**1. Introduction**

Providing energy and food with low environmental impact is considered as an urgent issue in order to meet demands of them for the growing global population. Alternative resources to replace fossil fuel for energy and chemical fertilizer production are required. Agricultural wastes, such as crop residues and livestock wastes, have been gathering attention as a source of renewable energy and nutrient [1]. Agricultural waste such as lignocellulosic biomass is available globally over 200 billion dry metric ton per year [2]. Livestock wastes such as manures are important nutrient source. Global estimates of nitrogen and phosphorus in the manures were 128 and 24 Tg for 2007, which are almost two times higher than those of fertilized chemical fertilizer [3].

Anaerobic digestion is a technology for treatment of organic wastes, which can biologically decompose carbohydrates, proteins, and lipids in the absence of oxygen and produce biogas (CH4 and CO2). In anaerobic digestion, nitrogen in protein and amino acids are mineralized and transformed into ammonium (NH4 + ). Total P and K are also not lost and retained in the digestate [4]. These nutrients are retained in the residue of anaerobic digestion, called digestate. Therefore, anaerobic digestion can produce both renewable energy and nutrients. In addition to organic waste treatment, anaerobic digestion can be utilized for effective biological pretreatment for anaerobic

biorefinery [5]. In the anaerobic biorefinery concept, biogas is further transformed into alcohol or syngas, etc., and digestate is utilized for algae, organic acid, and alcohol biopolymer productions [6]. Digestate can be also applied to agricultural land as a fertilizer [7] for production of crops or forages since it contains nutrients as noted above. Recycling digestate as a fertilizer can reduce chemical fertilizer production, hence reducing fossil fuel consumption and CO2 emission [8]. Harvested crop residues and collected manures from the livestock fed with the harvested forage can be used for substrate in anaerobic digestion. Thus, anaerobic digestion can be a key technology to recycle waste into value-added products and fertilizer.

Generally, anaerobic digestion is conducted in the form of liquid at low total solid (TS) content less than 15% [9], called wet anaerobic digestion. Wet anaerobic digestion is suitable for wastes with low TS contents (high-moisture contents) [10]. However, to maintain low TS content in the reactor, it requires a large amount of water if it treats wastes with high TS content, such as lignocellulosic biomass, resulting in increase in reactor volume as well as generation of a huge volume of wastewater to be treated [9]. In addition, digested slurry is subjected to solid–liquid separation process [11] after wet anaerobic digestion for further processing.

On the contrary to wet anaerobic digestion, operation at TS content of higher than 15% is classified as dry (solid-state) anaerobic digestion [9]. Dry anaerobic digestion has several advantages over wet anaerobic digestion such as less fresh water usage and favorable energy balance [12]. Agricultural waste such as lignocellulosic biomass has high TS content. For example, TS contents of the corn silage, grasses, and straw biomasses are 25–89% [13]. For livestock manure, depending on pretreatment (solid–liquid separation), TS contents of solid phase are 18–30% [13, 14]. Therefore, agricultural wastes are suitable in dry anaerobic digestion in terms of TS content. Total solid contents of the solid fraction after solid–liquid separation of wet digestate are 23–30% [15], which are comparable or slightly higher than TS content of the dry anaerobic digestate (TS content in the reactor) [16]. Therefore, it would be expected that dry anaerobic digestion would reduce post-digestate treatment such as solid–liquid separation and treatment of liquid fraction, which can reduce energy consumption and cost for plant construction and operation. Therefore, dry anaerobic digestion would have more advantages over wet anaerobic digestion for biorefinery of agricultural wastes.

Although dry anaerobic digestion has several benefits, still wet anaerobic digestion plants have more advantages in terms of energy balance and cost performance in practice [12], requiring more research on effective operation of dry anaerobic digestion. Operation parameters of dry anaerobic digestion should be carefully determined. In general, mass transfer in the dry digestion media is not adequate, and high organic loading would reduce degradation of substrate and biogas production [10]. In addition, treatment of waste with high nitrogen concentration, such as manure, would result in ammonia accumulation and failure [17].

Digestate from the anaerobic digester can be used as fertilizer as it contains nutrient for crop growth or further processed to produce value-added products as noted above. For digestate from wet anaerobic digestion, digestates are subjected to solid–liquid separation [18]. These liquid fraction and solid fraction can be used as fertilizer [18]. Numerous studies have been conducted to evaluate effect of digestate from the wet anaerobic digestion on crop production and environmental risks [15], while digestate from the dry anaerobic digestion has not been well studied.

In this chapter, we reviewed research progress in dry anaerobic digestion of agricultural waste. The key parameters and reactor types of dry anaerobic digestion were summarized. In terms of digestate recycling, we focused on the application of digestate in agricultural land. Especially, the effect of digestate from the wet and dry anaerobic digestion on soil nitrate leaching and root-knot nematodes was summarized.
