טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentIvnitsky Hanan
SubjectStudy of Membrane Biofouling in Nanofiltration of
Wastewater Treatment
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Emeritus Carlos Dosoretz
Professor Emeritus Raphael Semiat
Full Thesis textFull thesis text - English Version


Abstract

Pressure driven-membrane separation processes are primarily affected by fouling at the membrane surface. Biofouling, i.e., microbial adhesion and growth on membrane surfaces, is a highly complex fouling phenomenon which causes severe losses in performance and changes in membrane surface properties. Biofouling remains an unsolved problem because very little is understood about the fundamental nature of the growth processes.

The general objective of the research was to identify and characterize the structure, morphology and composition of the membrane biofouling layer and its repercussions on NF processes in wastewater treatment. Membranes were examined for bacterial enumeration, bacterial community composition (16S rRNA PCR-DGGE) and microscopy visualization (SEM, CLSM, AFM/NSOM). An empirical model was established to forecast the permeate flux and biofouling layer resistance in NF membranes treating tertiary wastewater. A data-driven modeling algorithm for forecasting the permeate flux and permeability reduction rate through membranes due to biofouling development is presented as well, delineating the most important parameters governing the fouling process.

Deposition of polysaccharides and the beginning of bacterial colonization were observed as early as 8 h into filtration, whereas developed biofilms that markedly affected membrane permeability were evident from day 2 or 3 onwards. The biofouling layer appears to develop to a convergent countable magnitude on the order of 107 CFU/cm2 and converging thickness value of 20 to 30 µm, regardless of operating conditions, probably as the result of a balance between available nutrients, shear forces and pressure drop across the membrane surface. Most bacterial species identified in the biofouling layer corresponded to Proteobacteria and only a small fraction corresponded to Gram-positive and other Gram-negative groups. b and g-Proteobacteria were found to be the most prevalent classes in all cases, while Pseudomonas/Burkholderia, Ralstonia, Bacteroidetes and Sphingomonas were the dominant groups found in most of them. Whereas the microbial diversity seems to be important to the biofouling pattern, the accumulation of extracellular polymeric substances (EPS) appears to be the main reason for the convergent nature of the permeability decline. A significant effect of biofouling on membrane surfaces appeared after approximately 3 days, when it began to be a dominant factor in membrane-fouling processes. Major efforts should be made to reduce fouling development on the membrane surface at early stages while operating with wastewater effluents. This should be achieved by a series of concerted actions including: operating at the lowest permissible pressure, reducing nutrient and organic carbon availability and preventing microorganisms from settling on the membrane surface.