Reverse Osmosis for Home Water: Worth It?

Reverse osmosis has been around in domestic settings for decades, but Johannesburg homeowners are increasingly curious about it — partly because of aging distribution infrastructure, partly because of the contaminants that can appear in water even after it meets the city’s treatment standards, and partly because load shedding and water interruptions have made people more conscious of what actually comes out of their taps. This article cuts through the marketing noise and gives you an honest picture of what RO does, what it costs, and whether it makes sense for your household.

How reverse osmosis actually works

RO pushes water under pressure through a semi-permeable membrane with pores small enough to block dissolved salts, heavy metals, and most molecular contaminants. The rejected material is flushed away as wastewater, and the treated water is stored in a small pressurised tank under your sink, ready to dispense through a dedicated tap.

A standard home RO unit has three to five stages:

1. Sediment pre-filter — removes particles that would clog the membrane
2. Activated carbon pre-filter(s) — removes chlorine and organic compounds that would degrade the membrane
3. RO membrane — the core filtration stage
4. Post-carbon polishing filter — improves taste before the water reaches your glass
5. Optional remineralisation stage — adds back calcium and magnesium (more on this below)

The membrane is the heart of the system. It is rated by rejection percentage for specific contaminants, and quality membranes from reputable brands typically reject 95–99% of dissolved solids — but that figure varies by contaminant, water temperature, and feed-water pressure, so treat manufacturer claims as indicative rather than absolute.

What RO removes — and what it does not

RO handles an unusually broad range of contaminants:

• Dissolved heavy metals (lead, arsenic, chromium)
• Nitrates and nitrites
• Fluoride
• Sodium and other dissolved salts — relevant if your area has high total dissolved solids (TDS)
• Many pesticides and industrial chemicals
• Chlorine by-products (the pre-carbon stage handles this first)
• Some pharmaceutical residues

What it does not reliably remove:

• Dissolved gases (a separate concern)
• Certain volatile organic compounds that pass through membranes more easily than water — hence the importance of the carbon pre-filter
• Bacteria and viruses in a single pass if the membrane is damaged (though an intact membrane does block most microorganisms)

For Johannesburg homes on municipal supply, water must meet SANS 241 (2024 edition), the South African national drinking-water quality standard. Municipal water that complies with SANS 241 is safe to drink, but compliance does not mean zero contaminants — it means contaminants are within permitted limits. An RO system takes you well below those limits for dissolved substances.

If you want a broader view of the filtration landscape before deciding, the water filtration guide covers the main technology types and how they compare.

The water-waste issue — and why it matters in Johannesburg

This is the honest downside. A conventional RO system produces wastewater — sometimes called brine or reject water — for every litre of purified water it makes. Older systems can have a ratio of 3:1 or worse (three litres rejected for every one litre purified). Newer high-efficiency membranes do better, with some approaching 1:1, but you need to verify this with your installer before buying.

In a city where Johannesburg Water reports non-revenue water of 44.8% — meaning nearly half the treated water in the distribution system is already lost before it reaches a paying customer — adding household water waste is worth thinking about carefully. It does not mean you should not install RO, but it does mean you should:

• Choose a higher-efficiency membrane
• Plumb the reject water line into your garden irrigation or greywater system rather than straight to drain
• Size the system to match actual drinking and cooking needs (typically two to four litres per person per day), not whole-house supply

RO is a point-of-use technology — it purifies water at one tap, not throughout the house. That is appropriate and efficient for drinking and cooking. Running your washing machine on RO water would be unnecessary and wasteful.

What it costs to buy and run

Pricing varies, so treat these as rough guides and confirm with quotes:

Item	Typical cost range
Under-sink RO unit (supply and install)	R3,500–R9,000
Replacement membrane (every 2–3 years typically)	R800–R2,500
Pre- and post-filter set (annually)	R400–R1,200
Professional service call	R500–R1,500

Running costs are modest compared to the capital outlay, but they are not zero. Membranes degrade over time and must be replaced on schedule — skipping this defeats the purpose of the system. Some installers offer annual service contracts; worth considering if you do not want to track filter lifespans yourself.

Electricity consumption is low. Most under-sink units use the existing mains pressure rather than a powered pump, though a booster pump may be fitted if your feed pressure is inconsistent — common during Johannesburg water interruptions.

Unsure which filtration technology suits your home and budget? Compare 3 free quotes — vetted installers, no obligation.

Remineralisation: do you need it?

RO water has very low TDS — it is effectively demineralised. Some people dislike the flat taste; others do not notice it. More substantively, there is ongoing discussion about whether regularly drinking very low-mineral water has health implications. The evidence is not settled, but if it concerns you, a remineralisation stage (a calcite or magnesium filter fitted after the membrane) adds back a portion of minerals and raises the pH slightly. This stage costs relatively little to add and can be retrofitted.

It is worth noting that the minerals removed by RO — calcium, magnesium — are also present in food and other drinks. For most people with a normal diet, the mineral contribution of drinking water is not nutritionally significant. But if you are considering RO for the household’s primary drinking source and have specific health considerations, speak to a registered dietitian rather than relying on a filter salesperson’s opinion.

RO versus other filtration options

RO is not the only sensible choice. Activated carbon filters are cheaper and remove chlorine, taste, and many organic compounds effectively — but they do not touch dissolved salts or heavy metals. Ultrafiltration removes bacteria and particles but again leaves dissolved solids. A combination system (carbon plus RO) covers the broadest range.

If your primary concern is taste and chlorine rather than dissolved heavy metals or nitrates, a quality carbon block filter may be sufficient and will cost less to buy and run. If your area has known lead pipe risk, older plumbing, or high TDS, RO is the more thorough option.

The water filtration guide sets out these trade-offs in detail if you want to compare before committing.

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Is it worth it for a Johannesburg home?

For most households: yes, with conditions.

An under-sink RO unit is one of the more thorough and relatively affordable ways to improve the quality of water you drink and cook with. It is not a luxury purchase — for families with young children, elderly members, or anyone with immune considerations, the reduction in dissolved contaminants is a reasonable precaution even when municipal water is technically compliant.

The conditions: buy from a reputable brand with local spare-parts availability, use a vetted installer who will set the system up correctly and can service it, factor in the annual filter replacement cost, and address the reject-water issue so it goes to something useful rather than straight down the drain.

What it will not do is solve water pressure problems, protect you during a boil-water notice if the membrane is not intact, or replace the need for a backup tank if you want water security during interruptions. Those are separate problems requiring separate solutions — but the RO system handles the quality side of the equation better than almost any other point-of-use technology available at the domestic scale.