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As part of our ongoing public commitment to address the pollution of the Diep River and Milnerton Lagoon, and the consequent negative environmental impact, the City has commenced with a six month trial of aerating the Diep River to address the loss of oxygen in the system caused by high pollution loads.

One of the most significant impacts of the pollution is the drop of dissolved oxygen levels in the river. This is caused by excessive particulate organic matter in the water which in turn results in the proliferation of bacteria which grow at such a rapid rate that they eventually deplete the oxygen levels to such an extent that that the river is anoxic in some locations. 

When this happens anaerobic respiration by sulphate reducing bacteria produces hydrogen sulphide. This is what causes the terrible odour that communities living along the Diep River and Milnerton Lagoon often experience.

The high rainfall in September resulted in significant changes to the Milnerton Lagoon mouth and provided far better tidal interchange leading to increases in oxygen rich marine water entering the lower reaches of the system. Unfortunately, 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.

A team of experts identified aeration of the Diep River as one of the remediating interventions to improve oxygenation. This is described as the most feasible short-term intervention to address the immediate impacts of anoxic sediment build-up in the lagoon and the consequent hydrogen sulphide production by bacteria in the lagoon.

‘I want to inform our residents that four aerators were installed on Wednesday last week to begin the six-month trial to assess whether actively adding oxygen to the system will have the expected and desired outcome of reducing hydrogen sulphide production, and hopefully reducing the odour as a result. 

‘The City has been planning meticulously for this installation. Over the past eight weeks, we have been recording dissolved oxygen levels at multiple locations across the river and lagoon system. 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,’ said the City’s Deputy Mayor and Mayoral Committee Member for Spatial Planning and Environment, Alderman Eddie Andrews. 

It is important to mention that this is a trial project. To our knowledge, this has not been done before in South Africa in an open river system.

‘Still, we are hopeful that this aeration trial will yield positive results, but we cannot be sure at this stage. Only by attempting this unique trial will we be able to determine benefit, no benefit, or even the small risk of complications. Should there be any negative impacts associated with the trial, the City will consider options, including stopping the trial,’ said Alderman Andrews. 

Diep River Estuary

The Diep River Estuary Management Plan was revised and approved by the City Council and the Western Cape Government in December 2022 and April 2023 respectively. The plan includes new information and recommendations on how the City can manage the estuary more effectively and guides interventions in addressing the longstanding environmental pressures in Milnerton, Table View and surrounds. 

The estuary covers an area of approximately 900 hectares and is the point at which the Diep River meets the ocean. The river originates in the Riebeek Kasteel Mountains north-east of Malmesbury, and flows for about 65km south-west towards Cape Town, before entering the sea some 5km north of the Port of Cape Town. 

As it flows towards the sea, the Diep River meanders through the Rietvlei and Milnerton Lagoon. The revised Diep River Estuary Management Plan identifies pragmatic and realistic management interventions that will be undertaken against short, medium and long-term time horizons to address the current pressures on the estuary, and within the context of a heavily altered and urbanised environment.

The plan was workshopped with a number of external experts and independent scientists in the field of coastal and estuarine science, and key City departments, and the public also had the opportunity to submit comments.

Issued by The City of Cape Town