A novel RO ex situ scale observation detector (EXSOD) for mineral scale characterization and early detection

A novel RO ex situ scale observation detector (EXSOD) was developed for direct visual real-time monitoring of mineral surface scaling on RO membranes. This system consists of a plate-and-frame RO cell with an optical window capable of withstanding pressures in excess of 2.4 MPa. Membrane surface imaging under RO operation is accomplished by high resolution digital photography using an optical microscope with lighting arrangement to enhance the boundaries of semi-transparent crystals. The utility of the direct membrane surface observation cell was demonstrated in a series of experiments conducted with a commercial brackish water RO membrane and with gypsum selected as the model scalant. The present study demonstrated, for the first time, a capability for early detection of mineral salt scaling at very early stages of formation, in advance of any measurable flux decline. The approach also illustrates that surface crystal development on RO membranes can be quantified with respect to both single crystal growth and the evolution of crystal number surface density. For the growth of gypsum, crystal surface number density evolved according to a sigmoidal growth curve, with crystal growth diminishing as gypsum rosette needles began to overlap. Dissolution of surface crystals occurred once solution at the membrane surface was undersaturated with respect to gypsum. It is suggested that the EXSOD system can be used for on-line monitoring of scaling and fouling in RO plants, evaluation of the impact of colloidal and biofouling as well as the effectiveness of antiscalants and membrane cleaning strategies.