Biofouling control of reverse osmosis membrane using free ammonia as a cleaning agent

Reverse osmosis (RO) is an important and widely-used membrane separation process for water recycling. However, biofouling is extensively considered a major problem for RO membranes due to the biofilm formation on the membrane surfaces. This study proposed and demonstrated a novel and sustainable chemical cleaning approach using a free ammonia (FA) solution for the removal of biofouling on RO membranes. The feasibility of FA solution for biofouling removal was investigated through a series of lab-scale soak cleaning tests and cross-flow cleaning tests on four fouled RO membranes (M1-M4) collected from municipal wastewater recycling plants. In soak cleaning tests on M1, FA concentrations of 65–560 mg NH3–N/L (pH = 8.9) can remove adenosine triphosphate (ATP) by 32–75 %, remove proteins by 18–47 % and remove polysaccharides by 31–74 %, which was up to 3.4 times of the removals by using NaOH solution under the same pH. FA solution of 310–560 mg NH3–N/L (pH = 8.9) even showed higher removals than NaOH solution with a higher pH of 11. In the cross-flow cleaning tests on M2-M4, FA solution of 310–560 mg NH3–N/L (pH = 8.9) removed the ATP by 82–100 %, removed proteins by 58–87 % and removed polysaccharides by 68–100 % in the fouling layers and increased the permeability by 8–16 %. Such cleaning effects in cross-flow tests were also positively correlated (R > 0.9, p < 0.05). Compared to the conventional anti-biofouling agent of NaOH solution (pH = 11), FA solution (310–560 mg NH3–N/L) showed significantly better cleaning performance. A high FA concentration of 560 NH3–N/L (pH = 8.9) could achieve comparable cleaning effect to 1 % EDTA solution (pH = 10). Additionally, increasing the frequency of cleaning or using a higher concentration of FA solution for biofouling removal will be more advantageous to prolong the membranes' lifespan. The findings provide a promising alternative to using FA as a cost-effective and environmentally friendly solution for cleaning biofouling on RO membranes.