Abstract : Membrane fouling due to natural organic matter (NOM) is the major issue in drinking water treatment using low-pressure membrane processes (MF/UF). Fouling due to NOM is more pronounced in the form of humic acid fouling, which is a degradation product of plant and animal residues in the environment. According to previous studies it was reported that humic acid fouling is mainly governed by cake formation mechanism which increases the total hydraulic resistance and there by induces flux decline. We have analyzed humic acid cake, formed on regenerated cellulose flat sheet membranes in different operating conditions like pH, trans-membrane pressure, molecular weight cut-off (MWCO) of the membranes and humic acid concentration to understand the fouling mechanisms. Different parameters like hydraulic permeability and hydraulic resistance, percentage of flux decline, modified fouling index (MFI-UF), streaming potential coefficients, and fractal dimensions of the surface cake were determined in order to assess the nature and extent of fouling. Using a combination of these macroscopic, microscopic and in situ tools, we have developed a new method for the analysis of humic acid cake and there by to assess the nature, mechanisms, and consequences of fouling due to NOM in low-pressure membrane processes. This method was applied to hollow fibers (virgin and fouled by natural surface water), combined with membrane autopsy. We have also conducted a pretreatment method for the removal of NOM using bentonite (mont-Al-CTAB). Pre-treatment method using bentonite was effective by reducing the total hydraulic resistance of the fouling layer.