English abstract
Membrane life time and permeate fluxes are primarily affected by the phenomena of concentration polarization (i.e. accumulation of solute) and fouling (i.e. microbial adhesion, gel layer formation and solute adhesion) at the membrane surface. A primary reason of flux decline during the initial period of a membrane separation process is concentration polarization of solute at the membrane surface. The main objective of the present study was to analyze and model concentration polarization in spiral wound membrane elements. In particular the influence of feed pressure, solute concentration, flow rate, temperature and spacer thickness in the membrane units on permeate flow and its salinity was evaluated. A laboratory. scale reverse osmosis unit, equipped with spiral wound seawater reverse Osmosis module was used to conduct the experiments. Our studies showed that the permeate flux increased with increasing feed/transmembrane pressure at a given feed solute concentration. The permeate flux also decreased considerably at higher feed solute concentration (i.e. feed salinity) suggesting the build up of a higher level of solute (i.e. concentration polarization) at the membrane surface. When the feed flow rate was decreased from 18 to 9 L/h, at constant temperature, the permeate flux decreased, but only at the higher feed pressures and higher salt concentrations. This suggested that the polarization of solute at the membrane surface was reduced at higher feed flow rates.
A major finding of our work was that the polymer membrane is very sensitive to changes in the feed temperature. There was up to a 60 % increase in the permeate flux when the feed temperature was increased from 20 to 40°C. This occurred both in the presence and absence of solute. Surprisingly, the permeate flux appears to go through a minimum at an intermediate temperature. There was up to a 100 % difference in the permeate flux between feed temperatures of 30 and 40°C. The differences were statistically significant (p<0.05). The temperature seemed to influence the membrane permeability hence the quality and quantity of the permeate. The effects of spacer thickness on permeate flux suggest that the observed flux decreases by up to 50% in going from a spacer thickness of 0.1168 to 0.0508 cm. The different geometry/configuration of the spacer may influence turbulence at the membrane surface that in turn affected concentration polarization. This suggests less turbulence with the smaller spacer thickness. The membrane module with an intermediate spacer thickness of 0.0711 cm was found to be the best economically since it gave the highest water production rate (L/hr). Membrane parameters were also estimated using an analytical osmotic pressure model (i.e. the combined film theory/Spiegler-Kedem model). The modeling studies showed that the membrane transport parameters were mainly influenced by the feed salt concentration and temperature
