Alternatives to Avoid Cape Town’s Day Zero – Desalination

In an attempt to find alternative sources of water to help meet the growing demands of Cape Town’s consumers, we examine various options. Geographically, Cape Town is situated alongside a seemingly inexhaustible supply of water from the Atlantic and Indian Oceans. However, the main problem is that in its current form, seawater is unusable for human consumption.

Drinking seawater is not safe and can lead to severe dehydration and serious health complications due to its high salt content of approximately 3.5%, comprised predominantly of sodium chloride. This concentration is far higher than what the human body can safely process. Your kidneys, which filter out excess salt, would require a greater quantity of fresh water than the volume of salt water consumed in order to flush out the minerals. Continued consumption of salt water results in progressive dehydration, a vicious cycle that makes you thirstier the more you drink, and which can ultimately lead to organ failure or even death.

Distillation and Solar Desalination

Reverse Osmosis

The more mainstream approach is to utilise some form of reverse osmosis or membrane technology, which can remove even dissolved salts, mineral ions, and heavy metals down to 0.0001 micron. The process involves the following steps:

• Abstraction: Seawater is drawn from tunnels or intake pipes, often located offshore below the low-tide sea level, usually in deep enough water to be free from sand and debris stirred up by wave movement, and to minimise environmental impact. These entry points have filtration sleeves to prevent aquatic organisms and large debris from entering the system.
• Pre-treatment and Suspended-particle Filtration: The water is initially processed through drum screens and then through one or a series of sediment filters of decreasing fineness, often down to 1 micron, to remove suspended particles, micro-organisms, suspended heavy metals, minerals, and other contaminants. These filtration banks can be self-cleaning, such as the vacuum technology used by Judo Wasseraufbereitung or Hectron, or a secondary bank of filtration units can be placed in series to accommodate the load while the primary bank is manually cleaned or its filtration media replaced (such as bag filters).
• Reverse Osmosis: The pre-treated seawater is then forced, under high pressure, through a series of semi-permeable membranes of decreasing fineness to remove minerals, salts, heavy metals, bacteria, and viruses. At the finest membrane level (usually around 0.0001 micrometres) even dissolved pollutants are removed. The resulting near-pure water is separated from the salt-enriched brine/wastewater, which is returned to the source, before progressing to the next stage.
• Conditioning (Remineralisation): The processed water, known as permeate, has been stripped of its chemicals and minerals and is slightly acidic. Not only is it corrosive in this state, but it is also not very palatable or healthy to consume. Measured health-safe quantities of minerals such as calcium, magnesium, potassium, and sodium are therefore added to remineralise the water, improving taste, pH balance, and overall hydration. Finally, the water is fluoridated and chlorinated to retard bacterial growth, and in certain cases where the CO2 level is high, it is also degassed.
• Distribution: The conditioned water enters the potable water system for distribution to the end user.

Disadvantages of Reverse Osmosis

The main disadvantage of reverse osmosis is the ratio of usable potable water to rejected brine water, much of which has twice the sodium concentration of the source water, along with other undesirable contaminants. There is also the question of where to return this brine waste. It cannot simply be flushed back into the ocean at a single point, as the salinity at the discharge point would be so high as to disrupt marine life. The waste stream therefore needs to be dispersed across various locations in order to spread the load.

Electrodialysis

The third main desalination option is electrodialysis, a water treatment process that uses an electrical current to separate ions within the source water. Like reverse osmosis, it is based on membrane technology, but this time it relies on ion exchange and electrical charges rather than pressure and physical barrier filtration.

How Electrodialysis Works

Electrodialysis operates by applying an electrical current through a series of ion exchange membranes, which selectively allow only positively charged ions (cations) or negatively charged ions (anions) to pass through while blocking ions of the opposite charge.

The process relies on the migration of charged ions towards the oppositely charged electrode, with anions moving to the anode and cations moving to the cathode. This movement is managed by the alternating placement of Cation Exchange Membranes (CEM) and Anion Exchange Membranes (AEM) in series. These are normally arranged in multiple parallel units to share the inlet volume load, offering multiple compartments and increasing efficiency in separating the diluted/desalinated water from the concentrated brine.

The process is as follows:

1. Two electrodes (a positive anode and a negative cathode) are placed within a repeating cycle of cation and anion exchange membranes.
2. An electrical current is applied between the two electrodes.
3. Negatively charged ions are drawn through the cation exchange membrane towards the cathode, while positively charged ions are simultaneously drawn through the anion exchange membrane towards the anode.
4. As the differently charged ions pass through the respective ion exchange membranes, the sodium ions in the saline source are separated from the water and the chlorine.
5. The layered compartments produce both diluted (desalinated) water and concentrated brine, which are extracted separately.
6. The concentrated brine is reintroduced into the ocean via staggered outlets, while the diluted water passes through a chlorine removal agent such as activated carbon to meet SABS standards, before entering the potable water distribution system for the end user.

Advantages of Electrodialysis

• High water recovery compared to reverse osmosis or solar distillation, with recovery rates of 75 to 90% under ideal conditions.
• An energy-efficient solution for moderate salinity, therefore mainly utilised where saltwater meets fresh water, such as estuaries, wetlands, or at the borders between ocean and land aquifers.
• Allows selective ion treatment, particularly useful in treating acidic effluents containing metals such as iron.
• Relatively environmentally friendly in comparison to other desalination methods.

Disadvantages of Electrodialysis

• At higher salinity levels (above 3 g/L), reverse osmosis becomes the more efficient option.
• Does not address bacteria, viruses, or organic particle contamination.
• Membrane scaling and fouling. As with reverse osmosis, the ion exchange membranes require regular maintenance and replacement. If a constant supply of treated water is required, a secondary backup system installed in parallel is advisable.
• Cost: as with reverse osmosis, and depending on the quality of the source water, replacement membranes and ongoing system oversight can be expensive.

We hope that you found this article informative. We welcome you to visit our website at www.amanzirms.co.za for further water-related articles in the Technical Insights and Articles sections.