To achieve economically competitive biological hydrogen production, it is crucial to consider inexpensive materials such as lignocellulosic substrate residues derived from agroindustrial activities. It is possible to use (1) lignocellulosic materials without any type of pretreatment, (2) lignocellulosic materials after a pretreatment step, and (3) lignocellulosic materials hydrolysates originating from a pretreatment step followed by enzymatic hydrolysis. According to the current literature data on fermentative H₂ production presented in this review, thermophilic conditions produce H₂ in yields approximately 75% higher than those obtained in mesophilic conditions using untreated lignocellulosic substrates. The average H₂ production from pretreated material is 3.17 ± 1.79 mmol of H₂/g of substrate, which is approximately 50% higher compared with the average yield achieved using untreated materials (2.17 ± 1.84 mmol of H₂/g of substrate). Biological pretreatment affords the highest average yield 4.54 ± 1.78 mmol of H₂/g of substrate compared with the acid and basic pretreatment - average yields of 2.94 ± 1.85 and 2.41 ± 1.52 mmol of H₂/g of substrate, respectively. The average H₂ yield from hydrolysates, obtained from a pretreatment step and enzymatic hydrolysis (3.78 ± 1.92 mmol of H₂/g), was lower compared with the yield of substrates pretreated by biological methods only, demonstrating that it is important to avoid the formation of inhibitors generated by chemical pretreatments. Based on this review, exploring other microorganisms and optimizing the pretreatment and hydrolysis conditions can make the use of lignocellulosic substrates a sustainable way to produce H₂.

In microbial fuel cells (MFCs), the oxidation of organic compounds catalyzed by microorganisms (anode) generates electricity via electron transfer to an external circuit that acts as an electron acceptor (cathode). Microbial fuel cells differ in terms of the microorganisms employed and the nature of the oxidized organic compound. In this study, a consortium of anaerobic microorganisms helped to treat the secondary sludge obtained from a sewage treatment plant. The microorganisms were grown in a 250 mL bioreactor containing a carbon cloth. The reactor was fed with media containing acetate (as the carbon source) for 48 days. Concomitantly, the electrochemical data were measured with the aid of a digital multimeter and data acquisition system. At the beginning of the MFC operation, power density was low, probably due to slow microorganism growth and adhesion. The power density increased from the 15^th day of operation, reaching a value of 13.5 μW cm^–2 after ca. 24 days of operation, and remained stable until the end of the process. Compared with data in the literature, this power density value is promising; improvements in the MFC design and operation could increase this value even further. The system investigated herein employed excess sludge as a biocatalyst in an MFC. This opens up the possibility of using organic acids and/or carbohydrate-rich effluents to feed MFCs, and thereby provide simultaneous effluent treatment and energy generation.