**1. Introduction**

Wood is a natural, renewable, and recyclable green material and bioenergy source. Under the circumstance of increasing depletion of non-renewable energy and material, getting the utmost out of wood is increasingly important. In general, almost 50% of a tree can converted to the final products, and the rest remain as wood waste (WW) [1]. WW mainly consist of the residues from tree felling and processing, as well as discarded furniture and building materials [2]. Among these processes, sawmills account for 40–60% of the total WW generation [3,4]. The total amount of China's WW is estimated to be 30.28 million tons in 2013 [5]. However, to date, only a minority of WW can be used for recycling and reusing, and an e ffective method to fully utilize these solid wastes has not been developed. Currently, the main treating options for WW are incineration for heating, thermal power

generation, and heat recovery due to its high calorific value. However, direct incineration for energy conversion is inefficient, and large amounts of greenhouse gases and volatile organic compounds can be released during the incineration, especially in small boilers or combustion chambers that often lack emission control systems. These emissions may cause serious environmental pollution.

The biofuels produced from lignocellulosic materials (e.g., wood and agricultural crops) are a green, sustainable, renewable energy, and biofuel production has become a Chinese national strategy [6]. Anaerobic digestion offers an attractive option for the utilization of WW for biogas production [7]. Nevertheless, a high carbon-to-nitrogen (C/N) ratio and high crystalline cellulosic structure make it difficult for WW to continuously and efficiently yielding biogas, which limits its large-scale application in anaerobic digestion technology [8].

Anaerobic co-digestion (AcoD) is a promising optimization technique in biogas production. This process lies in balancing the nutrients, bacterial diversity, pH, toxic compounds, and dry matter in different substrates to achieve a yield-increasing effect of CH4 [9–11]. The methane yield of different substrates can be remarkably elevated via the AcoD technique compared with single-substrate digestion [12]. Given the ability of the intrinsic characteristics of animal manures (low C/N ratio and high NH4–N content) to complement those of lignocellulosic biomass (high C/N ratio and rich lignocellulosic content) in digestion, mixing animal manures and lignocellulosic biomass (e.g., wood and straw) in AcoD to achieve production improvement for the biogas yield of different substrates is a common practice. For instance, Li et al. [6] recommended applying rice straw (RS) to pig manure (PM) in a (volatile solid) VS ratio of 1:1 or 1:2 can evidently increase biomethane production.

The high crystalline cellulosic structure is a substantial factor limiting the enzymatic degradation of lignocellulosic biomass during digestion [13]. To break down the linkages between lignin monomers or between lignin and polysaccharides to obtain lignocellulosic biomass, which is readily hydrolyzed, many pretreatment approaches have been established and normally categorized into three types: chemical (i.e., alkaline, acidic, and inorganic salts), physical (i.e., microwaves and liquid hot water), and biological (enzymatic and fungal). Within the pretreatment categories, NaOH pretreatment has been extensively applied to optimize biogas production of a wide range of lignocellulosic biomass [14].

Considering that the successful application of WW in anaerobic digestion to produce biomethane possibly brings considerable benefits because such feedstock is an abundant, cheap, and sustainable alternative, the utilization potentiality and optimizing strategy of WW in biogas production has to be evaluated. Although some studies have investigated the performance of biogas production from wood in anaerobic digestion [7,15,16], the information of the potential and optimizing strategy for WW to achieve high biogas production is still limited. For example, the optimizing strategy used for improving the biogas production of WW were scarce, and the comparisons of the improvements of biogas production between WW and conventional digestion substrates (e.g., lignocellulosic biomass and animal manure) under the same optimizing strategy, which can be used for more effectively assessing the optimization effect of biogas production of WW, were also lacking in previous studies. This study aimed to assess the performance and potential of methane production from WW in anaerobic digestion. The aim was achieved by (1) evaluating the disparity in biomethane productions between WW and the conventional digestion substrates (RS for lignocellulosic biomass and PM for animal manure), and (2) evaluating the improvement of biomethane production of WW under the effect of different optimizing strategies (NaOH pretreatment, AcoD technique, and their combination).
