Effects of temperature on lignocellulosic wastes hydrolysis and volatile fatty acids accumulation under neutral and strongly alkaline conditions

Abstract

The volatile fatty acids (VFAs) produced in the process of wetland plant litter (WPL) anaerobic digestion could be used as external carbon sources to enhance the removal efficiencies of oxidized contaminants in constructed wetlands. In this study, the effects of temperature on WPL hydrolysis and VFAs accumulation under neutral and strongly alkaline conditions were explored. In neutral (pH 7.0) fermentation, biotic factors were the leading reasons for WPL hydrolysis, and the maximal SCOD accumulation (2467 mg L-1) occurred at 35 C with a fermentation time of 20 days. In strongly alkaline (pH 12.0) fermentation, abiotic factors were the leading reasons for WPL hydrolysis, and SCOD concentrations increased with temperature at a given fermentation time. Further investigation showed that biotic release of carbohydrate was more sensitive to temperature change than abiotic release. 25 C was the optimal temperature for biotic release of carbohydrate, while abiotic release of carbohydrate slightly increased with temperature. From the results of linear regression, strong positive correlation was observed between VFAs production and the total release of carbohydrate. The optimal temperatures for VFAs accumulation under neutral and strongly alkaline conditions were respectively 35 and 25 C, both with a fermentation time of 20 days, and the VFAs concentrations were respectively 1890.1 mg COD L-1 and 1276.4 mg COD L-1. VFAs produced in all fermentations consisted of acetic, propionic, iso-butyric, n-butyric, iso-valeric and n-valeric acids, with acetic acid being the most prevalent product. The fermentation broth fermented at 35 C with a fermentation time of 20 days has the highest biological utilizability.