Critical flux, resistance, and removal of contaminants in ultrafiltration (UF) of natural organic materials

Critical flux is defined as the permeate flux of a membrane system under which no or little fouling is observed. Under sub-critical conditions, operating the system with constant flux mode resulted in better performance than with constant pressure mode. When the system was operated under super-critical conditions, the system with constant flux mode showed better performance at the early stage of the operation but as the system was operated in longer term, constant flux mode resulted in worse performance. In both the sub-critical and the super-critical flux conditions, the hydraulic resistances of the system with constant flux mode were less than those with constant pressure mode. It was because there was “overfouling” of the system at the beginning of the ultrafiltration under the constant pressure mode.; Fouling is one the biggest obstacles in using membrane filtration in water and wastewater treatment. Increasing inorganic particle concentration reduced fouling by acting as a “scrapper” or competent for adsorption sites with NOM.; Increasing NOM concentration increased the amount of NOM transported to the membrane which resulted in increased deposit layer or adsorption of NOM molecules and increased hydraulic resistance of the system.; Humic fraction of NOM, which is more hydrophobic and larger in its size, was found to cause more fouling because of its adsorptive capacity on the membrane. The soil-borne NOM caused more resistance than the water-borne NOM. Hydrophobicity and molecular weight can be the distinct difference between those two NOM types.; Dissolved organic matter has been known to be removed by coagulation with metal salts otherwise not be removed by the ultrafiltration alone. The hydraulic resistances of the membrane system with charge neutralization conditions were smaller than with sweep floc conditions. In some cases, the hydraulic resistance, an indicator of fouling, was lower with unconventional coagulation conditions than conventional ones.; It can be concluded that the optimum coagulation conditions in conventional water treatment are not always the optimum conditions for UF-with-coagulation system and the unconventional coagulation conditions can be much more effective than the conventional ones in the context of membrane filtration, depending on the raw water characteristics.