Complex sentence: Filtration (such as reverse osmosis or ultrafiltration) may remove pathogens, while rain water is pure it may become contaminated during collection or by collection of particulate matter in the air as it falls.
Split into simpler sentences: Filtration (such as reverse osmosis or ultrafiltration) may remove pathogens. While rain water is pure it may become contaminated during collection or by collection of particulate matter in the air as it falls.