Hard issue
Salt water could be the answer What is reverse osmosis?
While other countries and cities in Africa, most notably Cape
Town in South Africa, have struggled to overcome its water
provision challenges, the Erongo plant was built in two years.
Moreover, it is a simple and straightforward processing plant.
Although, understandably, the process requires a lot of electricity
and the initial capital costs are high, the long-term benefits far
outweigh the costs. Osmosis is the process where water moves from a dilute solution
on one side of a semi-permeable membrane to a concentrated
solution on the other side. In reverse osmosis (RO), this process
is reversed: water passes from the concentrated solution on one
side of the membrane to the dilute solution on the other side.
High pressure is required to drive the RO process — the feed
stream is pressurised to about 70 bar. Energy from the brine
stream is used to pressurise a portion of the feed stream, thus
reducing the electricity consumption of the process.
The EDP is not only a case study for mining companies operating
in desert or semi-desert areas, it is proof that salt water can
provide coastal populations with potable water. More than that,
it shows that if government and the private sector work together,
it is much easier to find solutions to what is not always such
complicated problems.
Although the plant is owned by Orano, it was designed, built, and
is operated and maintained by South African-based Aveng Water,
part of the Aveng group of companies. Aveng Water also operates
two acid mine drainage (AMD) treatment plants in Mpumalanga.
“There are major differences between a desalination plant and
an AMD plant,” says Dave Baillie, plant manager at the EDP.
Baillie worked at the Middelburg Water Treatment Plant before
he moved to Namibia. “It is a similar process in terms of ultra-
filtration (UF) and reverse osmosis (RO); however, at an AMD
treatment plant, there are large processing steps to precipitate out
36 AFRICAN MINING JANUARY - FEBRUARY 2019
To prevent fouling of the RO membranes, the seawater feed to the
RO units has to be filtered. This is done in three separate stages.
Source: Orano Mining
the magnesium and calcium before the water can be processed
through the RO membranes,” says Baillie.
Desalination process
“The first step,” explains Lazarus Gariseb, production
superintendent at EDP, “is the collection of seawater through an
intake unit anchored one kilometre off the coast at a depth of 10m.”
The seawater passes through a screen that catches anything larger
than 40mm in diameter, thus removing large debris, aquatic plants,
fish, and animals.
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