Characteristics And Mechanism Of Membrane Fouling In The Treatment Of Secondary Effluent With Ozonation-microfiltration Process

The reclamation of secondary effluent is an effective way to release the water stress facing China today. Microfiltration process offers a superior technology for reclamation of secondary effluent since it has the advantages of good effluent water quality, small footprint, and easy operation. However, membrane fouling is an obstacle hindering its large-scale application. Ozonation could be a measurement to control the membrane fouling. But its performance has not yet been predictable since the mechanisms have not been clarified. The purpose of this thesis was to study how secondary effluent fouls the microfiltration membrane, what happens in the pre-ozonation of the secondary effluent, and consequently its effects on membrane fouling.Based up on the analysis of the secondary effluent water quality and the operational tests of a pilot ozone-microfiltration equipment, dissolved organic matter (EfOM) and suspended solids (SS) in secondary effluent were identified as the main culprits causing membrane fouling.The characteristics of secondary effluent, EfOM, and SS, their variations in pre-ozonation reactions, and the effects on microfiltration membrane fouling were studied through a series of experiment with an ozonation equipment and a constant-flux microfiltration equipment.Among different fractions of EfOM, hydrophilic substances (HIS), making about 54% of EfOM in terms of DOC, has the strongest fouling potential. In regard to the hydrophilized PVDF membranes, characteristics of EfOM that result in greater fouling potential include, larger molecule weight, more hydrophilic properities, and higher carboxylic content. The pre-ozonation of EfOM reduces its fouling development in the following microfiltration process, owing to the 17% reduction of DOC (2.52mgO3/mgDOC) and decrease of molecules weight. For the prepared SS suspension, the decrease of its SS concentration, the increase of its particle size and a narrow PSD resulted in less membrane fouling, while lower SS concentration, larger particle size, and a wider PSD resulted in more severe membrane fouling. In purpose of membrane fouling control, there exists an optimum ozone dosage for the pre-ozonation unit. The effects of pre-ozonation on bacteria suspension include the changes of cell structure, and cell lysis. Pre-ozonation of bacteria suspension aggravated the membrane fouling with possible reasons of a denser cake layer resulting from the change of cell structure, and the slight increase of DOC from cell lysis.Models for predictingΨ, a membrane polluting index, and the best ozone dosage were established and could be used for guiding the determination on parameters in secondary effluent reclamation with ozone-microfiltration process.