Study On Membrane Fouling Control With Gas-Liquid Two-Phase In VT-MBR Reactor

Membrane bioreactor (MBR) has been widely used in water purification and wastewater treatment due to its advantages over the conventional activated sludge process such as better effluent quality, smaller footprint and higher organic loading. However, membrane fouling remains a major obstacle to the wide application of MBR, which causes permeate flux decline and operation costs increase. Recently, many researchers reported that cake formation on the membrane surface and concentration polarization could effectively be controlled by gas sparging, but which might lead to increased energy consumption. Consequently, the optimization of aeration amount has become a research hotpot at present. Vertical tubulant biological reactor (VTBR) not only supplys the gas to gas-liquid two-phase flow system, but also provides surplus pressure for membrane filtration, and lead to increased aeration efficiency and reduced energy consumption. In this study, a designed membrane module was installed behind the VTBR setup in order to improve the effluent quality and the residual gas of the VTBR reactor was used to gernerated the shear force to control membrane fouling.The experiment of tubular membrane microfiltration by kaolin suspension was carried out in order to confirm the feasibility of the combination with tubular membrane and VTBR. It was found that bubble rate and bubble frequency increased obviously as aeration increased yet membrane fouling rate decreased gradually so as to extend the membrane filtration operation time. The analysis of hydrodynamics characteristic showed that the gas injected can increase turbulence in the surface of membrane to slow down transmembrane pressure rate and keep membrane fouling in a low flow rate, thus saving the energy consumption.The impacts of gas flowrates on the performance of the combination systems were investigated treating synthetic wastewater under gas flowrate of6.9,11.4and15.0L/h. It was found that the removal of COD and NH3-N was significantly improved by increased gas flowrate and the effluent quality was better than the supernatant under the same gas flowrate. Membrane fouling could be effectively mitigated by increasing gas flowrate. Concentration polarization and cake deposition can be well dislodged by the shear force generated by bubble flow. In addition, the residual gas bubble flow promoted turbulence in the membrane tubes, resulting in significant enhancements of critical flux and there was a positive correlation between the critical flux and gas flowrate, R is about0.99. It was found that the growth characteristics of micro-organisms were significantly influenced by gas flowrate. The concentration of total EPS in the reactor decreased with the increase of gas flowrate. The quantity and composition of EPS in the supernatant and membrane surface were significantly different under the same gas flowrate. The accumulation of EPSp on the membrane surface might change the hydrophobicity of membrane tubes, which is not conducive to membrane fouling control. In addition, the increase of particle size might be attributed to the shed of biofilms, due to a better hydraulic environment acquired by increasing gas flowrate.