| Membrane treatment process is gradually applied inactual drinking water treatment project with its excellent performance,but the membrane fouling caused by the cake layer is always the bottleneck affecting the process.Coagulation is a cost-effective pre-treament process of membrane filtration.There are common combined coagulation and ultrafiltration techniques,including the traditional coagulation-membrane process,direct membrane filtration.Nevertheless,the concern is that the membrane module is generally static in the membrane tank,which allows the flocs to continuously precipitate on the membrane surface,leading to membrane fouling by thick cake layers.Therefore,the membrane module spriral rotation was investigated based on the floc looseness,aiming to strongly alleviate membrane fouling.The results indicate that 20 mg·L-1 humic acid?HA?is likely to induce severe UF membrane fouling,resulting in the sharp increase of the transmembrane pressure?TMP?to 61.1 kPa after 12 days of operation.The significant TMP reduction was observed after the injection of 13 mmol·L-1 iron-based flocs?Fe-flocs?or alum-based flocs?Al-flocs?,membrane foulingisalleviated.On day 12,the TMP was 28.4 kPa and 38.2kPa,respectively.The mitigation was further achieved by reciprocating the membrane module with a spiral height of 1 cm at 24 rpm.Because the loose flocs cake layer became thinner,the TMP reduced to 9.1 kPa and 11.5 kPa,respectively.As the spiral distance of the membrane module and the pH of feed water could change the thickness of the filter cake layer and the properties of the flocs,the impacts if these two factors were explored.The results from Computational Fluid Dynamics?CFD?confirmed that the greater the height of the spiral,the greater the surface shear force of the UF membrane.Hence,thinner filter cake layer,and lower membrane fouling were induced.When membrane module was rotating,Fe-flocs tended to stay away from the membrane surface because of the comparatively larger density,which could not effectively protect the ultrafiltration membrane,and the HA removal rate was low.Compared with Fe-flocs,Al-flocs owned lower density,suggesting less affected by the centrifugal motion.This property led to the tighter attachment of Al-flocs on the membrane surface,and the HA removal rate was higher.Moreover,when the pH of the feed water was acidic,HA has a better removal effect.Based on this,raw water from the South-to-North water?Beijing Section,China?was used to investigate the applicability of the process.Membrane fouling was severe after 8 days of operation and TMP was 82.1 kPa.When the13 mmol·L-1 Fe-flocs or Al-flocs were injected into the tank,the pH was adjusted to 6.0,and the membrane module was reciprocated with a spiral height of 1 cm at 24 rpm,the filter cake layer became thinner,and the degree of membrane fouling was significantly decreased.On day 12,the TMP was 14.2 kPa for Fe-flocs and 14.2 kPa for Al-flocs.The results further validated the practicality of this integrated coagulation-rotary membrane process,particularly with the injection of hydrolyzed Al-flocs. |
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