A new approach to evaluate membrane performance: COD and EC rejection rates

Olive mill wastewater (OMW), which contains abundant soluble and particulate matter, negatively influences the receiving environment. Membrane processes have the potential to treat OMW. Electrical conductivity (EC), which is very high in OMW, is one of the key factors of membrane processes. The EC value of the OMW sample used in this study was higher than the common values, and this situation resulted in high osmotic pressure. Several samples with different influent EC values were fed to a reverse osmosis (RO) system. The osmotic pressure values for each influent EC were calculated. The relationship between the osmotic pressure and influent EC values was nonlinear. The permeate flux could not be achieved for some applied pressures and influent EC values because of the high osmotic pressure of the feedwater. Moreover, the terms of the chemical oxygen demand (COD) rejection rate and EC rejection rate were developed and observed and predicted values were compared for these terms and permeate fluxes using linear and nonlinear regressions. It was determined that osmotic pressure is a limiting condition in OMW treatment using RO membranes and that COD and EC rejection rates are useful terms to evaluate membrane performance.