**2. Anaerobic digestion of lignocellulosic biomass**

Over the past decade, anaerobic digestion (AD) has been used effectively for the degradation of agricultural lignocellulosic biomass-maize straw and wheat straw-for the production of biogas, which could be used for combined heat and power (CHP) application [12–14]. Paddy residues, composed of lignocellulose, are difficult for anaerobic microorganisms to degrade as they have a complex polymeric carbohydrates that must be preprocessed into simpler monomers-called platform molecules-that can be further converted into bioenergy [15]. A number of researchers, however, have exclusively paid attention using rice straw for biogas generation through AD [14–17], but AD of pretreated paddy residues has rarely been reported.

Controlled delignification and depolymerization of paddy residues into simpler monomers, called platform molecules, are rather challenging and specifically mandatory on a technical scale and this problem is yet to be solved, for the synthesis of bioenergy. A variety of pretreatment methods have been applied for lignocellulose biomass [14, 18, 19]. It is worth noting that the pretreatment step not only helps to release platform molecules for higher degradation by anaerobic consortia but also helps to remove toxic metal elements from biomass, which are not biodegradable and hence long-term accumulation in anaerobic digesters inhibits stable digestion of biomass in the long run. In addition, a number of important limitations such as characteristics of the pretreated digestate, different solid loadings and carbon-tonitrogen (C/N) ratio to improve methane yield have to be investigated on immediate necessity base.

Several previous studies have reported that biogas produced from untreated rice straw is composed of methane (CH4, ~50–75%), carbon dioxide (CO2, ~25–50%), other impurities in small quantities such as water (H2O, ~5–10%), hydrogen sulfide (H2S, ~0.005–2%), siloxanes (0–0.02%), oxygen (O2, ~0–1%) and nitrogen (0–2%) [20–23]. Biogas is enriched by removing unwanted gases (CO2, H2S and water vapor) to increase the calorific value, so that it is economical to compress and transport to longer places for distribution or move to other area for multifaceted applications [24–26]. Biogas production by AD is an established technology that allows farmers to generate more income from biomass waste and closing nutrient cycles [25, 27]. These synergies can be extended even further if microalgal cultivation is added to produce algae-based bioproducts. Products from microalgae, such as feed and feed additives, can again be used in the agricultural sector, which closes

material cycles and extends the value chain for the biogas operator. Moreover, algae offer potential alternatives for multifaceted applications because of their high protein content and biomass yield, ability to be cultivate in their natural environment and zero effect on the food chain.
