Evaluation of the Anti-Fouling Effects of Micro-Nano Bubbles on the Performance of Reverse Osmosis Membrane

Colloidal fouling on a brackish water reverse osmosis (BWRO) membrane was simulated by a lab-scale plate-and-frame module in the presence and absence of air micro-nano bubbles (AMNBs). Synthetic feed water samples with the same physical and chemical properties, but various concentrations of colloidal silica (50, 100, 200, and $300\mathrm{mg}/\mathrm{L}$), were used for experiments. The results illustrate that colloidal fouling caused a severe decrease in permeate flux (24%–56%) and salt rejection (1.25% to 4.18%) in the absence of AMNBs. This reduction in membrane performance was attributed to the high hydraulic resistance and the cake-enhanced osmotic pressure (CEOP) of a fouled gel layer that were intensified with increase in the colloidal concentration. On the other hand, presence of AMNBs decreased the deposition rate of colloidal particles significantly and increased the porosity of fouling layer. Thus, an improvement in the membrane permeate flux (21%–40%) and salt rejection (1.2%–2.6%) were seen in the different concentrations of colloidal particles. The SEM images also confirmed the formation of loose fouling layers which were easily removed from the membrane surface by a clean-in-place (CIP) process. This research introduces AMNBs technology as an effective in-line method to control the adverse effects of colloidal fouling in reverse osmosis (RO) systems.

RO is a well-known process in the field of water and wastewater treatment. RO membranes have the ability to remove all pollutants, undesirable ions, and micro-organisms from the liquid phase that leads to their accumulation on the membrane surface. This phenomenon is called membrane fouling. Colloidal particles are one of the major fouling agents that are found in all water resources which has a direct adverse effect on the efficiency of RO systems. Various technics (based on the filtration process) are used to remove the colloidal particles in pretreatment stage of RO systems. In this research, the micro-nano bubbles (MNBs) technology was used to control the colloidal fouling in RO process. The results showed the presence of MNBs in feed water of RO could decrease the fouling of the membrane surface and improve the quality and quantity of produced water. Also, the energy consumption of this technology can be restored easily by the improvement in the quantity of the produced water. Thus, the MNBs technology could control the colloidal fouling of RO membranes with no adverse environmental effects.