Citric acid production from the integration of Spanish-style green olive processing wastewaters with white grape pomace by Aspergillus niger

[Display omitted] •White grape pomace provides appropriate sugars to GOW feedstock for A. niger.•Minimum regulation of sugar-enriched GOW is required for citric acid fermentation.•Advantages of the surface to submerged fermentation were demonstrated.•Citrate production using sugar-enriched GOW was comparable to that of other wastes.•The process enabled simultaneous valorization/detoxification of both streams. This study aimed to optimize an integrated simple process for citric acid production using Spanish-style green olive processing wastewaters enriched with sugars from white grape pomace and the robust Aspergillus niger B60. Mild mixing of equal quantities of the above streams governed satisfactory amount of appropriate carbon sources (equimolar mixture of glucose and fructose, 111.5 g/L) in the sugar-enriched wastewater and its neutralization. Various nutrients and fermentation conditions were investigated and maximum citric acid content (85 g/L) and yield (0.56 g/g) were obtained in liquid surface culture after minimum regulation by adding sucrose and NH4NO3 (100 g/L and 1.1 g/L, respectively). Scale-up experiments (5 L-scale) verified findings from small scale (250 mL). The chemical oxygen demand value and phenolic content of the treated wastewater were reduced by 78% and 64%, respectively. Findings support the potential for clustering the respective enterprises in a biorefinery plant for citric acid fermentation.