ABSTRACT
An ever-present problem in the use of commercial membranes for treatment of drinking water is fouling of the membranes by natural organic matter (NOM). This work describes a new approach to elimination or minimization of membrane fouling by NOM. When a 2% solution of polysulfone in NMP and propionic acid is slowly injected into water, approximately 50 nm polysulfone particles are spontaneously formed, and these hydrophobic particles quickly coagulate into approximately 12-microm diameter aggregates; the formed material has a surface area of approximately 100 m(2)/g and an equivalent "pore" size of 25 nm. When 50 mg/L of the new material is equilibrated with a local drinking water supply, virtually all adsorptive fouling of a 20-kDa molecular weight cutoff ultrafiltration membrane is eliminated. Interestingly, although only a very small percentage of the NOM is removed by adsorption on the polysulfone aggregates, it appears that exactly this small NOM component is responsible for nearly all of the membrane fouling. This paper describes the fabrication and characterization of the new polysulfone adsorbent and offers an hypothesis for the formation of the product via spontaneous emulsification and spinodal decomposition.
ABSTRACT
The effect of calcium on the stability of a commercial MWF is characterized through the experimental determination of the stability ratio, W. Three experimental methods of stability ratio evaluation are investigated. (1) The initial slope of the absorbance versus time curve is used to estimate the rate of coagulation. (2) Absorbance measurements are used to estimate N(0)/N with time. The stability ratio is determined from the slope of N(0)/N versus time. (3) Photon correlation spectroscopy (PCS) measurements of the volume distribution with time are used to estimate N(0)/N with time. Electrophoretic mobility was also measured and used to determine the fast coagulation concentration of the MWF. The accuracy of the experimentally determined stability ratios is evaluated using a population balance coagulation model. The model predicts the population distribution of a coagulating dispersion with time based on an initial particle size distribution and stability ratio. The model results were compared with the PCS-measured distributions to determine which stability ratio evaluation method best describes the stability of the MWF emulsion studied. Using the initial slope of the absorbance versus time curve to determine the fast coagulation concentration correlates well with electrophoretic mobility measurements. However, using absorbance measurements to determine the rate of coagulation underestimates the stability ratio of the MWF studied by orders of magnitude. N(0)/N values calculated from absorbance measurements provide a reasonable estimate of the stability ratio but inconsistencies in the method decrease its reliability. The stability ratio derived from PCS measurements appears to provide the most accurate, reliable description of MWF stability.
ABSTRACT
Membrane filtration (microfiltration and ultrafiltration) has become an accepted process for drinking water treatment, but membrane fouling remains a significant problem. The objective of this study was to systematically investigate the mechanisms and components in natural waters that contribute to fouling. Natural waters from five sources were filtered in a benchtop filtration system. A sequential filtration process was used in most experiments. The first filtration steps removed specific components from the water, and the latter filtration steps investigated membrane fouling by the remaining components. Particulate matter (larger than 0.45 microm) was relatively unimportant in fouling as compared to dissolved matter. Very small colloids, ranging from about 3-20 nm in diameter, appeared to be important membrane foulants based on this experimental protocol. The colloidal foulants included both inorganic and organic matter, but the greatest fraction of material was organic. When the colloidal fraction of material was removed, the remaining dissolved organic matter (DOM), which was smaller than about 3 nm and included about 85-90% of the total DOM, caused very little fouling. Thus, although other studies have identified DOM as a major foulant during filtration of natural waters, this work shows that a small fraction of DOM may be responsible for fouling. Adsorption was demonstrated to be an important mechanism for fouling by colloids.
