Establishment and assessment of an integrated citric acid–methane production process

To solve the problem of extraction wastewater in citric acid industrial production, an improved integrated citric acid–methane production process was established in this study. Extraction wastewater was treated by anaerobic digestion and then the anaerobic digestion effluent (ADE) was stripped by air to remove ammonia. Followed by solid–liquid separation to remove metal ion precipitation, the supernatant was recycled for the next batch of citric acid fermentation, thus eliminating wastewater discharge and reducing water consumption. 130 U/g glucoamylase was added to medium after inoculation and the recycling process performed for 10 batches. Fermentation time decreased by 20% in recycling and the average citric acid production (2nd–10th) was 145.9 ± 3.4 g/L, only 2.5% lower than that with tap water (149.6 g/L). The average methane production was 292.3 ± 25.1 mL/g CODremoved and stable in operation. Excessive Na+ concentration in ADE was confirmed to be the major challenge for the proposed process.