Ozone oxidation of actual waste leachate coupled with culture of microalgae for efficient lipid production under different temperatures

Abstract

The production of waste leachate (WL) has been increasing annually with the growth of population and the improvement of living standards, but it has become a difficult task to treat and resource it. Furthermore, the shortage of energy is becoming more serious, so the development of renewable energy instead of expensive fossil fuels is especially essential for productive life. This study constructed a system to oxidize WL by ozone at different temperatures and used it as a culture substrate for microalgae to produce biodiesel. It was shown that the biomass and lipid content of microalgae reached 420 ± 43.59 mg/L and 41 ± 2.2% at a low temperature of 15 °C, respectively. Compared with the reaction system at 5 °C, the oxidation of WL by ozone at 25-45 °C was more effective in removing ammonia nitrogen, total phosphorus, and chromaticity. Three-dimension excitation emission matrix (3D-EEM) fluorescence spectroscopy results showed that the fluorescence intensity of dissolved organic matter in WL was reduced by 59.4%-67.7% after the ozone oxidation, which improved the bioavailability of WL and laid a nutrient foundation for the growth of microalgae. At 45 °C, 72.7% of the chromaticity of WL was removed by ozone oxidation alone, and the ozone-coupled microalgae treatment system reduced ammonia nitrogen from 416.25 ± 1.05 to 214.6 ± 7.99 mg/L in WL. In addition, microalgae regulated the antioxidant system to mitigate oxidative damage induced by high concentrations of reactive oxygen species (ROS) caused by extreme temperatures by adjusting the levels of superoxide dismutase (SOD), catalase (CAT) and reduced glutathione (GSH). The lipids of microalgae cultured in WL were dominated by saturated and unsaturated fatty acids, and the saturated fatty acids content of lipids reached 60.8% at 15 °C, which was favorable for the production of biodiesel with better lubricating and combustion properties. This study provides a valuable theoretical basis for the resource utilization of WL and the practical production of microalgae biodiesel in cold regions.