Sanitizing an under sink system means treating the housings, tubing and storage tank, and the choice of product matters less than two rules: the membrane and carbon come out first, and the flush afterwards is longer than people expect. Get those right and almost any approved sanitizer does the job.
What to use
Most manufacturers specify a measured quantity of plain unscented household bleach introduced into the first housing, which is inexpensive and effective. Proprietary sanitizer sachets sold for the purpose are equally acceptable and remove the guesswork about dosing. Either way, follow the manufacturer's dose and contact time for your system rather than a general figure, since the volume of the housings and the tank vary and the point is to fill the whole system rather than to make a strong solution in one part of it.
What must come out first
The membrane, without exception, because chlorine based sanitizer destroys thin film composite material permanently and no flush undoes it. Carbon cartridges too, since carbon adsorbs the sanitizer and neutralises it, which leaves the rest of the system untreated while consuming the media. Store the membrane wet and covered if it is being reused. Reassemble the empty housings so the solution can circulate through them, the tubing and the tank, which is the whole point of the exercise.
What not to use
Scented or thickened bleach products, which contain additives that do not belong in a drinking water system and are difficult to flush. Anything containing surfactants or fragrance. Hydrogen peroxide in place of the specified product unless the manufacturer says so, since compatibility varies. And never mix sanitizers: bleach with an acidic cleaner produces chlorine gas, which is a genuine hazard in a kitchen cabinet. If in doubt, use the sachet sold for the system and follow the label.
Flushing until it is gone
Drain the storage tank completely, then refill and drain repeatedly until there is no smell of sanitizer anywhere in the system. This takes longer than most people allow and it is the step most often cut short. Only then fit the new cartridges and the membrane, and run the commissioning flush the manual specifies for new media on top of that, which is normally at least two full tank fills. Our whole house RO guide covers the same discipline at larger scale.
Questions people ask about reverse osmosis sanitizer
What sanitizer should I use?
Plain unscented household bleach at the manufacturer's dose, or a proprietary sachet sold for the purpose. Follow the system's own instructions.
Can the membrane stay in?
No. Chlorine based sanitizer destroys it permanently. Remove the membrane and any carbon cartridges before sanitizing.
Why remove the carbon too?
Carbon adsorbs the sanitizer and neutralises it, so the rest of the system goes untreated while the media is consumed.
How much flushing is enough?
Fill and drain the tank until no smell remains anywhere, then run the commissioning flush for the new cartridges on top of that.