African Mining January - February 2019 | Page 39

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