You need to define which aspects of WEOC were being analyzed in those studies that were compared so we can fully answer this question: concentration, composition, size distribution, etc?

Filtration provides a means of standardization. If you do not wish to filter, you should probably centrifuge the samples and pour off the supernatant to reduce the microbial cell count, especially if you plan on storing those samples for any length of time (even at 4 C).

Dear Thomas B Parr and Jonathan J Halvorson, thank you for your answers!