NANOFIBER AS FLOCCULANT OR MODIFIER IN MEMBRABE BIOREACTORS FOR WASTEWATER TREATMENT
The more stringent regulations for wastewater discharge present new technology challenges to wastewater treatment (WWT) plants. There is a particular pressing need for improving the hygienic quality of the treated water. Membrane bioreactors (MBRs) represent one of the most innovative approaches to restrain the release of pathogens from WWT plants. Using membranes with a pore-size of 0.1~0.5 mm or less, not only bacteria but also viruses are virtually completely retained. However, membrane fouling is a very serious problem faced by MBRs. Cake layer formation generates largest resistance for membrane filtrations. It was well known that adding flocculants could flocculate small sludge flocs and soluble EPS (Extracellular Polymeric Substances) into large flocs. Flocculated sludge flocs can form a more porous cake, which would enable a higher permeate flux. A procedure was developed to prepare glutaraldehyde-crosslinked chitosan nanofibers as a flocculant that is insoluble but well dispersible in water. Polyacrylonitrile (PAN) nanofiber serves as the reference for evaluating the nanofiber structure contribution to membrane filtration, because of its poor flocculation ability. Another commercial soluble flocculant MPE50 (Nalco Company, Naperville, Illinois) was also compared. The toxicity/inhibition test, turbidity reduction test, and short-time filtration test were conducted to evaluate the flocculant performance. There was no obvious inhibition to the growth of microorganisms by addition of 50~100 mg/L of any of the tested flocculants. It was also demonstrated that adding dissolved chitosan and MPE50 could help reduce the turbidity of the supernatant up by 80% and 55% respectively after allowing the sludge to settle for 45 minutes. Chitosan was effective in the pH range of 5-8, while MPE50 was effective in the wider range of 4-9. PAN nanofiber and crosslinked chitosan nanofiber showed low turbidity reduction ability at concentrations higher than 25 and 50 mg/L, respectively, after adjusting for the turbidity of nanofibers themselves. Total filtration resistance Rt and membrane fouling rate were calculated from the short-term filtration tests. Both dissolved chitosan and MPE50 could improve the filtration performance with lower transmembrane pressure (TMP) and higher permeate flux. However, the performance of PAN and crosslinked chitosan nanofibers was not very consistent from replicate experiments. It is demonstrated that adding nanofiber would not harm the filtration system but the crosslinked chitosan did not improve the filtration process as expected. The possible reasons are discussed for future improvements.
School:The University of Akron
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:flocculation fouling membrane bioreactor wastewater treatment
Date of Publication:01/01/2005