As part of its efforts to address the long-standing pollution issue at the Diep River and Milnerton Lagoon, the City announced the commencement of a six-month aeration trial at Diep River.
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Arguably the most polluted waterway in Cape Town, the Milnerton Lagoon has suffered multiple fish die-offs, sewage spills and a persistent stench as a result of the ongoing pollution of the Diep River.
The City explained that one of the most significant impacts of the high pollution loads is the loss of dissolved oxygen in the river system.
Excessive particulate organic matter in the water results in the rapid proliferation of bacteria, which eventually deplete the oxygen levels to such an extent that the river is anoxic in some locations.
‘When this happens, anaerobic respiration by sulphate-reducing bacteria produces hydrogen sulphide,’ the City said. ‘This is what causes the terrible odour that communities living along the Diep River and Milnerton Lagoon often experience.’
For years, residents in the surrounding areas have pleaded for urgent assistance from the City of Cape Town with the unacceptable pollution and subsequent health risks.
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Despite increases in oxygen-rich marine water entering the system’s lower reaches as a result of September’s heavy rainfall, the dissolved oxygen levels in the Diep River above Woodbridge Island remain extremely low.
‘It is also expected that during the summer months, the lagoon mouth will slowly revert to a less functional system and reduced tidal interchange,’ the City added.
Aiming to address the loss of oxygen, the City said a team of experts identified aeration of the Diep River as the most feasible short-term intervention to address the immediate impacts of anoxic sediment build-up and the consequent hydrogen sulphide production by bacteria in the lagoon.
Thus, four aerators were installed last week to begin the six-month trial to assess whether actively adding oxygen will reduce hydrogen sulphide production, thereby reducing the terrible odour.
‘We will continue with the monitoring of these sites so that we can quantitatively evaluate whether the aerators improve the dissolved oxygen levels over the next six months,’ the City’s Deputy Mayor and MMC for Spatial Planning and Environment Eddie Andrews said.
Andrews highlighted that this is a trial project, adding that aeration in an open river system has, to their knowledge, not been done before in South Africa.
‘Still, we are hopeful that this aeration trial will yield positive results, but we cannot be sure at this stage,’ Andrews added. ‘Should there be any negative impacts associated with the trial, the City will consider options, including stopping the trial.’
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