A capsule filter is a filter cartridge sealed inside its own disposable housing with inlet, outlet and vent connections, so there is nothing to clean and nothing to assemble. It has taken over sterile and clarifying duty at laboratory and pilot scale for that reason, and the decisions are the same three every filtration asks: the membrane, the rating and the area.
capsule filters against a cartridge in a housing
A cartridge in a stainless housing is cheaper per square metre and needs cleaning, sanitisation and an assembly step where an operator can pinch an O-ring. A capsule arrives sterilised, is connected by tubing and discarded, which removes both the cleaning validation and the assembly error, and costs more per batch. The crossover is volume and frequency: pilot and clinical work runs on capsules, and a plant filtering the same buffer every day runs on cartridges in housings.
Membrane and rating, and what the number means
A 0.2 micrometre rating is a sterilising grade claim validated by bacterial challenge, not a pore measurement, and a 0.45 is a clarifying grade. Membrane chemistry decides binding and compatibility: polyethersulfone for low protein binding, PVDF for low binding with solvent tolerance, nylon and cellulose for aqueous work, PTFE for gases and aggressive solvents. A prefilter layer in the same capsule protects the final membrane and is what makes a difficult feed practical, and it is specified as a combination rather than added later.
Sizing area from a flow decay test
Area is sized from the feed's own fouling behaviour rather than from a rule of thumb. Filter a measured volume through a small disc of the same membrane at the operating pressure, record volume against time until flow decays, and scale the throughput per square centimetre to the batch volume with margin. Two numbers come out of it: the area needed and whether a prefilter changes the answer, which it usually does. Sizing from the pump's flow alone ignores fouling and produces a filter that blocks mid-batch.
Integrity testing, and what it proves
A sterilising filter is integrity tested after use, and often before, by bubble point or diffusive flow against the manufacturer's specification for that membrane wetted with that fluid. A pass shows the membrane and its seals were intact, which is what supports the sterility claim for the filtrate; it does not show what was retained. The test needs a vent that can be operated aseptically and a record with the fluid, the temperature and the specification, which is why the capsule's vent design matters as much as its area.
Questions people ask about capsule filter
Capsule or cartridge in a housing?
Capsules for pilot, clinical and variable work, because there is nothing to clean or assemble. Cartridges where the same filtration runs often enough for the housing's cleaning validation to pay.
How much area do I need?
From a flow decay test on the actual feed, scaled to the batch with margin, and repeated with a prefilter to see whether the combination is cheaper.
Does an integrity test prove sterility?
It proves the membrane and seals were intact against the manufacturer's specification. Sterility of the filtrate rests on that plus the process around it.