Water. desalination + reuse

August-September 2012

Water. Desalination + reuse

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PROJECTS Using UF to pretreat Chile's SWRO mining desalination plants _________ Frans Knops, Manuel Garcia de la Mata, Cristian Mendoza Fajardo and Estanislao Kahne, X-Flow BV, Netherlands. ___ Editor's Note: Two desalination plants are described in this article to illustrate the use of ultrafiltration (UF) as pretreatment to seawater reverse-osmosis (RO) desalination in Chile. One plant (Gas Atacama) was commissioned in June 2011 and has since been operated successfully. A second plant is currently under construction. WATER IS an important component in the mining industry. With mines often located in remote and dry areas, a sustainable supply of water is crucial. The development of seawater desalination, reduced membrane costs and reduced energy consumption have made seawater a viable source of water supply for mining. Reliability of the desalination plant, quality of water and availability of sufficient water supplies are the most important requirements from the mining industry. Therefore best available and proven technologies should be used 1. The key for successfully operating seawater desalination based on reverseosmosis membranes (SWRO) is the pretreatment. Often dissolved air flotation (DAF) and/or dual media filters (DMF) are used, giving adequate protection of the SWRO most of the time 2. The waters off the Chilean coast are rich in nutrients, bacteria, algae, plankton and macro-organisms, and from time to time they face"red tides": a rapid increase or accumulation of algae. To maintain the SWRO feed water quality within specifications at all times, experienced and skilled operators are needed. Despite their awareness, many desalination plants with "conventional" pretreatment still suffer from SWRO downtime or increased operational costs due to unforeseen circumstances. Numerous tests around the world have proved that ultrafiltration (UF) provides optimum pretreatment for SWRO. UF will remove all suspended solids and provides a substantial reduction in microbiological activity. Plugging of SWRO spacers is completely eliminated, and the SWRO cleaning frequency can be substantially reduced 3,4,5. NORThERN ChilE Northern Chile consists of the Atacama Desert and the Andes Mountains. The Atacama Desert is a strip of land on the Pacific coast, west of the Andes mountains 1,000 km long and 105,000 km2 in area. It is, according to many publications 1, the driest desert in the world. Along the coast the aridity is, among other factors, the consequence of the Peru (Humboldt) Current, which is characterized by upwelling of cold water from the depths of the ocean. Discovery of mineral reserves led to development of the mining industry in Northern Chile, and copper mining is the region's chief source of revenue. Several new mines are being set up, while existing mines are increasing capacity. Ports were built at various towns, such as Antofagasta, Mejillones and Caldera (Figure 1). For mining, the availability and proper management of water is key to its long term sustainability. Desalination and specifically RO have been identified as a potential source of fresh water. The challenge for desalination is Figure 1: Map of Chile | 20 | Desalination & Water Reuse | August-September 2012 enormous, since the circumstances are among the most difficult globally: l Being the driest place on earth, absolute security of supply is of utmost importance. Due to the extreme and increasing scarcity, the political decision is to restrict fresh water consumption for industrial purposes l The ocean water experiences upwelling of cold water. Cold water has a higher viscosity than warm water, with associated higher pumping cost l The water in the coastal area could suffer, all year round, from high algae and TOC concentrations, with eg, red tide events. CONvENTiONal PRETREaTMENT South Pacific seawater is one of the most difficult to pretreat for RO desalination. Conventional treatment consisting of DAF followed by singe-stage sand filtration has been implemented, but also has its limits facing high peaks of silt density index (SDI). Especially during red tide events this technology is unable to maintain the required SDI. Typical SDI level is 4 – 5, which is unacceptable for proper operation of spiral-wound RO elements. In 2004, Degremont was awarded the construction contract for a 45,000 m3/d SWRO plant located at Port Coloso, 15 km south of Antofagasta. The necessity for an even more complete pretreatment line was anticipated. Degremont proposed high-rate DAF, two stages of pressurized dual-media filtration and cartridge filters with 5 micron cut-off. A six-month pilot test was conducted, which confirmed suitability of the selected treatment process: 90% of SDI readings were below 3. The Coloso plant has been in successful operation for a number of years and is used as a benchmark for future projects to be developed. Although conventional treatment

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