducted over sufficient time for the system to
reach steady state conditions.
SUMMARY
"It is an old axiom that the result of any testing
method can be no better than the sample on which it is performed." 1
Even state-of-the-art laboratories, with the use of calibrated
equipment by certified laboratory analysts, cannot provide accurate
data on treatment system performance if the effluent sample was
collected in a non-representative manner. Proper sampling is
the foundation on which laboratory analysis and data evaluation is
built. Improper sampling means laboratory analysis and data
evaluation is a total waste of time, money and resources. The
biggest problem is, once the sample is submitted, it is then too
late for anyone to determine if proper methods were used during
sample collection. The EPA states it in these terms: "It is
little wonder that there are so many disagreements among various
responsible Federal, state, city, and individual groups regarding
water chemistry characteristics and facility performance. When
variations in sampling methodology and laboratory systematic and
random errors are further compounded by errors in flow measurements,
differences can become astronomical. Without an adequate
monitoring program and tight controls on sampling techniques,
equipment, and laboratory procedures, data interpretation can be
reduced to little more than an exercise in futility."4
To represent the effluent flow stream over a
specific time, effluent samples must be flow-proportional composite
samples, collected over the same time from a properly designed
sampling port. This port must be properly located and
maintained to insure the sample submitted for laboratory analysis is
representative of the effluent flow stream. Only then will a
defensible sample be analyzed and evaluated. Refer to the
Norweco Technical Bulletin EFFLUENT SAMPLING PORT DESIGN FOR ONSITE
WASTEWATER TREATMENT for detailed information on the
proper construction of sampling ports.
REFERENCES
1. American Public Health Association, American
Water Works Association, Water Environment Federation. Standard
Methods for the Examination of Water and
Wastewater. 20th Ed. (1998).
2. U.S. Environmental Protection Agency. NPDES
Compliance Inspection Manual. (1985).
3. Water Environment Federation.
Wastewater Sampling for Process and Quality Control.
Manual of Practice No. OM-1. (1996).
4. U.S. Environmental Protection Agency, Region
VII, Field Investigations Section, Harris and Keffer.
Wastewater Sampling Methodologies and Flow Measurement Techniques.
(1974).
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Representative Sampling
Stricter regulations and management practices are
prompting increased effluent sampling programs for performance-based
onsite wastewater treatment systems. Effluent samples are
collected and then analyzed and averaged to evaluate system
performance. For onsite and other small flow wastewater
treatment systems, the design of the sampling port plays an
extremely important role in determining whether or not the sample
collected is actually representative of system effluent. "The
objective of sampling is to collect a portion of material small
enough in volume to be transported conveniently and yet large enough
for analytical purposes while still accurately representing the
material being sampled. This objective implies that the
relative proportions or concentrations of all pertinent components
will be the same in the samples as in the material being sampled…"1
If the sampling port is not properly designed, located and
maintained, the sample collected will not be representative of
system effluent. "Without proper sampling procedures, the
results of such monitoring programs are neither useful nor valid,
even with the most precise and accurate analytical measurement."2
Erroneous data as a result of analyzing a non-representative sample
will likely lead to an incorrect conclusion regarding system
performance and compliance. Only when proper scientific
sampling methods are being followed will a defensible sample be
obtained.
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