Supersaturated carbon-dioxide-enhanced backwash of ceramic microfiltration membranes

• CO 2 enhanced backwash cleans ceramic membranes better than water backwash. • Visualization shows bubbles do not form on permeate side of the membrane. • Higher TMP and lower flux during CO 2 backwash compared with DI water backwash. • TMP and flux data during backwash illustrated three stages of bubbles behavior. Supersaturated CO 2 -enhanced backwash (SCEB) was tested as an alternative to clean-water backwash to enhance the effectiveness of foulant removal for ceramic microfiltration membranes. Air bubbles are commonly used to tangentially scour the feed side surface of micro- and ultrafiltration membranes to add scouring and loosen cake layers; this is called air-assisted backwash. Unlike air-assisted backwash, SCEB forces dissolved CO 2 and CO 2 bubbles through membrane pores. In this study, transmembrane pressure (TMP) and flux profiles during SCEB were directly analyzed to reveal the exact nucleation location. The results from constant-flux filtration show that SCEB recovered more filtration TMP compared to DI water backwash, which saves 6.5%, 2.7%, and 6.9% of the energy during lake water, activated sludge, and rendering wastewater filtration, respectively. Visualization of the membrane permeate is done for the first time, confirming that CO 2 bubbles were not formed before entering the membrane matrix. The pressure and flux profiles of SCEB itself were directly monitored. The pressure profile from constant-flux backwash was analyzed to further understand the cleaning mechanism. The results from constant-flux SCEB showed more negative pressure than DI water backwash, which means that CO 2 bubbles went through the membrane pores and created more resistance during SCEB. The visualization confirms that CO 2 bubbles nucleated inside the membrane pores rather than on the permeate side to clear the foulant blocked deep inside the membrane matrix. The behavior of CO 2 bubbles can be divided into three stages during SCEB: i) Permeate flush out, ii) Bubble concentration growth, and iii) Bubble concentration equilibrium. The last stage has the highest concentration of CO 2 bubbles, which work as pore-scrubbing entities and play an important role in better cleaning effectiveness because of the higher nucleation rate, extra shear stress, and greater lifting force.