Hi Natalie,

Im doing something similar looking at ammoina and phosphate. When I carried out my contact time test, I used seperate samples for each time frame.  I had separate tubes (in triplicate with known concentration of ammoina and phosphate) for each time frame I was looking at.

Sinead

Hii. if u want to keep constant volume and study the effect of contact time you can do this by preparing different samples of same volume and study effect of time individually.Like one sample for 1 hour other for 2 hours and so on.

The above suggestions are best.  Amother method is begin with large volumes in test, and sampling out small insignificant subsamples.  As an example, if you begin with 1000 liters of sample, and remove 100 subsamples at various programmed times of 10 ml each, you have only removed 1 liter or 1/1000th of initial amount.  So you removals become relatively insignificant to the overall amount.  In addition if the initial concentrations are sufficiently high, you may be able to remove 1 ml and add in 9 ml to conduct test.  There are ways to mathematically address the insignificant removals, if you want to check how much of an effect you might be having in sampling, and adjust results if appropriate.  

Great, Thank you all for the suggestions!