Cape Town’s bold new water strategy
Combining the best elements of different technologies13th Nov 2020
In a document entitled ‘Our Shared Water Future – Cape Town’s Water Strategy’, the City of Cape Town has announced its intention to make a fundamental change in its management of risk – a clear indication that they are taking to heart the lessons learned from the Day Zero near-miss.
Whereas the traditional reliability standard for water supply for long-term decision-making was a 98% assurance of supply (AoS), the city has changed this to 99.5%. The traditional strategy implied that it was an acceptable risk to anticipate a shortfall in supply twice in a century, or once in 50 years, but we have learned that that is not acceptable – hence the bold new strategy that, by carefully balancing risks with cost, is also prudent.
Cape Town’s proposed water supply mix
Central to this new strategy is to alter the supply mix, with a planned profile for 2040 of:
- 75% from surface water
- 7% from wastewater reuse
- 7% from groundwater
- 11% from seawater desalination.
Surface water supply is the standard at present, but there is simply no more surface water reliable enough to develop. It’s also quite expensive, costing about R5.20 per cubic metre at present. Of course, as supply becomes more constrained, this cost is likely to rise.
Groundwater is also not a panacea. Contrary to popular perception, groundwater is not an inexhaustible resource, so any further extraction needs to take into consideration that there are already many private users obtaining water from the same – not inexhaustible – source. The Cape Flats Aquifer, in particular, supplies water to the Philippi Horticultural Area where most of Cape Town’s vegetables are grown. Putting extra strain on this aquifer could have dire consequences for Cape Town’s food security.
Reusing wastewater seems like a no-brainer, and it is. At present, Cape Town’s wastewater – including sewerage – is minimally treated, and then discharged into the sea. So reusing it is absolute common sense and logic, and all it requires is some re-engineering. And – because logic and common sense are not really that common – perhaps some good PR so that the fine people of Cape Town won’t lie awake at night under the misapprehension that the water coming out of their taps is the same water they flushed down the loo the day before.
Seawater desalination is another no-brainer, but it is also not that simple. It is costly both in terms of energy and money, and it necessitates the disposal of concentrated brine.
Is desalination really feasible?
At R12.00 per cubic metre, the cost of seawater reverse osmosis (SWRO) is more than twice the cost of surface water, but it is getting more affordable. For example, the latest SWRO contracts awarded in the Middle East, using fossil fuel as an energy source, come in at about R6.40 per cubic metre. The reason there is a difference between the two is related to volume throughput. The smaller the volume, the higher the price, with Cape Town’s planned 50 megalitre/day plant being on the small side for a utility-scale operation. So, if the city were to build a bigger desalination plant, the cost could get closer to the current bulk cost of water, and it will be driven down further with greater use of renewable energy.
New technology makes desalination more feasible
While it is tempting to look at the different options as competing for attention, it makes much more sense to look at them as complementary elements in an integrated strategy. The City of Durban, in conjunction with Hitachi, has recently trialled a technology based on the purification of wastewater through a reverse osmosis process similar to that used for SWRO, but at a cost of ±R8.00 per cubic metre. But here is where it becomes really interesting – this recovered wastewater is then blended with seawater, which creates a brine that is much less salty than seawater, so it will use less energy to desalinate. This is because reverse osmosis desalination works by forcing the salty water through a semi-permeable membrane that will not allow the salts through. So the amount of energy required is directly proportional to the salt concentration.
So that solves one problem – bringing down the cost of desalination. But what about the brine? There’s always a waste product that needs to be dealt with. Well, there’s good news there, too.
Waste not, want not
That old adage of ‘waste not, want not’ is a good one, and it should be engraved in one-metre-high letters on the municipal buildings of every city in the world. It’s all about attitude – looking at waste as a resource rather than as something to be metaphorically swept under the rapidly-becoming-seriously-tacky carpet. That brine that causes so many problems if it is irresponsibly disposed of actually contains many useful substances, but, until recently, it was not economically feasible to extract them. The key words here are ‘until recently’.
It is possible to ‘mine’ the waste brine for useful chemically pure substances by using a technology called eutectic freeze crystallisation (EFC), which was pioneered by the Department of Chemical Engineering at the University of Cape Town.
It works on the principle that different salts precipitate out at precise temperatures, and each salt has its own narrow range. So, by carefully controlling the temperature, chemically pure salts can be harvested from the brine at different stages. This technology is already being used in the Tweefontein Mine Water Reclamation Project in Ogies, Mpumalanga, where the objective is to have 98% water recovery from highly saline acidic mine waters. This locally developed technology has the potential to dramatically decrease the amount and/or concentration of waste brine, which will go a long way towards making the concept of desalination more sustainable, and thence politically acceptable.
Putting it all together
It’s clear that there is not one magic silver bullet that will instantly and easily fix Cape Town’s water problems – or that of any other city or country. But, by carefully evaluating the different options as elements in one holistic strategy, rather than pitting them against each other in a winner-takes-all bare-knuckle fist fight, it is possible to utilise the best of each approach and – hopefully – come up with a solution that really works. And the great thing is that it looks as if the City of Cape Town is doing just that.