Under-sink reverse osmosis units have become a common sight in Czech apartment kitchens over the last decade, particularly in Prague where high water hardness makes filtered water an attractive alternative to purchasing bottled water or relying solely on pitcher filters. Understanding how an RO system actually works — and what its limitations are — helps in evaluating whether the investment makes sense for a given household.

The Basic Principle

Osmosis, in its natural form, describes the movement of water molecules through a semi-permeable membrane from a region of lower solute concentration to one of higher concentration. Reverse osmosis works against this natural tendency: by applying pressure to the higher-concentration (more contaminated) side, water molecules are forced through the membrane while dissolved salts, minerals and larger molecules are rejected and flushed away as waste water.

The membrane itself is a thin-film composite (TFC) material, typically consisting of a polyamide active layer on a polysulfone support structure. Pore size is in the 0.0001 micron range — large enough to pass water molecules but small enough to block dissolved ions including calcium, magnesium, sodium, chloride, nitrates, lead, arsenic and most other dissolved solids.

A Typical Under-Sink System

Consumer RO units installed under kitchen sinks in Czech homes almost always use a multi-stage configuration. The exact stages vary by manufacturer and price point, but the most common arrangement is:

  1. Stage 1 — Sediment pre-filter (5 micron): Removes suspended particles, rust and sediment that would otherwise clog or damage the RO membrane. Polypropylene or melt-blown cartridge. Replacement every 6–12 months.
  2. Stage 2 — Granular activated carbon (GAC) pre-filter: Removes chlorine and chloramines, which degrade polyamide membranes. Also reduces some organic compounds affecting taste. Replacement every 6–12 months.
  3. Stage 3 — Carbon block pre-filter: A second carbon stage for finer chlorine removal and organic contaminant adsorption before the membrane.
  4. Stage 4 — RO membrane: The core stage. Removes 90–99% of dissolved solids depending on membrane quality and operating pressure. Produces permeate (filtered water) and concentrate (waste water). Membrane replacement every 2–5 years.
  5. Stage 5 — Post carbon (inline): Final taste polishing. Removes any residual odour from the storage tank. Replacement every 12 months.

Higher-end units add a mineralisation cartridge after the membrane (adding back calcium, magnesium and sometimes potassium to adjust taste and pH), a UV sterilisation stage, or a permeate pump to improve flow rate and waste water ratio.

Flow Rate and the Waste Water Question

This is the aspect of RO systems that generates the most discussion. For every litre of filtered water produced, a conventional RO unit without a permeate pump typically drains 3–5 litres of waste water to the sewer. This is sometimes described with a "recovery ratio" — a standard 1:4 system recovers 20% of feed water as permeate.

At Czech mains pressure (typically 3–4 bar in Prague apartments), a standard 50 GPD (gallons per day, approximately 190 litres/day) membrane fills a 10-litre pressurised storage tank over several hours. This is adequate for drinking and cooking water in a household of 2–4 people; it is not suitable for high-volume uses like filling a large cooking pot directly from the tap.

Permeate pumps significantly improve the ratio — down to approximately 1:1 or 1:2 — and also speed tank refill by boosting feed-side pressure. In Czech conditions, where water tariffs are modest (Prague water in 2025 was approximately 108 CZK/m³ including sewage), the waste water cost of a conventional system runs to roughly 200–400 CZK per year in added water/sewage charges for an average household. Worth considering, but rarely the deciding factor.

Prague mains pressure context: Most Prague apartments operate at 3–4 bar. Under 2.8 bar, standard RO membranes produce less permeate and waste ratio worsens. A booster pump resolves this but adds to cost and complexity.

What RO Removes — and What It Does Not

An RO membrane at standard ratings (typically ASTM D4516) rejects:

  • Dissolved minerals including calcium, magnesium, sodium, potassium: 95–99%
  • Nitrates: 85–95%
  • Lead: 95–98%
  • Fluoride: 85–95%
  • Arsenic (pentavalent): 90–95%
  • Chlorides: 95–99%
  • Most pharmaceuticals and personal care product compounds: variable, generally high

What RO does not remove: dissolved gases (hydrogen sulfide, radon, carbon dioxide), certain volatile organic compounds smaller than water molecules, and biological agents that may be present downstream of the membrane if post-membrane tubing is contaminated. The final carbon post-filter and a post-membrane UV stage address the last concern. The pre-carbon stages handle chlorine, so the membrane sees essentially no chlorine.

TDS and Mineralisation

RO-treated water typically exits the post-carbon stage at 10–30 mg/l total dissolved solids (TDS), compared to 200–350 mg/l in hard Prague tap water. That level is safe to drink; WHO guidance notes that "no reliable data suggests that demineralised water is harmful to health," though it also acknowledges that very low-mineral water may increase the leaching of minerals from cooking vessels and food.

For those preferring mineralised water, remineralisation cartridges using calcite or magnesium oxide media add back calcium and magnesium to a user-adjustable level. Some households use a blend valve that mixes a small proportion of untreated water with RO permeate to restore mineral content without a dedicated remineralisation stage.

Maintenance in Practice

An under-sink RO unit installed in Prague requires roughly two maintenance events per year:

  • Annual filter cartridge replacement (stages 1, 2, 3 and 5): typically 500–1200 CZK in cartridge costs depending on the system brand
  • Membrane replacement every 2–5 years: 800–2500 CZK depending on membrane type and flow rating

Systems should also be sanitised annually — a procedure involving running a dilute food-grade sanitiser through the system and flushing thoroughly — to prevent bacterial growth inside the storage tank and post-membrane tubing. Many installers offer annual maintenance contracts that bundle cartridge replacement and sanitation.

Is RO Suitable for Czech Households?

The case for an RO unit in a Prague household is strongest where one or more of the following apply: hardness above 20 °dH causes noticeable taste issues; the building is pre-1970 construction with uninspected pipes; the household includes infants or immunocompromised individuals; or the household currently spends significantly on bottled water. Where the concern is primarily hardness for appliance protection rather than drinking water quality, a whole-house softener is a more efficient solution and is covered in the companion article on softener comparisons.

For reference, independent test results on popular Czech-market RO units (brands including Atlas Filtri, Aquafilter and locally-assembled units) can be found through the Czech Association of Water Treatment Professionals and through consumer tests published by dTest, the Czech consumer testing organisation.