Performance of biosurfactants for online cleaning of hybrid ultrafiltration, nanofiltration, and reverse osmosis membrane process for multistage concentration of biogas slurry

Membrane processes are promising for concentrating biogas slurry, but suffer from severe fouling. Thus, this study aimed to evaluate the performance of two reusable biosurfactants, namely rhamnolipids and tea saponin, and their combination with conventional acidic and alkaline reagents for online cleaning of hybrid ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) process in multistage concentration of biogas slurry. Flux restored ratio and membrane integrity were assessed for screening suitable reagents. Results demonstrated that the combination of citric acid and tea saponin, potassium hydroxide and rhamnolipid, and citric acid and rhamnolipid was favorable for cleaning UF, NF, and RO membrane, respectively, to secure their concentration factors to 10 – 15 times. Membrane surface morphology and compositional analysis identified that organic foulants, primarily humic acids, were culprits to UF and NF membranes. Since their permeation through UF and NF barriers, inorganic salts, such as potassium and sodium crystals, were main contributors to RO membrane fouling. These foulants could be highly removed from the three membrane surfaces by their corresponding cleaning reagents as quantified by loss on ignition. As such, humic acids and nutrients were efficiently enriched, particularly in UF and NF retentates, to produce high quality liquid fertilizers and RO permeate for resource recycling. • Severe membrane fouling occurred in UF, NF, and RO concentration of biogas slurry. • Organic foulants, mainly humic acids, were culprits to UF and NF membrane fouling. • Potassium and sodium crystals were keys to RO membrane fouling. • Rhamnolipids and tea saponin were effective to clean UF, NF, and RO membranes. • UF-NF-RO produced high quality fertilizers and water from biogas slurry.