Water Journal : Water Journal April 2011
refereed paper membranes & desalination water APRIL 2011 101 inter-pass pump and reject recirculation from second pass into first-pass feed. The first-pass RO is capable of up to 35% recovery with three vessels by seven membranes and the second-pass RO is capable of 85% recovery with one vessel by seven membranes. The permanent plant underwent commissioning in August 2009. Figure 3: The JAA plant in South Australia. The design parameters for feed water quality for both plants are shown in Table 1. The challenge of desalinating these waters is not only their high salinity, but also relatively high heavy metals such as iron and aluminium. Challenges Ginkgo plant During commissioning of the Ginkgo plant the silt density index (SDI) was double checked to be < 1, which confirmed the decision to only have cartridge filters as the particulate removal process. However, approximately one month after commissioning, they were blocking up very rapidly, requiring cartridge change-out every two days instead of the expected monthly frequency. This required manning of the site on a continual basis (compared to normal remote operations and monthly service visits of up to a week) for the cartridge replacement, investigation of the root cause and ensuring the continuous operations of the RO plant to supply water to the mine. The used filter cartridges were fouled with fine brown solids (Figure 4). The SDI was checked for each bore and the results were generally >5 as per Table 2. The fact was that feed ferrous iron was well above the 0.315mg/L specification limit as per Table 2. Using a Hach DR890 colorimeter, the ferrous iron was 0.28mg/L at the plant and the groundwater was found to be milky in nature, indicating precipitated iron as the root cause. This was supported by an analysis of the foulants on the used filter cartridge which showed that iron was 31% dry weight. There were also iron bacteria in the groundwater. This and the higher iron levels also caused the RO membranes to foul more rapidly than the expected quarterly CIP frequency. This was managed via fortnightly CIP using citric acid and soda ash and EDTA, which only recovered to 80% of the new membrane's performance. Osmoflo carried out a particle size distribution analysis on the raw water and found the majority of suspended solids were greater than 20 μm in size. Pre-filtration tests were carried out on site in February 2006 to determine the best method for fine solids removal. An MMF and an Arkal disc filter rated at 22 micron were trialled upstream of a five-micron cartridge filter. The MMF at 15m/hr and the disc filter at lower velocities were able to remove most of the fine solids and prevent the cartridge filter from rapidly fouling. There was a concern with the dirt-holding capacity of the disc filter, with Osmoflo not having operated one before on such an application. Therefore, Osmoflo decided to Table 1: Design feed water quality for the plants. Parameter Units Ginkgo JAA pH 6.8 5--6 Conductivity uS/cm 57,500 50,000 -- 60,000 Turbidity NTU 4.0 < 5 (normally) Temperature 0C 23--27 16--28 DOC mg/L 3--6 Total hardness as CaCO3 mg/L 7,900 -- 8,900 TDS mg/L 38,000 45,900 -- 56,100 Aluminium mg/L 3--7 Boron mg/L 5--9 Calcium mg/L 810 405 -- 420 Iron mg/L 0.315 35--50 Ferrous Iron mg/L <0.005 0.1--10 Magnesium mg/L 1360 1,600 -- 2,000 Manganese mg/L 2--4 Potassium mg/L 62 390 -- 400 Sodium mg/L 12,800 <15,100 Chloride mg/L 14,200 15,000 -- 22,500 Fluoride mg/L <1.0 Silica mg/L 11 14--18 Ammonia as N mg/L 1.0 -- 3.0 Bicarbonate as CaCO3 mg/L 236 <10 Carbonate as CaCO3 mg/L <10 Nitrate mg/L 0 <0.5 Phosphate as P mg/L 0.05 <1.0 Sulfate mg/L 7,340 3,000 - 3,500 Sulphide mg/L <0.1 <0.1 Total alkalinity as CaCO3 mg/L 193 <20 Table 2: Feed water SDI and ferrous results. Bore no. SDI Fe2+ (mg/L) 1 8.3 (10mins) Not running 2 5.7 1.28 3 7.1 (10mins) 0.98 4 4.8 0.55 5 7.5 (10mins) 0.67 6 Not running 0.50 Figure 4: Some used filter cartridges.
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