Enhancement of volatile fatty acids production through anaerobic co-digestion of navel orange residue and waste activated sludge: Effect of pre-treatment and substrate proportions

In this study, the co-digestion system with Navel orange residues (NOR) and Waste activated sludge (WAS) was established, by pre-treating the NOR and setting different volatile solids (VS) ratios of NOR to WAS to motivate the production of volatile fatty acids (VFA). The pre-treatment method (pH 7 and temperature 70 °C) promoted the release of dissolved organic matter, and the concentration of soluble chemical oxygen demand (SCOD) increased by 45.56% compared with the untreated group (pH 3 and temperature 20 °C). In the co-digestion system, the highest VFA yield (5716.69 mg/L) was obtained at VS ratio of 2. When the VS ratio was increased to 4, the imbalance in proportions of carbon and nitrogen affected VFA production, and the high concentration of essential oils (EO) present in the NOR inhibited the methane production; the cumulative yield of methane gas decreased by 24.10% compared with the yield obtained when the VS ratio was 2. Analysis of microbial community revealed that an increase in the number of VFA-producing microbial populations and the abundance of Methanobacteria resulted in the accumulation of acetic acid. This study demonstrated that co-digestion of NOR with WAS improve VFA production, thus realizing the utilization of solid wastes and reducing environmental pollution.