Electricity + Control

by Hennie Pretorius, Endress+Hauser

South Africa, a water scarce country, should consider its water supply as its most valuable resource. All South Africans with access to piped water should always be responsible water users, and not only when a crisis looms, as is currently the case in the Western and Eastern Cape regions.

Cape Town waterWe focus on Cape Town and the Western Cape in order to indicate what possible solutions there are for drought stricken areas. The Cape Town region experiences a Mediterranean climate with warm dry summers and winter rainfall. It is dependent on water that comes from mostly six large dams of which Theewaterskloof is by far the largest. Since 2013 the stored volume of water has slowly been decreasing but three consecutive years of extremely low rainfall has accelerated the crisis. The period 2015 – 2017 is regarded as the driest three-year period in more than 80 years, and 2017 was the region’s driest year since 1933. Modelling by consultants indicates that this is a ‘one in 400-year’ event.

Cape Town has three main options to augment its water supply in times of drought, with the first being the large aquifers in the City and Cape Flats regions: The Cape Flats aquifer, the Table Mountain aquifer and the Atlantis aquifer. They can deliver, as per early estimates, 80, 40 and 30 Megalitres per day respectively. This water is, for the most part, treated in conventional water treatment plants.

The second option is desalination of abundant sea water. This process uses membrane technology to remove the salt from the water and to deliver potable water. It is, however a costly method because of the high energy demand.

Desalination process overview

  • Sea water is drawn from the sea through pipelines and enters the plant through screens to filter out larger material.

  • Pre-treatment filters, which may include ultra-filtration, remove smaller particles.

  • The filtered sea water is pumped to the Reverse Osmosis building where it is pushed through RO membranes at pressures of more than 60 bar.

  • These membranes remove the salts and only the water molecules are let through.

  • The ultra-pure water is then demineralised and disinfected to comply with local drinking water standards.

  • This potable water is stored in tanks from where it is pumped into the water distribution network.

  • The salt concentrate, known as brine, is returned to the ocean.

Source: Sydneydesal.com.au

The third option is direct potable reuse. This is defined as the injection of recycled water into the potable water network once it has been through a traditional water treatment plant, or into the raw water supply before passing through the traditional water treatment plant. Many people would like direct water reuse to be limited to agricultural or industrial use. Unfortunately, with the current and future water emergencies, using it as potable water would be essential. The purified municipal waste water is taken through various membrane filters and finally through RO membranes to get to the required quality. This is a viable solution, and unavoidable in future, for inland drought stricken areas such as Gauteng. Beaufort West’s water supply during the current drought has relied on its direct potable reuse plant.

The City of Cape Town will have to get the correct blend of water supply mix that will be cost-effective during times of plenty but can step up to supply more water during times of drought. Figure 1 indicates a possible water supply scenario from the available sources. (It will change depending on availability of surface water):

Currently the city has four desalination plants in construction phase, two groundwater projects and one water recycle (reuse) plant. The capacity of the desalination plants ranges from two to seven Megalitres/day. The plants are procured on a two-year lease contract, which includes the decommissioning as well as removal of these plants. These small plants are generally more expensive to operate in this finance option and the best option for Cape Town is to invest in the long term, in a permanent plant with a capacity of 150 to 200 Megalitres/day. The economy of scale ensures more cost effective desalinated water for the city.

Many companies in the Cape Town area have invested in their own desalination or borehole water treatment plants. The companies are made up of beverage suppliers, a major insurance company as well as a private hospital group.

Desalination experience in South Africa

South Africa is no stranger to desalination technology. Numerous mines use Reverse Osmosis, as is used in desalination plants, to clean up polluted water from mining activities as well as acid mine drainage water. Various coastal towns also use small or medium scale permanently installed plants for the supply of drinking water during times of drought:

  • Mossel Bay has a medium-sized desalination plant capable of producing 15 Megalitres/day. It is not being utilised at the moment, as the dams in the area are full. The plant requires continuous maintenance to keep it functional, and costs the municipality money even though it is not producing water.

  • Knysna and Plettenberg Bay in Bitou Munici- pality each have a two Megalitre/day plant. The Knysna plant is seldom used owing to high operational costs. In this area there is enough water available in traditional resources.

  • Lamberts Bay in Cederberg  Municipality  has a 1, 7 Megalitre/day plant (upgradable to five Megalitres/day) but it is not currently operational.

  • Richards Bay has a plant with a capacity of 10 Megalitres/day and is currently producing six Megalitres/day.

Saving water in your business or organisation

  • Make water saving tips visible in the workplace.

  • Encourage staff to reduce water use at work and at home.

  • Look for and fix all leaks on the property. Do regular water audits to see which areas of your business are using the most water, and monitor the effect of your water saving efforts.

  • Start a water saving project – it is good visibility for corporate social responsibility projects.

  • Install water efficient taps or install aerators on existing taps to reduce water flow.

  • Install water efficient shower heads and encourage two-minute showers.

  • Modify toilets to reduce flushing volumes.

  • Convert to water-wise landscaping.

  • If your school, organisation or business has a pool, fit a pool cover to reduce evaporation.

Source: City of Cape Town