*2.2. Biomethane*

Biogas is one of the most promising biofuels, with the ability to alleviate some of the rising worries about fossil fuels, such as the energy calamity and change in the weather [58,59]. Application of microalgae have been shown to be efficient, practical, and cost-effective in biogas generation [60–62]. Microalgae are particularly well suited for combined nutrient removal through wastewater treatment and carbon dioxide sequestration, due to their ability to assimilate large amounts of carbon dioxide and the possibility of blending microalgal cultivation with flue gas emissions or biogas upgrading, which involves removing carbondioxide (as biogas) to increase methane percentage [63–65]. Microalgae cultivation at a wastewater treatment facility offers a free source of water and nutrients, while also contributing to the wastewater treatment process and allowing the recycling of vital nutrients that would otherwise be lost to the environment. The resulting microalgal biomass can subsequently be processed to extract nutrients for fertiliser production or oils for biodiesel generation. Biogas can also be produced through anaerobic digestion of residual biomass [66–68]. Biomethane is generated via biochemical conversion of biomass, followed by gas upgrading, or by thermochemical conversion of solid biomass through gasification, followed by gas cleaning, methanation as the process of synthesis, and biogas upgrading of the product.

Anaerobic digestion by microalgae was first detailed by Golueke and his coworkers in 1957 by means of *Chlorella* sp. and *Scenedesmus* sp.,whichgenerated nearly 0.17–0.32 L CH4/gVS [69]. However, for two reasons, microalgae biomass is not currently considered a viable substrate for biogas production in the anaerobic digestion process: (i) the rigid cell wall of the microalgae confers a high level of resistance to microbial degradation, and (ii) due to the high protein content, the biomass has very low carbon-to-nitrogen (C/N) ratio, which is unfavourable [69,70]. Three different microalgae, belonging to the genus *Scenedesmus*, *Chlorella*, and *Chlamydomonas*, displayed higher carbon/nitrogen ratio in the biomass (24–26 on the basis of weight) when those microalgae were starved. Consecutively, they also lost their capacity to resist degradation by bacteria, thus leading to higher production of biomethane [71], with their conversion rates nearly equal to the theoretical threshold value [70].

By weakening or breaking the microalgae cell wall structure via pretreatment, we can improve the anaerobic biodegradability of microalgae; co-digestion also enhances the biogas yield by increasing the organic loading rate while controlling the concentration of ammonia. For a better understanding and optimization of the performance of the process, mathematical models and reactor design and operation techniques must be carefully studied. Finally, the biogas generated during the anaerobic digestion of microalgae should be improved before being burned on-site or injected into natural gas systems or utilized as liquefied petroleum gas.

To aid compression, to improve calorific value, and to avoid metal component corrosion, all hydrogen sulphide and other trace gases must be eliminated before biogas can be utilised in generators or fed to national gas networks. These steps can be through various chemical scrubbing methods [72,73]. However, the systems involved are huge and difficult to scale down, and can produce very hazardous by-products. Therefore, biotrickling and bioscrubbing have been developed as ecologically viable biological processes.

The use of the entire algal cell and the ability to employ low-quality algae sources, such as wastewater treatment or blooms, are the advantages of the biogas generation in general.
