AWWA WQTC69317

Minneapolis Water Works (MWW) is currently upgrading their Fridley Filtration Plant with pressurized membrane ultrafiltration (UF). The 95-mgd UF facility will treat Mississippi River water that has been pretreated by lime softening, recarbonation, coagulation, sedimentation and granular media filtration. The UF facility will treat settled or filtered water. Spent UF and granular media backwash water will be combined and recycled back to the softening plant (without additional treatment). A study was conducted concurrently with plant design to evaluate the operational and water quality impacts of backwash water recycle, with a particular focus on the fate of natural organic matter (NOM) that passes through the pretreatment processes, is retained on the UF membranes, and is displaced into the recycled backwash water. Specific concerns of the study included the following: partitioning of colloidal and dissolved NOM fractions within the liquid and solids streams from each treatment step and the long-term accumulation of these fractions within the liquid stream through backwash recycle; the potential for increased UF fouling from recycle of NOM fractions that have been demonstrated to be responsible for low-pressure membrane fouling (biopolymer (BP) fraction, including proteins and polysaccharides); and, disinfection byproduct (DBP) formation potential of re-chlorinated organics. Bench- and pilot-scale testing, combined with advanced NOM characterization techniques (fluorescence excitation emission matrix [EEM] and liquid chromatography with organic carbon detection [LC-OCD]), were used to quantify the amount of each NOM fraction present in the raw water and following each treatment process, from lime softening through UF, including UF and granular media filter residuals. Preliminary data show significant retention of biopolymer fractions by the hydrophilic UF membranes. Backwash combined with air scour was quite effective in removing both BP and humic fractions, especially when compared to backwash alone or high pH-chlorine clean, albeit a smaller DOC removal. Thus, the membrane fouling fractions are preferentially recycled to the front of the plant, making the removal of these fractions through the pretreatment processes more critical in order to prevent an increase in membrane fouling potential. Preliminary data show that the softening step provides the most significant removal of BP and humic substances with little additional removal achieved through coagulation and sedimentation. Includes 14 references, tables, figures.