Desalination won’t poison water supply

Xanthea Limberg, Mayoral committee member for informal settlements, water and waste services

Residents of Cape Town are closely following the progress of various water augmentation schemes that the City of Cape Town is pursuing, so it is important that commentators on the issue have their facts straight.

Firstly, Mr Herbert claims that the city’s marine outfall points rely “solely on the sun” to break down effluent.

While it is true that effluent is not treated to reduce bacterial counts before discharge, the effluent that is released is already diluted to a ratio of 100:1 in order to maximise the dispersion and assimilation of the effluent.

This reduces concentrations of bacteria to levels below what can be achieved with even advanced treatment processes (Roberts, 2010).

Furthermore, rigorous tests show that bacterial counts in the sea dissipate within 300m of the outfalls, confirming that the outfalls are working as designed and in accordance with global best practice in marine outfalls.

Mr Herbert also gives the impression that there is medical waste in the effluent. This is again misleading. The effluent disposed of at the marine outfalls in Cape Town has the chemical and biological characteristics of typical residential wastewater. It includes traces of the medicine that has passed through our bodies and excreted in our urine, but not medical waste.

It is illegal to dispose of medical waste via the wastewater system. Furthermore, our own test results do not in fact show excessive levels of hypertension medication in the tissue of black mussels as claimed.

Finally, Mr Herbert asserts that the City is neglecting checks and balances in terms of protecting public health. The recklessness of this claim is frankly indefensible. Prior to putting any of the desalination sites out for tender, the City and its consultants have run water and sediment analysis for all of the sites to assess their suitability for desalination.

These tests were outsourced to expert laboratories and were performed based on global best practice for pre-desalination analysis.

The City would like to reassure residents that all water introduced into Cape Town’s bulk water system is tested to ensure that it meets relevant standards. Rigorous water testing will be done at each stage of the desalination treatment process and the plants are designed so that water is released for consumption only once it has been confirmed as safe for drinking.

With regard to Mr Herbert’s suggestion that desalination rather be performed only in more rural areas, this is not practical. In order to reach required volumes within the required time-frame, a number of these small plants will be required and will need to be installed at various locations along our coastline.

There are also limited locations where this is feasible due to the layout of the reticulation system and the physical characteristics of our coastline.

As a representative of the local ratepayers’ association, it is understandable that Mr Herbert has an interest in preventing the possible development of a temporary desalination plant in his neighbourhood, but it doesn’t change the facts.

The marine environment around Mouille Point and Granger Bay is not “already highly polluted” as claimed, and there is no risk of “poisoning” our existing water supply.