I would suggest the final filtration be done with a 0.22 µm filter. In this way you are guaranteed to get rid of any solid particles that may create opalescence and interfere with the photometric determination. In addition, you will reduce the risk of microbial contamination of your analytical equipment

Do you have access to a centrifuge? You could spin down the macroscopic particles before filtering the finer sediments and reduce the amount of filtration. You would also have easier access to the particulate in case there is anything interesting in that material.

For filtering peatland water samples, use a stepwise approach: start with a 100-200 micrometer filter to remove larger particles, followed by 20-50 micrometers, and finish with a 1-5 micrometer filter to ensure clarity for chemical analysis. This method effectively reduces suspended substances while preparing the sample for photometric measurements.

Thank you all for your help. Matthew Wheal i do not have a centrifuge but i will try in my department. That might be a good strategy.