Cleaner production of citric acid by recycling its extraction wastewater treated with anaerobic digestion and electrodialysis in an integrated citric acid–methane production process

• An integrated citric acid–methane production process was proposed.
• Excessive Na+ in digestate could significantly inhibit citric acid fermentation.
• Digestate pretreated with air stripping improved the electrodialysis performance.
• Electrodialysis treatment could remove Na+ above 95% from pretreated digestate.
• Citric acid fermentation was not influenced by recycling the treated digestate.

To solve the pollution problem of extraction wastewater in citric acid production, an integrated citric acid–methane production process was proposed. Extraction wastewater was treated through anaerobic digestion and the anaerobic digestion effluent (ADE) was recycled for the next batch of citric acid fermentation, thus eliminating wastewater discharge and reducing water consumption. Excessive Na+ contained in ADE could significantly inhibit citric acid fermentation in recycling and was removed by electrodialysis in this paper. Electrodialysis performance was improved after pretreatment of ADE with air stripping and activated carbon adsorption to remove precipitable metal ions and pigments. Moreover, the concentrate water was recycled and mixed with feed to improve the water recovery rate above 95% in electrodialysis treatment, while the dilute water was collected for citric acid fermentation. The removal rate of Na+ in ADE was above 95% and the citric acid production was even higher than that with tap water.