DAP is usually performed over a temperature range of 120-210 °C, with acid con­centration typically less than 4 wt%, and residence time from several minutes to an hour [53]. In the DAP pretreatment, the combined severity (CS) is used for an easy comparison of pretreatment conditions and for facilitation of process control, which relates to the experimental effects of temperature, residence time, and acid concen­tration [54]. Lower CS is beneficial for the hemicellulose to hydrolyze to oligomers and monomers while higher CS could bring these monomers to furfurals, which are inhibitors for the subsequent enzymatic hydrolysis [55]. In order to maximize the efficiency of pretreatments, several studies have proposed a two-step procedure for DAP of SW [45, 56]. The conditions in the first step are less severe and serve to hydrolyze the hemicelluloses resulting in a high recovery of hemicellulose-derived fermentable sugars in the pretreatment effluent. By separating the solid and liquid phases after the first step, it is possible to minimize the degradation of hemicellulosic sugars to furfural and hydroxymethylfurfural (HMF). The solid material recovered from the first step is treated again under more severe conditions which promotes the enzymatic digestibility of cellulose fibers.

The DAP offers good performance in terms of recovering hemicellulose sugars but there are also some drawbacks. The dilute acid applied in the process could cause corrosion that mandates expensive materials of construction, such as hastelloy steel and ceramic valves. The neutralization of acid before the fermentation results in the formation of solid waste. In addition, the hemicellulose sugars might be further degraded to furfural and HMF, which are strong inhibitors to microbial fermentation [57]. Furthermore, most of the reported work used materials with significant size reduction, which consumes additional energy. Previous report indicated that grinding the materials to 1 mm accounted for 33 % of the power requirement of the entire process [58]. However, this is not practical in large-scale production. In addition, the detoxification step is required in DAP when running high solids pretreatment, which adds additional cost to the process.