World energy consumption is growing rapidly, while supplying the demand from traditional energy sources is not sustainable. The utilization of fossil fuels, including natural gas, oil, and coil is currently responsible for about 88% of this demand. Using non-renewable energy sources has resulted in greenhouse gas emissions, leading to global warming and climate changes. These facts promoted researchers to develop alternative energy-producing technologies, through which bioenergy has attracted substantial attention. Among the various biofuels, biogas is a widely used green fuel. It is produced through the microbial decomposition of organic matters under oxygen-free conditions. Through this process, the digestate is dewatered and used as a fertilizer or animal feed. On the other hand, bioethanol, the most important liquid biofuel, is widely used as a fuel or gasoline enhancer worldwide. Different renewable substrates can be used for biogas and bioethanol production. In contrast with first-generation biofuels produced from food crops, the production of second-generation biofuels uses non-edible resources. Lignocellulosic materials, as the main organic source on the earth, are considered as the most viable renewable energy feedstock for the production of second-generation biofuels. However, using lignocellulosic feedstock directly for biogas and bioethanol production is not possible due to the recalcitrant structure of these materials. Thus, an efficient pretreatment process is needed to accelerate the biomass degradation and improve the yield of biogas or bioethanol produced in the subsequent steps. In this study, the effluent of biogas production plant was used for the pretreatment of rice straw for the improvement of ethanol production. In addition, the organic active ingredients of the effluent, i.e., acetic, butyric, lactic and propionic acids (1–4%), as well as water were employed for the pretreatment at 100 and 140 ?C. The results indicated that the pretreatment at 100 ?C had no significant effect on the performance of subsequent enzymatic hydrolysis and ethanol production by simultaneous saccharification and fermentation (SSF). Among different types of organic acids presented in the biogas liquid waste, lactic acid showed a better performance. The highest concentration of glucose and ethanol were achieved after 72 h enzymatic hydrolysis and SSF from the straw pretreated at 140 ?C with 4% lactic acid. Applying biogas liquid waste for the straw pretreatment at 140 ?C resulted in an increase in glucose and ethanol concentrations by 42.4 and 47.5%, respectively, compared to those from untreated samples. SEM, FTIR, BET, BJH, and compositional analyses were used to characterize the changes in the structure and composition of rice straw by the pretreatment. Changes in the straw swelling, cellulose crystallinity, pore size distribution, and composition were responsible for the acquired improvements. Furthermore, the straw was pretreated at 130, 160, and 190 °C for 30 and 60 min and subjected to enzymatic hydrolysis, simultaneous saccharification and fermentation (SSF), liquid anaerobic digestion (L-AD), and dry anaerobic digestion (D-AD). The highest improvements in hydrolysis and ethanol yields were 100 and 125%, achieved from the straw pretreated with biogas liquid waste at 190 °C for 60 min. The best methane yield was obtained through L-AD and D-AD of straw pretreated at 190 °C and 30 min with biogas liquid waste, resulting in 24 and 26% enhancements in methane production. However, treatment with water had no significant effect on methane yield