## 1 Introduction  
Anaerobic digestion (AD) is an environment-friendly and sustainable technology for the treatment of agricultural residues for recovering energy in the form of biogas and organic fertilizer raw materials in the form of digestates [1, 2]. AD can be categorized according to the total solids (TS) content as liquid AD (L-AD) and solid-state AD (SS-AD) [3], and it can also be grouped into mesophilic AD (30-40 \({}^{\circ}\)C) and thermophilic AD (50-60 \({}^{\circ}\)C), according to the operating temperature [4, 5]. As less moisture increases equipment efficiency and reduces heating demand [6], while higher fermentation temperatures accelerate biogas production and reduce retention time [4], a growing corpus of studies has been focused on thermophilic SS-AD [7, 8, 9]. As a representative member of the agricultural residues, corn stover (CS) primarily comprises three types of polymers: cellulose, hemicellulose, and lignin. However, a rigid three-dimensional matrix formed by cross-linking the three components constrains the efficient biological deconstruction [6]. Thus, pretreatment is generally required prior to AD for enhancing the enzymatic accessibility and subsequent biodegradability of CS. Various pretreatment strategies have been previously used for lignocellulosic biomass, including biological, physical, chemical, physicochemical, and combined methods [10]. Among the existing pretreatment technologies, alkali pretreatment is considered one of the most promising options, considering its high efficiency in delignification and high final total sugar yields [11]. Calcium hydroxide/calcium oxide has been extensively studied as a potential reagent based on the cost-efficiency [12, 13, 14]. Ca(OH)\({}_{2}\) single-batch pretreatment and NaOH consecutive-batch pretreatment with leachate reuse have been compared for giant reed. The net benefit (revenue from increased energy production minus chemical cost) of Ca(OH)\({}_{2}\) pretreatment was positive at $1.1-5.8/ton dry biomass, whereas that of the NaOH was negative [15]. As the solubility of Ca(OH)\({}_{2}\) in water is considerably lower than that of NaOH or KOH, some of the unreacted solids remaining on the surface of the biomass were wasted. Therefore, a recirculating system for the saturated lime solution was introduced by researchers [16] to further reduce the chemical costs. However, the final pH after the alkali pretreatment process is typically >10.0, while the acceptable pH for AD is typically in the range of 6.5-8.5. Therefore, lowering the pH to the range where AD can be successfully initiated is the required step, including washing with deionized water, adding organic or mineral acids [17], or using gaseous carbon dioxide or biogas to neutralize alkali [18; 19]. These methods suffer from considerable limitations for practical applications, despite the fact that they are easily applicable in the laboratory conditions. The large volumes of wastewater generated during the washing process or bubbling gases and the extra chemical costs caused by adding acids should be considered by the industry. Liquid digestate (LD) from AD systems is rich in microbes for bioconversion. Previous studies have reported that LD could be utilized as a microbial agent for the bio-pretreatment of lignocellulosic biomass [20; 21; 22; 23]. Notably, acidogens from these microbes are also potential candidates for reducing the pH. Additionally, enzymes from microorganisms can attack inert lignocellulosic and convert it into volatile fatty acids (VFAs) that are easily fermented, which may accelerate the subsequent AD process. Hence, to the best of our knowledge, the use of LD to neutralize alkali-pretreated CS for the initiation of AD is reported herein for the first time. For alkali-treated materials immersed in liquids, gaseous CO\({}_{2}\) or biogas has been previously utilized as an effective aid for alkali neutralization [18; 19].