Hard issue
Two pipelines transfer the seawater from the intake structure to
a pump station located on the seashore. The seawater is pumped
to the plant through a single pipeline which is 1.2m in diameter.
The incoming seawater passes through a rotary screen fitted
with panels that remove particles larger than 60mm in diameter.
From the screening building, the water is collected in a tank that
feeds the UF trains. There are five installed rotary screens, with
provision for a further three if required.
The filtering in the UF process takes place in what can best be
described as horizontal pressure vessels, each one six metres long
and 200mm in diameter. Inside each pressure vessel, there are
four UF membranes. Each membrane consists of hundreds of
straws each about 0.5 µ m in diameter. The walls of the straws
are the filter medium. The water enters the inside of the straw
and passes through the pores in the wall. The solids in the water
collect in the straws as they are too big to pass through the
pores. The effective cut point of the UF membranes is 0.01 µ m.
The clean water that has filtered through the UF membranes is
collected in the RO feed tank.
The UF membranes are backwashed regularly to remove the
solid particles that build up in the straws. About once a month,
the trains are cleaned with a detergent to remove the solids not
cleaned out by the backwashing.
The plant was designed to have 14 UF trains at full capacity.
There are currently 11 trains installed and nine in operation. Each
train has 308 membranes installed, giving a total of 2 772 UF
membranes on the site. The typical membrane lifespan is five years,
though some of the membranes on the plant are still the original
ones installed eight years ago.
The next step in the desalination process is the RO unit. The
clean seawater is pumped up to 70 bar pressure and into the
RO vessels. These resemble the UF vessels, but they are eight
metres long and have six membranes per vessel. The RO
membranes consist of alternating layers of semi-permeable
membrane. The membranes are wrapped in a spiral around
the central collection pipe. Approximately 47% of the water
entering the RO vessels passes through membranes and out as
pure water. The remaining 53% (and all the dissolved solids)
leaves the membrane as brine. The brine is still at high pressure
and is used to pressurise a portion of the feed to the unit before
flowing back to the sea.
Pure water exits the plant.
Dave Baillie, plant manager at the Erongo Desalination Plant
(left), and Lazarus Gariseb, production superintendent at Erongo
Desalination Plant.
The plant was designed to have nine RO trains at full capacity.
Of the nine trains, eight are in operation at present. There are 512
membranes per train — 4 096 installed on the plant and again, the
expected life expectancy of a membrane is five years.
The permeate from the RO units is actually too pure and needs
to be re-mineralised before it can leave the plant. This is achieved
by passing the water through a bed of limestone where calcium
carbonate dissolves into the water. Chlorine is dosed in the water
to sterilise it and the pH is adjusted to the product specification
before the water is pumped into the NamWater supply line.
www.africanmining.co.za
After the water is screened in the screening plant, it is further
filtered in the ultra-filtration (UF) plant.
JANUARY - FEBRUARY 2019 AFRICAN MINING
37