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Abstract
The use of nanofiltration (NF) as a pre-treatment to reverse osmosis (RO) in a newsprint mill effluent recycling application was optimised to simultaneously minimise organic fouling on the RO and to produce a brine stream with a low sodium adsorption ratio (SAR) for land based disposal. Nine commercially available NF membranes were characterised and screened for use prior to RO in a dead-end filtration mode. A Salt-Organic-Separation (SOS) efficiency factor was developed to help rank the performance of the membranes. Three of the highest ranked membranes, NF-270, TFC-SR2 and ESNA1-LF2, were operated in cross-flow filtration mode to generate fouling, rejection and cleaning efficiency data for the purposes of building a MATLAB® model to simulate the effect of Recycled Fibre (RCF) content used in newsprint manufacture. NF-270 was selected as the basis for the simulation because of the high fouling propensity of the ESNA1-LF2 and low pH tolerance of TFC-SR2 at high pH necessary to remove the organic fouling layer.
Increasing the RCF content in newsprint manufacture from 0 to 50% significantly altered the sodium and silica content in the effluent which, in turn, limited the membrane’s operation. Residual silica in the biologically treated effluent is the recovery limiting constituent in the recycling process. In order to minimise silica fouling, it was necessary to reduce the silica content in the NF filtrate to 12 mg/l. This was achieved using a combination of calcium hydroxide and magnesium sulphate at pH 10.9. The presence of organic matter in the feed increased the operating pressure on the NF above that predicted by osmotic pressure using ROSA® by a factor of approximately two and reduced the efficiency of the silica removal process by 15%. Excess sodium in the effluent affected the brine management option for land disposal. Depending on the selection of the NF membrane, the sodium adsorption ratio (SAR) of the NF reject ranged from 1.6 to 3.9 which are 30 to 70% lower than the SAR of the original mill effluent. By blending of the biologically treated effluent with the NF brine and RO permeate it was possible to achieve a SAR of 5.