Why Does My Water Smell Like Rotten Eggs?

That sharp, unpleasant smell — like a struck match or a bad egg — is one of the more alarming things a Johannesburg homeowner can discover at the tap. The good news is that the cause is almost always identifiable with a few simple tests you can do yourself before calling anyone.

What Actually Causes the Smell?

The culprit is hydrogen sulphide (H₂S), a colourless gas that is detectable by the human nose at extremely low concentrations. It forms in one of three main ways in a residential water system:

1. Sulphur-reducing bacteria (SRB) in your geyser — the most common cause in Johannesburg municipal water systems.
2. Naturally occurring sulphur compounds in borehole water — common in certain geological zones across Gauteng.
3. A dry or blocked drain trap — which has nothing to do with your water quality at all, but is frequently mistaken for a water problem.

Understanding which of these is responsible changes the fix completely. Treating your geyser for a drain problem, or installing filtration when the issue is a dry P-trap, is wasted money.

How to Diagnose the Source in Ten Minutes

Work through this sequence before doing anything else.

Step 1 — Hot vs cold test. Fill a glass from the cold tap and another from the hot tap (let each run for ten seconds first). Smell both outside, away from the drain. If only the hot water smells, the problem is almost certainly your geyser. If both smell equally, the source is your incoming supply — either municipal water or your borehole.

Step 2 — Tap vs drain test. If you smell the odour at the basin but both glasses of water smell fine outdoors, the problem is your drain, not your water. Pour water down the drain and check whether the smell intensifies — a dry P-trap under the basin is the usual cause, especially in a bathroom that has been unused for a while.

Step 3 — Borehole vs municipal test. If you are on borehole water and the smell is in both hot and cold, the sulphur is most likely entering with the raw borehole water itself. If you are on municipal supply and both taps smell, contact Johannesburg Water to report it — persistent sulphur in municipal cold water is uncommon and worth logging.

Smell in hot water only	Smell in hot and cold	Smell at basin but not in glass
Geyser bacteria or anode rod	Borehole sulphur or supply issue	Dry or blocked drain trap
Flush and disinfect geyser	Test borehole water; consider filtration	Run water weekly; check trap

The Geyser Problem — Sulphur-Reducing Bacteria

This is the scenario most Johannesburg households encounter. Geysers set below about 60 °C create a warm, low-oxygen environment that sulphur-reducing bacteria find hospitable. These bacteria convert naturally occurring sulphates in the water into hydrogen sulphide gas. The smell is often worse first thing in the morning after the water has sat in the geyser overnight.

Geysers also contain a magnesium sacrificial anode rod designed to prevent tank corrosion. In some water chemistries, this rod accelerates bacterial hydrogen sulphide production. A plumber can replace it with an aluminium-zinc alternative, which tends to reduce the problem.

The standard first response is a thermal flush: raise your geyser thermostat to 65–70 °C for several hours to pasteurise the tank, then drain and refill it. If the smell returns within weeks, bacterial colonisation is likely persistent and you may need a UV disinfection unit on the geyser supply line or a different anode material. A qualified plumber should assess this — do not simply leave the thermostat high indefinitely, as scalding risk and energy use both increase.

If you're not sure whether the fix is a plumber, a filtration unit or both, it helps to hear from people who install all three. Compare 3 free quotes — vetted installers, no obligation.

Borehole Water and Sulphur Compounds

Johannesburg sits on geology that varies significantly across the city. Some aquifers carry naturally elevated sulphate levels, and borehole water drawn through certain rock formations can arrive at the pump with dissolved hydrogen sulphide already present, especially from deeper anaerobic zones.

If you are one of the growing number of Johannesburg households relying on a borehole — where typical complete systems run from R60,000 to R100,000, though the full range is R40,000–R150,000 depending on depth and equipment — water quality testing is not optional. SANS 241 (2024 edition) is the South African drinking-water quality standard against which your borehole water should be tested. The standard sets limits for aesthetics (including odour and taste) as well as health parameters.

Get a certified laboratory water test before deciding on any treatment. Hydrogen sulphide in borehole water is typically addressed with aeration (exposing the water to air to off-gas the H₂S), activated carbon filtration, or a combination of both. Our overview of water filtration options explains the main technologies and which problems each one addresses — it is worth reading before you commit to any equipment.

If your borehole tank water develops a smell after storage, that is a separate but related problem. Stagnant tank water creates its own bacterial issues — you can read more about that in our guide on what to do when tank water smells bad.

The Drain Red Herring

This one catches people out regularly. A P-trap — the curved pipe section under any basin, bath, or floor drain — is designed to hold a small amount of water that seals the drain and prevents sewer gases from entering the room. If a basin or shower has not been used for several weeks, that water evaporates and the seal disappears. Hydrogen sulphide and other sewer gases then drift up through the open pipe.

The fix costs nothing: run the tap for thirty seconds to refill the trap. If a drain is used regularly but still smells, the trap may be cracked, incorrectly installed, or the vent stack may be blocked — a plumber should check it. Do not confuse this with a water quality problem.

When to Be Concerned About Health

At the concentrations typically found in residential water, hydrogen sulphide is an aesthetic problem — unpleasant but not acutely dangerous for most people. However, any persistent chemical smell in your water warrants proper testing rather than assumptions. The National Institute for Communicable Diseases (NICD) provides guidance on waterborne illness in South Africa, and the SABS administers SANS 241, the standard your water should be tested against. If you are immunocompromised, pregnant, or have young children, do not delay testing.

For borehole users in particular, a single smell test is not sufficient — bacteria that cause illness are odourless, so a water test covering microbiological parameters is essential regardless of how the water smells.

Water testing and treatment quotes vary widely — getting more than one gives you a realistic price anchor and helps you spot outliers. Request quotes from three local specialists — no obligation, no pressure.

What to Do Next

The path forward depends on what the diagnosis reveals:

• Geyser bacteria: thermal flush first, then assess the anode rod and consider UV treatment if the problem recurs.
• Borehole sulphur: get a certified lab water test, then size filtration or aeration to the actual results — not to a generic product recommendation.
• Dry drain trap: run the tap. If that does not fix it, call a plumber to inspect the trap and vent stack.
• Municipal cold water: report to Johannesburg Water and request a written response; persistent odour in municipal cold water is uncommon and should be investigated.

In all cases, diagnose before you spend. The rotten-egg smell is specific enough that a methodical ten-minute test will usually point you to the right category of fix — and that saves you from buying treatment equipment for a problem that belongs to a different part of the system entirely.