Minneapolis Water Works (MWW) planned to upgrade their Fridley Filtration Plant with pressurized
membrane ultrafiltration (UF). The 95-mgd UF facility would treat Mississippi River water that has been
pre-treated by lime softening, recarbonation, and coagulation/settling. The UF facility will treat settled or
water filtered through existing granular media filters. Spent UF and granular media backwash water were
planned 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 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 by UF and is displaced through backwash into the recycled
waste. Specific concerns of the study included:
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 organics that will be chlorinated within the process.
Bench- and pilot-scale testing, combined with advanced NOM characterization techniques (primarily 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.
NOM characterization results showed significant retention of bio-polymer fractions by the hydrophilic UF
membranes. Both hydraulic backwash and backwash with air scour were shown to be effective in the
removal of bio-polymer and, to a lesser extent, humic fractions, indicating the membrane fouling fractions
will be preferentially recycled to the front of the treatment train. Jar tests conducted with the membrane
backwash water treated by either softening and coagulation or coagulation alone showed that optimized
coagulation is much more effective in removing BP and humic fractions in the recycled backwash waste than
the combination of softening and coagulation. Includes 14 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 2.4 MB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 15 |
| Published : | 11/01/2009 |
