Hydrogen (H₂) is a highly efficient and clean energy source with the potential for renewable energy. The production of H₂ from biological routes such as biophotolysis, photofermentation, dark fermentation, and bioelectrochemical production is characterized as a renewable alternative to current production, which is mainly based on energy-intensive electrochemical and thermochemical processes and responsible for the emission of high amounts of environmentally harmful compounds. Dark fermentation is the most efficient and cost-effective method for producing biohydrogen, making it a key research focus. This article offers a comprehensive overview of the dark fermentation process with the aim of enhancing hydrogen productivity and yields. Aspects related to the main substrates used, the inoculum sources and their pretreatment, and physical-chemical parameters of the process are covered. Furthermore, this manuscript addresses topics such as process integration, genetic and metabolic engineering of fermentative microorganisms, and the main types of bioreactors aimed at greater yields and productivity of biohydrogen to enable its production through dark fermentation on a larger scale.

The anaerobic digestion (AD) of a high diversity blend of fruit and vegetable waste (FVW) generated in tropical conditions as a single substrate was performed. A continuously stirred tank reactor (CSTR) operated in semi-continuous regime was used for AD. The reactor performance was monitored with gradually increasing organic loading rates (OLRs) from 0.5 up to 5.0 gVS L^–1 d^–1. The biochemical methane potential (BMP) of FVW determined by batch bottles was 360 L_N CH₄ kg_VS^–1, with a biodegradability of 79%. A stable pH with an adequate level of buffering capacity was observed during the entire experiment. Methane yield indicated the best performance at an OLR of 3.0 gVS L^–1 d^–1, with 285 L_N CH₄ kg_VS^–1 added, reaching 79% of BMP. At an OLR over 3.0 gVS L^–1 d^–1 accumulation of volatile fatty acids (VFA) was detected; in particular, propionic acid was monitored, and a decreased methane yield was detected. Biogas production rate was 1.55 L_N L^–1 d^–1 and showed linear increase according to increases in the OLR.

Water, energy, and food are essential elements for human life, but face constant pressure resulting from economic development, climate change, and other global processes. Predictions of rapid economic growth, increasing population, and urbanization in the coming decades point to rapidly increasing demand for all three. In this context, improved management of the interactions among water, energy, and food requires an integrated “nexus” approach. This paper focuses on a specific nexus case: biogas generated from organic waste, a renewable source of energy created in livestock production, which can have water-quality impacts if waste enters water bodies. An innovative model is presented to make biogas and biomethane systems feasible, termed “biogas condominiums” (based on collective action given that small- and medium-scale farms on their own cannot afford the necessary investments). Based on the “farm to fuel” concept, animal waste and manure are converted into electrical and thermal energy, biofuel for transportation, and high-quality biofertilizer. This nexus approach provides multiple economic, environmental, and social benefits in both rural and urban areas, including reduction of ground and surface water pollution, decrease of fossil fuels dependence, and mitigation of greenhouse gases emissions, among others. The research finds that biogas condominiums create benefits for the whole biogas supply chain, which includes farmers, agroindustry, input providers, and local communities. The study estimated that biomethane potential in Brazil could substitute the country’s entire diesel and gasoline imports as well as 44% of the total diesel demand. In the United States, biomethane potential can meet 16% of diesel demand and significantly diversify the energy matrix.

Sugarcane is the most traded crop in the world, with Brazil being the world’s largest producer. Sugarcane processing generates up to 28% of sugarcane bagasse (SB) from the entire plant, with only 50% of it used for energy generation. SB is a lignocellulosic biomass that can be converted into biogas. However, the optimization of pretreatment process parameters is essential for its successful scaling up. This study evaluated the effect of mild alkaline pretreatment of SB using NaOH and KOH at concentrations of 1–10% and exposure time of 1–12 hours) on the biochemical methane potential (BMP) under mesophilic temperature. The central composite rotatable design (CCRD) was applied as statistical tool to generate optimal operating pretreatment conditions. The tests were performed in triplicates totalizing 84 batch bottles. The BMP of the untreated SB varied between 297–306 L_N CH₄ kg VS⁻¹ while the BMP of the pretreated samples with NaOH and KOH were 19% and 20% higher. The optimized conditions were NaOH at 7.7% and KOH at 8.3% KOH for 12 hours. However, the range indicated by the statistical design with CCRD revealed that there was no statistical difference in terms of methane yield when concentrations between 4–10% NaOH and 6–10% KOH during 12 hours were applied, when compared to the specific optimized points. The optimization of the pretreatment parameters demonstrated to be a key-factor to improve the anaerobic digestion of lignocellulosic substrates, leading to a less chemically dependent and more sustainable approach, while allowing a more profitable process. GRAPHICAL ABSTRACT