# What Is Ultrafiltration (UF)? How It Works and What It Removes Author: Nitin Verma Author URL: https://insights.spans.co.in/author/nitin-verma Published: 2026-08-07 Meta Title: What Is Ultrafiltration (UF)? How It Works | Spans Meta Description: What ultrafiltration (UF) is and how it works — pore size, what it removes (turbidity, bacteria, viruses) and what it doesn't (dissolved salts), plus where UF fits. URL: https://insights.spans.co.in/what-is-ultrafiltration Ultrafiltration is one of the most useful and most under-explained steps in water treatment. People know it involves a membrane, and they often assume — wrongly — that it does roughly what reverse osmosis does. It does not. Understanding exactly what UF removes, and what it deliberately lets through, is what makes it the right tool in the many places it fits and the wrong one in the few it doesn't. ## What UF actually is Ultrafiltration is a physical barrier with pores in the range of about 0.01 to 0.1 micron — fine enough to be measured by molecular weight cut-off rather than a visible hole. Water is pushed through it at modest pressure, and anything larger than the pore is left behind. That is the whole mechanism: it is a very fine sieve, separating purely by size. What that pore size catches is a specific and valuable list: suspended solids, silt and turbidity; colloids too small to settle or be sand-filtered; bacteria; and most viruses, which are larger than the pores. Macromolecules and many large organic compounds are held back too. What comes out the other side is clarified, essentially disinfected water — clear, low in particles, and biologically clean. ## The thing UF does not do Here is the fact that trips people up, and it is worth stating plainly: **UF does not remove dissolved salts.** Anything genuinely in solution — the sodium, chloride, calcium, nitrate and the rest that make up your [TDS](https://insights.spans.co.in/tds-in-drinking-water) — is far smaller than a UF pore and passes straight through. Feed UF water at 1,200 mg/L TDS and the permeate is still about 1,200 mg/L TDS. It will be crystal clear and free of bacteria, but no less salty. This is the clean line between UF and RO. Reverse osmosis works at the molecular scale and rejects dissolved salts; UF works at the particle-and-microbe scale and ignores them. On the [membrane spectrum](https://insights.spans.co.in/membrane-filtration-spectrum), UF sits in the middle — much finer than sand filtration, much coarser than RO. Confusing the two leads to both classic mistakes: expecting UF to fix salinity (it can't), or reaching for RO when the problem was only turbidity and bacteria (expensive overkill). Our [RO-versus-UF comparison](https://spans.co.in/ro-vs-uf/) draws that line in full. ## Why low pressure changes the economics Because UF only has to overcome the resistance of a relatively open membrane — not osmotic pressure — it runs at low pressure and therefore low energy. It is typically built from hollow-fibre membranes with a large surface area packed into a small module, and it is backwashable: you periodically reverse the flow to lift the accumulated solids off the fibres, which keeps it running without constant chemical cleaning. Where fouling does build up, a periodic clean restores it, along the lines we describe for [membrane fouling generally](https://spans.co.in/knowledge/membrane-fouling-causes-prevention/). That low-cost, robust, backwashable character is why UF has quietly become the workhorse clarification step in modern plants. ## Where UF fits The applications follow directly from "removes particles and microbes, keeps salts": - **Drinking water and tertiary polishing** — turning clarified or secondary-treated water into a consistently clear, low-turbidity, low-pathogen product. - **Water reuse** — UF is often the barrier that makes treated wastewater fit for non-potable [reuse](https://spans.co.in/wastewater-reuse-india/), because it reliably removes suspended solids and pathogens. - **RO pre-treatment** — this is one of UF's most important industrial roles. By handing RO a low-turbidity, low-SDI feed, UF protects the expensive downstream membranes from fouling, a case we make in full in [UF as RO pre-treatment](https://insights.spans.co.in/uf-as-ro-pretreatment). - **Food, dairy and process streams** — concentrating or clarifying where the target is macromolecules, not salts. ## The one number that matters for your feed If you take one practical thing from UF, make it this: UF is defined by what it removes _and_ by what it passes. If your problem is that the water is dirty, cloudy, or microbiologically unsafe, UF is very likely the right, economical answer. If your problem is that the water is salty, hard, or too high in dissolved solids, UF will not touch it and you need NF or RO. Almost every UF mistake is a mismatch between the problem and that line. Get the match right and UF is one of the best-value barriers in water treatment — low energy, robust, and a reliable producer of clean feed for whatever comes next. If you want help deciding whether UF is the right step for your water, or how it should sit in the train, that is a short conversation to [have with us](https://spans.co.in/contact/), or reach us at bd@spans.co.in or +91-98100 00233. --- This blog is powered by Superblog. Visit https://superblog.ai to know more. ---