Improving rejection of organic fractions in reclaimed water based on intermolecular interaction effect.

The interactions of complex organic matters in reclaimed water were studied for the rejection mechanisms for organics removal by nanofiltration (NF) membrane. Rejection study on single organic fractions showed that base fractions are the most difficult ones to be removed, with the removal efficiencies of 40.08-47.73% for hydrophobic-base (Hpo-B) and 75.51-79.14% for hydrophilic-base (Hpi-B), respectively. Experimental results for interaction studies showed that with the presence of hydrophilic-acid (Hpi-A) and hydrophobic-acid (Hpo-A) at a concentration ratio of 1, the average rejections for acid + base fractions were 11-30% and 9-26% higher than those for the two corresponding single fractions, respectively. It was noted that after the ratio reaches a certain range (> 2 for Hpo-A in our case) the beneficial effects become less significant since the saturation of opportunities for interactions. With presence of acid and base fractions, the neutralization reactions and hydrophilic interactions would be the major beneficial interaction among different components. With the presence of hydrophobic-neutral (Hpo-N) at a concentration ratio of 1, the average rejections for neutral + base fractions were improved by 9-35% and when at a ratio of 2, the rejections only increased 2.28-8.87% more. The interaction between neutral organics and base organics would be due to the effect of coupling of different permeable components.

Treatability of organic fractions derived from secondary effluent by reverse osmosis membrane.

Dissolved organic matters (DOMs) from two batches of secondary effluent collected from a local water reclamation plant were fractionated using column chromatographic method with non-ionic resins XAD-8, AG MP-50 and IRA-96. Seven isolated fractions were obtained from the fractionation study and these fractions were quantified using DOC, UV(254) and SUVA values. The fractionation study revealed that the secondary effluent samples comprised about 47.3-60.6% of hydrophobic and 39.4-52.7% of hydrophilic solutes. The treatability of each isolated fraction was investigated by subjecting each fraction to reverse osmosis (RO) treatment individually. It was noted that RO process could achieve high DOC rejections for acid and neutral fractions (ranging from 80% to 98% removal) probably due to the negative charge of RO membrane. The results obtained also indicated that hydrophobicity of DOMs is significant in determining treatability of organic species by RO process. The performance of RO in terms of DOC rejection of un-fractionated secondary effluent was also investigated to assess possible effects of interactions among organic fractions on their treatability by RO process. It was noted that DOC rejection associated with the un-fractionated secondary effluent was generally higher (ranging from 2% to 45%) than the corresponding rejection obtained from each individual fraction isolated from the secondary effluent. This finding suggested there is a beneficial interaction among the fractions that in turn has contributed towards a better overall DOC rejection performance by RO treatment.