If you've landed in Mexico City for a stretch of remote work, tap water is one of the first practical calls you'll make, right after wifi and a decent chair. Ultrafiltration and reverse osmosis are the two technologies behind almost every drinking water system, and choosing between them is the single biggest decision you'll face when you put a filter on the kitchen tap. It's also the one most people get talked into rather than reasoned through.

This guide uses Mexico City as the working example, since it's where we install these systems every week and know the water in detail. But the physics doesn't change with the address. Whether you're filtering tap water here, in Chiang Mai, or in Lisbon, it comes down to the same question: what a UF or RO membrane blocks, and what it costs you to run one.

Here's the difference, and the honest tradeoff each way.

Ultrafiltration (UF) in a CDMX apartment

An ultrafiltration membrane has pores around 0.01 microns, roughly ten nanometers. That's small enough to physically block bacteria and parasitic cysts, which are the pathogens most people worry about.

What makes ultrafiltration appealing in a Mexico City apartment, particularly the older Roma Norte and Condesa buildings we work in most, is what it doesn't need. It runs on your building's existing water pressure, so there's no pump, no outlet, and nothing to plug in. It keeps working during a power outage. And it doesn't produce a waste stream — the water that goes in comes out.

The tradeoff: ultrafiltration doesn't remove dissolved solids. Heavy metals, fluoride, and anything else dissolved at the molecular level passes straight through, because those particles are smaller than the pores. It also does nothing about what happened upstream in your building's rooftop tinaco. No point-of-use system does, which is why that tank is worth cleaning on its own schedule.

What ultrafiltration doesn't ask of you is any routine upkeep. There's no monthly ritual and nothing to remember. A new system does need flushing through before you drink from it, to clear loose carbon fines and the initial taste off fresh media, but that happens when it's commissioned and again after each cartridge change, which are both visits we're already making. Between visits, you use the tap.

Reverse osmosis (RO)

A reverse osmosis membrane operates around 0.0001 microns, a thousand times finer. At that scale it stops essentially everything, including the dissolved solids that ultrafiltration lets through.

Some systems, like our 6-Stage Purikor, add one more stage after the membrane: a UV lamp that sterilizes the water in the last moments before it reaches your faucet. That placement is the point. Filtration removes what's in the water upstream; UV treats whatever might have found its way into the tubing or the storage tank afterward, right before it hits your glass. It's the one part of the system that addresses viruses directly rather than by size exclusion.

That thoroughness costs you three things. Reverse osmosis needs electricity, because the membrane works too slowly at household pressure and requires a booster pump and a storage tank to give you normal flow at the faucet. It wastes water: the residential Purikor units typically send about three liters down the drain for every liter of purified water they produce, depending on your incoming pressure and water quality. And it's more expensive, both to install and to maintain.

How to choose between a UF and RO water filter

The question isn't which is better. It's which problem you have.

If your concern is pathogens (bacteria, cysts, the things that make people sick), ultrafiltration handles that, without electricity or waste. For most Mexico City apartments this is the right answer, and it's the tier we recommend first.

If your concern includes dissolved solids (heavy metals, fluoride, high mineral content), ultrafiltration can't help you and reverse osmosis can. That's not a marketing upgrade, it's a genuine capability difference.

If you don't have an outlet under your kitchen sink, the decision is made for you. Reverse osmosis needs power. Ultrafiltration doesn't.

What we'd tell you if you asked us in person

Most people who get sold reverse osmosis didn't need it. It's the more expensive system, so it's the one that gets recommended, and “removes more” is an easy thing to say.

But more isn't automatically better when the extra capability addresses a problem you don't have, and it comes with a pump, a tank, a power requirement, and several liters down the drain for every liter you drink.

If you genuinely don't know what's in your water, that's a real question rather than a guessing game: lab testing answers it before you spend anything on a system.

Otherwise, our side-by-side comparison shows all four systems with the electricity and water-waste tradeoffs spelled out.

How does this compare to the garrafón you're buying now?

Worth asking, since that's the alternative for most households here, including most furnished rentals nomads move into. And the research on Mexican bottled water is not reassuring.

One study of 85 garrafón samples found 81% contaminated with total coliform bacteria and 23% contaminated with E. coli, both exceeding Mexican standards. Separate work sampling Mexico City's small purification plants (the neighborhood shops refilling 20-liter jugs) identified E. coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae among the contaminants. The number of those plants in Mexico City grew about 85% between 2012 and 2018, from roughly 1,200 to 2,300. Coliforms and E. coli are bacteria, the category of contaminant an ultrafiltration membrane is rated to stop.

Plenty of garrafón water is fine, but the quality varies with the operator, and you have no way to tell which one you got.

Sources

  • Bacteriological quality of bottled water from Mexico City small water purification plants, International Journal of Food Microbiology (via ScienceDirect).
  • Bacterial Contamination of Drinking Water in Guadalajara, Mexico, PubMed/PMC.
  • Membrane pore-size classifications and virus removal performance: Safe Drinking Water Foundation; NESC Tech Brief on Membrane Filtration.