Targeted chemical cleaning preserves high water flux and water quality in long-term pilot-scale membrane distillation for potable water reuse

Long-term, continuous operation of membrane distillation (MD) is limited by the lack of knowledge on scaling and wetting resistance. In this study, a pilot-scale vacuum-assisted air gap MD (V-AGMD) system with 25.92 m² of membrane area is used to purify reclaimed water from a membrane bioreactor. The V-AGMD system produced 600–700 L/day with a 75 % recovery rate of water for approximately three months. A decline in water flux from the initial value of 1.1 LMH was observed and routine chemical cleaning procedures were employed in response. Cleaning with hydrochloric acid (HCl) was effective at recovering water flux, but after one month of weekly acid cleaning, some evidence of fouling and scaling remained. Additional chemical cleaning procedures were tested, including chlorination and the use of chelating agents. Chlorination was ineffective because minimal organic fouling was present, and, without rinsing procedures, generated disinfection byproducts that were detected in the distillate. The chelating agent (Ethylenediaminetetraacetic acid, EDTA) recovered water flux by the removal of gypsum. Despite evidence of membrane scaling, the water quality of the distillate remained high. Rejection of dissolved contaminants temporarily decreased during the second month of operation but was recovered by cleaning with EDTA. The cleaning procedures were also critical in maintaining the energy efficiency of the V-AGMD system as they lowered the channel backpressure and improved the heat recovery after fouling and scaling. This study demonstrates that high water quality and water flux can be maintained in long-term MD operation through chemical cleaning that targets specific foulants and scalants.