Over the past weeks, water consumers in several parts of Cape Town have experienced an earthy taste and odour in their drinking water. The City would like to assure residents that despite the slight taste and odour, the water is absolutely safe to drink.
The taste and odour are caused by ‘Geosmin’ originating from several of the major dams supplying the city. The presence of Geosmin was noted earlier in the year, and has recently reoccurred. Treatment with activated carbon has been reintroduced.
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 causing the change in taste and smell?
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 does it occur?
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.
Is the water quality affected by Geosmin or MIB?
Neither of these compounds is toxic nor harmful.
What does it smell like?
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.
Why do we smell it?
The human nose is extremely sensitive to both Geosmin and MIB. If you poured a teaspoon of Geosmin into the equivalent of 200 Olympic-sized swimming pools, you would still be able to smell it.
The general threshold for human perception is about 15 ng/l. However people with sensitive noses can detect these compounds in drinking water when the concentration is as low as 8 ng/l (8 nanograms per litre = 8 parts per trillion).
Heating the water increases the volatility of these compounds, which explains why the smell is more easily detected when you are in the shower or when used for hot beverages.
How does the City detect it?
Apart from all of the usual monitoring and the analysis of water quality parameters in our treatment plants and downstream in the pipeline system, the City has a protocol of regular monitoring of both the raw and treated waters across Cape Town for naturally occurring organic compounds.
Can it be removed from the drinking 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.
Where does it originate and how does it get to my area?
The blue-green algae and resultant organic compounds have occurred in both the Theewaterskloof and Voёlvlei Dams in varying degrees over the last 12 to 15 years for a few weeks during the summer months.
How well-equipped is Cape Town to deal with the problems?
Cape Town has achieved a score within the top two position since inception three years ago of the nation Blue Drop drinking water quality rating system that assesses all water authorities in the country on an annual basis.
The City’s SANAS-accredited laboratory at the Scientific Services Branch is one of the best in the country, and is fully equipped to deal with all water quality-related issues. By way of example, the piece of equipment used to measure the level of Geosmin costs in excess of R1 million. The City has the added advantage of qualified, highly specialised and experienced staff to operate the equipment and to effectively analyse the results. The services of this division of the laboratory are also made available to many other municipalities.
Does Geosmin occur elsewhere?
Yes. The problem of Geosmin is not specific to Cape Town or to South Africa and we make use of the current world ‘best practice’ in terms of monitoring and treatment methods to manage the issue.
Are we trying to learn better ways to monitor and treat water?
Many of Scientific Services and Bulk Water Branch employees have been and are actively participating with other water authorities at a national level in research projects under the auspices of the Water Research Commission of South Africa.
Many of these research projects have resulted in refining treatment processes as well as improving methods of process control. The City also consults with other leading authorities on the subject in South Africa and abroad.