Micro-nanobubbles to alleviate ultrafiltration membrane fouling for biogas slurry concentration and nutrient enrichment: Performance and molecular mechanisms

This study evaluated the performance of micro-nanobubbles (MNBs) to mitigate membrane fouling during ultrafiltration (UF) concentration of chicken manure biogas slurry. Molecular characteristics of organic foulants in biogas slurry were examined to elucidate the underlying mechanisms. Results showed that MNBs pre-treatment of biogas slurry could reduce membrane fouling and improve fouling reversibility during subsequent UF concentration using oxygen as the gas source. MNBs increased the molecular number of dissolved organic matter (DOM) and thermodynamic instability, and reduced the particle size of biogas slurry. As a result, organic foulants, mainly including lignin, lipids, and protein/amino sugar, rapidly attached onto the membrane surface to form a thick but highly reversible fouling layer. Irreversible foulants mainly caused by inorganic foulants were alleviated due to MNB pretreatment to decrease carboxylic groups in biogas slurry to break the interaction between DOM and cations in inorganic foulants. The weakened cation-DOM interaction could be related to the reduction in calcium bridging with humic-like substances as well as electrostatic shielding between potassium and humic-like/fulvic-like substances. As such, the water flux and its recovery after regular cleaning were enhanced by 58.1% and 19.6% in UF operation, respectively, when MNBs was used for biogas slurry pre-treatment.