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Extended operation of a full scale immersed ultrafiltration membrane separation process in an industrial treatment facility revealed significant fouling episodes and reduced flux rates at certain operational parameters. Pilot testing conducted prior to full scale operation did not predict the accelerated rate of performance degradation experienced during full scale operation. Each fouling episode required a significant cleaning event, involving the addition of a citric acid immersion solution to soak the membranes. After these recovery cleans, the membrane filtration process would again return to a fouled state rapidly, often within 48 hours An extensive investigation was conducted with the assistance of the membrane manufacturer to determine the root cause of the membrane fouling.

Analysis of the acid bath after completion of the cleaning resulted in the increased concentration of several cations during the acid cleaning phase, however calcium was the predominant cation removed from the scale accumulation. Further investigation into the source of the calcium deposit revealed relatively low concentrations of calcium in the feedwater to the membrane tank. However, the substantial aeration process utilized to scour the membrane surfaces, in combination with the high alkalinity concentration in the wastestream, elevated the pH in the membrane process tank and consequently shifted the calcium from solution into calcium carbonate scale.

Elimination of the scale formation required a two phase approach. First, the process operation was modified to reduce the residence time in the membrane process tank, reducing the amount of carbon dioxide stripping which lead to the elevated pH levels. Second, the feedwater pH was trimmed utilizing a small dose of sulfuric acid to maintain the pH under that required to rapidly form calcium scale. This approach has dramatically improved plant performance, resulting in operational periods of up to four months without significant fouling. Further investigations will be conducted to determine the efficacy of carbon dioxide recovery for pH control, and the use of EDTA to prevent calcium scale through chelation.
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Document Type: Research Article

Publication date: 01 January 2004

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