Influence of flocs breakage process on submerged ultrafiltration membrane fouling

In order to explore the influence of floc characteristics on membrane fouling, flocs after breakage and re-growth were compared with those formed by conventional coagulation process when alum was used as coagulant. The results showed that, though the average size of flocs after breakage and re-growth was smaller than that without breakage, the fractal dimension of flocs after breakage and re-growth was lower than that without breakage independent of the dosage of alum. The increase of trans-membrane pressure (TMP) was lower when the flocs formed in breakage and re-growth process. When the dosage of alum increased, the fractal dimension (D2) of flocs without breakage decreased a little while D2 of flocs with breakage and re-growth was nearly the same. Moreover, lower dosage of alum (0.1 mM) caused higher increase of TMP with conventional coagulation process, comparing with higher dosage of alum (0.3 mM); while it was nearly the same when the flocs formed with breakage and re-growth process. The fractal dimension is an important factor influencing the increase of TMP compared with the average size of flocs, and a lower frequency of small flocs may cause a smaller increase of TMP. The characteristics of flocs surface changed after breakage and re-growth and this may also affect the increase of TMP.

► The increase of TMP with breakage process was much lower than that without breakage.
► The fractal dimension of flocs with breakage was lower than that without breakage.
► The average size of floc was not the key factor influencing the increase of TMP.
► The amount of small micro-flocs after flocs breakage and re-growth decreased.
► ‘Surface activation’ of the flocs may influence the TMP development.