The M-I SWACO dual-vessel filtration units can be used for a variety of filtration functions—from polishing after diatomaceous earth filtering to hydrocarbon removal. This unit effectively handles contaminants of both known and unknown particulate size and distribution. In applications in which contaminant size and distribution are unknown, the filtration process is stepped in stages. The modular cartridge and bag filter unit with optimized functionality significantly reduces footprint while eliminating the need for additional processing equipment in a wide range of filtration applications.