Selection of automatic samplers to ensure sample integrity

Selection of Automatic
Samplers to Ensure Sample Integrity
Traditional manual sampling techniques are often unacceptable for collecting wastewater samples for monitoring because of intricate routines required by permit or regulatory programs. Automatic samplers have resolved some issues about sample acceptability, but samplers offered in the marketplace differ in their ability to collect representative samples. Rick Sueverkruepp,
lsco, Inc.
Importance of
Collect and store aliquots that accurately reflect the composition of Transport Velocity
Collect a sufficient volume to represent accurate sample collection is the transport velocity. Of the many techniques used by differ in several ways, including intake samples that incorrectly represent certain design, constituents in the flow stream, pollutants in the flow stream. Many studies velocity of the flow stream, and transport reflects the concentrations of pertinent velocity. Additionally, differences spring Table 1. Densities of
for a single purpose or special sampling way that does not significantly alter the Wastewater Pollutants
sampling sludges, high suspended solids, extreme depths, or difficult location Material Specific
instrument selection process is a critical laboratory procedures cannot Grit 1.2-1.7 used by automatic samplers today, some deliver samples that incorrectly represent certain pollutants in the "capable of lifting a sample a vertical Peristaltic Pump Sample
Manual Sample
Mean of 4-hr interval
grab samples
Vacuum Pump Sample
BOD5 mg/L
Figure 1.
line velocity of 0.6 to 3.0 m/sec. (2 to 10 even under experimental conditions — to explains that "sample train velocities variations in transport velocity are not results are listed in Table 2, with averages should exceed 2 fps" Both statements are significant. In general, only researchers samples are clearly because of the different features of an ideal sampler, including that techniques used to automatically collect velocity adjustable from 0.61 to 3 m/sec more closely reflect the values that are analysis of loading effects These typically generated collecting samples manually. The are analyzed for BOD, COD, TSS and other Kirkpatrick [4] that "minimum (suction that contribute to these factors are almost line transport) velocities of 2 to 3 feet per differences in the sample values could be: second would appear warranted." It is velocity of 2 feet per second is required to with a specific gravity of 2.65 and a friction there would be less settling of solids during factor of 0.025." This particle is sand.
transport to the storage containers. The vertically, this particle would fall 0.61 m during transport. If the suction line used is representative sample. Transport velocity is also not a factor when dissolved materials would mean that the first 43.4 ml of the are concerned. In actual application, it has particles with these characteristics. If a Intake velocities. The intake velocity of would result in an understatement of the The differences in the sampler's ability to the intake velocity of the actual flow to take a representative sample was most transport velocity were raised to 3.0 fps, clearly pointed out in Harris and Keffer's concept of "iso-kinetic sampling" that the error would be 2.9%. At 1 fps, the error study. They noticed significant differences in the results obtained from using different These calculations seem to indicate that types of samplers to sample the influent at stream velocity are equal. It is theorized and a lower speed peristaltic pump. These velocity of 2 fps, and preferably 3 fps. units were installed so they could collect will not be forced into the intake line. A samplers agree that it is very difficult — stream. The intakes were tied together and Reprinted from The National Environmental Journal, March/April 1996, Adams Trade Press, Atlanta, GA Table 1. Comparison of Results from
Two Samplers on the Same Flow Stream
BOD5 COD NFS Test
BOD5 COD NFS
BOD5 COD NFS Average
BOD5 COD NFS
line velocity well above the flow stream volume to the sample container. After the velocity at lower head heights. This can cause a "scouring" of the channel floor chamber. This allow solids to settle to the collected from a "cloud" of settled out material that artificially enriches the sample for suspended solids content and Conclusions
References
and Quality Control," Water Environment Operational differences in the samplers. The samples collected by the peristaltic consistently reflects the results observed of liquid detectors and optical sensors, accuracy and repeatability. The operator collected samples data. Variations in the sample to be collected and this volume is then metered through a direct path to the Samplers: What to Look for When Buying," For more information, contact the author, Rick Sueverkruepp, Isco, Inc., P.O. Box 82531, Lincoln, NE, 68501-2351;

Source: http://www.clean.com.br/produtos/monitoramento_qualidade_agua/amostradores_automaticos/Selection_of_Automatic_Samplers_Article.pdf