## 1 Introduction
Furthermore, the calculated and measured biogas and methane yields of RS render it an attractive substrate for AD [22]. According to [PERSON] [23], biogas and methane potentials of RS can be calculated based on the crude nutrients (ash, fat, fiber, N-free extracts, protein), resulting in 409-415 and 207-211 L/kg VS, respectively [24]. The CH\({}_{4}\) yield measured under different experimental conditions were reported from 92 to 280 L/kg VS with various inocula and temperatures [11]. Other authors reported different CH\({}_{4}\) yields in their studies such as 226-281 L/kg VS [25] and 100-300 L/kg VS [26]. In addition, many emerging factors advance the utilization of RS for energy recovery through AD, like the usage of appropriate inocula and pretreatment methods. Researchers have become increasingly interested in the AD of RS as the Web of Science (r) shows an increasing trend of published papers with \"anaerobic digestion\" and \"rice straw\" as topics for the last 10 years as shown in Figure 1.  
Figure 1: Web of Science®bibliometric study with topics “anaerobic digestion” and “rice straw” from 1991 until December 2019. The number of publications from 1991 to 1999 was 6; from 2000 to 2008 was 7; and from 2009 to 2019 was 512. Apart from its availability and CH\({}_{4}\) potential, RS has a complex, lignocellulosic structure and ligno-carbohydrate complexes that create a barrier for the microbial conversion, which make it a difficult substrate to decompose by AD [4; 27]. Other challenges associated with mono-digestion of RS are the unbalanced nutritional composition (high C/N ratio) [28], low content of essential trace elements (TEs) [29], and its floatation behavior in wet AD systems [30]. Therefore, RS has not been used as an attractive substrate for the recovery of energy and nutrients through AD. Significant research and development have taken place in the last 10 years in the field of AD of RS as evidenced by the number of published papers in the scientific literature, as shown in Figure 1. In 2013, [PERSON] et al. [11] reviewed the AD of RS. This review provides insights only into certain aspects such as greenhouse gas emissions from rice fields, energy recovery potential of RS, various pretreatment strategies to enhance biogas from RS, optimal nutrient balance, and microbiological considerations. However, new knowledge and technologies are continuously unfolding in the field of AD of RS for energy and nutrients recovery. The process parameters such as temperature, organic loading rate (OLR), and mixing are fundamental in the AD and here referred to as \"factors\". The other process techniques, such as adding co-substrates, usage of appropriate inoculum in a suitable amount, recycling liquid digestate (LD), the addition of TEs, and their bioavailability, are also relevant to the AD of RS and termed as \"methods\" in this review. The objectives of this paper are to provide an overview of the effect of these factors and methods on the enhancement of energy recovery (biogas/methane yield) from RS through AD. Additionally, the mass balance of nutrients and their distribution into LD and solid digestate (SD) during AD are discussed in this review. Moreover, the digestion of RS at various scales is also part of the current paper. The focus is the detailed highlighting of these aspects in this review and those studies with other similar substrates have also been included for comparison where fewer studies with RS were available. The effects of various pretreatment techniques on the optimization of AD of RS have been discussed in the review conducted by [PERSON] et al. [11]. The enhancement in biogas yield from lignocellulosic substrates including RS by using various pretreatment methods (mechanical, chemical, biological, and physiochemical) has been reviewed recently by [PERSON] et al. [31]. Moreover, a recent review conducted by [PERSON] et al. [32] discusses the optimization of the AD of lignocellulosic substrates by hydrothermal pretreatment.