A reverse osmosis membrane will reject E coli, because bacteria are enormous compared with the dissolved ions the membrane already stops. That is not the same as saying an under-sink RO unit is a safe answer to a well that tests positive. Rejection happens at the membrane; contamination can happen after it, and nothing downstream disinfects.
Why the membrane stops bacteria
RO separation works on a dense film that passes water molecules and rejects almost everything dissolved. Bacterial cells are orders of magnitude larger than the ions being rejected, so they cannot pass an intact membrane. Laboratories rely on that behaviour every day. The physics is not in doubt, and it is why RO appears in specifications alongside removal of dissolved solids, metals and many organic compounds.
Where a household unit falls down
The membrane is one component inside a system full of fittings, tubing, a storage tank with a bladder, and a faucet. A pinhole, a poorly seated O-ring or a bypass around the element puts feed water into the product side. Bacteria that get past, or that enter during a filter change, can then grow in the tank and the tubing where there is no residual disinfectant at all. That is why RO is not certified as a microbiological purifier by itself.
What to do when a well tests positive
Treat it as a well problem, not a faucet problem. Shock chlorinate the well and the plumbing, find and fix the entry point, which is often a bad cap, a shallow casing or surface water reaching the annulus, and retest. If positives recur, fit a continuous barrier for the whole house, typically ultraviolet disinfection sized to the flow with adequate prefiltration for clarity. Our well water testing guide sets out the sampling sequence.
The responsibility sits with the well owner
Private wells are not federally regulated under the Safe Drinking Water Act, so nobody is testing your water unless you arrange it. The EPA reports that around 15 percent of the US population, over 43 million people and more than 23 million households, rely on private wells, and that a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. Routine testing is the only way to know.
Questions people ask about does reverse osmosis remove e coli
Can I drink RO water from a well that failed a bacteria test?
No. Treat the well itself with disinfection and fix the entry point. An under-sink RO unit is not certified as a microbiological barrier and offers no protection downstream of the membrane.
Does UV or RO come first in a well system?
Ultraviolet disinfection normally treats the whole incoming supply after prefiltration, with any RO unit sitting downstream at the kitchen tap for dissolved contaminant reduction.
How often should a private well be tested for bacteria?
Test at least annually, and always after flooding, well work or any change in taste, odor or appearance, because there is no utility monitoring a private supply for you.