Why Does My Water Smell Like Rotten Eggs?

A rotten-egg or sulphur smell in water is caused by dissolved hydrogen sulphide gas (H₂S). This is a common occurrence in Kenyan borehole water, particularly in areas with volcanic geology or organic-rich sediments.

The Chemistry Behind the Smell

Hydrogen sulphide (H₂S) is a naturally occurring dissolved gas formed when sulphate-reducing bacteria decompose organic matter in oxygen-depleted (anaerobic) groundwater environments. The characteristic smell is detectable by the human nose at concentrations as low as 0.0005 mg/L, making it one of the most smell-detectable water quality issues long before it reaches concerning health levels.

In Kenya, H₂S is particularly common in:

• Rift Valley boreholes with volcanic geology and associated sulphur deposits.
• Deep boreholes in stagnant aquifer zones with high organic matter content.
• Water heaters and geysers, where warm water accelerates bacterial activity and H₂S production.

Is Hydrogen Sulphide Harmful?

At the concentrations found in household water supplies (typically <0.05 mg/L), H₂S is not acutely harmful. The WHO has not set a health-based guideline value for H₂S in drinking water because the smell threshold is encountered at levels far below those associated with toxicity. However:

• H₂S is corrosive to copper, silver and certain metals, it causes blackening of silverware and tarnishing of copper fixtures.
• Sulphate-reducing bacteria in the system may indicate an anaerobic biofilm community that could include other, more harmful bacteria.
• The smell makes water unpalatable and can indicate the need for broader borehole maintenance.

Treatment Options by Severity

A Note on Hot Water Systems

If the smell is only present in hot water and not cold, the source is almost certainly sulphate-reducing bacteria in the hot water storage tank. The solution is to temporarily raise the hot water temperature to 60°C for several hours to pasteurise the tank, then flush thoroughly. This should be done carefully to avoid scalding risk and is not a permanent fix if the incoming cold water has elevated H₂S, the bacteria will re-establish.

Q: Will a standard RO system remove hydrogen sulphide?

A: At trace levels, activated carbon (present as a pre-filter in multi-stage RO systems) effectively removes H₂S. At moderate to high levels, additional pre-treatment, aeration or chlorination, is needed upstream of the RO system. iClear can assess your specific level and recommend the right configuration.

Q: Could my sulphur smell be from something other than H₂S?

A: Rarely, a sulphur-like smell can come from organic compounds produced by certain algal blooms. A simple water test distinguishes the two.

Diagnose and fix your rotten-egg water problem

iClear's technicians can test your water and configure the right treatment for hydrogen sulphide at any level. Find out more.