In this study, flux, retention, and permeate quality of various nanofiltration and low-pressure reverse osmosis membranes were investigated at two temperatures and by using a low shear (DSS Labstak M20 ® ) and a high shear (CR250/2) filter. The overall aim was to study the suitability of nanofiltration in purification of the discharge water from external activated sludge processes in the pulp and paper industry for reuse in the paper manufacturing process and to compare the results to nanofiltration of paper machine process waters.
In this study the discharge waters were nanofiltered at a higher flux than paper machine process waters. The permeate was almost free of colour and organic compounds but contained significantly more inorganic compounds than the permeate from the filtration of process waters. To successfully remove monovalent anions and inorganic carbon from the discharge water a low-pressure reverse osmosis membrane such as the TFC ULP membrane is needed. With that membrane the permeate flux is lower than for nanofiltration membranes but the permeate quality is significantly better when considering inorganic ions such as sodium, chloride, nitrate and inorganic carbon (bicarbonate). The permeate flux was two times higher in the high shear filter than in the low shear filter but the retentions were significantly lower.