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Minneapolis Water Works (MWW) planned to upgrade their Fridley Filtration Plant with pressurizedmembrane ultrafiltration (UF). The 95-mgd UF facility would treat Mississippi River water that has beenpre-treated by lime softening, recarbonation, and coagulation/settling. The UF facility will treat settled orwater filtered through existing granular media filters. Spent UF and granular media backwash water wereplanned to be combined and recycled to the head of the softening plant.A study was conducted concurrently with plant design to evaluate the operational and water quality impactsof backwash water recycle, with a particular focus on the fate of natural organic matter (NOM) that passesthrough the pretreatment processes, is retained by UF and is displaced through backwash into the recycledwaste. Specific concerns of the study included:partitioning of colloidal and dissolved NOM fractions within the liquid and solids streams from eachtreatment step and the long-term accumulation of these fractions within the liquid stream throughbackwash recycle;the potential for increased UF fouling from recycle of NOM fractions that have been demonstrated to beresponsible for low-pressure membrane fouling (biopolymer (BP) fraction, including proteins andpolysaccharides); and, disinfection byproduct(DBP) formation potential of organics that will be chlorinated within the process.Bench- and pilot-scale testing, combined with advanced NOM characterization techniques (primarily liquidchromatography with organic carbon detection [LC-OCD]) were used to quantify the amount of each NOMfraction present in the raw water and following each treatment process.NOM characterization results showed significant retention of bio-polymer fractions by the hydrophilic UFmembranes. Both hydraulic backwash and backwash with air scour were shown to be effective in theremoval of bio-polymer and, to a lesser extent, humic fractions, indicating the membrane fouling fractionswill be preferentially recycled to the front of the treatment train. Jar tests conducted with the membranebackwash water treated by either softening and coagulation or coagulation alone showed that optimizedcoagulation is much more effective in removing BP and humic fractions in the recycled backwash waste thanthe combination of softening and coagulation. Includes 14 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2009 Number of Pages: 15File Size: 1 file , 2.4 MB