**1. Introduction**

Agriculture is a potential sector of the global economy, and paddy production has risen nearly 4 times in the last 60 years. India is the second largest paddy producer after China in the world, making more than 10% of the global share and thereby non-edible paddy residues, namely rice straw and rice husk [1]. The farmers are burning million of tons of paddy residues that create smog that contributes to heavy pollution in India. Furthermore, it also reduces the nutrients in the crop soil and damages desired microbial populations [1]. Therefore, management of the large quantities of paddy residues is becoming a significant environmental issue. This and the global warming effect have emerged as a research interest to utilize paddy residues as a feedstock for renewable energy like biogas production. Thus, biogas generation from paddy residues can be a major step toward harnessing one of the world's most prevalent, yet fully unutilized, renewable energy resource.

Carbon dioxide (CO2) is a key greenhouse gas mainly releases from burning of fossil fuels namely oil, coal, and lignocellulosic biomass [2–4]. Although conventional physio-chemical and thermal approaches to reduce CO2 have their own advantages, there is an urge to focus on biological transformation of CO2 to energy and products of interest [5, 6]. Algae use CO2 as a carbon source during photosynthesis and release oxygen into the atmosphere [7, 8]. In parallel to CO2 mitigation, algal biomass has applications in production of single cell protein, bioactive compounds, pigments, cosmetics, pharmaceuticals, biogas, biofertilizer, bioplastics, biohydrogen and phytoremediation [6, 9–11]. Thus, application of micro algal biorefinery concept to produce renewable energy will enhance the economics of bioenergy production by means of circular bio-economy. The current chapter focusses on overview on anaerobic digestion, different conventional methods and microalgal biorefinery for the enrichment of biogas. In addition, the last section of the chapter discusses in short about the algal biomass recovery and its potential applications by means of circular bioeconomy.
