Forward osmosis coupled with lime-soda ash softening for volume minimization of reverse osmosis concentrate and CaCO₃ recovery: A case study on the coal chemical industry

Highlights

• Forward osmosis (FO) coupled with chemical softening for CCI ROC minimization

• Effective removal of scale precursor ions by lime-soda ash softening

• Enhanced water recovery from 54% to 86% by mitigation of FO membrane scaling

• High-purity CaCO₃ was recovered from the softening sludge

• Membrane cleaning efficiency of 88.5% was obtained by EDTA for softened ROC

Abstract

Reverse osmosis (RO) is frequently used for water reclamation from treated wastewater or desalination plants. The RO concentrate (ROC) produced from the coal chemical industry (CCI) generally contains refractory organic pollutants and extremely high-concentration inorganic salts with a dissolved solids content of more than 20 g/L contributed by inorganic ions, such as Na⁺, Ca²⁺, Mg²⁺, Cl⁻, and SO₄²⁻. To address this issue, in this study, we focused on coupling forward osmosis (FO) with chemical softening (FO-CS) for the volume minimization of CCI ROC and the recovery of valuable resources in the form of CaCO₃. In the case of the real raw CCI ROC, softening treatment by lime-soda ash was shown to effectively remove Ca²⁺/Ba²⁺ (>98.5%) and Mg²⁺/Sr²⁺/Si (>80%), as well as significantly mitigate membrane scaling during FO. The softened ROC and raw ROC corresponded to a maximum water recovery of 86% and 54%, respectively. During cyclic FO tests (4 × 10 h), a 27% decline in the water flux was observed for raw ROC, whereas only 4% was observed for softened ROC. The cleaning efficiency using EDTA was also found to be considerably higher for softened ROC (88.5%) than that for raw ROC (49.0%). In addition, CaCO₃ (92.2% purity) was recovered from the softening sludge with an average yield of 5.6 kg/m³ treated ROC. This study provides a proof-of-concept demonstration of the FO-CS coupling process for ROC volume minimization and valuable resources recovery, which makes the treatment of CCI ROC more efficient and more economical.

Graphical abstract

Keywords

Coal chemical industry / Forward osmosis / Chemical softening / Reverse osmosis concentrate

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Acknowledgements

This work was supported by the National Key Technology R&D Program (No. 2019YFC0408503) and State Key Laboratory of Urban Water Resource and Environment (No. 20180X09).

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2020 Higher Education Press