## 1 Introduction  
The main component of agro-industrial waste is lignocellulosic, which is constituted by three fractions--lignin, hemicellulose and cellulose [1]. Lignocellulosic biomass can be converted into different products, including biofuels and fine chemicals with a high added-value, and can also be used as a cheap substrate source in microbial fermentation and enzyme production [2]. An interesting by-product generated in the process of sugar extraction from sugar beet (_Beta vulgaris_), is the exhausted sugar beet pulp (ESBP), which has the following composition--24%-32% of hemicellulose, 22%-30% of cellulose, 3%-4% of lignin, and 24%-32% of pectin [3]. Therefore, ESBP is a suitable substrate for biogas production, through the anaerobic digestion process (AD) [4; 5; 6]. The use of biogas produced from ESBP could generate economic and environmental benefits for energy-intensive consumers, such as sugar beet industries and, hence, contribute to the circular economy. AD is a mature, highly efficient, and economically viable technology for transforming organic wastes into methane [7; 8; 9]. The process occurs under anaerobic conditions, requiring considerably less energy with respect to the aerobic treatment processes [10]. Agro-industrial by-products could release, depending on their nature and origin, toxic or inhibitory compounds for microorganisms, such as ammonia, sulphides, heavy metals, long chain fatty acids (LCFAs), and other organic compounds. The intensification of the microorganisms activity in the system, especially the increase of their hydrolytic ability, could be counterproductive if the breakdown of the lignocellulosic material resulted in the release of intermediate inhibitory compounds. The accumulation of these compounds would lead to the destabilization of the AD process [11]. Therefore, in order to improve the efficiency and stability of the AD process, a deep knowledge of the critical process parameters, affecting the different microbial groups is required [12; 13]. According to the literature, operations in thermophilic temperature ranges (50-60 \({}^{\circ}\)C) lead to a higher biogas production than in mesophilic range (30-40 \({}^{\circ}\)C), but compromises the stability of the process [14; 15]. However, the decomposition of agro-industrial by-products could present limitations due to the presence of non-biodegradable lignocellulosic and the deficit in the nitrogen source, with respect to the high content in organic matter [1; 16]. [PERSON] et al. [11] have reported that thermophilic or hypertherophilic acidogenic anaerobic digestion of ESBP led to the accumulation of LCFAs, such as palmitate and lignoacetate. The use of the effluents from these reactors for the feeding of two subsequent mesophilic reactors led to the inhibition of methanogenic archaea population. LCFAs are considered inhibitory compounds for the AD process, especially for methanogenic microorganisms [17]. With respect to the deficit of nitrogen in ESBP, several authors [18; 19] have shown that the anaerobic co-digestion (AcoD) of ESBP with animal manure, solves this problem. Thus, animal manure supplies the necessary nutrients, but it also provides alkalinity and specific microorganisms, originating from the digestive tract of animals, which are capable of degrading vegetal fibers [20; 21]. Therefore, animal manure improves, both, the biodegradability of the lignocellulosic material and the stability of the AD process. [PERSON] et al. [22], have studied the AcoD of ESBP with pig manure (PM) in continuous stirred tank reactor (CSTR) reactors operating in the mesophilic range of temperature. Other authors [23; 24; 25] have studied the AcoD of different agricultural wastes with animal manure and have shown that co-digestion improves the anaerobic process and waste digestibility. The main objective of this study was to determine the optimal mixture ratio of ESBP and PM for the thermophilic anaerobic co-digestion of both wastes.