Silica removal in industrial effluents with high silica content and low hardness
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High silica content of de-inked paper mill effluents is limiting their regeneration and reuse after membrane treatments such as reverse osmosis (RO). Silica removal during softening processes is a common treatment; however, the effluent from the paper mill studied has a low hardness content, which makes the addition of magnesium compounds necessary to increase silica removal. Two soluble magnesium compounds (MgCl2∙6H2O and MgSO4∙7H2O) were tested at five dosages (250–1,500 mg/L) and different initial pH values. High removal rates (80–90 %) were obtained with both products at the highest pH tested (11.5). With these removal efficiencies, it is possible to work at high RO recoveries (75–85 %) without silica scaling. Although pH regulation significantly increased the conductivity of the waters (at pH 11.5 from 2.1 to 3.7–4.0 mS/cm), this could be partially solved by using Ca(OH)2 instead of NaOH as pH regulator (final conductivity around 3.0 mS/cm). Maximum chemical oxygen demand (COD) removal obtained with caustic soda was lower than with lime (15 vs. 30 %). Additionally, the combined use of a polyaluminum coagulant during the softening process was studied; the coagulant, however, did not significantly improve silica removal, obtaining a maximum increase of only 10 %.
KeywordsSilica removal Magnesium Softening Coprecipitation Membranes fouling Effluent reuse Paper recycling
The authors wish to acknowledge the financial support of the European Commission through the project “AQUAFIT4USE” (ref. 211534), the Community of Madrid through the program “PROLIPAPEL II-CM” (S-2009AMB-1480), and the Spanish Ministry of Education for the doctoral grant of I. Latour (AP2009-4197). The authors would also like to thank Sachtleben Wasserchemie GmbH for supplying the coagulant used in this study and Holmen Paper Madrid for the water samples used in this work. Finally, the collaboration of Patricia García and Maria Balmaseda during the lab work is deeply acknowledged.
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