UF vs RO: what each membrane process actually removes, and when to use which

Ultrafiltration uses a membrane with pores fine enough to retain suspended matter, colloids and many microorganisms, while allowing dissolved salts and smaller molecules to pass through. It operates at relatively low pressure compared to RO and does not reduce dissolved solids.

Reverse osmosis uses a much tighter, semi-permeable membrane and higher applied pressure to reject a large proportion of dissolved salts, along with particles and most microorganisms. This makes it the appropriate choice when the water-quality problem is elevated TDS, hardness, or specific dissolved contaminants rather than just suspended solids.

UF is commonly used as a pretreatment stage ahead of RO when particle or colloidal fouling risk is a concern, since protecting the RO membrane from fouling extends its service life and maintains consistent performance between cleanings.

RO systems produce a reject (concentrate) stream containing the rejected dissolved solids, and typical recovery rates are meaningfully lower than UF, which generally operates at higher recovery since it isn't rejecting dissolved salts. This affects both raw-water demand and reject/wastewater management planning.

If a water test shows the primary issue is turbidity, suspended solids or microbiological contamination with acceptable dissolved-solids levels, UF alone may be sufficient. If TDS, hardness or specific dissolved contaminants exceed the required outlet quality, RO — often with UF or another pretreatment stage ahead of it — becomes the relevant option.