A novel recycling process using the treated citric acid wastewater as ingredients water for citric acid production

• The integrated citric acid–methane fermentation process was investigated.
• The inhibitors could reach their equilibrium levels after recycling 2–7 batches.
• The accumulation patterns of inhibitors could be simulated in the recycling system.
• The integrated process could save fresh water and eliminate wastewater pollution.

In this study, an integrated process coupling citric acid and methane fermentations was proposed to solve severe wastewater pollution problem in cassava-based citric acid production. The accumulation patterns of the potential and major inhibitors in this process, including organic compounds, volatile fatty acids (VFAs), total ions and pigments were investigated. Both simulation and experimental results indicated that these inhibitors could reach their equilibrium levels after 3–7 fermentation runs when reutilizing the treated citric acid wastewater. As a result, the proposed citric acid fermentation process by recycling the wastewater treated in methane fermentation could be stably operated for more than 15 runs, which could save a large amount of fresh water and relieve the severe wastewater pollution in citric acid production potentially.