A Three Year Long Experience of Effective Fouling Inhibition by Threshold Flux Based Optimization Methods on a NF Membrane Module for Olive Mill Wastewater Treatment

The operation of a spiral-wounded nanofiltration membrane module used continuously for three years in order to purify different pretreated olive mill wastewater streams will be discussed. The membrane module was for the first time used at the beginning of year 2006 on a batch pilot scale plant for critical flux studies and wastewater treatment in our laboratories. The olive mill wastewater is a waste stream produced by the olive oil mill factories, characterized by very high organic matter load and polyphenols concentration. Without fouling inhibition at all, nanofiltration membranes will reach zero-flux conditions within days. This is not the case of this nanofiltration membrane module, which was successfully operated continuously for three years during our laboratory work. This result was reached by proper fouling inhibition control, relaying on both critical flux measurements and the development of an optimized operation method. Although the critical flux theory was successfully applied to this system, it was not capable to explain the observed fouling behavior of the examined membrane system. The doubt to work on a membrane system that does not follow perfectly the critical flux laws grow throughout the years. In year 2011, Field et al. introduced the threshold flux concept as an extension to particular membrane systems treating real wastewater streams, and this latter theory fits to the observations made on olive mill wastewater. In this work, a revision of previously obtained results in terms of critical flux will be performed, using the threshold flux theory as discussion basis. In the examined system, both critical and threshold points were found at 7-8 bar depending of the used feedstock and membrane condition. Moreover, it will be checked why the adopted "critical flux" approach was successful in inhibiting fouling for so many years despite it was not the correct approach.