Membrane fouling control through aggregate design and trans-membrane pressure selection

Abstract

Treatment of waters and wastewaters by microfiltration (MF) requires the addition of chemical coagulants to enhance the removal of dissolved substances. Under these conditions the feed to the MF contains flocculated particulates which must be retained by the membrane. While an extensive knowledge base on the effect of dispersed particles on membrane cake formation and fouling exists, much less information is available on the impact of aggregates on cake characteristics. Results of impact of the size and structure (as characterized by the fractal dimension) of particulate aggregates on microfiltration membrane fouling are in qualitative agreement with a simple model based on the Carman-Kozeny equation. Larger flocs form a cake with large inter-floc porosity which results in a significantly higher permeate flux than achieved for smaller flocs. Concomitantly, looser flocs (of low fractal dimension) are likely to form a cake that has higher intra-floc voidage thus flux is higher than a cake made of compact flocs of similar size. Analysis of cake compression indicates that compressibility is strongly influenced by trans-membrane pressure (TMP). The placement of highly porous aggregates onto the membrane results in formation of a highly porous cake layer provided a low TMP is maintained. Rapid compression of the cake occurs at higher TMPs as shown by the significantly lower porosity of the cake. Under high TMP conditions, the cake porosity exhibits a strong size dependence with larger floc sizes yielding higher porosities. This result possibly indicates formation of relatively impermeable assemblages (as a result of significant compaction) with flux controlled by inter-aggregate flow, i.e. flow around compressed flocs. In comparison, the marked lack of size dependence of porosity at low TMP suggests that permeate flux is dominated by flow through (rather than around) the highly permeable flocs. These results suggest that it should be possible to control both operating conditions (such as TMP) and floc characteristics such that high permeate flux at a given TMP or low cake resistance at a fixed flux is achievable.