**2.3 Total solid content**

High TS content can reduce reactor volume and capital cost [9]. However, in dry anaerobic digestion, higher TS content reduces methane production. Xu et al. reported that maximum methane production rates were proportionally increased with TS content between 0 and 20% while gradually decreased from 20% TS to 30% TS content in mesophilic digestion of corn stover [33]. For mesophilic dry digestion of empty fruit bunch and oil palm trunk, methane yields at 16 and 25% TS contents were 250–350 mL g<sup>−</sup><sup>1</sup> VS. At 35% TS content, however methane yields were less than 100 mL g<sup>−</sup><sup>1</sup> VS with some exception [31]. In semi-batch dry thermophilic co-digestion of pig manure and rice straw, biogas yields were around 600 mL g<sup>−</sup><sup>1</sup> VS, and no VFAs accumulation was observed between 18% and 27% of TS content in the reactor [16]. However, biogas production was decreased concomitantly with VFAs accumulation when TS content in the reactor exceeded 28% [16]. Therefore, TS content should be carefully chosen and managed.

According to Le Hyaric et al., increasing TS content resulted in linear decrease in methane production from acetate, propionate, and cellulose [34]. They pointed out that acetate removal is a rate-limiting step in dry anaerobic digestion since H2 produced from cellulose degradation was rapidly consumed and showed higher methane production than degradation of acetate [34]. However, there have been less information on rate-limiting step at high TS content. More study is required.

*Dry Anaerobic Digestion for Agricultural Waste Recycling DOI: http://dx.doi.org/10.5772/intechopen.91229*

It has been thought that slow solute transport would cause reduction of biogas production at high TS content. In the dry anaerobic digestion, molecular diffusion is thought to control solute transport within the digestion medium because mixing is limited [35]. Solute flux by molecular diffusion is proportional to solute concentration gradient. And its proportional constant, called diffusion coefficient, characterizes the extent of the solute transport by molecular diffusion. Less information are available on the measurement of diffusion coefficient in the dry anaerobic digestion media. Several studies measured diffusion coefficient at high TS content. According to Bollon et al., diffusion coefficient of the solutes in the water is in the order of 10<sup>−</sup><sup>9</sup> m2 s<sup>−</sup><sup>1</sup> while in the order of 10–11 m2 s<sup>−</sup><sup>1</sup> at 8–25% of TS in digestate of the biowaste [35]. Similar results were also obtained by Zhang et al., who measured dewatered and digested sludge at 6–15% of TS content. Abbassi-Guendouz et al. demonstrated that limiting the overall mass transfer resulted in lower cumulative methane production [36].
