Reclamation of backwash water by cross flow microfiltration- a case study

Cross flow microfiltration (MF) for reclaiming backwash water from two water treatment plants was studied. The results showed that both transmembrane pressure (TMP) and cross flow velocity affected the permeability significantly. Cake resistance (R_c) contributed to the majority of total filtration resistance among all MF experiments. It was found that higher solid loading of backwash water did not lead to lower permeability. On the contrary, size distribution and fractal dimension of particulate matters in backwash water were more important in determining specific cake resistance and permeability. Packing of particulate matters with higher fractal dimension induced more compact structure of cake layer, which resulted in higher specific cake resistance. It was found that the effect of fractal dimension on cake compressibility was insignificant, probably because of the decrease in cake deposition during turbulent cross flow MF. Theoretical analysis on the size distribution of deposited particulate matters indicated that the proportion of submicron to micron particulate matters deposited became higher when cross flow velocity was increased. As a result, cake porosity became lower when under turbulent cross flow. Permeate quality was satisfactory in meeting drinking water standards.