Selective separation of contaminants from paper mill effluent using nanofiltration

Abstract

Biologically treated newsprint mill effluent containing 57mgL−1 DOC and 1430 TDS was used in a screening study of nine commercial NF membranes for use as pretreatment for reverse osmosis in an end of pipe water recycling application. A salt-organic-separation (SOS) efficiency factor was developed to help rank the performance of the membranes. The SOS measures the ratio of the sum of the percentage rejection of organics and divalent cations over the percentage rejection of monvalents. It can be used to discriminate between NF membranes that are not too permeable to divalent cations or organics in which case the NF permeate will have a high chlorine demand due to the carryover of organics, or too retentive in which case all the material in the effluent will be retained and fouling problems are likely to occur. The optimum SOS efficiently for this study appeared to range from 3.5 to 5.6 for six membranes, DK, HPA-150, ESNA1-LF2, DL, TFC-SR2 and NF-270, which were categorised as membranes with an intermediate rejection. Out of these membranes ESNA1-LF2, TFC-SR2 and NF-270 were capable of operating up to 90% recovery with high permeabilities ranging from 17.7 to 22.3Lm−2h−1bar−1.Additionally, impact of membrane surface properties, such as molecular weight cut-off, surface charge and hydrophobicity, was assessed on the fouling and SOS efficiency. The molecular weight cut-off was found to have a correlation with the fouling potential of the membranes, while none of the surface properties had any correlation with the SOS efficiency.