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This paper presents the findings of pilot study work which was carried out primarily toestablish cleaning regimes for a submerged microfiltration installation treating water that hasseasonally high organic loadings.In parts of the United Kingdom it is illegal to supply water containing more than one Cryptosporidiumoocyst in 10 litres, yet there have still been outbreaks with suggested links to supplies wheredetected oocyst concentrations were significantly below this limit. One such supply wasfrom an 18.5 mgd plant where treatment was ozonation (for color reduction) followed byslow sand filtration. Due to the political sensitivity of the situation it was imperative toprovide as sure a barrier as possible against oocysts, quickly and economically. Submergedmembrane microfiltration was the chosen solution, placed after the existing slow sandfiltration.A pilot filter was set up at Invercannie to prove the process and to confirm the cleaningregimes that should be applied on the full scale plant. Due to the fast track program, thepilot testing was carried out in parallel with construction.The membrane feedwater was ozonated and slow sand filtered, but still contained TotalOrganic Carbon (TOC) in the range 0.75 mg/l to over 7 mg/l. Such levels of TOC can lead tosevere organic or biological fouling on membranes, unless the cleaning regime is highlyeffective. Ozonation in the process train is also thought to exacerbate the fouling potential bythe breakdown of the natural organic matter to assimilable organic carbon.Normal backwashing with air and water had to be enhanced by the use of a maintenance cleanalso known as a chemically enhanced backwash (CEB). Here sodium hypochlorite isintroduced during a modified backwash. The membrane is allowed to soak in the sodiumhypochlorite solution for 10 minutes before the cell is rinsed twice with membrane filteredwater. In order to meet the performance criteria of 28 days between cleaning in place (CIP)washes the Chemically Enhanced Backwash frequency required varied between one every 2days with 25 mg/l chlorine up to once per day with 150 mg/l chlorine.This is a demanding environment for the membranes, as they are frequently exposed to veryhigh levels of chlorine. If chlorine levels were not kept high enough during the(approximately) daily chemically enhanced washes, the membrane's performance fell belowthe specified requirements. The membranes must operate with feedwater temperatures downto 4 C, at an external area flux of 43.1 L/(msup2/sup.h) (25.3 gal/ftsup2/sup/d) with a trans-membranepressure not exceeding 85 kPa (12.3 psig). In addition, the significant levels of chlorine in theCEBs led to the formation of trihalomethanes (THMs), although a clear correlation betweenthe two was not always evident.The full scale plant has operated successfully since commissioning. The TMPs at full scalehave been closely comparable to those found for the corresponding time of year in the pilotplant. Resistance at full scale has been greater. At the time of writing, the full scale plant hadyet to operate over the full winter period. The data gained from pilot Runs 17 and 18 from January to March 2004 do however give confidence that appropriate Chemically EnhancedBackwashes, with chlorine levels up to 150 mg/l if necessary, should enable satisfactoryoperation to be maintained. Includes 2 references, tables, figures.

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Edition: Vol. - No. Published: 03/01/2005 Number of Pages: 19File Size: 1 file , 720 KB