**3. Biogas enrichment**

Removal of CO2 increases the percentage of biomethane in biogas. The processes involving CO2 capture and storage are gaining attention as an alternative for reducing CO2 concentration in the enrichment of biogas [22, 28, 29]. Several physiochemical [22, 30–32], biological [33, 34] and thermal methods [29, 35] for biogas enrichment process or purification of biogas have been reported [20, 29] (**Table 1**). For example, purification of biogas using calcium hydroxide (Ca(OH)2) solution, thus CO2 would be reacted with Ca(OH)2 solution to form the precipitate of calcium carbonate (CaCO3) has been reported [36]. Potential adsorbents such as activated carbon, silica gel, clay, alumina and zeolite have been reported for gas purification [37–40].

Although every adsorbent system and conventional methods have their own advantages, these technologies are considered as expensive and environmentally hazardous short-term solutions, as there are still concerns about the environmental sustainability of these processes. Alternatively, microalgal sequestration of CO2 and its transformation to value-added biomass could be a potential solution due to its feasible and unmatched advantages over current approaches [40–42]. The waste product at the end of algal CO2 sequestration is oxygen, clean air. Microalgae will utilize inorganic carbon from CO2 into lipids under sunlight and increase the accumulation of biomass and algal oil [42]. As photosynthesis is a key process for microalgae metabolism and their growth, these systems are suitable even for regions with high temperatures and sunlight exposure. The effectiveness of CO2 mitigation and its consumption by algae can modify according to the state of the algal physiology, gas residence times, light intensity, nutrient availability


### **Table 1.**

*Different methodologies for biogas purification.*

and temperature. It has been reported that CO2 sequestration as high as 99% is attainable upon defined environmental and nutritional conditions, and with gas residence times as low as two seconds [42–45]. In addition, microalgal system combined with AD systems and the synergy between algae-bacteria can help to avoid the power demands from aeration, which actually represent almost 60% of the total energy requirement of waste effluent treatment plants in industries. During photosynthesis, algal system provides oxygen that is necessary for aerobic microbes to digest and biodegrade organic effluents, consuming in turn the CO2 released due to bacterial growth [44].
