Sterile filtration is how heat-sensitive liquids are sterilized, and it is one of the steps in the manufacture of bacteriostatic water. A 0.22 µm (or 0.2 µm) membrane is called sterilizing-grade, but the name describes what it retains, not everything a laboratory might want removed.
What sterilizing-grade means
A filter earns the description by a challenge test: a suspension of the small bacterium Brevundimonas diminuta at not less than 107 cells per square centimetre of membrane is passed through, and the filtrate must be sterile. The nominal 0.22 µm rating is shorthand for passing that test rather than a measured pore diameter; retention happens by size exclusion and by adsorption in the tortuous membrane structure.
What it removes and what it does not
| Removed | Not removed |
|---|---|
| Bacteria | Bacterial endotoxin (the lipopolysaccharide is far smaller than the pores) |
| Yeasts and molds, including spores | Viruses (most are 20–300 nm) |
| Particulate matter above the rating | Mycoplasma (a 0.1 µm membrane is needed) |
| Cell debris and precipitates | Dissolved substances of any kind, including benzyl alcohol, salts and degradation products |
The endotoxin line is the important one for water products: filtration sterilizes, but it does not depyrogenate, which is why endotoxin is controlled upstream in the water system and confirmed by testing rather than removed at the filter. See what is an endotoxin unit? and sterile vs. nonpyrogenic.
Why filtering a cloudy vial is not a fix
Passing contaminated bacteriostatic water through a 0.22 µm laboratory filter unit removes the organisms but leaves behind their endotoxin and metabolic products, adds extractables from the filter, and produces a liquid with no documentation at all. The vial is discarded instead. See why is my bacteriostatic water cloudy?
Membrane materials
- Polyethersulfone (PES): high flow, low protein binding; the usual choice for aqueous solutions.
- Polyvinylidene fluoride (PVDF): very low binding; used where the solution contains proteins.
- Nylon: robust and solvent-tolerant, but binds proteins.
- Cellulose acetate and mixed esters: traditional, low cost, less solvent-tolerant.
- PTFE (hydrophobic): for gases and organic solvents, not aqueous solutions.
Integrity testing
In manufacturing, every sterilizing filter is integrity-tested after use (and usually before) by a bubble-point, diffusion or pressure-hold test that confirms the membrane is intact and correlates with the bacterial challenge. A failed integrity test means the batch is not released. A 0.45 µm prefilter is often placed upstream to protect the sterilizing filter from clogging; it is not itself sterilizing-grade. See how bacteriostatic water is manufactured and what is bioburden?
Related
Sterility assurance level explained · Sterility testing (USP <71>)
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