Can fungal degradation replace conventional biological processes for treatment of highly acidic and saline preserved fruit processing wastewater by virtue of Candida, Pichia and Saccharomyces?

Preserved fruit processing wastewater, characterized by high levels of salinity, acidity, and organic load, presents significant challenges to conventional biological treatment technologies, which typically necessitate pH adjustment and salinity reduction. For the first time, a fungal consortium predominantly composed of Candida, Pichia, and Saccharomyces was employed in this study to directly deal with highly saline and acidic preserved plum processing wastewater, in order to achieve COD removal and pH enhancement. COD fractionation in preserved plum processing wastewater by virtue of particle size distribution and oxygen uptake rate shows that 84% of COD was soluble, and 97% of COD were degradable for the inoculated fungal consortium which was characterized by an extremely low biomass yield coefficient of 0.28 mg COD-biomass∙(mg COD-wastewater)-1. In contrast to the conventional COD:N:P ratio of 100:5:1, fungal degradation utilized significantly less nitrogen and phosphorus, exhibiting a COD:N:P ratio of 386-770:4.4:1. Based on the results of pH and salinity effects, this fungal consortium demonstrated optimal performance within a pH range of 3-4 and a salinity range of 3%-5%, which aligns well with the typical conditions of preserved fruit processing wastewater. In the full-scale preserved plum processing wastewater treatment facility with a capacity of 50 m3∙d-1 employing a novel combined process of fungal degradation and activated sludge treatment, the COD removal efficiencies during fungal degradation and subsequent activated sludge treatment were 67% and 27%, respectively, with pH being elevated from 3.42 to 6.52 solely through fungal degradation. Microbial structure analysis indicated that the inoculated fungal consortium was firmly established in the fungal degradation tank, constituting a substantial proportion of the microbial community. Species interaction network analysis revealed that positive correlations predominated within fungal and bacterial communities, particularly among Candida, Pichia, and Saccharomyces, suggesting a collaborative relationship among these species. The findings of this study confirm that fungal degradation is a viable and promising alternative to traditional biological treatment technologies for highly saline and acidic wastewater from preserved fruit processing.