WEF Discussion Forums
Laboratory Management and Technical Issues
I have to do a 24 hour flow proportional composite sample, for the analysis of cbod. I have an automatic sampler that takes a 4-20 ma input from a Q-meter. I have to program my sampler to take a sample after so many gallons pass through the Q-meter. How do I determine after how many gallons it should take a sample?
For example I have a 0.5 MGD flow. Should I program the sampler to take a sample every 5,000 gallons for 100 aliquots? Or should I program it to take a sample every 20,000 gallons for 25 aliquots? I have a 5 gallon bucket and can take a sample from 20-500 ml (sample size is programable)? Does anyone know where I can find the rules for composite sampling (flow proportional)? Standard Methods?
I would set it at 20000 gal. for 25 aliquotes and 200 ml each for 5 L of sample. This should be representative and enough sample for analysis. About 1 aliquote per hour and at least 100 ml aliquote minimum would be good.
We all know how to run the test, but we don't know how to take represenatative sampls! So what good is the test anyway?
I agree with James below but there are other variables you need to be aware of.
1. You need to know or have input on how the flow meter is set up. ISCO and Sigma samplers are designed to communicate through pulses. This is set up in the flow meter as gallons per pulse. If your average flow is 0.5 MGD, the flow meter needs to be set up where you have the most flexibility such as 1000 gallons per pulse. If you collect after every 20,000 gallons, set the sampler to collect after every 20 pulses (20 x 1000= 20,000).
2. Calculate how much volume is needed for all tests and QC requirements - dups and spikes - Five liters is not very much sample especially if you have a clean effluent and perform QC. This will determine the volume per each aliquot collected.
3. Instantaneoud flow must be checked daily. Technicians calculate how many aliquots should have been collected using the flow volume discharged divided by the volume at which was determined to collect each aliquot. (Ex. 500,000 gal / 20,000 gallons = 25 samples should have been collected)This value is then compared to the actual number of aliquots collected. If only 14 were collected then the flow meter and sampler need to be checked.
4. Does your flow change substantially during the seasons - rainy and dry. If the flows increase dramatically during the rainy season because of inflow and infiltration you want to adjust the sampler to collect less volume of sample so that you do not overfill the composite container. If it is during the dry season and flow goes down substantially, the volume will need to be increased so that you collect a sufficient volume of sample for testing and QC.
5. Most sampling equipment offers options to purchase discrete sample basins so that sample can be collected 1 every hour and the proportioned by flow based on the instantaneous flow at the time that sample is collected. This is more dependable; however it is most time consuming for personnel.