Comparison of Permeability and Fouling Trends for Two Different Generations of Norit Membranes at the Columbia Heights UF Plant

摘要

The City of Minneapolis operates a 70 MGD Ultrafiltration (UF) system at Columbia Heights to provide a physical barrier to potential waterborne pathogens in the treatment of the drinking water supply. The GE plant uses Norit membranes, and was commissioned in 2005. During 2010 the original membranes were replaced with a new generation of Norit membranes which had a higher permeability but the same cut off (based on LRV accreditation from the California Department of Public Health). The original membranes at Columbia Heights operated with a fairly stable permeability, normally starting at around 10-14 gfd/psi (250-350 lmh/bar) after a chemical clean, gradually falling to between 6 and 8 gfd/psi (150-200 lmh.bar) in a period of about 90 days. Fouling rates increased during times of poor feed quality, sometimes occurring in late spring with snow melt run off, or autumn, due to heavy rains after the summer. During these periods, unstable performance suggested that a change in CEB chemicals or a greater CEB frequency was required. Also, mechanical problems increased fouling rates sharply. The new generation membranes have started operation with higher permeability levels, and have stabilized at a permeability 1.5-2.0 times that of the original membranes. Although the fouling rate within a CEB cycle is similar, it has been possible to extend the CEB interval by using an improved CEB procedure, and also due to the higher starting level. The new membranes respond well to the cleaning procedures used. The long term downward drift of permeability which characterized the original membranes has not so far occurred with the new membranes. As a result, more extensive clean-in-place procedures (CIP) have been unnecessary in the first 6 months of operation. It is likely that the higher permeability of the new membranes has reduced the tendency for pore plugging and reduced the fouling propensity of the membranes.