A field team choosing a water filtration membrane often frames the question as which technology is "strongest." That is the wrong question. The right one is what, physically, a given pore size can and cannot stop — because the membrane in a field hospital’s intake line is doing one very specific job, and buying more filtration than the water source requires wastes power, pressure, and maintenance budget without adding protection.
This piece works through the two membrane classes most often debated for field and clinical water systems — ultrafiltration and reverse osmosis — and where each one actually belongs.
What Pore Size Actually Buys You
Membrane filtration works by physical exclusion: anything larger than the pore diameter is rejected. The pathogens a field program worries about span a wide size range. Bacteria typically run from about 0.5 to 10 microns; protozoan cysts such as Cryptosporidium and Giardia are generally on the larger end of that range; viruses are far smaller, often cited between 0.01 and 0.3 microns. A membrane rated at 0.22 microns is considered "sterilizing grade" against most bacteria — but that same pore size does nothing reliable against viruses, which are simply too small to be caught by exclusion alone.
That size gap is the entire logic behind choosing between membrane classes.
Microfiltration and Ultrafiltration: The Field Workhorses
Microfiltration (MF) membranes typically run from about 0.1 to 10 microns, with common cutoffs around 0.22, 0.45, and 1.0 microns. MF reliably removes bacteria and protozoan cysts, but because most viruses fall below its pore size, MF alone is not considered a virus barrier.
Ultrafiltration (UF) tightens the pore size to roughly 0.01 to 0.1 microns. That narrower range is what closes the gap MF leaves open: UF is generally regarded as effective against bacteria, protozoan cysts, and viruses in a single pass. In the United States, NSF/ANSI 419 sets performance-testing protocols specifically for microfiltration and ultrafiltration membrane modules seeking Cryptosporidium removal credit, and NSF/ANSI 53 covers certification for filters reducing contaminants with direct health effects, including Cryptosporidium. For a field hospital drawing from surface water, shallow wells, or any source with an uncertain microbial load, UF is usually the membrane doing the actual pathogen-removal work — and it typically runs at low enough pressure to be gravity-fed or hand-pumped, which matters when grid power is not guaranteed.
Where Reverse Osmosis Fits — and Its Tradeoffs
Reverse osmosis (RO) membranes sit at the far end of the scale, with pore sizes cited around 0.0001 microns. At that resolution, RO rejects not just bacteria, protozoa, and viruses but dissolved salts and many dissolved organics as well — which is precisely what RO is built for. NSF/ANSI 58 is the standard specific to RO systems, and it centers on total dissolved solids (TDS) reduction and membrane rejection performance, not primarily on microbial log-removal claims the way NSF/ANSI 53 and 419 do.
The practical catch is that RO membranes are considerably more vulnerable to fouling from suspended solids and microbial buildup than UF membranes are, and they require meaningfully higher operating pressure. For that reason, RO is generally recommended for use downstream of water that is already coliform-free rather than as the frontline barrier against an unknown microbial load — running raw, untreated field water directly through an RO membrane risks fouling and shortening its service life. In a field hospital, RO’s real job is usually desalination or dissolved-solids reduction for brackish or high-TDS sources, paired after MF or UF pretreatment, not pathogen removal on its own.
Matching the Membrane to the Water Source
| Membrane | Typical pore size | Reliably removes | Best field role |
|---|---|---|---|
| Microfiltration (MF) | 0.1–10 microns | Bacteria, protozoan cysts | Pre-treatment or bacteria/cyst-only threats |
| Ultrafiltration (UF) | 0.01–0.1 microns | Bacteria, protozoan cysts, viruses | Primary barrier for most field hospital intake water |
| Nanofiltration (NF) | ~0.001–0.01 microns | Bacteria, protozoa, viruses, some dissolved organics | Hardness/organics reduction plus microbial barrier |
| Reverse osmosis (RO) | ~0.0001 microns | Near-total rejection, including dissolved salts | Desalination/high-TDS sources, staged after UF pretreatment |
The Buyer Question That Actually Matters
The specification question worth asking is not which membrane is strongest in the abstract, but what is actually in the water source. If the concern is bacterial and viral contamination in fresh surface water, a shallow well, or a river intake, ultrafiltration alone typically covers that threat profile at a fraction of the pressure, power draw, and fouling risk that reverse osmosis carries. RO earns its place when the water itself is saline, brackish, or carries dissolved-solids levels that no membrane working by pore size alone can address — a different problem than microbial contamination entirely. Specifying RO for a straightforward pathogen problem adds cost, energy demand, and maintenance burden without adding protection a correctly sized UF system did not already provide. It’s also worth remembering that pore size only does its job once turbidity is under control: general guidance in humanitarian water treatment holds that water above roughly 5 NTU should be settled or pre-filtered before any membrane or disinfection step is expected to perform reliably.
Conclusion
For most field hospital and camp settings drawing on fresh surface or groundwater, ultrafiltration — not reverse osmosis — is the membrane doing the actual microbial work. Reverse osmosis belongs downstream, brought in when the problem is salinity or dissolved solids rather than pathogens. Matching the membrane to the water source, rather than defaulting to the most aggressive technology available, is what keeps a field filtration spec both effective and sustainable to operate.