Effects of molecular weight and powder-activated carbon (PAC) pre-treatment

Fouling characteristics of membranes with various molecular weight cut-offs (MWCO) were investigated. The effects of the molecular weights (MW) of humic acids and pre-treatment with PAC on membrane fouling were studied. It was found that the hydraulic resistance caused by fouling materials calculated using cake resistance in series model is a better indicator than the percentage of flux decline to assess the fouling of membranes with various MWCO. The effects of MWCO of membranes and MW of humic acids on membrane fouling can be explained by the different types of fouling mechanisms.

Fig. 8. Resistance, Rf, for various samples treated with YM10. a solution without PAC. It is also interesting to note that the fraction of NOM that is not removed by PAC, i.e., NOMnonads caused fouling as much as or even worse than that caused by sample I, indicating the fraction of NOM that is adsorbed by PAC might not induce much fouling on membranes. According to Lin et al. [13] and Crozes et al. [24], the smallest MW fraction that was not selectively removed by PAC and was the dominant fraction after being pre-treated with PAC, was responsible for membrane fouling. These findings are consistent with the results shown above that organics of the smaller MW fraction can enter the membrane pore causing pore blocking and adsorption. As indicated previously, these types of fouling are a much worse fouling In the current study, the fouling resistance of membranes is a better parameter than the percentage of flux decline for comparing fouling characteristics of membranes with different MWCO. The YM3 membrane had the worst organic fouling. The different fouling mechanisms encountered by membranes of different MWCO were used to explain the experimental data. From these results, one can infer that the fouling of the YM3 membrane was caused by pore blocking and pore adsorption mechanisms, which might cause a higher rate of flux decline than the gel layer formation and concentration polarization mechanisms. Various MW of humic acids were obtained by pre-filtering humic acid solutions through membranes with specific MWCO. Although the DOC concentration of sample IV (MW <3 KDa) was only 0.63 mg/L, this sample induced the highest flux decline and fouling on the YM1 membrane than other samples that had DOC concentrations ranging from 5 to 2.52 mg/L. This can be explained by the fact that humic acids with a low MW might enter the pore, causing pore blockage and adsorption type of fouling while humic acids with high MW form a layer of gel or concentration polarization on top of the membrane surface, preventing small MW organics from entering the membrane pores. Results from PAC pre-treatment indicate that fractions not adsorbed by PAC caused higher fouling than fractions adsorbed by PAC. PAC alone did not impose significant fouling on the membrane. PAC pretreatment would enhance the organic removal but at the same time increase membrane fouling.