Funny Tasting Tap Water – Geosmin in South African Bulk Water Supply
Despite the slight taste and odour, municipal water is absolutely safe to drink. Over the past weeks, water consumers in several parts of the country have experienced an earthy taste and odour in their drinking water. This taste and odour is caused by ‘Geosmin’ – originating from several of the major dams supplying the cities. The presence of Geosmin was noted earlier in the year, and has recently reoccurred.
Municipalities continue to add powdered activated carbon dosing during the water treatment process in an attempt to absorb this compound and to reduce the unpleasant smell and taste. This situation is likely to persist until the onset of the winter rains.
The water is perfectly fit for human consumption. Continuous water quality monitoring by the City’s South African National Accreditation System (SANAS) accredited laboratory has revealed elevated Geosmin concentrations, greater than 10 nanograms per litre (ng/l). In general, the human perception threshold is about 15 and 20 ng/l for Geosmin odour and taste respectively and people with a heightened sense of smell and taste would be the first to notice the presence of Geosmin in their water supply. These are extremely low concentrations and it should be noted that a nanogram is a billionth of a gram.
What is Geosmin?
Geosmin and MIB (2-Methyl-iso-Borneol) are naturally occurring compounds found in surface waters (rivers and dams) as organic molecules produced by blue-green algae. Bright sun, warm temperatures and nutrients result in ideal growing conditions for the algae. The compounds are produced inside the algae cells and are only released when the algae die.
Why is Geosmin Released?
Apart from the sunlight and raised temperatures, algae require nutrients to grow and some of the dams have suffered from upstream negative environmental impacts that have raised the nutrient levels, which support these algal blooms. This phenomenon usually only occurs once a year for a few weeks during the summer months, but has again reoccurred this year.
What Does Geosmin Smell Of?
Both Geosmin and MIB are typically earthy or musty in character and are sometimes described as smelling of Carbaspray or fresh green mielies (corn). These odours can be very persistent and although the algae are removed, the organic compounds, Geosmin and MIB are not removed by conventional water treatment processes. It requires treatment with activated carbon to reduce the smell to below threshold levels.
How is Geosmin Removed From Tap Water?
Yes, Cape Town follows universal best practice, which is to treat the water with powdered activated carbon. The very tiny organic molecules are absorbed by the carbon particles and the carbon is then removed from the water. It is not possible to remove all molecules, but the aim is to maintain the concentration below the normal human taste and smell threshold level.
While Cape Town and some parts of Gauteng have encountered this occurrence, municipal bodies have aimed to reassure us that the presence of Geosmin is not harmful to our health in any way.
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