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Drinking water algal bloom poses no medical risk

The City of Cape Town’s Scientific Services Branch has isolated and identified an algal species called Melosira in the water of the Molteno Reservoir, and is assuring the public that it poses no medical risk to public health.

Melosira falls into the class Bacillariophyceae which are diatoms. In drinking water can give the water a grassy or fishy odour

Melosira are commonly found in drinking water reservoirs worldwide. The Melosira species belongs to the family of algae called Bacillariophyceae and is a diatom that forms long filaments. They are known for clogging filters in water systems.

The mechanism of the sudden and unexpected bloom of this species in the reservoir is being carefully examined to determine the causal factors so that appropriate corrective and preventative action can be taken.

Molteno Reservoir is a balancing reservoir situated above the city in Oranjezicht and is supplied by a blend of water from the Theewaterskloof, Wemmershoek, Steenbras and Voëlvlei Dams. The algae have not been detected in the incoming water supply. The water level in the reservoir rises at night when the demand in the city area is low and falls during the day while the peak demands prevail. The reservoir is cleaned annually during low demand periods in winter.

In order to reduce the effects of the Melosira, the Water and Sanitation Department’s Bulk Water Branch is adjusting the supply system to minimise the flow of water through Molteno Reservoir by maximising alternative systems. This will reduce the area of supply from Molteno Reservoir until the branch is able to lower the water level reservoir and clear it of the algal growth.

The procedure to empty the reservoir requires significant resources and careful logistical planning. It is also weather-dependant as the reservoir plays a vital role in balancing the supply and demand. It cannot be removed from service during these periods of hot weather and the associated high water demands.

The exercise will begin with the lowering of the water level towards the end of next week when cooler weather is expected and once all logistical considerations have been taken care of.

The City wishes to apologise for any inconvenience that this situation has caused and wishes to reassure the public that there is no health risk and that every effort is being made to rectify the situation as quickly as possible.

Source: City of Cape Town

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