Analysis Of The Biofouling Behavior Induced By NaClO In-Situ Cleaningin In Membrane Bioreactor

Membrane bioreactor(MBR)has become a competitive technology for wastewater treatment and reuse due to many advantages compared with the conventional activated sludge process.However,the membrane fouling can reduce the membrane flux,shorten the service life of the membrane,and increase the operation cost during MBR operation.Compared with the physical cleaning,chemical cleaning can effectively remove reversible and irreversible fouling and maintain the stable membrane flux.However,the changes in the characteristics of sludge mixed liquor caused by NaClO leakage will lead to serious biofouling during in-situ cleaning.In this study,the analysis of the biofouling behavior induced by NaClO in-situ cleaning in MBR were investigated systematically.This work was performed as follow:(1)The fate of NaClO and detached membrane foulants and their impacts on the thermodynamic properties of raw sludge during MBR in-situ cleaning were investigated.It was found that about 20.3% of the total NaClO was potentially released into the bulk sludge,which significantly altered the raw sludge surface properties,such as a reduction in the number of surface electron donor components(?-)and an increase in surface hydrophobicity.After exposure to NaClO,the adhesion energy with the membrane and self-cohesive ability of the treated sludge increased by 3.20 and 8.19 times,respectively,compared to the raw sludge.Based on the extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory and the batch filtration test results,the detached foulants preferentially aggregated with the treated sludge and form the reconstructed sludge,rather than adhere to the membrane surface.The reconstructed sludge had a high protein(PN)level in the extracellular polymeric substances(EPS),with a higher membrane fouling potential compared with the raw sludge.(2)The reason of the rapid formation of initial membrane fouling with in-situ cleaning was investigated.The activated sludge after NaClO-shock promoted the rapid formation of initial membrane fouling(23.80 and 10.60 kPa/d).After in-situ cleaning,the EPS spatial distribution of the sludge significantly changed,and had a high PN content in different EPS layers and a substantial deterioration in sludge filterability compared with raw sludge.Based on the XDLVO theory,NaClO shock significantly changed the surface properties of raw sludge.It reduced the surface ?-and increased surface hydrophobicity.The sludge after NaClO-shock exhibited a stronger adhesive energy with the membrane and higher self-cohesive ability,resulting in severe membrane fouling.(3)Effects of NaClO shock on MBR performance under long-term operating conditions were investigated.The experimental results showed that the membrane fouling rate(13.70,23.50 and 23.30 kPa/d)increased significantly after cleaning,while the membrane fouling rate(6.40,-0.20 and 3.50 kPa/d)decreased rapidly after cleaning for 2 d,indicating that the NaClO-shocked sludge had a rapid self-recovery ability.With the increase of operating time,the EPS contents of sludge also showed a trend of increasing first and then decreasing,and with the gradual improvement of sludge filterability,which reducing membrane fouling.Thermodynamic analysis showed that the surface properties of treated-sludge,including the surface ?-,hydrophobicity,adhesive energy and self-cohesive ability,were restored to the raw sludge level after cleaning for 14 d,which reducing the membrane fouling potential.