Cross-flow filtration with different ceramic membranes for polishing wastewater treatment plant effluent

Nowadays the need for sustainable water treatment is essential because water shortages are increasing. Depending on the wastewater treatment plant (WWTP) effluent constituents, the effluent cannot be simply discharged to environment because it contains toxic ions and organic micropollutants which are harmful for aquatic organism. A possible strategy to avoid this is to polish the effluent by membrane processes. Different ceramic membranes were studied to test their ability to remove inorganic and organic compounds from the effluent. Hence, various active layers such as mesoporous TiO2 (average nominal pore size is 15 nm), mesoporous γ-alumina (5 nm), microporous TiO2 (1nm) and microporous hybrid silica (<1nm) on macroporous α-alumina support (~100nm) were used. The total ions and specified toxic ions (e. g. Cu2+) rejections were measured using conductivity measurements and atomic adsorption spectroscopy, respectively. The type and the molecular size of removed organic compounds were determined using pH, full spectrum UV and size exclusion HPLC. Inorganic N-compound rejections were calculated by N-autoanalyzer. The retention of humic like substances measured by UV254 (Fig.1) decreased almost linearly by membrane permeability because of pore size (Fig. 2). The ions rejection decreased nonlinearly with membrane permeability and it was not high for MF, UF and even NF membranes (Fig. 2). Hybrid silica membrane removed almost 95% of NH4+ and 85% of NO2- whereas the others membranes were less efficient (Fig. 3). NO3- rejection was to some extent a logarithmic function of membranes’ permeability. The results showed that γ-alumina membrane can be the optimum choice to polish the WWTP effluent compare to the others based on the membrane permeability and selectivity. Bioassays with Daphnia magna and Vibrio fischeri suggested that effluent polishing with γ-alumina membrane reduced overall acute toxicity of the treated water.