A rotten egg smell in water is usually hydrogen sulfide gas. It can occur naturally in groundwater, be produced by sulfate-reducing bacteria in a well or plumbing system, or be generated inside a water heater by a reaction involving the anode rod. Which of those it is determines the fix — and the quickest way to narrow it down is to check whether the smell is in all your water or only the hot.

Do this before you call anyone

It takes ninety seconds and it will save you a considerable amount of money.

  1. Run a cold tap for a minute. Smell it.
  2. Run the hot tap at the same fixture. Smell it.
  3. Repeat at a second fixture on a different part of the house.

If only the hot water smells, your water heater is almost certainly the source and whole-house water treatment will not fix it.

If both smell, the source is your supply and treatment is the right conversation.

We've been called out to homes where a whole-house system was already installed for a smell that was coming from the water heater. The system worked perfectly. It just wasn't treating the thing causing the problem.

Cause one: hydrogen sulfide occurring naturally in groundwater

Some groundwater contains dissolved hydrogen sulfide as a result of geological conditions. While the water is under pressure in your plumbing, the gas stays in solution. The moment it reaches an open tap and the pressure drops, the gas comes out — which is why the smell is strongest in the first few seconds and why showers are where most people notice it. Warm water and steam release far more of it into the air.

This is a supply-side problem, and it's treatable.

Cause two: sulfate-reducing bacteria

Certain naturally occurring, non-pathogenic bacteria produce hydrogen sulfide as a byproduct of their metabolism. They can establish themselves in a well, in a pressure tank, or in sections of plumbing where water sits and moves slowly.

The tell is usually inconsistency. The odor fluctuates, it's worse at fixtures you rarely use, or it's noticeably stronger after the house has been empty for a few days. Where bacteria are a factor, filtration alone often doesn't hold — the well and plumbing generally need disinfecting as well, or the problem returns.

Cause three: a reaction inside the water heater

This is the one behind most hot-water-only complaints, and it's the one most likely to get treated incorrectly.

Water heaters contain a sacrificial anode rod — a metal rod that corrodes deliberately so the tank doesn't. Under certain water conditions, a reaction involving that rod can generate hydrogen sulfide inside the tank. The water arriving at your house may be perfectly fine.

The fix is at the heater: replacing the anode rod with a different material, and sanitising the tank. That's plumbing work rather than water treatment, and when that's what you need, we'll tell you.

A water softener will not fix this

Worth stating plainly, because it comes up constantly. A water softener reduces hardness minerals through ion exchange. It is not designed to address hydrogen sulfide and it will not remove the smell.

If a company suggests a softener for a sulfur odor, it's reasonable to ask what they're basing that on.

What testing shows

Hydrogen sulfide is volatile — it dissipates quickly, which means it often needs evaluating on site rather than from a sample shipped somewhere. Beyond confirming it's present, evaluation typically looks at:

  • Where it's coming from, by testing at different points in the system
  • Sulfate levels, and whether sulfate-reducing bacteria are a likely factor
  • Iron and manganese, which frequently occur alongside it and affect media selection
  • pH, which determines which oxidation approaches will actually work

More on scope: how water testing works.

Treatment approaches for supply-side odor

  • Oxidation followed by filtration. The most common approach — the gas is oxidised and the resulting product is filtered out. Media and method depend on concentration, pH and what else is present.
  • Aeration. Releasing the gas from the water before filtration. Effective in the right circumstances, particularly where concentrations are meaningful.
  • Well disinfection. Where bacteria appear to be a factor, disinfecting the well and plumbing is usually part of the approach rather than an alternative to filtration.

Detail on each: specialty water treatment.

Why it sometimes comes and goes

Hydrogen sulfide levels in a well can vary seasonally or with water table movement. If the odor is intermittent, mention that when you call — it means a sample taken on a good day may not reflect the conditions you're actually calling about, and we'll know to account for it.

The short version

  1. Check hot versus cold. That single test routes the whole problem.
  2. Hot only → water heater. Not a treatment job.
  3. Both → supply. Get it evaluated before anyone specifies equipment.
  4. A softener is not the answer either way.