Ultrafiltration and reverse osmosis treatment of an acid stage wastewater

The Weyerhaeuser Grande Prairie pulp mill produces 300,000 tons per year of bleached kraft for the tissue and specialty grade paper markets. Alberta Environmental Protection has given the mill stringent limits for Color (90 kilograms/admt) BOD (3 kg/admt), TSS (5 kg/admt), and AOX (1.5 kg/admt). Several technologies and combinations are being studied to ensure that the mill meets future requirements to minimize environmental impact including: oxygen delignification, ozone treatment, chemical coagulation, and membrane separation. The specific research objectives were to: (1) employ ultrafiltration techniques to separate the D_o wastewater into various molecular size fractions and to characterize each in terms of total organic carbon (TOC) and color, (2) to quantify the ability of a UF membrane with a molecular weight cut-off of 8,000 Daltons (D) to reject TOC and color at three temperatures (20°, 30° and 40°C) and three wastewater pH levels (2.4, 5.3 and 7.0), and (3) to quantify the effect of 5 μm cartridge filtration and UF pretreatment on the flux and rejection characteristics of a reverse osmosis (RO) membrane. The following summarizes the results of this research. 1) Chemical characterization after separation showed that 59% of the TOC is comprised of molecules with a molecular size of less than 1,000 D but, only 20% of the color is due to these molecules. 2) Increasing the processing temperature in the range 20 to 40°C positively impacted permeate flux rate, however, water quality was not significantly affected. 3) UF processing at pH 7.0 above the pKa (5.3), increased the permeation rate but at a wastewater pH below (2.4), the converse was true. 4) Pretreatment by either 5 μm filtration or UF followed by RO yielded a permeate equal in quality, however, permeate flux rates were higher with UF treatment.