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.” 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.


“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.

  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).

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.