**2.11 Hydrolysis**

Hydrolysis is probably the most prevalent chemical reaction in multiple WtE and WtL technologies. Hydrolysis is the chemical reaction where the addition of

**43**

**Figure 7.**

*Example scheme of solvent extraction technology.*

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

tieth century, with maximum yields of 190 L Mg<sup>−</sup><sup>1</sup>

molecules bind to H+

**2.12 Solvent extraction**

a water molecule breaks the chemical bond of another molecule and the resulting

during the 1930s, the industrial growth at the time needed to develop processes of ethanol production that did not use food sources. In 1934, six pilot reactors were built with the objective of optimizing different hydrolysis technologies, not only to produce ethanol but also other products such as xylitol and furfural. After the First World War, this process was no longer economically viable against more conventional methods. With the advances of the last two decades, enzymatic hydrolysis seems to be the most promising application regarding hydrolysis techniques.

Solvent extraction is a relatively modern technology used in the extraction of products from its substrates (**Figure 7**). By choosing a solvent that best dissolves the wanted product, this process usually results in higher yields when compared with other methods. The separation is quick and efficient and most of the solvent can be reused. Extraction of oils via this technology is a common application in the industry, normally used after mechanical extraction. Hexane is the most used solvent, but ethanol and isopropanol have also been proposed as alternative options. The Soxhlet extractor is often the preferred method for lipid extraction due to the simplicity of operation, relative safety, and ease of replicating results on an industrial scale [76]. From research, organic solvents such as chloroform, ethanol, and hexane were found to produce the best results when performing lipid extraction from microalgae. Solvent mixtures were also observed to yield better results

could produce sugars from cellulose through hydrolysis with sulfuric acid. This hydrolyzed sugar could then be processed and fermented to produce ethanol. The production of ethanol by hydrolysis began extensively at the beginning of the twen-

and OH<sup>−</sup> ions. In 1819, Henri Braconnot discovered that he

of biomass. In the former USSR

**Figure 6.** *Example scheme of microbial electrolysis technology.*

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

*Elements of Bioeconomy*

**2.10 Microbial electrolysis**

and new reactor configurations.

**2.11 Hydrolysis**

the need for optimization depending on the waste to be transformed [65]. Other experiments have focused on process enhancement via salt pretreatment. Addition of inorganic salts, for instance, has been reported to improve reducing sugar yields

Microbial electrolysis is a bioelectrochemical transformation where hydrogen or methane is produced from various wastes and wastewaters. Microbial electrolysis cells (MEC) use the metabolic activity of exoelectrogenic bacteria to catalyze redox reactions and promote the flow of electrons between the electrodes [68]. Specifically, the bacteria convert biodegradable substrates at the anode, releasing electrons and protons (**Figure 6**). The electrons are then transferred to the cathode (where hydrogen is produced) inducing an electrical current with electrical potential values (0.2–0.8 V) lower than in traditional electrolysis (1.8–3.5 V) [69]. Microbial electrolysis cells (MEC) have the potential to become one of the most important WtE technologies. However, electrode materials are still costly, and further developments are needed. In this regard, the use of biochar-based electrodes seems to compose an interesting research route [70–72]. Currently, coupling with other technologies for energy generation seems to be its leading application. The use of microbial electrolysis as a pretreatment for AD, for example, has been explored recently with interesting results. In a study focused on the valorization of highly concentrated FW [73], MEC was found to accelerate methane production rate and stabilization. As another example, post-processing of wastewater resulting from hydrothermal liquefaction for recovered hydrogen has also been demonstrated with effective results [74, 75]. As a technology, MEC are still in the early phase of development, and further progress is expected with the use of novel electrode materials

Hydrolysis is probably the most prevalent chemical reaction in multiple WtE and WtL technologies. Hydrolysis is the chemical reaction where the addition of

of sugarcane leaf wastes and mustard stalk and straw [66, 67].

**42**

**Figure 6.**

*Example scheme of microbial electrolysis technology.*

a water molecule breaks the chemical bond of another molecule and the resulting molecules bind to H+ and OH<sup>−</sup> ions. In 1819, Henri Braconnot discovered that he could produce sugars from cellulose through hydrolysis with sulfuric acid. This hydrolyzed sugar could then be processed and fermented to produce ethanol. The production of ethanol by hydrolysis began extensively at the beginning of the twentieth century, with maximum yields of 190 L Mg<sup>−</sup><sup>1</sup> of biomass. In the former USSR during the 1930s, the industrial growth at the time needed to develop processes of ethanol production that did not use food sources. In 1934, six pilot reactors were built with the objective of optimizing different hydrolysis technologies, not only to produce ethanol but also other products such as xylitol and furfural. After the First World War, this process was no longer economically viable against more conventional methods. With the advances of the last two decades, enzymatic hydrolysis seems to be the most promising application regarding hydrolysis techniques.
