Flat sheet ceramic membrane and its surface modification for direct sewage filtration, electrically-assisted fouling control and catalytic ozonation

Abstract

Flat sheet ceramic membrane (FSCM) has been increasingly used in water and wastewater treatment owning to its unique properties such as high permeability, excellent mechanical strength and chemical stability. This research was conducted on FSCM for wastewater treatment, focusing on direct membrane filtration (DMF) of domestic wastewater and the membrane surface modification for electrically-assisted fouling control and catalytic ozonation. The different membrane surface modification methods were developed, namely the liquid-phase chemical coagulation, carbon nanotubes (CNTs) deposition and bi-metal oxide catalyst coating on the membranes, for the different functions of the membranes, including the direct wastewater filtration, electrically enhanced membrane filtration, and membrane catalytic ozonation. The performance, influential factors, and the mechanisms of the new processes and new materials were investigated. An innovative coagulation-FSCM (C-FSCM) filtration process was developed for the DMF of municipal wastewater, to remove a majority of the pollutants from the filtrate effluent and concentrate organics and nutrients into sludge for potential resource recovery. FeCl3 (20 mg Fe/L) or Poly-aluminum chloride (PACl, 15 mg Al/L) was used for wastewater pre-coagulation, and the integrated C-FSCM process was operated at a high filtration flux of 41.7 LMH (1.0 m/d) and removed organics and total phosphorus by up to 90.0% and 99.9%, respectively. PACl was more effective than FeCl3 as the coagulant for membrane fouling control. The fouled membrane could be readily recovered by chemical backwashing with NaOH or HCl for 10 min every a few days to achieve long-term operation of C-FSCM for rapid and direct filtration of municipal wastewater. A novel conductive ceramic membrane was fabricated for the electrically-assisted membrane fouling control during wastewater filtration. The ceramic membrane support was coated firstly with a dopamine layer and then with oxidized CNTs through vacuum filtration followed by heating in the Ar atmosphere. The coated membrane had a good conductivity with a resistance of 0.8 kΩ-cm and was charged as a cathode in the filtration system. The conductive membrane performed well for treatment of a synthetic wastewater containing the inorganic matters (kaolin suspension) or organic pollutants (oil emulsion). By charging the membrane with DC at 2.0 V, the membrane fouling rate decreased by about 50%. The rate of energy consumption for membrane fouling control was only 22 ×10-3 kWh/m3 for the high frequency paused charge mode. By in-situ coprecipitation to coat metal oxide catalysts on the FSCM support, catalytic ceramic membranes (CCM) were made for catalytic ozonation, including the Mg-Ce membrane and Mg-Mn membrane. The system combined ceramic membrane filtration with heterogeneous catalytic ozonation for advanced wastewater treatment and membrane fouling control. Compared to solo ozonation, catalytic ozonation with CCM improved the organic removal efficiencies by more than 40% for treating a dye solution and the secondary wastewater effluent. Based on the •OH quenching tests, ozonation with CCM was able to produce much more •OH radicals than ozonation only for organic degradation and membrane fouling control. The membrane catalytic ozonation system is simple and effective for treating a wide range of toxic and persistent organic pollutants in wastewater.