Water. desalination + reuse

teChnology Case study In this project, raw feed water comes from a canal. The RO plant has a feed capacity of 960 m3/d. Product flow is 25 m3/hour. Feed water Raw water treatment scheme: Treatment Treatment Treatment Treatment Treatment Treatment Treatment Sand filter Reservoir settling pits Multimedia filter Carbon filter Bag filter Cartridge filter RO membranes SDI after cartridge filter: Unlimited. Problems: Membranes choke within a week, flow decreases more than 60% from the original specification, feed pressure doubles and flux decreases. Cause: Clay and organics cause a bad blockage of the membrane elements. Remedies and Results Various remedies were introduced to try to cope with this: Remedy 1 Membrane cleaning with conventional chemicals and the ordinary cleaning procedure Maintain high pH (11.5) with caustic soda, or ethylenediaminetetraacetic acid (EDTA) and sodium dodecylbenzene sulphonate (a surfactant). Recirculate that solution through the membrane for 4-6 hours, then flush with fresh water for 30 minutes. After that, maintain low pH (2-3) with hydrochloric acid (HCl) or sulphamic acid and citric acid and recirculation that solution for 2-3 hours. This produced no significant result in increasing flux. Remedy 2 Branded chemicals; same procedure Many imported European cleaning chemicals (which shall be nameless) were used to try to restore flux, with the same cleaning procedure as mentioned before. These produced unremarkable results for both high and low pH chemicals. Remedy 3 Cleaning with Genesol 703; same procedure After continuously cleaning the membranes on a weekly or bi-monthly basis using many different chemicals, we selected a product Genesol 703 made by Genesys International of the UK. This product showed some improved results for clay removal from membranes and restored the membranes flux to 50-60% after a 4-6 hour circulation of Genesol 703 in 2% solution at 40��C on a weekly basis. After ten months of difficult and hectic operation, however, the product flows came down to 5m3/h, and cleaning with Genesol 703 no longer gave significant results. At this point, it was decided to replace the membranes, because operation was not feasible. Because of fouling and the inability to consume more energy to increase the flow rate and feed pressure, the plant was producing only 20% of its design product flow and was not economical. Also, due to high pressure, the membranes had compressed physically. Output from Step 2 before and after settling. Results The results were amazing. Membrane flux was restored to the original design specification at the design feed pressure on membranes that were going to be thrown away and there was no change in the salt passage. 120 100 80 Flux % Restore 60 40 20 0 1st New Remedy: Step 1 After 15 days of stoppage of the RO plant, we undertook a new experiment on the old membranes using Genesol-703. The membranes were soaked with a 4% solution of 703 under 10-20 psi (0.69-1.38 bar) pressure. After 24 hours of soaking the membranes, they were flushed with fresh water. The solution which came out from membranes contained a large quantity of clay/silt and was a muddy color. The output from Step 1 New Remedy: Step 2 Again the membranes were soaked, with a 4% solution of 703 under the same pressure. After 72 hours, the membranes were flushed and the solution which came out was like sludge. After flushing the membranes with fresh water we found over 250 g clay in one liter of water. 2nd 3rd New 120 100 1st 80 2nd 60 3rd 40 New 20 0 Clean Restore Clean Restore ConClusions l Genesol 703 was effective at removing clay and silt from fouled membranes in conventional cleaning protocol. l In extreme fouling conditions Genesol 703 when left to soak for 1-3 days had a surprising and dramatic effect on removing clay and silt deposits bringing membranes back from the dead and restoring flux. Water Regime (Pvt) Ltd is a distributor for Genesys International, manufacturer of Genesol 703. The Pakistan company independently initiated and wrote this case study due to the surprising nature of the results. l February-March 2012 | Desalination & Water Reuse | 35 |

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