Crossflow reversal effect on removal of RO particulate fouling: a comparison of two spacers

Abstract

Worldwide fresh water scarcity is nowadays becoming a reality rather than a futuristic concern. Non-conventional sources of wastewater are being considered as a solution for this problem, since fresh water sources will not be sufficient to satisfy the world’s water demand in the following years. Extensive research is being done on different wastewater treatment technologies and methodologies, and lately, reverse osmosis (RO) membrane filtration has caught more attention as a solution for reclaiming and reusing wastewater. Nevertheless, the biggest problem RO membranes face is fouling, which increases energy consumption, and therefore, costs. Organic/particulate fouling affects the mass transfer of an RO membrane, but it can also contribute to biofouling, worsening the mass transfer and causing pressure drop on the RO system. For this study, a RO crossflow cell was used in order to investigate the effect of crossflow reversal on particulate fouling removal by testing two different feed spacers: (a) 1.25-mm high cavity spacer and (b) 0.71-mm high zigzag spacer. The experiments were carried out with an average crossflow velocity of 0.36 m/s, an average permeate flux of 20 L/m2 hr (LMH), an average feed electroconductivity (EC) of 14.90 mS/cm, and a feed temperature in the range of 21-25 C. Four runs were done, each with an average runtime of 1050 minutes. Two runs tested the cavity spacer, one with a crossflow reversal done at minute 806, and the other run with a crossflow reversal done twice every working day. The last two runs follow the same methodology but using the zigzag spacer. Each run used a new membrane and spacer. The mass transfer was graphed as a Mass Transfer Coefficient (MTC), and its development with time was analysed for identifying fouling, as well as particulate fouling when applying the crossflow reversal method. Crossflow reversal proved to be a reliable method for removing organic/particulate fouling. The cavity spacer caused a more severe organic/particulate fouling, but allowed it to be removed by crossflow reversal. The cavity spacer also proved to be less energy-consuming if applied to a full-scale RO facility. On the other hand, the zigzag showed to have a better effect on flow mixing and concentration polarization disruption inside the feed channel, thus keeping a higher mass transfer for a longer period of time. A more frequent crossflow reversal showed was not more efficient in removing organic/particulate fouling.